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57 (2011) 1 3

Platnica SV-JME 3-2010_04.ai 1 9.3.2011 7:37:15

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Journal of Mechanical Engineering - Strojniški vestnik

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3 year 2011 volume 57 no.


Platnica SV-JME 3-2010_04.ai 2 9.3.2011 7:37:25

Strojniški vestnik – Journal of Mechanical Engineering (SV-JME) Aim and Scope The international journal publishes original and (mini)review articles covering the concepts of materials science, mechanics, kinematics, thermodynamics, energy and environment, mechatronics and robotics, fluid mechanics, tribology, cybernetics, industrial engineering and structural analysis. The journal follows new trends and progress proven practice in the mechanical engineering and also in the closely related sciences as are electrical, civil and process engineering, medicine, microbiology, ecology, agriculture, transport systems, aviation, and others, thus creating a unique forum for interdisciplinary or multidisciplinary dialogue. The international conferences selected papers are welcome for publishing as a special issue of SV-JME with invited co-editor(s).

Editor in Chief Vincenc Butala University of Ljubljana Faculty of Mechanical Engineering, Slovenia Co-Editor Borut Buchmeister University of Maribor Faculty of Mechanical Engineering, Slovenia Technical Editor Pika Škraba University of Ljubljana Faculty of Mechanical Engineering, Slovenia

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Editorial Office University of Ljubljana (UL) Faculty of Mechanical Engineering SV-JME Aškerčeva 6, SI-1000 Ljubljana, Slovenia Phone: 386-(0)1-4771 137 Fax: 386-(0)1-2518 567 E-mail: info@sv-jme.eu http://www.sv-jme.eu Founders and Publishers University of Ljubljana (UL) Faculty of Mechanical Engineering, Slovenia University of Maribor (UM) Faculty of Mechanical Engineering, Slovenia Association of Mechanical Engineers of Slovenia Chamber of Commerce and Industry of Slovenia Metal Processing Industry Association Cover: Acoustic Emission (AE) monitoring is a promising method of optimisation of the laser cutting process. The continuous AE signals captured during the laser cutting process and signals of AE bursts immediately after termination of laser cutting allow the estimation of laser cut quality defined by the state of the laser cut surface and the occurrence of dross at the lower cut edge. Image courtesy: Laboratory for Metals Testing and Heat Treatment, University of Ljubljana, Faculty of Mechanical Engineering

ISSN 0039-2480 © 2011 Strojniški vestnik - Journal of Mechanical Engineering. All rights reserved. SV-JME is indexed / abstracted in: SCI-Expanded, Compendex, Inspec, ProQuest-CSA, SCOPUS, TEMA. The list of the remaining bases, in which SV-JME is indexed, is available on the website. The journal is subsidized by Slovenian Book Agency.

International Editorial Board Koshi Adachi, Graduate School of Engineering,Tohoku University, Japan Bikramjit Basu, Indian Institute of Technology, Kanpur, India Anton Bergant, Litostroj Power, Slovenia Franci Čuš, UM, Faculty of Mech. Engineering, Slovenia Narendra B. Dahotre, University of Tennessee, Knoxville, USA Matija Fajdiga, UL, Faculty of Mech. Engineering, Slovenia Imre Felde, Bay Zoltan Inst. for Mater. Sci. and Techn., Hungary Jože Flašker, UM, Faculty of Mech. Engineering, Slovenia Bernard Franković, Faculty of Engineering Rijeka, Croatia Janez Grum, UL, Faculty of Mech. Engineering, Slovenia Imre Horvath, Delft University of Technology, Netherlands Julius Kaplunov, Brunel University, West London, UK Milan Kljajin, J.J. Strossmayer University of Osijek, Croatia Janez Kopač, UL, Faculty of Mech. Engineering, Slovenia Franc Kosel, UL, Faculty of Mech. Engineering, Slovenia Thomas Lübben, University of Bremen, Germany Janez Možina, UL, Faculty of Mech. Engineering, Slovenia Miroslav Plančak, University of Novi Sad, Serbia Brian Prasad, California Institute of Technology, Pasadena, USA Bernd Sauer, University of Kaiserlautern, Germany Brane Širok, UL, Faculty of Mech. Engineering, Slovenia Leopold Škerget, UM, Faculty of Mech. Engineering, Slovenia George E. Totten, Portland State University, USA Nikos C. Tsourveloudis, Technical University of Crete, Greece Toma Udiljak, University of Zagreb, Croatia Arkady Voloshin, Lehigh University, Bethlehem, USA President of Publishing Council Jože Duhovnik UL, Faculty of Mechanical Engineering, Slovenia Print Tiskarna Present d.o.o., Ižanska cesta 383, Ljubljana, Slovenia General information Strojniški vestnik – The Journal of Mechanical Engineering is published in 11 issues per year (July and August is a double issue). Institutional prices include print & online access: institutional subscription price €100,00, general public subscription €25,00, student subscription €10,00, foreign subscription €100,00 per year. The price of a single issue is €5,00. Prices are exclusive of tax. Delivery is included in the price. The recipient is responsible for paying any import duties or taxes. Legal title passes to the customer on dispatch by our distributor. Single issues from current and recent volumes are available at the current single-issue price. To order the journal, please complete the form on our website. For submissions, subscriptions and all other information please visit: http://en.sv-jme.eu/ You can advertise on the inner and outer side of the back cover of the magazine. We would like to thank the reviewers who have taken part in the peer-review process.

Strojniški vestnik - Journal of Mechanical Engineering is also available on http://www.sv-jme.eu, where you access also to papers’ supplements, such as simulations, etc.


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3 Contents

Contents Strojniški vestnik - Journal of Mechanical Engineering volume 57, (2011), number 3 Ljubljana, March 2011 ISSN 0039-2480 Published monthly

Editorial 167 Papers Igor Solodov, Daniel Döring, Gerd Busse: New Opportunities for NDT Using Non-Linear Interaction of Elastic Waves with Defects Philipp Menner, Henry Gerhard, Gerd Busse: Lockin-Interferometry: Principle and Applications in NDE Theodoros Hasiotis, Efstratios Badogiannis, Nicolaos Georgios Tsouvalis: Application of Ultrasonic C-Scan Techniques for Tracing Defects in Laminated Composite Materials Raimond Grimberg: Electromagnetic Nondestructive Evaluation: Present and Future Bernd Wolter, Gerd Dobmann, Christian Boller: NDT Based Process Monitoring and Control Gerhard Mook, Fritz Michel, Jouri Simonin: Electromagnetic Imaging Using Probe Arrays Antonios Kyriazopoulos, Ilias Stavrakas, Cimon Anastasiadis, Dimos Triantis: Study of Weak Electric Current Emissions on Cement Mortar under Uniaxial Compressional Mechanical Stress up to the Vicinity of Fracture Reinhard Danzl, Franz Helmli, Stefan Scherer: Focus Variation – a Robust Technology for High Resolution Optical 3D Surface Metrology Drago Bračun, Boštjan Perdan, Janez Diaci: Surface Defect Detection on Power Transmission Belts Using Laser Profilometry Jožef Horvat, Jurij Prezelj, Ivan Polajnar, Mirko Čudina: Monitoring Gas Metal Arc Welding Process by Using Audible Sound Signal

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, 167

Editorial

Special issue: Non destructive testing & evaluation Material inspection is an extremely important task in the modern industrial production, which requires the machine elements and parts of tools to be made with required properties and free of defects. The state-of-the-art in the production demonstrates an ever stronger computer support, providing for online monitoring of product quality and statistical processing of data on the quality of production over longer periods. The material inspection discipline includes destructive testing and in the last decades also non-destructive testing methods. Non-destructive testing of materials and structures is therefore gaining on significance both for the manufacturing of different parts and structures and for the testing of these same parts and structures during operation. The use of non-destructive methods is very significant for periodic testing of means of public transportation in the civil aviation and rail traffic, for nondestructive testing in thermal power stations, and especially for testing objects in nuclear power plants. Periodic inspections are often prescribed by the equipment manufacturer, who determines the necessary testing method, supported by corresponding devices and accessories. Today product quality assurance demands the product control to be aligned with the level of manufacturing. Automated manufacturing requires as thorough as possible control over the quality of materials entering the manufacturing process and as thorough as possible control over the state of materials during the manufacturing, meaning an uninterrupted production of high-quality products. We must therefore be aware that the introduction of automated and computer-supported manufacturing systems also requires automated material inspection, and that the detection of individual types of defects present in the material requires the selection of an appropriate method of inspection. The final decision to reject a workpiece is made on the basis of determined deviations from the required material characteristics and/or a detected defect in the material. After a defect is detected, it must be determined how serious it may be according to the criteria set for the acceptability or non-acceptability of a machine element. Various devices are available today for automated computer-supported detection and evaluation of defects in materials using penetrant, ultrasonic,

electromagnetic and radiographic examinations. These systems provide numerous advantages compared to the classic non-destructive material inspection methods, such as a considerably faster pace of testing, removal of subjective human operator bias during the testing of material properties and during the assessment of defect size or seriousness, providing for a reliable insight into the product quality. In the last decade, numerous nondestructive material testing methods have undergone intense development and gained a wide recognition, mainly due to the progress in the development of various types of sensors and accompanying computer support, including the visualisation of states of materials and defects in the materials. The current development of non-destructive material inspection methods is primarily influenced by the automation of manufacturing, which must guarantee production with no rejected parts. The process of automation runs aligned with the development and introduction of non-destructive material tests, owing to the synergic effects of development of sensor technology, electronics, microprocessor technology and computer technology. The basic characteristics of inspection devices are important for the individual non-destructive testing methods, enabling the determination of individual material characteristics as well as the detection of various types of detects with corresponding visualisation and registration. The articles presented in this special issue have been presented at the jubilee 10. International conference on non-destructive testing, which took place from September, 1st - 3rd 2009 in Ljubljana. 60 papers were presented at this occasion and the conference scientific board selected only 15 of them to undergo a repeated review, yielding articles in an extended and updated form. Ten articles are presented in this issue and the remaining five papers have been published in the regular issue of our Journal of Mechanical Engineering, vol. 56, no. 9 and 10, 2010; and vol. 57, no. 2, 2011, accessible at the journal web site (http://www.sv-jme.eu). Janez Grum, Guest editor 167


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, 169-182 DOI:10.5545/sv-jme.2010.168

Paper received: 22.08.2009 Paper accepted: 04.03.2010

New Opportunities for NDT Using Non-Linear Interaction of Elastic Waves with Defects Solodov, I. - Döring, D. - Busse, G. Igor Solodov - Daniel Döring - Gerd Busse* Institute of Polymer Technology (IKT), Non-Destructive Testing (IKT-ZfP), Stuttgart University, Germany

When material is subjected to an intense load, its stiffness changes due to elastic nonlinearity. This effect is especially pronounced in damaged materials, so that nonlinearity can be used as an indication of incipient damage. Under dynamic load, mechanical constraint between the fragments of planar defects provides gigantic nonlinearity in finite-amplitude contact vibrations. The local vibration spectra acquire a number of new frequency components which are used as signatures of damage. The experimental implementation of nonlinear NDT relies on the use of laser interferometers (Nonlinear Laser Vibrometry, NLV) to detect the nonlinear waves that are generated selectively by defects. In addition, the planar defects as localized sources of nonlinear vibrations efficiently radiate nonlinear airborne ultrasound (Nonlinear Air-Coupled Emission, NACE). The frequency conversion mechanism concerned with contact nonlinearity of the defect vibrations provides an efficient generation of air-coupled higher-order ultra-harmonics, ultrasubharmonics, and combination frequencies. Both the NLV and NACE are proposed for remote scanning and high contrast defect-selective imaging in nonlinear NDT. ©2011 Journal of Mechanical Engineering. All rights reserved. Keywords: Nonlinear NDT, defect-selective imaging, nonlinear air-coupled emission 0 INTRODUCTION Conventional ultrasonic Non-Destructive Testing (NDT) is normally based on the scattering of acoustic waves by defects leading to the amplitude and phase variations of the input signal due to the wave-defect interaction. The efficiency of the interaction depends on the size of the defects and a degradation of linear material properties (that is, a local stiffness) caused by the damage. For incipient damage, the contribution of both factors is negligible and determines unacceptably low sensitivity of the technique for NDT of this type of defects. The nonlinear approach to NDT (NNDT) makes use of the fact that the nonlinear part of the stress-strain diagram is a sensitive indicator of the presence of defects. The dynamic nonhookean behaviour of the material is concerned with non-linear response of defects, which is related to the frequency changes of the input signal. These spectral changes are caused by anomalously nonlinear local dynamics of defects of various scale and nature. Monitoring of the local nonlinearity makes NNDT defect-selective,

i.e. it reveals directly the vulnerable (faulty) areas within material or a product. In damaged materials, the nonlinear response is provided by the Contact Acoustic Nonlinearity (CAN) [1]: strongly nonlinear local vibrations of defects due to mechanical constraint of their fragments, which efficiently generate multiple ultra-harmonics and support multi-wave interactions. Another contribution to the nonlinear spectrum comes from resonance properties of planar defects [2]. Similar to the resonance behaviour of an air bubble in a liquid, vibrations of a certain mass of material around a cracked defect are managed by reduced stiffness which provides a specific characteristic frequency of the defect and brings the nonlinear resonance scenario into elastic wave interaction with defects. In this paper, basic mechanisms responsible for frequency conversion in the nonlinear elastic wave-defect interaction are reviewed and major features of CAN spectra are discussed. Experimental methodologies of nonlinear laser vibrometry (NLV) and nonlinear air-coupled emission (NACE) are introduced and used to study the nonlinear spectra of defects. Applications for defect-selective imaging and

*Corr. Author’s Address: Institute of Polymer Technology (IKT), Non-Destructive Testing (IKT-ZfP), Stuttgart University, Pfaffenwaldring 32, 70569 Stuttgart, Germany, gerd.busse@ikt.uni-stuttgart.de

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NNDT are demonstrated for a series of hi-tech materials and industrial components. 1 PHENOMENOLOGY OF HIGHER HARMONIC GENERATION VIA CAN The CAN generally manifests in a wide class of damaged materials and is caused by mechanical constraint between the fragments of planar defects which makes their vibrations extremely nonlinear. The CAN develops diversely for longitudinal and shear driving tractions which activate different mechanisms of nonlinearity. 1.1 “Clapping” Mechanism Consider a pre-stressed crack (static stress σ0) driven with longitudinal acoustic traction σ~ (Fig. 1) which is strong enough to provide clapping of the crack interface. The clapping nonlinearity is due to asymmetrical dynamics of the contact stiffness: the latter is, apparently, higher in a compression phase (due to clapping) than that for tensile stress when the crack is assumed to be supported only by edge-stresses. Such behavior of a clapping interface can be approximated by a piece-wise stress (σ)-strain (ε) relation [3]: σ = C [1 - H(ε) (DC/C)] ε ,

(1)

where H(ε) is the Heaviside unit step function; DC = [C - (dσ/dε)ε>0], and C is the intact material (linear) stiffness.

Fig. 1. Model of a clapping crack The bi-modular pre-stressed contact driven by a harmonic acoustic strain ε(t) = ε0cosνt is similar to a “mechanical diode” and results in a pulse-type modulation of its stiffness C(t) and a half-period rectified output (Fig. 2). Since C(t) is a pulse-type periodic function of the driving frequency ν (Fig. 2b), the nonlinear part of the spectrum induced in the damaged area (σNL(t) = DC(t)·ε(t)) contains a number of its higher harmonics nν (both odd and even orders) whose amplitudes are modulated by the sincenvelope function [4]: An = ΔCΔτε0[sinc((n+1)Dt) - 2cos(pDt) sinc(nDt) +sinc((n-1)Dt)],

where, Dτ = τ / T τ = (T / π)Arc cos(ε0 / ε0) is the normalized modulation pulse length. The spectrum of the nonlinear vibrations (2) is illustrated in Fig. 3a and contains a number of both odd and even higher harmonics arising simultaneously as soon as ε>ε0 (threshold of clapping). The sinc-modulation in Eq. (2) is amplitude dependent: as the wave amplitude

Fig. 2. a) Mechanical diode model, b) stiffness modulation and waveform distortion 170

(2)

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Fig. 3. a) CAN higher harmonic spectrum, b) dynamic characteristics ε0 increases, τ grows from 0 to T/2. This affects dynamic characteristics of the higher harmonics (Fig. 3b) and provides their amplitude oscillations for ε0>ε0 due to the DC(t) spectrum “compression” effect. 1.2 Nonlinear Friction Mechanisms [4] For a shear wave drive, the surfaces of the contact interface are mechanically coupled by the friction force caused by the interaction between asperities (Fig. 4). If the driving amplitude is small enough, the interface shear motion is constrained by the interaction between neighboring asperities which prevents the contact surfaces from sliding (micro-slip mode). The mechanical diode model for the micro-slip motion is shown in Fig. 5a and demonstrates a step-wise increase in tangential stiffness as the neighboring asperities interact. This interaction is independent of the direction of shear motion and causes stiffness variation twice for the input signal period (Fig. 5b). Similar to the hysteresis case, such a constraint introduces a symmetrical nonlinearity and provides only odd harmonic generation. Similar to the clapping mechanism, their amplitudes are sinc-modulated due to pulse-type stiffness variation [5] (Fig. 6): τ A2 N +1 = 2∆Cε 0  T   2 Nτ sinc   T 

   2( N + 1)τ     + sinc T  

and exhibit a similar non-power dynamics.

(3)

Fig. 4. Crack interface in shear drive When the amplitude of tangential traction is greater than the contact static friction force, the micro-slip motion changes for sliding. An oscillating shear wave drive is accompanied by a cyclic transition between static and kinematic friction (stick-and-slide mode), so that the contact stress-strain relation follows a hysteretic loop [4]. The contact tangential stiffness changes symmetrically (independent of the direction of motion) between the static (for a stick phase) and dynamic values (in slide phase) twice over the input strain period which provides odd higher harmonics generation. Similarly, the CAN features sinc-spectrum modulation and non-power dynamics. 2 EXPERIMENTS ON HIGHER HARMONIC GENERATION BY DEFECTS Some experimental results which confirm the CAN spectral properties are presented in Figs. 7 to 9. In the experiments, the methodology of NLV [6] that includes an intense CW acoustic excitation (flexural waves) in the kHz-frequency range combined with a laser probing of the spectra

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Fig. 5. a) Mechanical diode model, b) stiffness modulation and waveform distortion in micro-slip mode and temporal vibration patterns was used. The specimen of multi-ply carbon-fibre-reinforced plastic (CFRP) with an impact damage area (Fig. 7a) was measured.

Fig. 6. Sinc-modulated odd higher harmonic CAN spectrum in micro-slip mode

a)

The temporal vibration pattern measured outside the damage is a sinusoidal signal with a negligible distortion (Fig. 7b). The waveform changes dramatically in the damaged area: the half-period rectified signal is observed (Fig. 8b) in perfect agreement with the diode model predictions for the clapping CAN mechanism. The higher harmonic spectrum (Fig. 8a) exhibits an evident sinc-amplitude modulation which is also typical for the clapping contact. The mechanism of friction nonlinearity was found to prevail in wood which is a natural fibre-reinforced composite. In intact wood, the nonlinear spectrum averaged over the specimen surface exhibits an evident odd harmonic domination (Fig. 9a). Then, the specimen was damaged (crack made by cutting), and the spectrum measured in the neighborhood of the

b)

Fig. 7. a) CFRP-specimen with impact damage area, b) temporal vibration pattern outside damage 172

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a)

b)

Fig. 8. a) Higher harmonic spectrum, b) temporal waveform pattern in the damaged area a)

b)

Fig. 9. a) Odd, b) odd/even higher harmonic spectrum in intact and damaged wood respectively crack acquires even higher harmonics (Fig. 9b) obviously generated due to contact clapping in the crack. The change of type nonlinearity enables, therefore, the use of the harmonic signature for discerning and imaging flaws in wood and wood composites [7]. 3 ULTRA-SUBHARMONIC AND ULTRAFREQUENCY PAIR NONLINEAR MODES 3.1 Phenomenology In addition to the higher harmonic generation, the experiments [2] and [8] also revealed different scenarios of CAN dynamics, which expand considerably nonlinear spectra of cracked defects. These scenarios exhibit forms of dynamic instability, i.e. an abrupt change of the output for a slight variation of the input parameters. To illustrate the feasibility of the new nonlinear vibration modes and ascertain their basic spectral patterns, it is assumed that the crack exhibits both resonance and nonlinear

properties and is thus identified as a nonlinear oscillator [2]. Its characteristic frequency (w0) is determined by a linear stiffness and an associated mass of the material inside the damaged area (Fig. 10). The contact nonlinearity is introduced as displacement (X) dependent nonlinear interaction force FNL (X). The driven vibrations (driving force t(t) = f0 cos νt)) of the nonlinear oscillator are found as a solution to the nonlinear Eq.: X + ω02 X = f (t ) + F NL ( X ) .

(4)

In the second order of the perturbation approach FNL ~ cos(ν ‒ ω0)t that accounts for the interaction between driving and natural frequency vibrations. If ν ‒ ω0 ≈ ω0, the resonance increase in the output at ω0 ≈ ν / 2 is observed (subharmonic generation). The higher-order terms correspond to the frequency relation mν ‒ nω0 that provides resonance output at ω0 ≈ mν / (n+1). For n = 1, the crack generates ultra-subharmonics (USB) of

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the second order mν / 2; the higher order USB correspond to the higher values of n.

Fig. 10. Resonance model of a clapping crack

Fig. 11. Crack as a set of coupled nonlinear oscillators In reality, a damaged area has a more complicated structure that can be conceived as a set of coupled nonlinear oscillators (a pair of those with normal frequencies wα and wβ is shown in Fig. 11). If the frequency of the driving acoustic wave is ν ≈ ωα + ωβ, the difference frequency components ν ‒ ωα ≈ ωβ and ν ‒ ωβ ≈ ωα provide cross excitation of the coupled oscillators. It results in a resonant generation of the frequency pair ωα , ωβ centred around the subharmonic position. The higher-order nonlinear terms in Eq. (4) expand the CAN spectrum, which comprises multiple ultra-frequency pairs (UFP) centred around the higher harmonics and USB [2]. USB and UFP belong to the class of the instability modes and can be interpreted, respectively, as a half-frequency and combination frequency decay of a high-frequency phonon (driving frequency signal). The resonance growth 174

of these modes is possible only if the input excitation exceeds a certain threshold; it is then affected by the amplitude and frequency instability and hysteresis [9]. The resonance instability manifests itself in the avalanche-like amplitude growth beyond the input threshold. The reverse amplitude excursion results in bistability: the input amplitudes for the up and down transitions are different (amplitude hysteresis). Such a dynamics is a distinctive signature of the nonlinear acoustic phenomena associated with nonlinear resonance. 3.2 Experimental Observation of USB and UFP Spectra Examples of the USB and UFP spectra observed in damaged materials by using NLV are shown in Figs. 12 and 13. The USB spectrum in Fig. 12 is measured in a cracked area of a polystyrene plate driven at ~1.3 kHz with a shaker. The higher harmonic pattern changes abruptly for the USB spectrum as the driving amplitude grows beyond a certain threshold value. The “wavy” amplitude modulation in Fig. 12 indicates the involvement of the CAN mechanisms in USB generation. Fig. 13 shows a section of the nonlinear spectrum measured in a glass-fibre reinforced composite (GFRP) with an impact damage for a 20 kHz excitation beyond the UFP-threshold. The positions of the second (40 kHz) and third (60 kHz) harmonics as well as ultra-subharmonics (50 and 70 kHz) can be clearly identified. The UFP lines are centred around the USB positions and distanced by D @ 1.2 kHz. The UFP signals with larger D and smaller amplitude are also seen in Fig. 13. The dynamic properties of the nonlinear resonance modes are illustrated in Fig. 14a. The figure shows the amplitude of the 3ω  /  2 -subharmonic wave generated in the reflection of 30-MHz acoustic waves from a crack in LiNbO3 crystal [8], as a function of the input voltage. The step-like thresholds ((VIN)1 ≥ 3V; (VIN)2 ≥ 4V) followed by the stable plateaus can clearly be seen. The sharp increase at the thresholds confirms a transition into the instability region where the avalanche-like development of nonlinear oscillations takes place. The hysteresis

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Fig. 12. USB spectrum in cracked area of polystyrene plate; the arrow indicates driving frequency

Fig. 13. UFP spectrum in impact damaged area of GFRP-specimen;driving frequency is 20 kHz a)

b)

Fig. 14. a) Threshold and bistability of USB mode, b) schematic of nonlinear dynamics of a cracked defect presented as a pair of coupled nonlinear oscillators of the curves in Fig. 14a is the evidence of bistability in the crack signature [8]. The experimental results on the nonlinear dynamics of fractured flaws are summarized schematically in Fig. 14b for a defect represented by a pair of coupled oscillators (normal frequencies w1 and w2) [10]. At low amplitudes of the driving excitation (frequency ν), the nonlinear

spectrum follows the non-resonant scenario of the previous section and comprises the higher harmonic (HH) and the wave modulation (WM) frequency components. As the input amplitude exceeds the threshold value, resonance instability generally results in the activation of the USB components first. The threshold amplitude depends on the driving frequency: a minimal

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threshold requires frequency matching to the main subharmonic resonance (ν = 2ω). The frequency zones for the USB generation expand readily as the excitation amplitude increases. A further increase of acoustic excitation above a given threshold gives rise to the UFP spectra. A direct transition from the non-resonance modes to the UFP-instability is also possible when the sumfrequency resonance matching conditions are satisfied. The V-shaped zones in Fig. 14 are typical of parametric resonance modes [9] and indicate that the frequency matching is not required for high driving amplitudes. Finally, the multiple UFP bring the system to a quasi-continuous spectrum, which indicates a build-up of chaotic vibrations. 4 CAN APPLICATIONS IN NDT: NLV FOR DEFECT-SELECTIVE IMAGING The nonlinear spectra discussed above are produced locally in the damaged area while an intact part of material outside the defects vibrates linearly, i.e. with no frequency variation in the output spectrum. Thus, nonlinear defects are active sources of new frequency components rather than passive scatterers in conventional ultrasonic testing. This makes nonlinearity a a)

defect-selective indicator of damage presence and development. The high localization of nonlinear spectral components around the origin is a basis for nonlinear imaging of damage. The NLV [6] uses a sensitive scanning laser interferometer for detecting nonlinear vibrations of defects. The excitation system includes piezo-stack transducers operating at 20 and 40 kHz. After a 2D-scan and FFT of the signal received, the C-scan images of the sample area are obtained for any spectral line within the frequency bandwidth of 1 MHz. 4.1 Higher Harmonic Imaging Higher harmonics in damaged materials are readily generated at reasonably small input amplitudes (strain ~ 10-7 to 10-6) (Fig. 14b), so that even internal or other low-amplitude vibration sources provide enough energy for the higher harmonic imaging. The internal excitation activates a clapping mechanism of higher harmonic generation in a number of mechanical units which use gear boxes and push-pull components. In many cases, the presence of clapping may indicate a faulty part of an instrument. Fig. 15 [11] illustrates the feasibility b)

Fig. 15. a) Linear and b) third harmonic NLV-images of saw-cutting tool

Fig. 16. Fundamental frequency (w) and higher harmonic imaging of a delamination in a “smart” structure 176

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“dry-contact” portable piezoelectric transducers attached to the specimen by vacuum suction.

of detection of the clapping components in a commercial saw-cutting tool. The 3D harmonic image of the motor rotation frequency clearly locates the position of the worm-gear transmission (Fig. 15b) which moves in a nonlinear (clapping) way. Fig. 16 shows imaging results for an oval delamination on top of a piezo-actuator embedded into a glass fibre-reinforced composite (GFRP). Such “smart” structures are likely to be used for active structural health monitoring of aerospace components. The actuator itself was used as an internal excitation source fed with a few volt input. The higher harmonic images selectively reveal the boundary ring of the delamination where clapping and rubbing of the contact surfaces are, apparently, expected. On the contrary, the driving frequency (50 kHz) image indicates only a standing wave pattern over the area of the actuator. A flexible operation in the higher harmonic mode can be achieved even for large industrial parts by using portable ultrasonic transducers. The example in Fig. 17 shows nonlinear imaging of a fatigue crack in a riveted aviation component. The 20-kHz excitation was implemented with a)

4.2 Nonlinear Imaging in USB- and UFP-Modes [10] The input acoustic power for the USB- and UFP-modes is somewhat higher than that for the higher harmonic experiments. In practical terms, this requires an acoustic intensity of a few W/cm2 in the high-MHz-frequency range and the driving amplitudes of μm-scale in the low-kHz range. A strong increase of the instability modes beyond the threshold leads to distinctive nonlinear spectra with multiple USB- and UFP-components located exclusively in damaged areas. A few examples of nonlinear NDE using these modes are given below to demonstrate their applicability and superior performance in cases where the data obtained by non-resonant modes are insufficient. Fatigue loads in metals (rotors, turbines, etc.) cause minute cracks of micro-meter scale which may gradually develop into major cracking and initiate an abrupt material fracture. Linear ultrasound is virtually unable to detect a fatigue

b)

Fig. 17. a) Higher harmonic imaging of a fatigue crack in aviation component, b) excitation with portable ultrasonic transducers a)

b)

Fig. 18. a) crack photo, b) USB-image of 5 μm-wide fatigue crack in Ni-base super-alloy New Opportunities for NDT Using Non-Linear Interaction of Elastic Waves with Defects

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crack at the early stage of their development. Examples of nonlinear imaging of fatigue induced micro-flaws and degradation of metal microstructure using the USB-modes are given in Figs. 18 and 19. Fig. 18 shows fatigue cracking produced by cyclic loading in Ni-base super-alloys. Such a crack of ~ 1.5 mm length, with average distance between the edges of only ≈ 5 mm, is clearly detected in the USB-image (7ν/2), whereas traditional linear NDE by using slanted ultrasonic reflection failed to work with such small cracks. Plastic deformation is known to change the metal micro-structure by generating clusters of dislocations, which are the forerunners of micro-cracking. The feasibility of NNDE for such delicate defects in the USB-mode is illustrated in Fig. 19 for a steel auto-component (diameter 0.8 cm; length 6.5 cm) subject to 6% tensile deformation. Some necking initiated in its central part indicates that the plastic deformation was mostly concentrated in its central part. The nonlinear image reveals the area of deteriorated material properties by an evident increase in the USB amplitude. Fiber reinforced composites constitute a class of hi-tech engineering materials whose application area is rapidly expanding in aerospace and automotive industries. Fibre metal laminates

Fig. 19. USB-image of plastic deformation area in a steel component are new materials with excellent tolerance to impact, corrosion and lightning stroke, low flammability and low weight. Fig. 20 displays an example of NNDE of such an advanced material for aircraft industry: glass fibre reinforced aluminium laminate (Glare®). More specifically, it shows the USB-images of a Glare® plate with two inserted circular Teflon-foils to simulate local delaminations. In the images, the defect can be recognized fairly well, and the quality is enhanced for the higher orders of the USB. We believe that the latter is associated with a peculiar distribution of subharmonics over the delamination area and stronger acoustic dissipation outside the defect at higher frequencies.

Fig. 20. USB-images of a pair of simulated delaminations in a Glare® plate a)

b)

c)

Fig. 21. a) Nonlinear imaging of impact damage in GFRP, b) linear (20 kHz-image), c)5th harmonic image UFP – image 178

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Mechanical impacts in multi-ply composites produce a fracture which is a combination of matrix-fibre debonding, cracking, and delaminations. Such a combination of fractured defects makes the impact area strongly nonlinear and, normally, all nonlinear modes can be traced in the spectra observed. Similarly to all nonlinear modes discussed, the UFP-components generally display a strong spatial localization around the defects and are applicable for the detection of damage. The benefit of the UFP-mode is illustrated in Fig. 21 for a 14-ply epoxy based glass-fiber reinforced composite (GFRP) with a 9.5 J-impact damage. The linear image taken at the driving frequency of 20 kHz reveals only a standing wave pattern over the whole sample (Fig. 21a). The higher harmonic image is also corrupted by the standing wave pattern (centre), whereas the image at the first UFP-side-lobe of the 10th harmonic of the driving frequency (198.8 kHz) yields a clear indication of the damaged area (Fig. 21b). An example of nonlinear monitoring of macro-defects in constructional materials is

shown in Fig. 22b for a slab of GFR-concrete (15 x 30 x 1.5 cm). Production technology of this material often suffers from internal delaminations induced by fibre-matrix debonding. For an intense 20 kHz-excitation, multiple UFP-components were measured in vibration spectra which clearly indicate the delamination areas (dark, Fig. 22a).

a)

b)

5 DEFECT-SELECTIVE IMAGING VIA NONLINEAR AIR-COUPLED EMISSION (NACE) The scanning laser vibrometer suffers from variation of optical reflectivity; e.g. the measurements fail in damaged areas with a particularly strong scattering of laser light. Our experiments have demonstrated that planar defects as localized sources of nonlinear vibrations efficiently radiate nonlinear airborne ultrasound. Such a nonlinear air-coupled emission (NACE) is proposed as an alternative (and in many cases superior) methodology to locate and visualize defects in NNDE [12].

Fig. 22. UFP-image; b) of delamination area (dark) in a) GFR-concrete slab a)

b)

Fig. 23. a) Third harmonic air-borne radiation from delamination area, b) set-up for NACE NNDE New Opportunities for NDT Using Non-Linear Interaction of Elastic Waves with Defects

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a)

b)

Fig. 24. Nonlinear imaging of an impact induced damage in multi-ply (+450; -450) GFR-plate: a) laser vibrometry (second harmonic image), b) NACE (9th-11th) higher harmonic image a)

b)

Fig. 25. NACE image (b) of 5 x 15 mm delamination (a) in GFRP multi-ply plate a)

b)

Fig. 26. NACE imaging in steel specimens (40 kHz-excitation): (9th to 11th) harmonic imaging of 50 μma) wide fatigue crack, b) (5 × 40 mm) hammer peening area in steel plate a)

b)

Fig. 27. a) photo of the measured laser welded joint in steel, b) NACE image of the welded join 180

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The feasibility of NACE by nonlinear defects is illustrated in Fig. 23a, which shows the third harmonic airborne radiation field from the delamination area in the “smart structure” specimen for piezo-excitation at 50 kHz and 20 Vrms input. The image obtained by using air-coupled vibrometry [13] indicates that the source of nonlinear radiation is strictly confined in the defect area and NACE exhibits an evident directivity pattern. The practical version of the NACE capable of locating and imaging the defects uses a highfrequency focused air-coupled (AC-) ultrasonic transducer as a receiver (Fig. 23b) [14]. Its bandpass frequency response combined with high-pass filtering circuit provides a strong rejection of the low-frequency excitation signal. The altitude angle varies to match the radiation directivity pattern. The nonlinear spectral components are then used as an input to AC-scanning equipment (AirTech 4000) for computer imaging of the NACE amplitude distribution over a specimen surface (C-scan). In the experiments, a pair of focused (focus spot ~2 to 3 mm, focus distance 40 mm) AC-transducers with frequency responses centred at ~400 kHz and ~450 kHz (3 dB-bandwidth of ~20 kHz) were alternately used as receivers in the C-scan mode. Two excitation frequencies around 40 and 20 kHz were used to maximize the NACE around (9th to 11th) and (23rd to 24th) higher harmonics, respectively. Unlike NLV, which analyzes the light reflected from the specimen, the NACE-imaging relies on the direct nonlinear acoustic radiation by the defects. For the weakly-focused ACtransducers with cm-range depth of focus, the receiver is insensitive to medium scale variations of the surface profile. Our experiments show that NACE operates well in various constructional materials (wood, concrete, metals) with raw surfaces and rugged defects in components, like bolted joints, arc welds, etc. In Fig. 24, the NACE imaging results are compared with NLV of multiple impact damage on the rear surface of a carbon fibre-reinforced (CFR-) multi-ply (+45°; -45°) composite plate (175 x 100 x 1 mm). Both techniques reliably visualize the defects with similar sensitivity.

The high lateral resolution of NACE is illustrated in Fig. 25 which shows a photo (a) of (5 x 15 mm) delamination of specific shape in multiply GFRP plate. The mm-size contour details are reproduced closely in (9th to 11th) NACE image (right). Fig. 26a shows the (9th to 11th) harmonic NACE image of a 50 μm-wide fatigue crack in a steel plate (150 × 75 × 5 mm) with two horizontally located grip holes for cyclic loading at some distance from the crack. The image reveals that the NACE detects not only the crack itself but also the fatigue structural damage in the plasticity areas between the crack and the grip holes. To verify the NACE sensitivity to microdamage induced by plastic deformation, the NACE inspection was implemented for a steel specimen with a cold work area (5 × 40 mm) produced by hammer peening. The image in Fig. 26b confirms that the NACE develops even without serious cracked defects and clearly discerns the microdamage induced by plastic deformation. The NACE NDE-application was found to be particularly beneficial in metallic components where low acoustic damping facilitates the formation of standing waves, which produce a strong spurious background in the NLV. In particular, the images in Fig. 27 show that the NACE pattern reproduces well the quality of a laser weld-line between two steel components. 6 SUMMARY The interaction of an ultrasonic wave with fractured defects is accompanied by frequency conversion, which is determined by nonlinear contact dynamics and strongly depends on the amplitude of the acoustic wave. At moderate driving amplitude, the CAN suggests a fully deterministic scenario with higher harmonic generation and/or wave modulation. These effects feature anomalously high efficiency, specific dynamic characteristics, modulated spectra, and unconventional waveform distortion. For the higher excitation, the family of nonlinear contact phenomena includes generation of ultrasubharmonics and frequency pair components, hysteresis, instabilities, transition to chaos, etc.

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that are well known in other branches of nonlinear physics. The intact parts of the material outside the defect vibrate linearly, i.e. with no frequency variation in the output spectrum. Thus, in NNDT, a small cracked defect (transparent in the linear ultrasonic NDT) behaves as an active radiation source of new frequency components rather than a passive scatterer in conventional (linear) ultrasonic testing. This makes NNDT a unique defectselective instrument for localising and imaging of nonlinear flaws. The latter include a numerous class of contact defects, scaled from dislocations (nano-scale) to fatigue (micro-) cracks and macro-debonds in joints. Since the micro-contact (nonlinear) defects are only the forerunners of further major damage, the NNDT is thus capable of early recognition of material degradation and “predicting” the oncoming fracture. Numerous case studies demonstrate the NNDE and defect-selective imaging by using scanning NLV and NACE. Particularly successful examples include hi-tech and constructional materials: impact damage and delaminations in fibre-reinforced plastics, fatigue micro-cracking and cold work in metals, delaminations in fibrereinforced metal laminates and concrete. Multiple nonlinear frequency components generated in imperfect materials provide abundant information on the properties and location of defects. 7 REFERENCES [1] Solodov, I.Y. (1998). Ultrasonics of nonlinear contacts: propagation, reflection and NDE-applications. Ultrasonics, vol. 36, no. 1-5, p. 383-390. [2] Solodov, I., Wackerl, J., Pfleiderer, K., Busse, G. (2004). Nonlinear self-modulation and subharmonic acoustic spectroscopy for damage detection and location. Appl. Phys. Lett., vol. 84, no. 26, p. 5386-5388, doi:10.1063/1.1767283. [3] Solodov, I.Y., Krohn, N., Busse, G. (2002). CAN: An example of nonclassical acoustic nonlinearity in solids. Ultrasonics, vol. 40, no. 1-8, p. 621-625. [4] Pecorary, C., Solodov, I. (2006). Nonclassical nonlinear dynamics of solid interfaces in partial contact for NDE 182

[5]

[6]

[7]

[8]

[9] [10]

[11]

[12]

[13]

[14]

applications. Universality of Non-Classical Nonlinearity with Application to NDE and Ultrasonics, Delsanto, P. (ed.), Chapter 19, p. 307-324, Springer Verlag, New York. Pfleiderer, K. (2006). Frequenzkonversion aufgrund nichtlinearer akustischer Phänomene: Grundlagen und Anwendung zur defektselektiven zerstörungsfreien Prüfung. Ph.D. Thesis, Stuttgart University, Sttutgart. Krohn, N., Pfleiderer, K., Solodov, I., Busse, G. (2002). Rev. Progress, QNDE, Thompson, D.O., Chimenti, D. (eds.) vol. 22, p. 981-988. Solodov, I., Pfleiderer, K., Busse, G. (2004). Nondestructive characterization of wood by monitoring of local elastic anisotropy and dynamic nonlinearity. Holzforschung, vol. 58, no. 3, p. 504-510. Solodov, I., Korshak, B. (2002). Instability, chaos, and “memory” in acoustic wavecrack interaction. Phys. Rev. Lett., vol. 88, 014303, p. 1-3. Minorsky, N. (1962). Nonlinear Oscillations. D. Van Nostrand Co. Inc., Princeton. Solodov, I., Pfleiderer, K., Busse, G. (2006). Nonlinear acoustic NDE: inherent potential of complete non-classical spectra. Universality of Non-Classical Nonlinearity with Application to NDE and Ultrasonics, Delsanto, P. (ed.), Chapter 29, p. 465-484, Springer Verlag, New York. Solodov, I., Pfleiderer, K., Busse, G. (2006). Nonlinear acoustic NDE using complete non-classical spectrum. Innovations in Nonlinear Acoustics, 17th ISNA, AIP, p. 35Solodov, I., Busse, G. (2007). Nonlinear aircoupled emission: the signature to reveal and image micro-damage in solid materials. Appl. Phys. Lett., vol. 91, vol. 91, 251910, 2007. Solodov, I., Döring, D., Busse, G. (2009). Air-coupled laser vibrometry: analysis and applications. Appl. Optics, vol. 48, no. 7, p. C33-C37. Solodov, I., Busse, G. (2008). Listening for nonlinear defects: A new methodology for nonlinear NDE. Nonlinear Acoustics – Fundamentals and Applications, 18th ISNA, AIP, p. 569-573.

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, 183-191 DOI:10.5545/sv-jme.2010.169

Paper received: 22.08.2009 Paper accepted: 04.03.2010

Lockin-Interferometry: Principle and Applications in NDE Menner, P. - Gerhard, H. - Busse, G. Philipp Menner - Henry Gerhard - Gerd Busse* Institute of Polymer Technology (IKT), Non-Destructive Testing (IKT-ZfP), Stuttgart University, Germany

Interferometry is relevant for non-destructive evaluation (NDE) since dimensional changes much smaller than an optical wavelength result in detectable signals. Fringe images obtained with ElectronicSpeckle-Pattern-Interferometry (ESPI) or shearography display changes of surface topography between two states of an object, usually using a static load. Usually, hidden defects are found by comparing the observed fringe pattern to the one obtained on an intact reference component and to attribute observed differences to a defect. Our approach is a periodical object illumination with light that is absorbed in the surface to generate heat and a corresponding modulation of thermal expansion. At the same time fringe images are recorded (either with ESPI or shearography) to give a stack. Subsequently, each image is unwrapped and thereafter the time-dependent content of each pixel is Fourier transformed at the excitation frequency, so the result is local amplitude and phase of the modulated response at this frequency. The phase image displays local delay between excitation and response. This phase change depends on the depth where the defect is located since thermal waves are involved. In this paper, NDE-examples obtained using this new technique are presented. It is also shown how the achieved improvement as compared to conventional interferometry is up to an order of magnitude. ©2011 Journal of Mechanical Engineering. All rights reserved. Keywords: Lockin-ESPI, Lockin-Shearography, defect-selective imaging, interferometric depth profiling

0 INTRODUCTION Shearography is a speckle-interferometrical method which measures the derivative of object deformation along a certain (“shearing”) direction. It compares two different object states by superposition of the speckle pattern of the sample, which is a pattern of bright and dark spots (“speckle”) that appear when an optically rough surface is illuminated with coherent light. This pattern is a fingerprint of the surface, i.e. changes of the surface (caused by loading the object) affect the speckle pattern. A comparison of the two speckle pattern gives an image with which local strain changes can be revealed. The object is loaded by mechanical force, by variation of internal or ambient pressure (e.g. of tires, pipes etc.), or by heating of the object surface. The latter can be done easily with spotlights: The light of the lamps is absorbed at the surface, and the induced temperature gradient causes buckling of the object. As defects change the local mechanical

properties, they can be detected as a result of the locally changed deformation. 1 METHOD: THEORY AND EXPERIMENTAL SETUP 1.1 Conventional Shearography The test object is illuminated by coherent laser light which is scattered at the object surface and passes a shearing device (a modified Michelson setup). As one of the mirrors is slightly tilted, a light ray from one point of the object surface is superimposed to the light ray from a neighbor point. The resulting intensity of the superposition of the two light waves depends on their relative phase difference, which changes when the object is deformed. Since two neighbouring points are compared, the measurement does not show the deformation itself, but its derivative in the direction where the mirror is tilted (“shearing direction”) [1]. By superposition of the speckle patterns of two different object states (e.g. by

*Corr. Author’s Address: Institute of Polymer Technology (IKT), Non-Destructive Testing (IKT-ZfP), Stuttgart University, Pfaffenwaldring 32, 70569 Stuttgart, Germany, gerd.busse@ikt.uni-stuttgart.de

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pixelwise subtraction), a fringe pattern that reveals local strain changes appears. By temporal phase shifting, the contrast of the fringe pattern can be improved significantly, and it allows revealing the sign of deformation. Shearography is less sensitive to vibrations than other interferometrical methods (e.g. ESPI) and can therefore be used in industrial environments, but it still cannot distinguish defects lying in different depths, and suffers from detecting the deformation of the whole test object, which can hide defects. Fig. 1 illustrates the optical setup of shearography.

Fig. 1. Setup of a conventional shearography system 1.2 Optically Excited Lockin Shearography Optically excited Lockin shearography is based on the temporal deformation of an object which is illuminated with intensity modulated lamps. Due to absorption of this modulated radiation, the surface temperature changes periodically and drives periodical heat diffusion whose propagation into the interior of the object is described by a “thermal wave”. At thermal boundaries, the waves are reflected back to the surface where they are superimposed to the initial thermal wave. In this way, the local phase angle and amplitude of the modulated temperature field change. The defect-induced change of deformation, which is oscillating at the frequency of the lamp modulation, is the effect that is analysed with this method (Fig. 2). The principle of signal evaluation of an image stack is well known from optically excited 184

Lockin thermography [2] and [3] and could be transferred successfully to the ESPI method [4] and [5] and to shearography [6]. As it was expected from our experience with the other Lockin methods, thermal phase angle images ‒ which do not show the height of an effect, but its local temporal delay with respect to the modulated excitation – are very insensitive to external perturbations. This robustness is based on an internal normalization: the phase angle image results from a Fourier transformation which results in a ratio, so that many artefacts cancel each other. The idea is illustrated by a simplified scheme of a Lockin measurement (Fig. 3).

Fig. 2. Setup of Lockin shearography The test object is optically excited by intensity modulated lamps (I) while the camera continuously records images of the speckle pattern of the object surface (II). In order to determine the optical phase distribution, a piezo actuator changes the optical path length of one of the two partial images by π/4 between the images. A computer calculates optical phase images out of every four speckle pattern images in real time (III). The first optical phase image is used as a reference and is being subtracted from all subsequent optical phase images. This temporal phase shifting results in fringe images with a much higher contrast. In Fig. 3 (III), there are four optical phase images per excitation period. In practice, up to 100 or more can be obtained. A phase unwrapping algorithm converts the fringe pattern into the height profile of the deformation gradient (IV). This stack of demodulated optical phase images contains the information of how the deformation gradient of each pixel changes in time at the modulation frequency (V). To extract this information, the

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image stack at each pixel is analyzed for temporal signal changes at the modulation frequency of excitation, while all other changes in the signal are ignored. This filtering is done at each pixel along a vertical column of the image stack (“pixel rod”) using a discrete Fourier transformation at the excitation frequency (VI). In this way, the information that is contained in the whole image stack is finally compressed to only two images (VII): the Lockin amplitude image (showing the local height of the modulation effect) and the Lockin phase image, displaying the local thermal phase delay between excitation and object response. It is crucial to distinguish between optical phase images and thermal Lockin phase images: optical phase images are interferometric high-contrast images which are obtained by temporal phase shifting, i.e. by systematic changes of optical path lengths and therewith the phase difference of two light waves. These are the images that the image stack consists of. The

Lockin phase image, however, is the result of temporal analysis of the image stack, i.e. it shows the phase angle for each pixel, which corresponds to the local temporal delay of modulated heat flow between excitation and object response. Therefore, this Lockin phase image is the final result, which contains information from the whole image stack. Fig. 4 illustrates the signal at one pixel both along the time axis and in the frequency domain. The sine modulation of the deformation gradient is seen superimposed to the generally increasing signal, which reaches a plateau towards the end (Fig. 4a). The spectrum of this signal shows two peaks (Fig. 4b). The left peak (at the minimal frequency) is related to the slow warming of the object because the heat that is induced by the lamps cannot be dissipated to the surrounding of the object rapidly enough; it needs some time until a stationary state is reached. The second peak is related to the modulation of the lamps at the excitation (“Lockin”) frequency. At this

Fig. 3. Procedure of a Lockin measurement

a)

b) Fig. 4. Signal at one pixel rod; a) time domain, b) frequency domain Lockin-Interferometry: Principle and Applications in NDE

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frequency, the discrete Fourier transformation is performed. Due to this spectral separation the overall deformation of the object, caused by the slow warming, is eliminated, so that only signal changes matching to the modulation frequency contribute to the phase image. The excitation frequencies used in this paper are usually below 1 Hz, so the deformation is far away from any mechanical resonance, and there are no inertia effects involved. Therefore, all areas of the object are deforming simultaneously (with a constant temporal delay to the excitation), so the phase angle is constant all over the object, except in case of a defect. The region around the defect is deforming periodically as well, but with a different temporal delay, caused by the reflection of the thermal wave. In this way, defects can be detected very easily, since the intact areas of the object give a constant background, which makes the method defect selective (Fig. 5).

To avoid misunderstanding, it needs to be emphasised that this method differs substantionally from the well-known interferometrical methods used for vibration analysis. While such methods use elastic waves and show vibration nodes, optically excited Lockin Shearography is based on dynamic heat conduction (thermal waves), and at much lower frequencies. It shows local thermal expansion, so the contrast mechanism (the physical principle) is completely different. 1.3 Experimental Setup For our Lockin shearography system (Fig. 6), a conventional out-of-plane sensor head is used. An array of up to 16 laser diode modules illuminates the sample. The lasers are mounted variably on four arms, so that the object can be illuminated homogeneously, and the sensitivity vector is constant. For thermal excitation, up to four 1 kW spotlights with filters can be used. Disturbances due to convection of warm air are minimized by applying a steady air flow.

Fig. 5. Influence of defects on the thermal phase angle under periodical deformation Another advantage of Lockin shearography is also directly linked to the use of thermal waves. The thermal diffusion length μ, which is the depth to which the amplitude of the thermal wave has decreased 1/e (about 37%), depends on the excitation frequency:

µ=

2λ , ω ⋅ ρ ⋅ cp

(1)

where λ is thermal conductivity; ρ is density; cp is specific heat capacity and ω is excitation frequency. The compression of the image stack to an amplitude- and a phase image via Fourier transformation complies with a weighted averaging, which results in an increased signal-tonoise ratio. 186

Fig. 6. Lockin shearography system 2 RESULTS 2.1 Defect Selectivity Into a blackened disk of acrylic glass (polymethylmethacrylate, PMMA) with 12 mm in diameter, 16 holes were drilled from the rear

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side; the remaining wall thickness ranged from 0.6 to 3.6 mm. After a short and constant excitation of the front side (several seconds), an image sequence was recorded, while the sample cooled down. Fig. 7b illustrates the unwrapped image with the best contrast of the sequence, i.e. this is an optimal conventional shearography image.The top row of the simulated defects is visible well and the row beneath less. The superposed deformation of the whole disk makes the detection of defects more difficult. The same sample was excited periodically at a frequency of 0.05 Hz. Fig. 7c shows the Lockin phase image that was calculated from the sequence. The deformation of the whole disk is eliminated by the Fourier transformation, therefore the holes stand out clearly against the constant background which makes the phase image defect selective (if the sample features a

constant thickness). The lowest row can hardly be detected in a phase image at this frequency. 2.2 Signal-to-Noise Ratio For the determination of the signal-to-noise ration (SNR), another blackened PMMA disk was used containing two pairs of rows of flat bottom holes drilled from the rear side. The pair on the left has a remaining wall thickness of 0.7 mm and the pair on the right 1.4 mm. It was measured at an excitation frequency of 0.04 Hz. The results are illustrated in Fig. 8. The demodulated conventional image with the best contrast of the sequence (Fig. 8b) displays the left pair of holes and barely the right pair, while the Lockin phase image (Fig. 8c) clearly shows both pairs of hidden holes. The SNR of the image from the sequence is about 3, the SNR of

a) b) c)

Fig. 7. a) PMMA sample with simulated defects, b) measured with conventional optically excited shearography, c) Lockin phase image at 0.05 Hz

a) b) c)

Fig. 8 a) scheme of PMMA sample, b) best image of Lockin sequence (=conventional result), c) Lockin shearography phase image derived from stack of images Lockin-Interferometry: Principle and Applications in NDE

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the Lockin phase is almost 30. Therefore, also the second pair of holes deeper underneath the surface is now clearly seen. The significant improvement by one order of magnitude is based on the phase analysis of the effect coding which is performed by dynamic excitation and the Fourier analysis. Fig. 9. Aluminum wedge embedded in epoxy resin

2.3 Depth Resolution According to Eq. (1), the diffusion length of thermal waves depends on their frequency. Therefore, the thickness of the layer that causes the periodic buckling of an object by its thermal expansion can be adjusted.

A series of measurements at various excitation frequencies were taken on an aluminum wedge embedded in epoxy resin (Fig. 9). Therefore, the thickness of the epoxy on top of the aluminum wedge increases continuously

Fig. 10. Sample of an aluminum wedge embedded in epoxy resin; phase images at various modulation frequencies, measured with optically excited Lockin shearography

a)

b)

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Fig. 11. a) Phase image at 0.075 Hz, b) phase angle along dashed line


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along the sample. The results of the measurements are shown in Fig. 10. As the diffusion length of the thermal waves increases with decreasing frequency, more and more of the hidden wedge becomes visible. The point of the phase reversal moves continuously downwards. From 0.05 Hz on, another (unwanted) effect can be observed: during the manufacturing process of the sample two delaminations between the epoxy layer and the aluminum block occurred; one on the left side and one on the right side, next to the wedge. These delaminations can be observed only at lower frequencies (starting from 0.05 Hz) when the diffusion length is large enough. In Fig. 11, the phase angle at one excitation frequency (here 0.075 Hz) along the dashed line is illustrated in the chart on the right. This kind of curve progression is well-known from Lockin thermography, but while that method is limited to a depth range of about two thermal diffusion lengths, Lockin interferometry provides a larger depth range. In the example shown in Fig. 11, the depth range is about 2.5 the thermal diffusion length, but it may reach values of up to 3. 3 APPLICATIONS Lockin shearography is well suited to reveal defects which affect the deformation behaviour. This is especially important for components which are safety-relevant, e.g. for aerospace components. Since access to the rear side of components during maintenance is mostly limited, all measurements were performed from the front side. Three typical examples are shown. 3.1 Carbon Fiber Reinforced Plastic Laminate with Impact Damage A carbon fiber reinforced plastic (CFRP) plate made of multiaxial canvas (Fig. 12a) was damaged with a 5 J impact and inspected with Lockin shearography with an excitation frequency of 0.1 Hz from the impact site. The best image from the sequence (Fig. 12b) displays only the deformation of the whole plate and there is no indication of the impact. As this deformation of the whole body is eliminated in the Lockin phase image (Fig. 12c), and the SNR enhanced due to the Lockin-method, the

impact damage stands out clearly in the middle of this image. 3.2 Debonding in Ultralight Aircraft Wing The airframe of ultralight aircrafts usually consists of sandwich material like foam core and GFRP cover. Elastic and thermal waves are heavily damped and scattered in such materials, which makes non-destructive testing very difficult. Shearography is a good choice for inspection, as it can reveal defects by monitoring the deformation of the structure under load. Even though thermal waves cannot diffuse deep enough into the structure to reach all defects, Lockin shearography still improves the probability of detection. The Lockin phase image does not show defects located deeply under the surface, but since the amplitude image shows the local modulation height, it can be used to find debondings. The skin of the wing is bonded at two ribs and at the spar and the leading/trailing edge. Therefore, it deforms periodically in the middle, just like a membrane (though the excitation frequencies are far away from mechanical resonance). The gradient of displacement ranging from black to white should be observable in every field between two fins, the spar and one edge (Fig. 13c, between spar and leading edge). If it ranges over two of these fields, there must be a debonding between a rib and the skin (Fig. 13c, between spar and trailing edge). This can be observed with conventional shearography as well, but since the discrete Fourier transformation works as a weighted averaging, the signal-to-noise-ratio of the Lockin amplitude image is much better. 3.3 Stringer Break in Fuselage Panel of Commuter Airliner During the development of the Dornier 328, a panel similar to the planned fuselage structure was built to be tested in a buckling mode (Figs. 14a and b). During the test, a debonding of several stringers occurred. This local debonding can be easily observed in the Lockin phase image (Fig. 14c) in the right, inner part of the panel, since the background signal of the intact area is nearly constant.

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4 CONCLUSION

5 ACKNOWLEDGEMENTS

Conventional shearography is a fast and easy method for remote full field non-destructive testing. The combination with the Lockin technique improves shearography significantly: defect selectivity simplifies the interpretation of the results, and the improvement of signal-tonoise ratio by up to one magnitude increases the probability of detection. This reliable method is best applicable to polymer and composite structures since their thermal diffusivity is low and their thermal expansion high.

The authors are grateful to the Institute of Aircraft Design (IFB) of the University of Stuttgart for kindly providing samples, and the Waiblingen aero club for permitting the publication of the results. 6 REFERENCES [1] Leendertz, J.A., Butters, J.N. (1973). An image-shearing speckle-pattern interferometer for measuring bending moments. Journal of

a) b) c)

Fig. 12. a) CFRP plate with impact damage, 150 x 100 x 4 mm, b) best image of image sequence, c) Lockin phase image at 0.1 Hz

a) b) c)

Fig. 13. a) Ultralight aircraft, b) debonding in wing structure, c) Lockin shearography amplitude image at 0.25 Hz

a) b) c)

Fig. 14. a) Fairchild Dornier 328, b) CFRP panel used for buckling test, 700 x 1070 x 3.5 mm, c) Lockin phase image at 0.008 Hz

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Physics E: Scientific Instrument 6, p. 11071110. [2] Busse, G. (1992) Patent no. DE 4203272- C2, 1-3. [3] Wu, D. (1996). Lockin-Thermography for non-destructive material testing and material characterisation. PhD thesis, University of Stuttgart, p. 24-26. (In German) [4] Gerhard, H., Busse, G. (2003). Use of ultrasound excitation and optical Lockin method for speckle interferometry deformation measurement. Proceedings of Nondestructive Characterisation of Materials XI, Berlin, Springer-Verlag, p. 525-534.

[5] Gerhard, H. (2007). Development and test of new dynamic speckle methods for nondestructive material and component testing. PhD thesis, University of Stuttgart, p. 59-61. (In German) [6] Gerhard, H., Menner, P., Busse, G. (2007). New opportunities and applications of Lockin-Speckle-Interferometry in nondestructive testing of polymers. Proceedings of Stuttgarter Kunststoff-Kolloquium 20, 5V3, p. 1-8. [7] Rosencwaig, A., Gersho, A. (1976). Theory of the photo-acoustic effect with solids. Journal of Applied Physics, vol. 47, p. 64-69.

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, 192-203 DOI:10.5545/sv-jme.2010.170

Paper received: 28.05.2009 Paper accepted: 08.07.2009

Application of Ultrasonic C-Scan Techniques for Tracing Defects in Laminated Composite Materials

Hasiotis, T. – Badogiannis, E. – Tsouvalis, N.G. Theodoros Hasiotis – Efstratios Badogiannis* – Nicolaos Georgios Tsouvalis School of Naval Architecture and Marine Engineering, National Technical University of Athens, Greece In this paper practical ultrasonic C-scan techniques for NDT of laminated composite materials are developed and applied, with an aim to trace specific artificial defects. Two types of materials are examined; an advanced carbon/epoxy system and a typical marine type glass/polyester system. Both were constructed with two manufacturing methods (Hand Lay-Up and Vacuum Infusion). Several artificial defects were embedded into the test plates, varying in shape, magnitude and through thickness position. Test plates were C-scanned using ULTRAPAC II ultrasonic system with ULTRAWIN software and typical examination techniques (layer to layer examination, full width examination, etc.) were used to determine and characterize defects. In addition, appropriate software tuning procedures and examination strategies were applied, which further developed and optimised tthe scanning procedure. These efforts resulted in effective C-scan images, allowing the determination of the position and even the shape of the defects in some cases. Finally, precise determination of specimens’ thickness was achieved. ©2011 Journal of Mechanical Engineering. All rights reserved. Keywords: composites, ultrasonic inspection, c-scan techniques, defects, delamination, flaw detection, wrinkle 0 INTRODUCTION In the last 30 to 40 years fiber composite materials have seen a growing popularity in a wide spectrum of industries. The pleasure boats industry has been extensively using composites for some decades. Areas of higher-technology applications include aircraft and spacecraft structures. With a decreasing material price for the most commonly used fiber and resin types, composite materials have more recently been applied on a larger scale in ships and marine structures in general. It is the need for reducing the weight of the structure, in order to increase the strength-to-weight and stiffness-to-weight ratios, which raises the use of composite materials. Other advantageous properties include good thermal and acoustic insulation, low fatigue and corrosion and easy manufacturing of aero and hydrodynamically optimised shapes [1]. Defects are defined as any deviation from the nominal, ideal or specified geometric and/ or physical make-up of a structure or component and, for the laminated composite materials, most frequently arise during the manufacturing procedure. Manufacturing defects can evolve to damage during the service life of a structure. 192

Delamination is a defect type frequently met in composite materials, described as the separation of a layer or group of layers from their adjacent ones, due to failure of the internal bonding between the layers. Delamination can be either local or covering a wide area. It can occur either during the curing phase of the resin in the manufacturing stage or during the subsequent service life of the laminated part. Delaminations constitute a severe discontinuity because they do not transfer interlaminar shear stresses and, under compressive loads, they can cause rapid and catastrophic buckling failure [1] and [2]. Ultrasonic inspection is considered as the most efficient method used for quality control and materials quality inspection in all major industries. This includes electrical and electronic components manufacturing, production of metallic and composite materials and fabrication of structures such as airframes, piping and pressure vessels, ships, bridges, motor vehicles, machinery and jet engines. In-service ultrasonic inspection for preventive maintenance is also used for detecting any defects and impending failure in various structures like railroad-rolling-stock axles, press columns, earthmoving equipment, mill rolls,

*Corr. Author’s Address: School of Naval Architecture and Marine Engineering, National Technical University of Athens, Heroon Polytechniou 9, GR-157 73 Zografos, Athens, Grece, nautheo@yahoo.gr


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mining equipment, nuclear systems and other machines. ASM Handbook [3] presents the pulseecho method as the most widely used ultrasonic method, which involves the detection of echoes produced when an ultrasonic pulse is reflected from a discontinuity or an interface of a test piece. This method is very often used for flaws location and thickness measurements. C-scan display records echoes from the internal portions of test pieces as a function of the position of each reflecting interface within an area. Flaws are shown on a read-out, superimposed on a plan view of the test piece and both flaw size (flaw area) and position within the plan view can be recorded. Flaw depth is normally not recorded, although a relatively accurate estimate can be made by restricting the range of depths (gates) within the test piece that is covered in a given scan. In the literature there are many applications of the ultrasonic C-scan technique for the inspection of composite materials, mainly in carbon/epoxy systems, the use of which in advanced structures justifies the cost of such inspections. To name a few of these applications, the technique has been used to characterize artificial delaminations [4], to detect impact damage in carbon/epoxy composite plates [5], to characterize the distribution, size and shape of voids in composite materials [6] and to reveal some special features of the fiber/matrix interface [7]. The clustering procedure was applied in the present study in order to facilitate dimensioning of the detected defects. ASNT Non-destructive Testing Handbook [2] defines clustering as an algorithm and thus, a software supported process, in which a set of data is organized in groups that have strong similarities. The objective of clustering procedures is to find natural groupings of the data under study. Scarponi and Briotti [8] have also evaluated artificial defects in composite materials by means of ultrasonic inspection. Their inspection was aiming only at locating the depth position of the defects and not their size, whereas they report that difficulties were met in the inspection of GFRP materials. [9] on the other hand, proposed guidelines for the ultrasonic inspection of composite materials, in an effort to estimate the

size of the defects in materials of this type. The same authors concluded also that the sizing of the defects is not efficient, especially in multi layered materials with large thickness. In this paper ultrasonic inspection of composite specimens is performed in order to locate several artificial defects, made of Upilex polyamide material. Two material types and two manufacturing methods were used in order to investigate their effect on the effectiveness of the inspection method. In parallel, the two different materials would reveal any possible special features of the U/S software calibration that are needed for an effective inspection. The defects are simulating typical composite delaminations and they were embedded into the 14 layers test laminates. Their varying shape and size, as well as their varying location in the thickness direction of the laminate (overlapping between each other in one case), have as a result several delamination types as a result, which in turn constitute different ultrasonic tracking cases. Appropriate tuning of the device software resulted in effective C-scan images, allowing the determination of the position and, in some cases, of the shape and size of the defects, as well as the determination of the specimen thickness. 1 EXPERIMENTAL 1.1 Materials Two types of materials were investigated, namely a typical marine glass/polyester composite (GFRP) and an advanced carbon/epoxy system (CFRP). In addition, two manufacturing methods were used; the simple and conventional in the marine industry Hand Lay-Up method (HLU) and the more advanced Vacuum Infusion method (VI). The combination of the aforementioned two materials and two manufacturing methods resulted in the preparation of totally four test laminates. GFRP test laminates were manufactured using the typical polyester resin NORSODYNE G 703 from GRAY VALLEY, exhibiting a viscosity of 3200 mPa.s at 25 °C and a specific weight of 1.17 g/cm3, in association with a woven roving glass reinforcement having a weight of 500 g/m2. CFRP test laminates were manufactured using the cold cured epoxy resin DER 358 from DOW,

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Fig. 1. Layout of specimens with artificial defects exhibiting a viscosity of 600 to 700 mPa.s at 25 °C and a specific weight of 1.14 g/cm3, in association with a 205 g/m2 unidirectional carbon fibers reinforcement from HEXCEL (type G 1059). The yarns type of this carbon fiber unidirectional reinforcement is 6K HR in warp and EC5 11 in the weft direction, respectively, whereas the corresponding weight distribution is 98 and 2%. The same polyester resin and the same epoxy resin were used to manufacture the corresponding test laminates with both manufacturing methods. All four resulting laminates had 14 layers each. The artificial defects that were embedded inbetween the layers of the test laminates were in the form of a very thin (12.7 μm) sheet, made of Upilex polyamide material.

(on the right in Fig. 1) included all the embedded polyamide artificial defects. In GFRP laminates, the woven roving fiber reinforcements were layered with their warp direction along y-axis, whereas in CFRP laminates, the unidirectional fiber reinforcements were layered with their warp direction along x-axis.

1.2 Geometry of Specimens

In total eight separate polyamide artificial defects were embedded in each one of the defected specimens. These defects, designated D1 to D8 (see Fig. 1), had the specific dimensions and were placed at the specific x-y positions shown in Fig. 1. The distances between these defects were chosen so that their presence does not influence the detection of their neighbour ones. More specifically, the detailed characteristics of all these defects are as follows:

The four test laminates had dimensions 300 × 250 mm, as it is shown in Fig. 1. The average uniform thickness of the test laminates are listed in Table 1, measured by conventional methods (calliper and micrometer). Two specimens were cut from each test laminate, each one with dimensions 100 × 150 mm. The first specimen (on the left in Fig. 1) is a reference specimen without any polyamide artificial defects and the second 194

Table 1. Thickness of specimens Average uniform thickness [mm] CFRP-HLU 7.49 CFRP-VI 3.00 GFRP-HLU 8.28 GFRP-VI 5.45 Specimen

Hasiotis, T. – Badogiannis, E. – Tsouvalis, N.G.

Thickness at wrinkle [mm] 7.84 3.28 8.86 5.84


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D1: circular shape, radius is 5 mm, embedded between layers No. 4 and 5 (layers numbering starts from the mould side of the test laminate). • D2: square shape, side is 10 mm, embedded between layers No. 4 and 5. • D3: circular shape, radius is 10 mm, embedded between layers No. 4 and 5. • D4: square shape, side is 20 mm, embedded between layers No. 4 and 5. • D5: circular shape, radius is 10 mm, embedded between layers No. 3 and 4 (solid line in Fig. 1). • D6: circular shape, radius is 5 mm, embedded between layers No. 7 and 8 (dashed line in Fig. 1). • D7: circular shape, radius is 5 mm, embedded between layers No. 3 and 4 (solid line in Fig. 1). • D8: circular shape, radius is 10 mm, embedded between layers No. 7 and 8 (dashed line in Fig. 1). Defects D1 to D4 were placed at the same position through thickness, at a distance of approximately 30% of the total thickness from the mould surface of the laminate. Their objective was to investigate the capability of the method and the software settings required to detect the shape of the defects (square versus circular) for two different magnitudes (smaller and larger). Moreover, their non-symmetric placement through thickness would enable the investigation of the effectiveness of the method when trying to detect these defects from both sides of the laminate. Unfortunately, the relatively rough free surface of all test laminates (the surface not in contact with the mould) did not enable the performance of ultrasonic inspections from this side, due to the many signal reflections. Thus, all ultrasonic measurements and inspections reported in the following were performed from the smooth side of the specimens that was in contact with the mould. The objective of defects D5 to D8 was somewhat different. Defects D5 and D6 are placed through thickness in reverse order with respect to defects D7 and D8. The aim here was to investigate the capability of the method to detect and distinguish two overlapping defects of different size. These latter four defects were on purpose not placed at symmetric through thickness

positions, in order to try to detect them from both sides of the laminate. In addition to the above eight polyamide defects, a ninth one was manufactured (D9), consisting of a wrinkle of the sixth layer from the mould surface, along the full length of the test laminate. The width of this wrinkle varied from 10 to 15 mm. In the GFRP test laminates, this type of defect was manufactured by cutting the sixth layer in two pieces and overlapping them, since it was very difficult to create such a small wrinkle with woven roving glass reinforcement. The objective of defect D9 was to investigate the capability of the method to detect and dimensionalise such types of defects, which are common in typical marine composite structures. Thickness values are given in Table 1, for both the nominal average uniform thickness of each test laminate, as well as for the increased thickness at the area of the wrinkle. The listed thicknesses are average values from five measurements in the case of the uniform thickness and from two measurements in the case of the thickness at the wrinkle. 1.3 Device and Equipment Set-Up The ultrasonic inspection of the specimens was made by applying the Pulse – Echo method. A ¼ʺ diameter single crystal pulse-receiver flat transducer of 5 MHz from PANAMETRICS was used and the inspection was made with the specimens immersed in distilled water. The ultrasonic device used is ULTRAPAC II system (automated immersion system), in association with ULTRAWIN software for data acquisition, control and imaging. It is an automated immersion system, equipped with high speed computercontrolled stepped motor drivers. X-Y axis scanning speed exceeds 20 in/s (500 mm/s), with a resolution of 0.0027 in (0.07 mm). The direct vertical axis driver has a maximum speed of 1 in/s (25 mm/s) and a resolution better than 0.001 in (<0.025 mm). ULTRAPAC ΙΙ system uses a heavy duty Plexiglas tank supported by an also heavy duty aluminium framing system. A classic PC controls ULTRAPAC ΙΙ system, which employs a Pulse/Receiver card for the generation and receiving of the ultrasonic waves. A different card is also used for the analog to digital conversion of

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Fig. 2. Schematic view of the ultrasonics system the incoming signal (A/D). A schematic view of the whole ultrasonics system can be seen in Fig. 2. The software used (ULTRAWIN) is a data acquisition, control and imaging software, running under Microsoft Windows XP. It has a graphical user interface and is packed with features including real time A/B/C-scan imaging, powerful post processing modes like zoom pan cursors and waveform capture and display on sample location. The experimental system used has additional features to enhance ULTRAPAC’s capabilities, as they are described in [10]. Amongst others, clustering software (CLUSTWIN) and 3-D image software (3-D WIN) was used to conduct this research. Clustering software helps the user to locate certain areas of interest by establishing a specific parameter like cluster or defect size or upper and lower threshold of amplitude signal. In 3-D image software, any standard two dimensional C-scan image can be displayed as a 3-D projection. ULTRAWIN is also equipped with a six axes motion control, with multiple gate settings and a flexible hardware configuration. Special techniques and tuning procedures were applied for the ultrasonic inspection of the various specimens. As described in [2], it is common practice to set the distance between the transducer and the material (water path) as close as possible to the end of the near field value of the transducer used (which is 33.2 mm in our case), in order to avoid the fluctuation of the acoustic pressure, which takes place into the near field zone. In addition, as [11] suggests, the specimens were placed on a glass plate, which was used as a reflective plane in order to distinguish the backwall echo from any other one. 196

The four reference specimens without defects were used for the determination of the sound velocity inside all types of materials. For this to be done an initial constant sound velocity was considered (i.e. that of steel) and a U/S dummy thickness measurement was performed at each specimen. [12] suggests that the actual sound velocity could be calculated by comparing this dummy thickness to the actual thickness of each specimen (see Table 1). The sound velocity calculated in this way was equal to 2.9 mm/ms in the case of the CFRP specimens and 2.79 mm/ms in the case of the GFRP ones. A linear Distance Amplitude Correction (DAC) curve was used in order to increase the ultrasonic signal amplitude, thus facing the signal losses owing to factors like scattering, absorption, etc [13]. The gates were synchronized with the first echo from the specimen and the detection threshold was adjusted to a value above 20% of the Full Screen Height (FSH), thus avoiding the produced noise echoes. Depending on the nature of each defect to be detected, as well as on the area of interest to be inspected (i.e. layer by layer or backwall inspection), a detailed study was done aiming at selecting the most effective detection strategy. Four common strategies were studied; max-peak, max-thres, first-peak and first-thres. 2 RESULTS AND DISCUSSION In the present study, results are presented only for the CFRP and the GFRP specimens manufactured with the vacuum infusion method.

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Fig. 3. TOF C-scan of CFRP-VI specimen focused at backwall

Fig. 4. Amplitude C-scan of CFRP-VI specimen focused between the 3rd and the 5th layer Application of Ultrasonic C-Scan Techniques for Tracing Defects in Laminated Composite Materials

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2.1 CFRP Specimens Fig. 3 presents the Time of Flight (TOF) C-scan of the CFRP specimen with defects. The gate was placed at backwall echo and the first-peak strategy was used in order to measure the thickness of the specimen. The thickness variation is presented in Fig. 3 with different colours. The gate width was properly adjusted in order to monitor the maximum thickness value of the wrinkle (see Table 1). Fig. 3 shows that the majority of the specimen thickness is around the average value of 3 mm, however there are areas where thickness is larger, like those around and on the artificial defects and at the wrinkle. The horizontal orthogonal white colour zone indicates that the wrinkle area has thickness greater than 3.28 mm. B and B’-scans shown in the horizontal (below) and the vertical (on the right side) axis of the image, respectively, present the variation of the TOF parameter (thickness variation) within the thickness range illustrated at the legend on the left side of Fig. 3 (2.99 to 3.28 mm). B-scan displays the thickness variation along the horizontal cross-section of the specimen where the crosshair is placed and, similarly, B’-scan displays the thickness variation along the corresponding vertical cross-section. It is clearly shown that the existence of the artificial defects affect the local thickness of the specimen in the areas where the defects are located. This is recorded by an increase in TOF. It also becomes clear from B and B’-scans that defects D6 and D7 which overlap defects D5 and D8 cause high attenuation of the ultrasonic beam. Evidential of this high attenuation is the existence of white areas (no data points), which are present and clearly shown in the C-scan image. These white areas are also shown in the B and B’scans as areas with no thickness. In correspondence with this behaviour, the same pattern is illustrated in the B’-scan for the wrinkle defect. An area of no data points is present, which is indicative of high attenuation. This is due to the construction method of the wrinkle itself, where the 6th layer was folded in order to create the wrinkle. As a result, two additional layers exist at the area of the wrinkle. This fact is the cause for the excessive laminate thickness above the wrinkle which is reported in Table 1 and is almost 198

9% of the nominal thickness of the specimen. This increased thickness causes an increase in the attenuation at this specific region. In Fig. 4 the amplitude C-scan of the CFRP specimen with defects is shown. The gate was properly adjusted in order to inspect the area from the top of the 3rd layer to the bottom of the 5th layer. The max-peak strategy has been applied in order to monitor all defects which have initially been placed between the 3rd and the 4th layer that is all defects except D6 and D8. These six defects are clearly shown in Fig. 4. The same figure also clearly shows the direction of the fibers, which is parallel to the short side of the specimen (x-axis of the test laminate, see Fig. 1). In the sequence, the cluster analysis utility of the software was used, in order to determine the dimensions and the position of each defect and compare these values to the initially defined ones before the construction (Fig. 1). The clustering procedure was applied to isolate the group of data with amplitude greater than 70% of FSH, which correspond to defects. The aim of this analysis is to conclude on whether the applied procedure is able to accurately estimate the defects dimensions, as well as to define if the manufacturing procedures influenced the final position of the defects. The last two columns of the “Cluster Results” window in Fig. 4 present the monitored data for the co-ordinates of the center of each defect (defects numbering is shown in the figure), where x- and y-axes origin is at the lower left corner of the picture. The comparison of these data to the corresponding initial values shown in Fig. 1 leads to the conclusion that the location of the defects was very accurately identified. Moreover, it is shown that the applied method scanned accurately the shape of each defect. The “Cluster Results” window in Fig. 4 presents also the ultrasonically monitored area of each defect (fourth column). By comparing these values to the original defect areas that can be calculated from Fig. 1, it can be seen that the ultrasonic method significantly overestimates the area of all defects, from 14 to 73%. These differences may be due to the fact that all layers above the artificial defects exhibit a small bending over the defects. As a result, echoes from the areas adjacent to the defects are amplified up to 70% of FSH and they are recognized as defect areas.

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Fig. 5. 3-D amplitude C-scan of CFRP-VI specimen focused between the 3rd and the 5th layer In Fig. 5, a 3-D amplitude C-scan of the CFRP specimen is given, focused from the 3rd down to the 5th layer. The six defects of the specimen D1 to D5 and D7 are clearly shown in this figure. They are coloured in red, since these areas are highly reflective to the acoustic energy. 2.2 GFRP Specimens For the GFRP specimen, a max-peak strategy was applied and TOF was measured in order to determine the through thickness position of the peaks with maximum amplitude. In general, increased amplitude of peaks implies the existence of defects. On the other hand, increased amplitude areas are also monitored because of the high reflectivity of glass fibers. Fig. 6 shows the TOF C-scan of the GFRP specimen, focused at the 3rd layer in order to detect defects D5 and D7. Similarly to Fig. 3, the histograms in the horizontal (below) and the vertical (on the right side) axis of the image show the B and Bâ&#x20AC;&#x2122;-scans along the two corresponding axes of the crosshair, presenting the variation of the TOF parameter (thickness variation) within

the thickness range illustrated at the legend on the left side of Fig. 6 (0.95 to 1.34 mm). The echoes coming from the glass fibers result in a more or less uniform colour motive of the inspected surface. In this respect, Fig. 6 clearly shows the general pattern of the glass yarns, constituting the woven roving reinforcement. The orientation of the fibers is clearly shown. The echoes coming from the defects have the same TOF and, therefore, are much more concentrated and correspond to a single colour (red areas in the figure). In order to better define these areas, it has been also taken into account that their size must be considerably larger than the size of the repeated square crossings of the fibers in the warp and fill directions, whose dimension is approximately 4 mm. However, in the B and BÎ&#x201E;-scan images it is shown that these red areas are not clearly defined, but they are mixed together with the echoes coming from the glass fibers. Therefore, these areas do not have a specific shape and well defined borders, as it is evident from Fig. 6. Fig. 7 shows the TOF C-scan of the GFRP specimen, focused at the area between the 4th and the 5th layer, in order to detect defects D1 to D4,

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Fig. 6. TOF C-scan of GFRP-VI specimen focused at the 3rd layer

Fig. 7. TOF C-scan of GFRP-VI specimen focused between the 4th and the 5th layer 200

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together with the corresponding B-scan along the horizontal crosshair. The red coloured areas, which correspond to the two aforementioned defects, are reported to a depth of approximately 1.6 mm, which is very close to the actual through thickness position of these defects. Although the existence of the defects is indicated in Fig. 7, their geometry is once more not clearly defined, as was also the case in Fig. 6. This was due to the high attenuation that the glass fibers demonstrate, masking the reflections from the artificial defects. It is clearly illustrated in Fig. 7 that the reflections coming from the fibers are superimposed to the reflections coming from the defects. Spots of different colour and no data areas can also be observed in the area of the defects. This phenomenon is more severe at the boundaries of the defects. As far as the wrinkle is concerned, it is also not precisely defined in the GFRP specimen. Intense echoes as well as high attenuation areas generated from the woven roving fabrics mask the echoes from the wrinkle in the GFRP specimen. Moreover, in contrast to the case of the CFRP specimens, it was here impossible to fold the 6th layer in order to construct the wrinkle; it was created by simply cutting the layer in two pieces and overlapping them. As a result, the percentage thickness increase of the wrinkle in the case of the GFRP specimens was smaller than that of the CFRP specimens. The echoes generated from the woven roving fabrics are more intense at the intersection of the glass yarns in the fill and warp directions. That is why spots of different colour are displayed in GFRP C-scans. The different colour spots correspond to echoes coming from different depths of the material. The green spots mask the blue areas and the blue spots mask the red areas as shown in Figs. 6 and 7. It must be noted that, in general, the echoes coming from the fibers are more intense compared to the echoes coming from the defects or other areas. This is due to the reported elliptic shape of the glass yarns’ cross section (see Fig. 8). As [7] mention, a convex or a concave shape reflects a big portion of ultrasonic energy. A 3-D TOF C-scan of the GFRP specimen is given in Fig. 9, focused from the 4th down to the 5th layer. Contrary to the CFRP specimen case, the four defects D1 to D4 are not clearly defined

in this figure. Echoes with high amplitude at the same TOF are monitored, but the shape of the resulting area is not geometrically specific. The above results verify the difficulty in detecting the shape and size of defects in glass reinforced composite thick laminates, as it is also reported in the literature [8] and [9]. 3 CONCLUSIONS The present study investigated the efficiency of the ultrasonic inspection method for detecting defects in laminated composite fibrous materials. The study involved two different materials. The artificial defects were of various shapes and sizes and were placed at various through thickness positions. The main conclusions that can be drawn out from this study are the following: • The equipment used and the procedures applied proved much more efficient in the case of the CFRP specimens than when inspecting the GFRP ones. This happened due to the intense echoes that were reflected by the glass fibers, which were frequently overlapping the echoes coming from the actual defects. This phenomenon did not happen in the case of the carbon fibers. • The method proved capable of accurately defining the position of the artificial embedded defects in the CFRP specimens. In the case of the GFRP specimens, position detection of these defects was less accurate, however satisfactory. • The shape of the embedded defects was also accurately monitored in the case of the CFRP specimens. On the contrary, the shape of the defects in the GFRP specimens was not very well defined. • Regarding the size of the defects, the method significantly overestimated their values in the case of the CFRP specimens for reasons explained in section 2.1, whereas it was not possible to estimate any size at all in the case of the GFRP specimens. • The wrinkle defect was accurately detected in the case of the CFRP specimens, whereas it was not detected at all in the case of the GFRP ones. The reason for this is the fact that the echoes from the wrinkle of the GFRP

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Fig. 8. Cross section (x100) of GFRP specimen showing the elliptic shape of the fiber yarns

Fig. 9. 3-D TOF C-scan of GFRP-VI specimen focused between the 4th and the 5th layer specimen were masked from intense echoes and high attenuation areas that were generated by the glass fibers.

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The orientation of the fibers was detected in both types of specimens. This detection became possible since the high attenuation areas of the fibers correspond to no data

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points (white) areas, which are clearly visible in the c-scan images, taking also into account that these no data points are grouped in lines. The detection was much clearer in the case of the GFRP specimens than in the case of the CFRP ones due to the fact that the attenuation of the glass fibers is much higher than that of the carbon ones and moreover, since the glass woven roving reinforcement presented a much more coarse weaving in comparison to the more dense carbon unidirectional cloth. • The ultrasonic inspection of the GFRP specimens would probably resulted in a much better representation of the actual situation if another sensor was used, having lower frequency (i.e. 3.5 MHz) and being focused. In this way, the scattering of echoes coming from the glass fibers would be considerably decreased. Recapitulating, the currently followed procedure and the equipment used led to accurate predictions of the position and shape of the defects embedded in the CFRP specimens, even in the case of overlapping defects. The prediction was less accurate for the size of the defects. In the case of the GFRP specimens, the present method led only to a rough estimation of the position and the shape of the defects. 4 ACKNOWLEDGMENTS The authors gratefully acknowledge Messrs A. Markoulis and H. Xanthis, technical personnel of Shipbuilding Technology Laboratory, for their valuable contribution in the construction of the test laminates. 5 REFERENCES [1] Hayman, B., Berggreen, C., Tsouvalis, N.G. (2007). A review of the causes of production defects in marine composite structures and their influence on performance. Proceedings of the 1st International Conference on Marine Structures, Glasgow. [2] ASNT, Non-destructive testing handbook (1991). Volume 7: Ultrasonic testing, American Society for Non-destructive Testing, 2nd Ed., p. 151, 197, 244.

[3] ASM Handbook (1992). Volume 17: Nondestructive Evaluation and Quality Control. American Society for Metals, 9th Ed., p. 516. [4] Lloyd, P.A. (1989). Ultrasonic system for imaging delaminations. Composite Materials, Ultrasonics, vol. 27, p. 8-18. [5] Imielinska, K., Castaings, M., Wojtyra, R., Haras, J., Le Clezio, Hosten, B. (2004). Air-coupled ultrasonic c-scan technique in impact response testing of carbon fibre and hybrid: glass, carbon and kevlar/epoxy composites. Journal of Materials Processing Technology, vol. 157-158, p. 513-522. [6] Liu, L., Zhang, B.M., Wang, D.F., Wu, Z.J. (2006). Effects of cure cycles on void content and mechanical properties of composite laminates. Composite Structures, vol. 73, p. 303-309. [7] Chang, J., Zheng, C., Ni, Q.-Q. (2006). The ultrasonic wave propagation in composite materials and its characteristic evaluation, Composite Structures, vol. 75, p. 451-456. [8] Scarponi, C., Briotti, G. (2000). Ultrasonic technique for the evaluation of delaminations on CFRP, GFRP, KFRP. Composite Materials, Composites: Part B, vol. 31, p. 237-243. [9] Tittmann, B.R., Crane, R. (2000). Ultrasonic inspection of composites. Kelly, A., Zweben, C. (Eds.), Comprehensive Composite Materials, Elsevier, New York, p. 259-320. [10] ULTRAWIN Software (2002). User’s Manual, Rev. 2.58. [11] Roth, D.J. (1997). Using a single transducer ultrasonic imaging method to eliminate the effect of thickness variation, image of ceramic and composite plates. Journal of Non-destructive Evaluation, vol. 16, no. 2, p. 101-119. [12] Prassianakis, I. (2003). NDT of materials: ultrasonics method. National Technical University of Athens Press, Athens, p. 192. (in Greek) [13] Pfeiffer, U., Hillger, W. (2006). Spectral distance amplitude control for ultrasonic inspection of composite components. Proceedings of the European Conference of Non-destructive Testing ECNDT 2006, Berlin.

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, 204-217 DOI:10.5545/sv-jme.2010.171

Paper received: 22.08.2009 Paper accepted: 04.03.2010

Electromagnetic Nondestructive Evaluation: Present and Future

Grimberg, R. Raimond Grimberg NDT Department, National Institute of Research and Development for Technical Physics, Romania Appearing more than 125 years ago, the electromagnetic nondestructive evaluation has transformed form “art” to an engineering science, which is recognized. This development supposed the elaboration of theories based on Maxwell equations, of adequate transducers and afferent measurement electronics. Together with the development of computer science, the domain has made a spectacular leap. This paper presents a review of theoretical principles, numbering a few methods for solving forward and inverse problems, a review of the principal transducers types, electronics and the possibility of automatic interpretation of control results. Few directions where the domain might develop are sketched, starting from the observation that new types of materials, structures, complex equipments that shall be controlled, permanently appear. The purpose is constituted by obtaining a much higher probability of detecting for the highest possible reliability coefficient for the electromagnetic nondestructive evaluation of materials. ©2010 Journal of Mechanical Engineering. All rights reserved. Keywords: electromagnetic non-destructive evaluation, theory, forward problem, inverse problem, eddy current transducers, instrumentation, signal processing

0 INTRODUCTION Eddy current examination has its origin with Michael Faraday’s discovery of electromagnetic induction in 1831. Faraday was a chemist in England during the early 1800s and is credited with the discovery of electromagnetic induction, electromagnetic rotations as the magneto-optical effect, diamagnetism and other phenomena [1]. Faraday discovered that when a magnetic field passes through a conductor or when a conductor passes through a magnetic field, an electric current will flow through conductor if there is a closed path through which the current can circulate. The phenomenon of eddy currents was discovered by French physicist Leon Foucault in 1851, and for this reason eddy currents are sometimes called Foucault currents. Foucault built a device that used a copper disk moving in a strong magnetic field to show that eddy currents are generated when a material moves within an applied magnetic field [2]. When J.C. Maxwell died in 1879, at the time when many still doubted his theories 204

and eight years before, Hertz demonstrated the existence of the electromagnetic waves, D.E. Huges distinguished different metals and alloys from one another by means of an induced eddy current. Locking an electronic oscillator, Hughes used the ticks of a clock falling on a microphone to produce the exciting signal. The resulting electrical impulses passed through a pair of identical coils and induced eddy currents in conductive objects placed within the coils. Listening to the ticks with a telephone receiver (invented by A.G. Bell two years earlier), Hughes adjusted a system of balancing coils until the sound disappeared [3]. Hughes measured the conducting of various metals on his induction balance, using copper as a reference standard. This standard, “the International Annealed Copper Standard” (IACS) survives today as a common conductive measure; values are given as a percentage of the conductivity of copper. A relative scale, %IACS, appears in much of the literature on eddy current [4]. For the next fifty years, no significant advances in eddy current testing were reported. At the end of 1922s, eddy current devices begun to appear in the steel industry for measurements on

*Corr. Author’s Address: NDT Department, National Institute of Research and Development for Technical Physics, 47 D.Mangeron Blvd., Iasi, 70050, Romania, grimberg@phys-iasi.ro


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billets, round stock and tubing. But the limitations of electronic instrumentation allowed no more than some simple sorting applications. Instrumentation and electromagnetic theories that developed during the Second World War, primarily for the development and detection of magnetic mines, paved the way for the robust testing methods and equipments that allowed eddy current its entry into mainstream industry. In the early 1950s, Friedrich Förster presented developments that introduced the modern era of eddy current NDE [5]. Förster combined precise theoretical and experimental work with practical instrumentations. Clever experiment using liquid mercury and small insulating tabs allowed accurate discontinuity measurements. Förster produced precise theoretical solutions for a number of probe and materials geometries. In a major development in quantitative EC testing, Förster adapted complex notation for sinusoidal signals to his phase-sensitive analysis. The EC response was displayed on a complex inductance plane, inductive reactance plotted against real resistance. Conventional EC testing and analysis rely on this basic impedance plane method [6]. In addition to theoretical development proven by clear and detailed experimentation, Förster and his colleagues designed capable measuring equipments. During the 1950s and 1960s, Förster equipments and methods made eddy current an accepted industrial tool. Förster’s work has rightly identified him as the father of modern EC testing. The progress in theoretical and practical uses of the EC testing advanced the technology from am empirical art to an accepted engineering discipline. During that time, other nondestructive test techniques such as ultrasonic and radiography became well established and eddy current testing played a secondary role, mainly in the aircraft industry. North American Aviation was a prime contractor to NASA during the Apollo program. It was responsible for building the SII stage of the Saturn V as well as the Apollo Command and Services Modules, and its Rocketdyne division manufactured the F-1 and J-2 racket engines that, between them, powered all three stages of the launch vehicle. The EC testing has been utilized

only for the sorting of materials for the F-1 and J-2 rocket engines [7]. Relative recent requirements – particularly for the heat exchanger tube inspection and pressure tube examinations in the nuclear industry, for a lot of aircraft components, etc. – have contributed significantly to further developments of EC as a fast, accurate and reproducible nondestructive examination technique. One the major advantages of EC as an NDE tool is the variety of inspections and measurements that can be preformed. In the proper circumstances, EC can be used for: • crack detection and characterization, • material thickness measurements, • nonconductive coating thickness measurements, • conductivity measurements for: • material identification, • heat damage detection, • case depth determination, • heat treatment monitoring. Some of the advantages of eddy current inspection include: • sensitivity to small cracks and other defects, • detects surface and near surface defects, • inspection can give immediate results, • recent equipment is portable, • minimum part preparation is required, • test probe does not need to contact the part, • “hot” products can be tested, • inspects complex shape and size of conductive materials. Some limitations of eddy current include: • only conductive materials can be inspected, • surface must be accessible to the probe, • skills and training required are more extensive than other techniques, • reference standards needed for setup, • depth of penetration is limited, • flaw that lie parallel the probe coils windings and probe scan direction are undetectable. 1 THEORETICAL BACKGROUND Eddy currents are electrical currents induced in massive conductors placed in time variable magnetic (or electric) fields. Due to this

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fact, the eddy current examination operation can be schematically represented (see Fig. 1). The situation presented in Fig. 1a is named the forward problem for eddy current examination. This is frequently implemented due to two reasons: • Facilitate the interpretation of the eddy current control’s results, no need of a lot of test pieces in which many types of defects shall be practiced. • Allows the optimization of eddy current transducers for certain geometries, material properties and flaws possible to appear in the materials to be tested.

The principal methods for solving the forward problem for the case of harmonic fields will be presented here. The electromagnetic field is described by Maxwell’s equations: ∂B , ∂t ∂D ∇× H = − + J, ∂t ∇ ⋅ D = ρ, ∇× E = −

∇ ⋅ B = 0,

at which constitutive equations are added: D = ε E,

Fig. 1. General presentation of electromagnetic examinations, a) forward problem, b) inverse problem The situation presented in Fig. 1b is named the inverse problem for eddy current examination. Solving it allows the quantitative evaluation of emphasized flaws. The electromagnetic field applied to the examined piece can be: • harmonic

E ( r , t ) = E0 ( r ) e jωt H ( r , t ) = H 0 ( r ) e jωt ,

(1)

where r is the position vector, j= −1 , ω is the angular frequency, E0 and H 0 are the amplitudes of electrical and respective magnetic fields. This case is known as mono-frequency examination and is extremely researched and utilized [8] and [9]. • a source of harmonic components with angular frequencies ω1, ω2, ...

E ( r , t ) = E1 ( r ) e jω1t + E2 ( r ) e jω2t + ... , H ( r , t ) = H1 ( r ) e jω1t + H 2 ( r ) e jω2t + ... .

(2)

This eddy current examination method is named multi-frequency technique [9] with different shapes of impulses. This method is named impulse technique [10]. 206

(3)

B = µH,

(4)

where ρ is electrical charge, ε is dielectric permittivity, µ is magnetic permeability, E is the electric field, D is dielectric polarization, B is magnetic induction, J is the density of induced current. Considering the temporal dependency given by Eq. (1), Maxwell’s equations are written as: ∇ × E = − jωµ H , ∇ × H = (σ + jωε 0 ) E ,

ρ , ε ∇ ⋅ B = 0,

∇⋅E =

(5)

taken into account that J = σ E. (6) A vector wave equation can be developed for E :

∇ × ∇ × E = − jωµ∇ × H = − jωµ (σ + jωε 0 ) E

(

)

= ω 2 µε 0 − jωµσ E.

(7)

The factor of E from the last equality of Eq. (7) represents the square of the complex wave number: k 2 = ω 2 µε 0 − jωµσ . (8) For the materials with high conductivity, σ>>ωε0, the propagation constant of the electric field is: ωµσ ωµσ γ = −k 2 = +j . (9) 2 2

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The real term from Eq. (9) represents the attenuation constant of the field in the conductive material and the imaginary term represents the phase constant, both being equal in modulus. We can define the standard penetration depth as: 2 , (10) δ= ωµσ and represents the distance at which the amplitude of field is e times attenuated (e is the natural logarithm basis). The detection of flaws, or anomalies, by means of eddy current depends upon the fact that flaws are not electrically conducting and that the eddy current flow is interrupted at the boundary of the flaw. The flaw, therefore, can be considered to be an inhomogeneity, which consists of conductivity, σt , is known a priori. The electric permittivity and magnetic permeability of each region are those of free space, ε0, and µ0. Hence, the first two Maxwell equations for the two regions are: known region

∇ × E0 = − jωµ0 H 0 , (11) ∇ × H 0 = (σ 0 + jωε 0 ) E0 ,

flawed region

∇ × E f = − jωµ0 H f (12) ∇ × H f = σ f + jωε 0 E f .

(

)

Upon subtracting (12) from (11), we get:

(

) ( ) (13) ∇ × ( H 0 − H f ) = σ 0 ( E0 − E f ) + + jωε 0 ( E0 − E f ) + (σ 0 − σ f ) E f , ∇ × E0 − E f = − jωµ0 H 0 − H f ,

where we have added and subtracted σ 0 E f to get the final form. Thus, the perturbation of the H − H E − E electromagnetic field 0 0 f satisfy f , the same equation as the original electromagnetic field within the known region, except for the presence of the anomalous region, or flaw. This term, which is equivalent to a current source, J a , represents the presence of the anomalous region, or flaw. It is important to note that J a vanishes off at the flaw, because there σt = σ0. In the usual way a vector wave equation for E0 − E f can be derived from Eq. (13):

(

)

(

)

∇ × ∇ × E0 − E f = − jωµ0∇ × H 0 − H f =

(

= ω 2 µε 0 − jωµ0σ 0

(

)(E

0

)

− E f − (14)

)

− jωµ0 σ 0 − σ f E f .

Eq. (14) can be considered as a fundamental equation of eddy current nondestructive testing and, in principle, can be solved through two procedures: • analytical, • numerical. From the analytical procedures, the Green’s function method [11] and [12] will be presented and from the numerical ones, the finite element method [13] will also be presented. 2 ANALYTICAL SOLVING OF FORWARD PROBLEM USING DYADIC GREEN’S FUNCTIONS METHOD The dyadic Green’s function establishes a bi-univocity relationship between a current source with J (r ') density and the field E (r ) created by this source in the observation point r :  E ( r ) = ∫ G ( r , r ')J ( r ') dr ', (15) Vsource

where the integral extends on the source volume and G (r , r ') is the dyadic Green’s function. Analyzing Eq. (14) it can be observed that the last term from the right member can be considered as a source that extends on the flaw volume and thus it can be write immediately as a formal solution of Eq. (14) for the perturbed field E0 − E f : E0 ( r ) − E f ( r ) =

jωµ0

 G ( r , r ')E f ( r ') σ 0 − σ f dr '. (16)

V flaw

(

)

Let us consider a simple planar geometry to can exemplify the method. This is presented in Fig. 2.

Fig. 2. Simple planar geometry Electromagnetic Nondestructive Evaluation: Present and Future

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The geometry of the problem allows the decomposition of the space in three regions: • region 1, free space over the piece, • region 2, the conductive piece, • region 3, free space under the piece. The following notations will be introduced:  • for the dyadic Green’s functions Gij ( r , r ') the field produced in the point r from region i, due to a point source r ' placed into region j, with i, j = 1,2,3, • for the electrical fields E1,2 ( r ) the electrical field in region 1 or 2 with flaw present; E0 (r ) the incident electrical field. Eq. (16) allows the calculation of E in the zone of flaw (region 2): E0 ( r ) − E1 ( r ) =

jωµ0σ 0

  σ f G12 ( r , r ')E2 ( r ')  − 1 dr '. (17) σ 0   V flaw

An integral equation that allows the calculation of perturbed electric field in region 1 where both the source of electric field and the device for measurement of the field are placed is disposed at the same time. E0 ( r ) − E1 ( r ) =

jωµ0σ 0

  σ f G12 ( r , r ')E2 ( r ')  − 1 dr '. (18) σ   0 V flaw

If we consider that the incident electrical field E0 ( r ) is produced, for example, by a coil through which an alternative electrical current with J 0 ( r ') density circulates. E0 ( r ) = jωµ0 2π

∫∫

 G21 ( r , r ')J 0 ( r ') dr '. (19)

the mesh cells might be considered equal with the field in the center of the cell. σ

Nc

E 2 ( r ) = ∑ E j ( r )Pj ( r ) ,

208

j =1

σf σ0

(20)

Nc

− 1 = ∑ σ j Pj ( r ), j =1

where Pj ( r ) are basis functions that can be chosen in different ways [15] and Nc is the number of cells from mesh. Replacing Eq. (20) in Eq. (17), finally the following is obtained: Nc

Nc

∑ E j ( r )Pj ( r ) + jωµ0σ 0 ∑ E j ( r )σ j ⋅

j =1

j =1

 G22 ( r , r ') Pj ( r ') dr ′ = E0 ( r ') .

(21)

V flaw

Taking the moments of Eq. (21), meaning the multiplying of Eq. (21) with weighting functions Qi ( r ) , i = 1, 2, ..., Nc and integrating over the flaw: Nc

∑ E j (r ) ∫ j =1

Pj ( r ) Qi ( r ) d ( r ) +

V flaw

Nc

Vexciting _ coil

+ ∑ E j ( r )σ j jωµ0σ 0 j =1

dr

V flaw

⋅Pj ( r ') dr ' Qi ( r ) dr ' =

Introducing Eq. (19) in Eq. (18), the electric field E1 ( r ) due to the presence of flaw in region 2 can be calculated. The integral equations which result are Fredholm equation, 2nd range that has not exact solutions. From this reason, a discretization procedure shall be used, allowing in the same time the transformation of Fredholm integral equation into a system of algebraic equations. The one most frequently used is represented by the method of moments [14] and [15]. To apply this method, a mesh is applied over the interest zone of the piece presented in Fig. 2 (the zone containing the flaw). The cells must be small enough, so that the field in the interior of

f Decomposing E 2 ( r ) and  − 1 after   σ0 base function:

 G22 ( r , r ') ⋅

V flaw

(22)

E0 ( r ) Qi ( r ) dr ,

V flaw

i = 1, 2,.., N c .

The vector matrix version of Eq. (22) is:

( A + jωµ σ G ) E = F , (23)

0 0

where the two supralines means matrix. Aij =

Gij = σ j

Grimberg, R.

flaw

Qi ( r ) Pj ( r ) dr , (24)

Qi ( r ) dr

V flaw

Fi =

G22 ( r , r ′ ) Pj ( r ') dr ', (25)

V flaw

V flaw

E0 ( r ) Q ( r ) dr. (26)


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If the bases function Pj ( r ) re chosen as pulse function:

j 1 r ∈ cell (27) Pj ( r ) =  otherwise 0 

and weight function Qi ( r ) as delta function:

Qi ( r ) =

δ (r ) . (28) r

The variant of the method of moments used above is named point matching. 3 SOLVING OF FORWARD PROBLEM USING FINITE ELEMENT METHOD The finite element method attempts to approximate the continuous problem in a rather straightforward way. The steps involved are the following [13]: • Discretization: the solution space is discretized into finite elements. Finite elements are either linear, surfaces or volumes. The symmetry properties can be utilized for reducing the number of elements. • Approximation: an approximation over a finite element is defined and has some required properties. • Minimization: a method of minimizing the error due to the discretization must be included. • Solution: the previous steps result in a system of equations (linear or nonlinear) which must be solved. • Post processing of data may be required, depending on the function calculated in the previous step. The finite element method (FEM) approximates the solution rather than the partial derivates in the differential equation. It is a volumetric method in which the approximation is valid at any point in the solution domain, not only at discrete points. The process calls for discretization of the continuum into any number of finite volumes subspaces over which the approximation is valid. These subspaces or finite elements are of any well-defined geometrical shape while the approximation is normally a polynomial interpolated over a finite number of points that define the shape of the finite element. Since the restriction on their shape and

size are minimal, finite elements are well suited for discretization of awkward geometries. The elements may not be uniformly distributed and can be of any size. Furthermore, it is possible to mix different types of elements to increase the FEM ability to handle a given geometry. A number of commercial finite element and volume integral codes capable of simulating arbitrary shaped defects and tests geometries are currently available. An interesting variation of the FEM that relies on the underlying mesh and element mode connectivity is the class of meshless methods which dicretizes the domain by a set of nodes alone [16]. One of most interesting applications involve the use of numerical models for calculating the probability of detection and the generation of receiver operating characteristics for a given set of conditions [17]. Although such probability of detection (POD) models cannot account for human factors, they are very useful for isolating physical factors that contribute to variations in test results and the impact of such models will become more commonplace as industry begins to embrace the concept of design for testability and life cycle management in the future.

Y = AX , (29)

where Y is a unicolumn vector which represents the response of the assembly eddy current transducer-equipment, X is unicolumn vector which represents the parameters of the piece’s degradation and is unknown and A is the model matrix. The solution of the inverse problem consists in determining the vector X elements knowing the elements of Y from measurements and the matrix A due to the analytical or/and numerical modelling of the phenomena. Formally, the solution of the problem is written:

−1

X = A Y , (30)

where A -1 is the inverse of matrix A . Since matrix A is not compulsory squared, the pseudo inversion or the inversion in Moore-Penrose sense can be used. The difficulty consists in the fact that the problem for solving Eq. (30) is, in general, illposed because the matrix A has a small condition number (condition number represents the ratio between the smallest singular value of the matrix

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and the bigger one), so that a small variation of data can produce a completely different solution [18]. In these conditions, the matrix A must be preconditioned using different regularization algorithms. Simulation models are increasingly being used as a basis for solving inverse problems. The most straightforward method is to use an iterative scheme where an appropriate norm of error between the output of a simulation model is used and the test signal to adjust the material parameters until agreement between the signals is obtained. A number of schemes to reduce the computational effort ranging from table look-up schemes to methods that improve the convergence rate have been proposed [19] to [21]. The principal disadvantage lies in the computational border associated with the implementation of those inversion schemes although some of the newer methods appear to be successful in overcoming this problem. These models based inversion schemes will become more popular as computational power becomes less expensive. Such inversion schemes offer the obvious advantage of not requiring any training data unlike systems based approaches which typically need substantial amounts of data. Model based inversion schemes may be the only choice in situations where training data is either scarce or not of the quality required to design robust defect characterization systems. 4 THE INVERSE PROBLEM At the core of those presented in Fig. 1, the inverse problem consists in the evaluation of the characteristics of the physical system made from the piece to be examined which contains flaws from the knowledge about electromagnetic field applied in different points of the piece and the response of the eddy current equipment in the same points. From the physical model which has been developed (indifferent that represents analytical or numerical solutions), linearized and digitized, the eddy current control operation can be represented through a matrix equation 5 EDDY CURRENT TRANSDUCERS The eddy current accomplish two roles: 210

transducers

must

shall induce eddy current into the conductive material to be examined, • shall emphasize their flow modifications due to material degradations. The simplest method to create time variable magnetic fluxes which shall induce eddy current into the material to be examined is represented by the coils crossed by alternative currents, by current impulses or more alternative currents with different frequencies. This coil is named the emission part of the eddy current transducer. In Fig. 3 are presented, for planar geometry, few types of emission coils and the electric field created by them, at the level of examined piece. The frequency of alternative current is 100 kHz and amplitude is 0.1 A. To emphasize the induced eddy current and the effect of material degradation over their propagation, sensors sensitive to the variation of magnetic field can be used: bobbins, sensors with Hall effect, sensors based on quantum effect – SQUID [22], sensors based on magneto-resistive effect –GMR [23]. The most utilized sensors for emphasizing eddy currents are the bobbins with and without magnetic core. Other types of sensors are less utilized due to certain limitations as frequency range that can be detected, (usually until few tens of kHz) as well as their costs, especially for SQUID. The eddy current transducers can be: • absolute – the signal delivered by the transducer depends by the state of the piece to be tested in the point in which the transducer was fixed, • differential – the signal delivered by the transducer represents the difference between two neighbor region of the piece to be tested. The simplest type of absolute eddy current transducer is represented by the transducer with a unique coil in which the same coil generates time variable magnetic field that induces eddy current in the tested piece and at the same time, due to the modification of resistance and inductive reactance emphasizes the eddy current. In Fig. 4 a few types of eddy current transducers destined to the control of tubes, both through outside and inside are presented.

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Fig. 3a. Circular coil with 100 turns, inner diameter 1.8 cm, outer diameter 4 cm, height 4 mm;

Fig. 3b. Rectangular square coil with 100 turns, side of 4 cm and section of winding 4 mm;

Fig. 3c. Rectangular spiral coil with 100 turns, side 4 cm and step of 0.1 mm

The absolute transducers emphasize the material degradation on the entire surface but they have a low signal to noise ratio. In adition, they are sensible to the variation of lift-off. The differential transducers emphasize only the modifications in the profile of the degradation, they are less sensible to the lift-off and have lower signal to noise ratio. The inventiveness in the domain of eddy current transducer is very large. Thus, the eddy

current transducer with orthogonal coils has been developed (Fig. 5a). It is made from a ferrite cup core inside which the emission coil is inserted and orthogonal on it is wounded the reception coil. The transducer is a send-receive type, absolute but is relatively less influenced by the modification of lift-off and presents good signal to noise ratio [24]. With its help, plates form carbon-epoxy composite were tested, the structure of the reinforcement

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fibers as well as the delamination due to impact were emphasized [25].

Fig. 4. Eddy current transducer for the control of tubes In Fig. 5b the amplitude of signal delivered by the transducer at the scanning of a region from the composite plate that contains a delamination due to an impact with 4 J energy is presented. Since with the occasion of periodical outage at the nuclear power plants, the tubular bundles of steam generators are completely examined by eddy current, a special attention was accorded to the development of adequate transducers. The pancake rotating coil type transducer was developed for the confirmation of flaw indications delivered by the differential and absolute transducers that pass inside the tubes as well as for a better evaluation of the emphasized discontinuities severity. The transducer rotates around the tube’s axis with relatively high revolution speed (60 to 120 rot/min) and at the same time goes forward inside the tube (axial speed 2 to 3 mm/s). Thus, by correlation revolution speed – transducer diameter – advance speed, the entire surface of the tube can be scanned. In Fig. 6 the amplitude delivered by this type of transducer is presented. The emphasized flaws are marked in the image. In this way, the orientation of flaws can be evidenced. The pancake rotating coil transducer type, besides the clearly advantages, presents a series of limitations as: low axial advance speed, thus an increasing of the inspection time, the existence of rotating parts and the difficulty in transmitting 212

the signal from the transducer to the equipment. The eddy current transducer with rotating magnetic field was developed to overtake these disadvantages [26] and [27]. The transducer is a send receiver absolute type and does not contain parts in rotation.

Fig. 5a. Eddy current transducer with orthogonal coils

Fig. 5b. The amplitude of the signal delivered by the transducer at the scanning of a region from the composite plate that contains a delamination due to an impact with 4 J energy

Fig. 6. The signal delivered by the pancake rotating probe

The emission part is made from three rectangular coils making a 120° angle between them, fed with a system of three phased electrical

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currents. Composing the phase of the fields created by the three coils, a radial rotating magnetic field is created, having the same angular frequency as of the three phased currents. The reception part of the transducer is made from a circumferential array of reception coils (Fig. 7a). The physical realization of the inner eddy current transducer with a rotating magnetic field for the inspection of pressure tubes from pressurized heavy water reactors is presented in Fig. 7b and the amplitude of the signal delivered by the reception array is presented in Fig. 7c.

It is possible that the future of eddy current transducers will be the use of sensors array. These have started to be initially used for the increasing of the control speed [28]. Using a sensor array made from an emission coil and an array of reception coils, together with a super resolution procedure, very small discontinuities as fatigue cracks lattices have been emphasized [29] and [30].

Fig. 8a. EC sensors array

Fig. 7a. Scheme of inner eddy current transducer with rotating magnetic field

Fig. 8b. Fatigue crack lattice emphasized by penetrant liquid

Fig. 7b. Physical realization

Fig. 8c. Signal delivered by the EC sensors array

Fig. 7c. Signal delivered by the transducer

In Fig. 8 the sensor array used, the fatigue cracks lattice emphasized with penetrant liquid and the signal delivered by the array, according to [30] is presented.

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6 INSTRUMENTATION The arrival of the microprocessor in the 1970s and the availability of inexpensive analog to digital (A/D) converters in recent years have had, perhaps, the most impact on instrumentation. Signals can now be routinely be sampled and quantized with 16 bit precision and as a consequence, a bulk of the processing can be performed in discrete-time. Sources of noise introduced in the signal, excluding noise introduced by the transducer, from start (transducer output) to finish (digital output of the instrument) include quantization noise, error introduced due to finite word length and algorithmic errors. The quantization noise is closely related to the number of bits associated with the A/D conversion process. A crude rule of thumb is to assume an improvement of 6 dB in the signal-to-noise ratio (SNR) for every additional bit. Quantization noise is seldom a concern these days with the ready availability of 16 bit converters in the case of conventional eddy current applications. Round-off errors due to finite word length were a matter of concern when microprocessors could not handle fast floating point calculations. The availability of relatively inexpensive 32bit floating point digital signal processors with cycle time as low as 3.5 ns has rendered this issue moot. The overall SNR of eddy current instruments have leapfrogged as a consequence and most instruments offer performance levels that were simply not possible a few years ago. Ready access to inexpensive computational horsepower with massive amounts of storage within the instrument has also revolutionized our ability to extract and process information in numerous ways. Since most functions are now implemented in software, programmability has become a common place feature. High, low and band pass filters are routinely digital in nature. Features such as rotation, translating and gain and most importantly graphics are all implemented in software. This allows the user to tailor the instrument characteristics to the application far more effectively. Prognostication in a rapidly changing world is very difficult. However, it is easy to see that spectacular improvements with respect to computation speed and memory will continue to 214

have an impact on the industry. It is more than likely that this will affect the way in which the inspection data is interpreted. Current industrial practice is to rely either on manual interpretation or simple calibration based approaches to estimate the size, shape and location of the flaw. Access to vast amounts of computation power would allow instrument manufacturers to incorporate sophisticated signal interpretation algorithms. A number of three-dimensional defect characterization approaches, both model and system based, have been proposed in recent years. It is relatively safe to assume that such defect characterization algorithms would become an integral part of the instrument menu. In the case of specific geometries, it may even be possible to use the defect profile estimate to calculate its impact on the structural integrity of the test component within the instrument. In short, we will very likely see the migration of activities that have hitherto been performed off-line to the eddy current instrument. The ease with which application specific integrated circuits (ASIC) can be designed and manufactured today as well the emphasis on miniaturization will inevitably result in smaller instrument footprints. The limitation is, of course, the size of the display required to present large amounts of information. This is being addressed through the use of head mounted displays. Such displays are also likely to become commonplace for displaying data in a virtual reality environment. In the future it should routinely be possible to â&#x20AC;&#x153;navigateâ&#x20AC;? through a virtual world allowing the user to examine a defect profile estimate in 3D from any arbitrary perspective using a stereoscopic head display. Multifrequency eddy current systems have become far more sophisticated as circuit speeds have improved and our ability to build high quality filters and multiplexers/switches have grown. Both time and frequency division multiplexed systems are now routinely available with the latter offering significantly higher eddy current signal bandwidths. This trend is likely to continue into the foreseeable future resulting in much higher inspection speeds. Mixing and other signal processing algorithms for suppressing artefacts are also growing in sophistication and it should be possible to suppress artefacts more effectively in the future.

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The availability of high resolution (18 bits and higher) A/D converters will have an impact on pulsed eddy current and remote field eddy current methods. These inspection methods will be able to make use of the large dynamic range and low noise floor of such converters. The circuits preceding the A/D converter (such as the antialiasing filter) in the processing scheme will have to be designed very carefully, of course, to ensure full exploitation of their low noise characteristics.

the data obtained from eddy current holography at the scanning of a region from a carbon epoxy composite plate [33]. 8 CONCLUSIONS

7 SIGNAL PROCESSING Signal processing techniques represent a powerful approach for improving the probability of detection. Such techniques are also very often used to enhance the quality of the signal by reducing noise and other artefacts that detract from our ability to detect and characterize the flaw. Signal enhancement procedures are of interest in situations where the signal is interpreted manually as well as in case where it is analyzed using an automatic system. Signal quality can be enhanced in a variety of ways ranging from simple filtering schemes to those that adapt with noise statistics. The latter are particularly useful when the noise is highly correlated with the signal. A number of new approaches have been proposed in recent years for the classification of defect signals. A variety of pattern recognition techniques based on statistical as well as trainable approaches have shown promise [31]. Prominent among the latter category are those that make use of neural networks. Equally interesting developments are occurring with regard to the development of systems based methods for estimating defect profiles. One of the more exciting developments in the field involves the concept of data fusion. The basic premise is that it is possible to garner additional information concerning the state of piece by combining information from multiple sensors. Sensors could be of the same type (operating at different positions and/or excitation frequency) or they could be entirely different from each other. Fig. 9 shows, after [32], the data fusion based on the theory of evidence between the data from ultrasound control using Lamb waves generated by hertzian contact and

Fig. 9a. US

Fig. 9b. EC

Fig. 9c. Data fusion It is impossible for a paper such as this to capture the essence of all that has been done in a

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field that is over a hundred years old. However, it is safe to say that some of the most exciting developments are ahead of us. A confluence of developments in the fields of electronics, computer technology, simulation tools and signal processing is contributing to the excitement and fuelling some of the most compelling advances. Technology is indeed breathing new life into the field and there is much to look forward to in this important scientific endeavour. Now, the electromagnetic nondestructive evaluation had been transformed from “art” to an absolutely necessarily engineering science. The actual quality requirements impose a rigorous control on the basis of quality codes (ASME), standards (EN, ASTM, etc), or, according to ISO standards, of understanding between the producers and the beneficiaries. The development of new theories, transducers, equipments and adequate analysis software will lead to an increase in the probability of detection for the highest possible reliability coefficient. The development of new types of materials and their different applications (different classes of materials and structures in the construction of International Spatial Station, the superconductors used in ITER reactor, usage of known materials in special purposes as components of The Large Hadron Collider from CERN) made that the eddy current examination shall represent not a close domain, but, on the contrary, a dynamic domain, a fully evolving one. 9 ACKNOWLEDGEMENTS This paper has been supported by the Romanian Ministry of Research, Development and Innovation under National Plan II Contracts: No.71-016/2007 MODIS, No.32-134/2008 SYSARR and Nucleus Program, Contract no. No.09430104 10 REFERENCES [1] Faraday, M. (1885). Experimental researches in electricity. vols. I. and II. Richard and John Edward Taylor, vol. III. Richard Taylor and William Francis. 216

[2] Foucault, J.B.L. (1913). Catholic Encyclopaedia. New York: Robert Appleton Company. [3] Shull, P.J. (2002). Nondestructive evaluation: Theory, techniques and application. Technology&Engineering, NY. [4] Cecco, V.S., Drunen, G. van, Sharp, F.L. (1983). Eddy current manual. vol. 1, AECL 7523, rev. 1, Chalk River. [5] Forster, F., Breitfeld, H. (1952). Theoretische und experimentelle Grundlagen der Zerstorungsfreien Werkstoffprufung mit Wirbelstromverfahren parts I und II, Z Metallkunde, vol. 43, no. 5, p. 163-180. [6] Forster, F., Breitfeld, H., Stombke, K. (1954). Theoretische und experimentelle Grundlagen der Zerstorungsfreien Werkstoffprufung mit Wirbelstromverfahren parts III und IV, Z Metallkunde, vol. 48, no. 5, p. 166-199, 221-226. [7] Young, A. (2008). The Saturn V F-1 engine, powering Apollo into history. Springer, Berlin. [8] Dood, C.V., Deeds, W.E. (1968). Analytical solutions to eddy current probe coil problems. J Appl. Phys., vol. 39, p. 28292838. [9] Moore, P.O., Udpa, S.S. (2004). Nondestructive Testing Handbook. 3rd Ed., vol. 5, Electromagnetic testing, ASNT, OH. [10] Lepine, B.A., Wallace, B.P., Forsyth, D.S., Wyglinshi, A. (1999). Pulsed eddy current method. NDT.Net, 4,1. [11] Tai, C.T. (1994). Dyadic green’s functions in electromagnetic theory. IEEE Press, NY [12] Chew, W.C. (1999). Waves and fields in inhomogeneous media. Wiley-IEEE Press. [13] Ida, N. (1995). Numerical modeling for electromagnetic nondestructive evaluation. Chapman & Hall, London. [14] Harrington, F. (1968). Field computation by moment methods. Mc.Millan, NY. [15] Balanis, C.A. (1989). Advanced engineering electromagnetics. J. Wiley & Sons, NY. [16] Xuan, L., Zeng, Z., Balasubramaniam, S., Udpa, L. (2004). Element-free Galerkin method for static and quasi-static electromagnetic field computation. IEEE Transactions on Magnetics, vol. 40, no. 1, p. 12-20.

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[17] Rajesh, S.N., Udpa, L., Udpa, S.S. (1993). Numerical model based approach for estimating probability of detection in nde application. IEEE Trans. on Mag., vol. 29, no. 2, p.1857-1861. [18] Bertero, M.E., Pike, R. (1993). Signal processing for linear instrumental systems with noise, signal processing and its applications. Bose, N.K, Rao, C.R. (Eds.) North Holland, Amsterdam. [19] Kojima, F., Okajima, N. (2001). Crack profiles identification of steam generator tubes in PWR plants using database. Electromagnetic Nondestructive Evaluation (V), Pavo, J. (ed.) IOS Press, p. 97-104. [20] Li, Y., Liu, G., Shanker, B.S., Sun, Y., Sacks, P., Udpa, L., Udpa, S.S. (2001). An adjoint equation based method for 3D eddy current signal inversion electromagnetic nondestructive evaluation. (V), Pavo, J. (Ed.), IOS Press, p. 89-96. [21] Grimberg, R., Udpa, L., Udpa, S.S. (2008). Electromagnetic transducer for the determination of soil condition. International Journal of Applied Electromagnetics and Mechanics, vol. 28, no. 1-2, p. 201-210. [22] Pandney, W. (1998). Response function of an electromagnetic microscope. Review of Progress in QNDE, Thompson, D.O., Chimenti, E.E. (Eds.), 17, Plenum Press, NY, p. 1025-1031. [23] Danghton, J., Braun, J., Chen, E. (1994). Magnetic field sensors using GMR multilayer. IEEE Trans. on Magnetics, vol. 30, no. 6, p. 4608-4610. [24] Savin, A., Grimberg, R., Mihalache, O. (1997). Analytical solutions describing the operation of rotating magnetic field transducer. IEEE Trans. on Magnetics, vol. 33, no. 1, p. 697-702. [25] Grimberg, R., Savin, A., Steigmann, R., Bruma, A., Barsanescu, P. (2009). Ultrasound and eddy current data fusion for evaluation of carbon epoxy composite

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delaminations. INSIGHT, vol. 51, no. 1, p. 25-32. Grimberg, R., Udpa, L., Savin, A., Steigmann, R., Udpa, S.S. (2005). Innereddy-current transducer with rotating magnetic field: theoretical model, forward problem. Research in Nodestructive Evaluation, vol. 16, no. 2, p. 65-77. Grimberg, R., Udpa, L., Savin, A., Steigmann, R., Udpa, S.S. (2005). Innereddy-current transducer with rotating magnetic field, experimental results: application to nondestructive examination of pressure tubes in PHWR nuclear power plants. Research in Nodestructive Evaluation, vol. 16, no. 2, p. 79-100. Mook, G., Michel, F., Simonin, J. (2008). Electromagnetic imaging using probe arrays. 17th World Conference on Nondestructive Testing, Shanghai. Grimberg, R., Wooh, S.C., Savin, A., Steigmann, R., Prémel, D. (2002). Linear Eddy-Current Array Transducer. INSIGHT, vol. 44, no. 5, p. 289-293. Grimberg, R., Udpa, L., Savin, A., Steigmann, R., Palihovici, V., Udpa, S.S. (2006). 2D eddy current sensor array. NDT & E International, vol. 39, no. 4, p. 264-271. Udpa, L., Udpa, S.S. (1996). Application of signal processing and pattern recognition techniques to inverse problems in NDE. Int. J. of Appl. Electromagnetics and Mechanics, vol. 9, no. 1, p. 1-20. Grimberg, R., Steigmann, R., Leitoiu, S., Andreescu, A., Savin, A. (2008). Ultrasound and eddy current data fusion evaluation of carbon – epoxy composites delaminations. Emerging Technologies in Nondestructive Testing, Busse, G. et al. (eds.), p. 349-355. Grimberg, R., Prémel, D., Savin, A., Bihan, Y. Le, Placko, D. (2001). Eddy current holography evaluation of delamination in carbon – epoxy composite. INSIGHT, vol. 43, no. 4, p. 260-264.

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, 218-226 DOI:10.5545/sv-jme.2010.172

Paper received: 22.08.2009 Paper accepted: 08.04.2010

NDT Based Process Monitoring and Control Wolter, B. - Dobmann, G. - Boller, C. Bernd Wolter* - Gerd Dobmann - Christian Boller Fraunhofer Institute for Non-Destructive Testing (IZFP), Germany

The old focus on using automation was simply to increase productivity and reduce costs. Today, the purpose of automation has shifted to broader issues. Highly automated processes ensure high quality on a constant level, if it is connected with a high degree of (automated) monitoring and control. Meanwhile, continuous process and quality monitoring by non-destructive testing (NDT) is an accepted procedure to early diagnosis of irregular process conditions, followed by an NDT-based feed back control and optimization. Consequently, the development of process integrated NDT is an important scientific task. Such developments have to fulfil the requirements of today’s industrial production concerning integrability, automation, speed, reliability and profitability. Flat steel production and processing, gas assisted injection moulding and cold joining processes are some examples of a successful application of the process integrated NDT. ©2011 Journal of Mechanical Engineering. All rights reserved. Keywords: automation, mechanical material properties, micro-magnetic method, microwaves, nondestructive testing (NDT), process integrated NDT, process control, process monitoring, residual stress, tool-integrated sensor, ultrasound 0 INTRODUCTION Automation was developed in order to avoid hazardous or unpleasant manual operations and to increase productivity. On the other hand, automation should also be considered as a social issue since the early days of industrialization because manual labour was replaced with lessexpensive machines. Today, automation of the workforce is quite advanced and is encroaching on ever more skilled jobs. If the old focus on using automation was simply to increase productivity and reduce costs, currently the purpose of automation has shifted to broader issues. Automation is now often applied primarily to increase quality in the manufacturing process. By replacing manual work with an automated process, error rates can be reduced drastically. Even if automation is viewed as a way to minimize human error in a system, increasing the degree and levels of automation also increases the consequences of error. With increasing levels of automation the consequences of an error rapidly approach a catastrophe. It is for this reason that a high degree of process automation requires a high degree of (automated) monitoring and control of process and product quality. Currently, the manufacturing industry is facing significant new challenges. Competitive producers have to be able to offer a broad range 218

of products with specific, custom-tailored technological properties in a short time. This requires an increasing number of production processes of high complexity. Reduced development times and increased diversity of products and processes also means that there is less time to ensure that the manufacturing processes is capable of producing appropriate product quality before high-volume production starts. Nevertheless, manufacturers are becoming less vertically integrated and increasingly rely on external suppliers even with highly sophisticated parts and components. These new challenges of automated manufacturing result in a steadily increasing need for non-destructive testing (NDT), which as an integrated component of the process, allows its continuous monitoring and qualitybased control. Meanwhile, a variety of examples for in-process NDT can be found in beam welding processes. In case of laser beam welding, optical sensors are used to adaptively control the focus position and laser power by monitoring the weld seam line and its geometry or the light reflected from the weld seem. For example, Kaierle et al. [1] have proposed such an on-line quality control scheme. More sophisticated NDT, like laser-ultrasound and acoustic emission allow to determine penetration depth and to detect weld seam errors in real time, as proposed by Satoru

*Corr. Author’s Address: Fraunhofer Institute for Non-destructive testing, Campus E3 1, D-66123 Saarbruecken, Germany, bernd.wolter@izfp.fraunhofer.de


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[2]. The use of tool-integrated ultrasound and acoustic emission for endpoint control and process optimization during resistance spot welding was reported from several authors, for example from Waschkies [3] and Dennison et al. [4]. Schneider et al. [5] have used tool-integrated ultrasound for end-point control during the screw driving (see section 3.3). Acoustic emission is also well known as a tool for detecting tool wear and break and other irregular process conditions during machining process, as for example reported from Guo and Ammula [6] or Chen and Griffin [7]. In order to determine variations of mechanical properties or residual stress during machining, micro-magnetic sensors can be used, which are integrated into the machining tool, as proposed by Wolter et al. [8]. As described in section 3.1, another application of micro-magnetic methods is the monitoring of quality characteristics in flat steel products during producing and processing (Altpeter et al. [9], Dobmann et al. [10], Wolter et al. [11]). Other NDT methods are needed if the processing of non-metallic materials, like polymers should be monitored and controlled. An example, reported by Sklarczyk et al. [12] will be described in section 3.2. 1 BENEFITS FROM NDT BASED PROCESS MONITORING AND CONTROL In addition to the final inspection of products, NDT can be used during manufacturing in terms of monitoring and even control of process quality, as described in Fig. 1. Quality control with

NDT combines the advantages of both classical methods of process control, which are statistical process control (SPC) on the one hand and monitoring of process variables on the other. The intent of SPC is to monitor product quality and maintain processes to fixed targets. It aims to get and keep processes under control. However, - due to its probabilistic nature - SPC can keep a process under control only to the extent that it can indicate when a process might have gone out of control. Therefore, process readjustments or repairs are always delayed. In principle, a continuous preservation of process quality is not possible with SPC. Another approach is the real-time model-based quality control by monitoring of process measurable (force, pressure, temperature, etc.). A process disturbance can be identified and based on a known control model and the process can be readjusted or adapted immediately although the real-time quality feedback to this disturbance is lacking. An additional quality model is necessary, in order to determine to what extent quality has been affected. Automated NDT is capable of performing all SPC functions in real-time or at least several times faster than standard SPC. As a monitoring tool, it allows to mark and discard all nonconforming parts. Only good material goes â&#x20AC;&#x153;out the back doorâ&#x20AC;?. Consequently, the process limits can be exhausted, allowing increased productivity. On the other hand, process disturbances affecting the product quality can be detected and

Fig. 1. Quality control with NDT NDT Based Process Monitoring and Control

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located directly. The process can be controlled using its quality characteristics as control variables directly. 2 REQUIREMENTS FOR PROCESS INTEGRATED NDT Process-integrated NDT has to meet a variety of requirements. Customers ask for NDT systems, which are integrable, automatable, realtime capable, reliable, profitable and flexible. 2.1 Integrable, Mountable Miniaturized, rugged, low-maintenance NDT sensors at moderate costs are required. An example is described in section 3.2 and was presented by Sklarczyk et al. [12]. As a further example, Kloster et al. [13] have shown that GMR (giant magneto-resistance) gradiometers allow to design very compact stray flux sensors with integrated pre-magnetization device. Dobmann [14] has reported that the general trend for process sensors evolving from simple analogous measuring recorders to the so-called “smart sensors” or “intelligent sensors” with integrated signal processing or even with integrated actuator will be transferred to NDT sensors too. Bräuer et al. [15] have used thin-film technology to integrate temperature or force sensors as a “sensitive skin” on the surface of a machine or a tool. It is expected that the combination of thinfilm technology with other physical measurement effects permits the development of new toolintegrated NDT sensors. The communication with process integrated NDT systems has to account for established industrial interface standards. Industrial process control systems commonly make use of field-buses, which are specialized for the process control environment (Profi-bus, LonTalk, SDS, etc.). 2.2 Automatable For most manufacturing processes the degree of automation is permanently increasing. The vision of an “autonomous production” includes the idea that all desired tasks can be performed without or at least with a minimum human interaction. This is true also for tasks of 220

quality monitoring and control. Therefore, not only sophisticated mechatronical layouts for sensor and / or part manipulation have to be developed, but also intelligent concepts for automated acquisition, evaluation, transfer and management of inspection data, right up to autonomous procedures for selfcalibration and self-maintenance. 2.3 Real-Time Capable For a variety of NDT techniques (e.g. micro-magnetic methods, nuclear magnetic resonance - NMR) the inspection speed is limited by physical constraints. With these methods, only relatively slow processes can be monitored in realtime. Often not only a slow data acquisition rate, but also the subsequent data processing is limiting the testing rate. Especially in case of fast image reconstruction, evaluation and presentation, fast transfer and computation of the raw measuring data is required. “Multi-Link G-bit Ethernet”, “Routed Fiber-channel Link”, or “Multi-channel LVDS” are non-standard solutions for fast data transfer. Sophisticated data processing algorithms as well as fast devices for parallel computing like Graphics Processing Units (GPU) and Field Programmable Gate Arrays (FPGA). Such approaches have been used for NDT systems since Bulavinov et al. [16]. 2.4 Reliable and Comprehensive The integrated NDT technique has to provide the measurable with reliability, which is predetermined by the quality requirements of the process, which means that the NDT technique itself has to meet the criteria of measuring gauge capability standards in order to monitor and control the quality capability of the process. In order to achieve a required process capability index CP, the NDT measurement uncertainty must not exceed a defined percentage of the process tolerance range. In some cases a single NDT method is not sufficient to acquire all relevant quality information of the process with the obligatory measuring uncertainty. Therefore, the trend can be recognized to combine several NDT methods with partly divers and partly redundant information.

Wolter, B. - Dobmann, G. - Boller, C.


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Fig. 2. Sensor holder and table carrier of the micro-magnetic in-line inspection system 2.5 Profitable and Flexible Nondestructive testing systems are not mass-produced products. The small number of marketable systems and the specifics of individual test requirements lead to high engineering expenditures at low sales volumes. Especially process integrated NDT systems are often â&#x20AC;&#x153;unique itemsâ&#x20AC;?, leading to high costs for purchase and maintenance. However, these costs are faced with a variety of savings, which are often overlooked. These include not only the saved non-conformity costs (costs due to further processing, eliminating the nonconforming material, call-back, product liability, etc.) but also the saved costs for manual tests (destructive and nondestructive) and the possibility to increase productivity and yield. On the other hand, ways have to be found to reduce the engineering expenditures for highly sophisticated NDT systems. As claimed by Kroening et al. [17], flexible, cost-effective solutions for NDT hardware and software could be developed based on modular structured platforms, which support a further development of a preferably large field of individual NDT techniques. 3 CASE STUDIES 3.1 Quality Control in Flat Steel Production and Processing The mechanical-technological properties characterize the fitness for use of a material under various conditions. In addition to thickness,

width, surface finish and flatness, these properties are of central importance to the quality of hot and cold rolled steel products. These material properties are adjusted during several production steps. Therefore, there is the need to monitor and control these quality parameters continuously. State of the art to determine yield and tensile strength is the selection of standardized specimens at the end of the process and destructive testing according to the definition in the inspection laboratory using standard tensile testing machines. Hardness measurements are performed by using standardized indentation techniques according to Brinell or Vickers. This destructive testing is obviously not an adequate solution for online monitoring and control. The traditionally process orientated steel industry has a strong interest to replace these time-consuming and expensive destructive tests by more appropriate methods. Micro-magnetic testing, i.e. nondestructive (ND) material characterization with electromagnetic methods is generally suited for this purpose. In a ferromagnetic steel mechanical and magnetic properties of are influenced by the same microstructural parameters (lattice defects). Therefore, correlations between both, which can be used to predict values of mechanical properties from measured values of magnetic properties, can be observed. A technically mature application of micro-magnetic NDT is the inline monitoring of mechanical parameters in strip steel during production, as shown in Fig. 2. This application was described by Wolter and Dobmann in 2006.

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Another possibility is to attach the micromagnetic sensor to a robotic arm, as it is shown in Fig. 3 (Dobmann et al. [14]). This allows the maximum possible flexibility regarding the sensor application to the strip surface.

Based on this method the manipulated variables of the process (stamp force, path, etc.) can be optimized for different conditions (sheet thickness, steel grade, etc.) and furthermore, it allows to prevent process malfunctions (rip-off, spring-back of the sheet) by a feed-back control based on the detected strain hardening in the material.

Fig. 3. Sensor attached to a robot for gradual inspection of several parallel running strips Micro-magnetic testing can be used not only for monitoring and control during steel sheet production, but also during sheet processing, i.e. deep-drawing. Altpeter et al. [9] have developed a miniaturized sensor (see Fig. 4), which can be integrated into the stamp of a deep-drawing machine.

Fig. 5. Heavy plate inspection with micromagnetic techniques, a) inspection trolley, b) remote control desk

Fig. 4. Miniaturized micro-magnetic sensor, integratable into a deep-drawing stamp With this tool, material behaviour during the deep-drawing process can be observed. It allows the modification of mechanical properties as well as the arising stresses and strains in the material to be monitored during the process. 222

Mechanical-technological properties are also important for heavy plate production. The customer asks for geometrical and mechanical properties, which are uniform across product length and width, especially for high-value grades. For a plate with a length of several meters, the boarders are usually subjected to other cooling conditions than the rest. Indeed, especially the plate ends are known to cool faster, generating an undesired increase in Rm and Rp0.2. State-of-the-art is to determine this so-called â&#x20AC;&#x153;cold endsâ&#x20AC;? based on empirical values and cut them off. The destructive

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testing of the cold ends follows this cutting. Therefore, the cutting itself is not a controlled process and if too much or to less material has been cut-off, this will only be realized in hindsight. This lack of knowledge results in enormous costs due to reworking, pseudo-scrap and delayed shipment release. The European steel producers put their annual costs at 11 million Euros. For that reason an NDT solution to locate the cold ends of the plate was developed. Knowing exactly the contour of the zone with unacceptable material allows an open loop control for the cutting process. It has been established that for determination of mechanical properties in highly texturized materials, it is beneficial to combine micro-magnetic methods with ultrasonic time-offlight measurements in order to get more accurate and reproducible results. The NDT equipment was integrated into a remote controlled trolley, allowing the sensor to be half-automatically moved along the surface of the plate (Wolter et al. [8]). The left picture in Fig. 5 shows the trolley in operation on a heavy plate on the roller conveyor and the right picture shows the control desk for remote control.

In contrast to conventional microwave sensors, this module is capable of measuring not only amplitude but also phase and frequency. The entire size of this module is small enough to be mounted into the gas-assisted injection moulder. A high temperature resistant plastics window transparent to microwaves is used as a dielectric antenna and separates the cavity from the waveguide. Fig. 7a gives the scheme of the measurement arrangement. When the cavity is filled with liquid plastics and when a gas bubble passes over the position of the window the wave propagation inside the cavity is modified resulting in a measurable change of the reflection and scattering behaviour inside the cavity.

3.2 Microwave Monitoring of Gas Assisted Injection Moulding The plastics industry has an increasing need for online monitoring of injection moulding processes. Gas-assisted processes (gas injection technique GIT) are applied to save plastics material and to assure the constancy of the shape of the produced part. The aim of the process monitoring and surveillance is to assure the proper sequence of process steps and the correct position of the gas bubble inside the cavity. Existing monitoring methods like the measurement of temperature or pressure inside the cavity provide only indirect information or are often not quick and specific enough with regard to gas-assisted processes. As it has been reported by Sklarczyk et al. [12], Fraunhofer IZFP and ICT have developed a miniaturized GIT monitoring system based on the 94 GHz radar sensor of Fraunhofer IAF (see Fig. 6).

Fig. 6. Radar module for monitoring of GIT process, a) flanged horn antenna, b) inside view of the radar module Fig. 7b presents the result gained during an injection moulding test with gas assistance. The important process steps like the passage of the plastics melt and of the gas bubble over the

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position of the millimetre wave window could be found by abrupt changes of the measuring quantities amplitude, phase and frequency of the IF-signal. They could be identified unequivocally by comparison with a video movie taken during the test through an optical window. The three measuring quantities can be combined to improve the validity of the method. Theoretical calculations using ANSYS FE code confirmed the findings and can be used in the future to optimize the measuring parameters like frequency or type of the millimetre wave window. Thus, the process sequence can be controlled in a quick and direct way. 3.3 Tool-Integrated Ultrasonic Systems for Control of Cold Joining Processes In case of screw joints the pre-stressing force is significant for the joint strength and therefore, this force can be used to control the screwing process. Screw break or joint release after applied load are extreme consequences of an improper adjusted pre-stressing force. In order to control the screwing process, usually the torque moment or the angle of rotation is measured during the process. Both control variables could be incorrect in predicting the pre-stressing force due to the a priori unknown influence of friction loss between the screw head and support surface or between the screw thread and the mating thread. It is well known that the screw elongation caused by pre-stressing can be determined by ultrasonic time-of-flight (TOF) measurements. Not only the increase of macroscopic screw length but also a decrease in ultrasonic velocity due to the acoustic-elastic effect result in a rising

TOF, when pre-stressing force is extended. Commercially available screwing control devices based on ultrasound measure the TOF in the screw only before and after but not during the screw drilling process. Afterwards the screw elongation or the pre-stressing force is determined by means of stored calibration tables. Therefore, these conventional devices only allow the final inspection of the screw joint and if necessary the correcting of an improper prestressing force. Measuring the ultrasonic TOF during screwing, as it has been described by Schneider et al. [5] allows a closed loop control of the process. For this purpose, the ultrasonic sensor and part of the electronics have to be integrated within the power screwdriver (see Fig. 8). Furthermore, the screw itself has to be applied with an ultrasonic coupling / reflection coating on its head / shank. This results in increased piece costs for the screws, impairing the market acceptance of the method. To overcome this drawback, the screwdriver can be equipped with a special coupling sheet in order to avoid a special pre-treatment of the screws. Clinching, or press joining, is a highspeed mechanical fastening technique for point joining of sheet metal. It is a fast and simple single-step technique requiring no consumables or pre-drilled holes. Clinching can be used on coated and painted materials, and is suitable for joining dissimilar materials. During clinching, the sheets are squeezed between a punch and a die. Due to the high local pressure, the material start to yield, whereby material is expelled sideways forming an interlocking button (see Fig. 9a). It is this interlocking button that holds the sheets together. Currently, the clinching process is

Fig. 7. GIT monitoring with radar, a) scheme GIT monitoring, b) radar measuring quantities versus time 224

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usually controlled by force-travel monitoring. The force-travel curve is controlled in order to follow a predefined target curve. However, this method is generally insensitive since local variations in undercut, neck or residual bottom thickness, which can considerably affect the joint strength, are hardly detectable in the force-travel curve.

production, assuring product and process quality with a minimum of man-machine interaction. Consequently, NDT techniques have to evolve from simple measuring machines to intelligent control systems. Today, the technologies to enhance the automation dregree and the “intelligence” of NDT techniques are available. Advanced miniaturized sensors with integrated signal evaluation techniques, fast electronics and algorithms for real-time data processing along with comprehensive information on the processing conditions and the properties and characteristics of the materials being processed, will provide new opportunities for efficient NDT-based real-time process monitoring and control in near future. a)

Fig. 8. Closed loop control of screwing process with screw nut integrated ultrasound A new approach is to continuously measure the residual thickness of the punch-sided and the die-sided sheet with tool-integrated ultrasonic sensors (see Fig. 9b). Based on these measuring data, the residual bottom thickness and with it, the thickness of the undercut can be determined during the process. Determining this quality characteristics in-situ allows a reliable control of the clinching process.

b)

4 CONCLUSIONS Case studies have shown that NDT aided production provides a significant contribution in order to improve product quality, reduce scatter in properties, minimise scrap and improve process economy. Therefore, it is not surprising that the traditional tasks of NDT - preventive maintenance and off-line quality control - are more and more extended towards process integrated monitoring of quality characteristics and open / closed loop process control. Μοdern NDT techniques used in the manufacturing industry have to follow the still strengthening trend to ever higher degrees of automation. In the future NDT will be an essential component of a more or less autonomously running

Fig. 9. Monitoring of clinch point quality with tool integrated ultrasound, a) quality characteristic of the clinch point, b) ultrasonic sensors integrated into punch and die 5 REFERENCES [1] Kaierle, S., Abels, P., Fiedler, W., Mann, S., Regaard, B. (2008). OnlineQualitätssicherung für das Laserstrahlschweissen. Maschinenbau - Das Schweizer Industrie-Magazin, Jahres-

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Hauptausgabe MB-Revue, p. 132-135. (In German) Satoru, S. (2006). Sensing technology for the welding process. Welding International vol. 20, no. 3, p. 183-196. Waschkies, E. (1997). Process-integrated resistance spot welding testing using ultrasonic technique. Welding in the world. vol. 39, no. 6, p. 345-350. Dennison, A.V., Toncich, D.J., Masood S. (1997). Control and Process-Based Optimisation of Spot-Welding in Manufacturing Systems. International Journal of Advanced Manufacturing Technology, vol. 13, p. 256-263. Schneider, E., Herzer, H.R., Braunbach, K.H. (2005). Ultraschall-System zur on-line Bestimmung der Schraubenvorspannkraft und zur Schraubersteuerung, Proceedings of the DGZfP-Jahrestagung 2005, Berlin, Deutsche Gesellschaft für zerstörungsfreie Prüfung (DGZfP), V45. (In German) Guo, Y.B., Ammula, S.C. (2005). Real-time acoustic emission monitoring for surface damage in hard machining. International Journal of Machine Tools & Manufacture vol. 45, p. 1622-1627. Chen, X., Griffin, J. (2007). Grinding Acoustic Emission Classification in Terms of Mechanical Behaviours. Key Engineering Materials, vol. 329, p. 15-20. Wolter, B., Kern, R., Kopp, H. (2005) Schleifwerkzeug mit integrierten mikromagnetischen Sensoren. European patent EP 1604782 A1. Altpeter, I., Kopp, M., Kröning, M., Milch, M., Schäffner, C., Behrens, B.A. (2006). Influences on the part quality in conventional deep drawing processes. Proceedings of the 9th European Conference on Nondestructive Testing, Deutsche Gesellschaft für zerstörungsfreie Prüfung (DGZfP), Berlin, Fr.1.7.3. Dobmann, G., Altpeter, I., Wolter, B., Kern, R. (2008). Industrial applications of 3MA - micromagnetic multiparameter microstructure and stress analysis. Proceedings of the 5th International Conference Structural Integrity of Welded Structures: ISCS 2007, Timisoara. Wolter, B., Kern, R., Schneider, E., Bucholtz, O.W., Hofmann, U., Meilland, P. (2005). Zerstörungsfreie Bestimmung

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von Qualitätsmerkmalen bei der Grobblechfertigung, Proceedings of the DGZfP-Jahrestagung, Berlin, Deutsche Gesellschaft für zerstörungsfreie Prüfung (DGZfP), V41. (In German) Sklarczyk, C., Surkov, A.S., Langenberg, K.J., Mayer, K. (2006). 94 GHz radar sensor for process control and imaging. Proceedings of the 9th European Conference on Nondestructive Testing, Deutsche Gesellschaft für zerstörungsfreie Prüfung (DGZfP), Berlin, We.2.8.4. Kloster, A., Kroening, M., Smorodinsky, J., Ustinov, V. (2006). A linear magnetic stray flux array based on GMR gradiometers. Electromagnetic Nondestructive Evaluation VII - eNDE, IOS Press, Amsterdam, p. 173179. Dobmann, G. (2007). Sensoren in der zerstörungsfreien Prüfung: Ziel ist die Charakterisierung eines Werkstoffes oder Bauteils, Sensors, p. 18-21. (In German) Bräuer, G., Bandorf, R., Biehl, S., Dietz, A., Lüthje, H., Vergöhel, M. (2006). Intelligente Schichten für denkende Oberflächen. Vakuum in Forschung und Praxis, vol. 18, no. 6, p. 25-29. (In German) Bulavinov, A., Kroening, M., Reddy, K.M., Ribeiro, J.G. (2007). Real-time quantitative ultrasonic inspection. Proc. of the 4th PanAmerican Conference for Non-destructive Inspection, Buenoes Aires: Asociación Argentia de Ensayos No Destructivos y Estructurales (AAENDE), p. 69. Kroening, M., Ribeiro, J.G., Vidal, A. (2007). Progress in NDT system engineering through sensor physics and integrated efficient computing. Proc. of the 4th PanAmerican Conference for Nondestructive Inspection, Buenoes Aires: Asociación Argentia de Ensayos No Destructivos y Estructurales (AAENDE), p. 70. Wolter, B., Dobmann, G. (2006). Micromagnetic Testing for Rolled Steel. Proceedings of the 9th European Conference on Nondestructive Testing, Deutsche Gesellschaft für zerstörungsfreie Prüfung (DGZfP), Berlin, Th.3.7.1.

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, 227-236 DOI:10.5545/sv-jme.2010.173

Paper received: 22.08.2009 Paper accepted: 19.03.2010

Electromagnetic Imaging Using Probe Arrays

Mook, G. - Michel, F. - Simonin, J. Gerhard Mook* - Fritz Michel - Jouri Simonin Institute of materials and joining technology, Otto-von-Guericke-University Magdeburg, Germany Electromagnetic methods like eddy current technique do not provide images but merely produce local signals in a difficult-to- understand measurement plane. Trials to generate images comparable to x-rays are mostly based on costly and time consuming mechanical surface scanning. The paper presents theoretical and experimental steps ahead to modular probe arrays for eddy current inspection. In contrary to other attempts, these arrays work at low frequencies able to penetrate below the surface and provide good lateral resolution. In this way, they bring up not only surface defects but also hidden defects like 1 mm pores with 0.5 mm underlying. Potential industrial applications are outlined. ©2011 Journal of Mechanical Engineering. All rights reserved. Keywords: digital eddy current instrument, eddy current imaging, probe arrays, low frequency application 0 INTRODUCTION An open coil system fed by an alternating current induces eddy currents in a conductive material. The density and spatial distribution of these currents depends on the coil geometry, the frequency and some material properties like conductivity, magnetic permeability and geometry. The eddy currents build up a responding electromagnetic field interfering with the exciting field. Depending on the resultant field, the coil system provides complex valued signals carrying information about the material properties. To increase the low frequency performance of the receiver coil more windings, high permeable ferrite cores or sophisticated balancing of two or more coils can be used. On the other hand, [1] used anisotropic magneto-resistors (AMR), [2] tried giant magneto-resistors (GMR) and [3] tested GMR arrays to substitute inductive receivers by new magneto-resistors can be used. These elements are able to sense even DC fields. Their drawbacks are non-linearity, saturation, hysteresis effects and the demand for DC offset. For eddy current applications, newly developed inductive receivers perform not worse than magnetoresistors and are easier to handle. Therefore, the following paper focuses on inductive systems.

1 SENSOR ELEMENTS 1.1 Imaging by Scanning To generate a fingerprint of the electromagnetic properties of the material a single probe (coil system) scans the surface track by track. This way, [4] to [6] record the real and imaginary part of the probe’s signal, they are processed according to the material properties of interest. The resultant complex signal is displayed correspondingly to the probe position. Within one scan of the probe, two pictures may be recorded; one represents the real part and the other represents the imaginary part of the signal. Fig. 1 illustrates this method. The papers [7] and [8] demonstrated the advantage of this single probe scanning: high quality of imaging due to the constant probe characteristics. The drawbacks are the expensive scanners and the time consuming imaging process. Additionally, scanners are limited to simply shaped objects like flat or uniaxial bent surfaces. More complicated surfaces have to be inspected manually without imaging. The idea of scanning the surface using an electronically moved field was presented by [9] to [20] who developed this idea using different probe modifications for dedicated applications.

*Corr. Author’s Address: Institute of materials and joining technology, Otto-von-Guericke-University Magdeburg, POB 4120, D-39016 Magdeburg, Germany, mook@ovgu.de

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1.2 Point Spread Functions For imaging applications, the Point Spread Function of the probe is of most interest. Fig. 2 displays the Point Spread Function of some conventional probes widely used in eddy current inspection. With absolute probe a point-like defect is spread to a crater indicating low sensitivity directly below the probe, called the blind spot. The best sensitivity is observed below the edge of the ferrite core. The differential probe provides a bipolar Point Spread Function with a blind line along the gap between the receiver coils. Multidifferential probes are well suited for crack detecting of unknown orientation but provide the most complicated and less suited Point Spread Function for imaging.

Fig. 1. Electromagnetic imaging using a single scanning eddy current probe

Fig. 2. Point Spread Functions of conventional eddy current probes One kind of inductive probes with increased inspection depth is the non-axial transmit-receive probe sometimes called pitchcatch, half transmission or even remote field probe deeply investigated by [21]. These probes offer the opportunity to optimize the distance between the transmitting and the receiving coil. Fig. 3 brings up the principle of those probes. The magnetic 228

field of the exciting coil penetrates accordingly to the well known rules of alternating field spreading into the material. The receiving coil only picks up this part of the flux, which has penetrated deeply into the material. The larger the spacing between the two coils the deeper the detected flux lines have penetrated into the material but the lower becomes the measurement signal. This system of two non-axial coils may be considered as an axial coil system with a diameter corresponding to the coil distance of the non-axial system. With increasing distance (or diameter) of the coils the defect volume decreases relatively to the volume of interaction lowering the signal amplitude. One has to trade off between these parameters.

Fig. 3. Non-axial probe selects deep penetration field trajectories Fig. 4 displays the Point Spread Function of this probe. It differs significantly from that of common axial eddy current probes, presented by [22] and is most suitable for imaging applications. The shape resembles a Mexican hat. A sharp positive maximum is surrounded by a negative brim. This shape does not seriously change with increasing spacing between the coils. How does the signal magnitude depends on the spacing? To answer this question, calculations were conducted with different spacing values. Fig. 5 presents the result in relation to the lift-off signal of 0.01 mm. With increasing spacing, the signal ratio only slowly decreases. For hidden defects with more than 2 mm underlying, the signal ratio gains its maximum at a certain value beyond zero. For assessing the chances for defect characterization the complex values of the measurement voltage were calculated for different

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Fig. 4. Calculated signal distribution of a small hidden pore, x and y - position of the pore (calculated using VIC-3D Volume Integral Code for three dimensional calculations by Sabbagh Ass., Inc.) inspection situations. Commonly all defect signals are referred to the lift-off signal. Fig. 6 brings up the signal behaviour of a pore with defined underlying. Over the sound material the signal is centred at the balance point. When approximating a defect, the signal trajectory starts with negative y-values. Just like axial probes the signal turns to positive y values over the defect. The defect signal turns clockwise with increasing defect underlying. This circumstance opens up the opportunity of assessing the defect underlying. The signal magnitude mirrors the defect volume.

pitch-catch probe shown in the left column of Fig. 7 is based on one central transmitting coil and six receiving neighbouring coils. The number of probes formed by this method is more than twice the number of coils. The single core probe in the right column of Fig. 7 spreads its magnetic field also to all neighbours but is recorded by the transmitting element. The number of probes equals the number of cores. This way, two operation modes may be used: the pitch-catch mode and the single core mode. In any mode, a transmitter separator switches the transmitting coils and a receiver multiplexer switches the receiver coils. Both work independently from each other. At any time slot only one probe is active. a)

Fig. 5. Relative signal magnitude vs. coil spacing and defect underlying 1.3 Cascading and Operation Modes Fig. 7 compares the Point Spread Function of both probe types cascadable to an array. The

b)

Fig. 6. Complex measurement signal of a pitchcatch-probe moving over hidden pores, a) calculated signals, b) measured signals Contrary to single probes used in conventional applications the probes in an array interact. Intended or unintended the magnetic field of a transmitting coil is guided by the neighboured cores. Fig. 8 shows this situation for the pitchcatch mode. The field of a single pitch-catch probe

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in Fig. 8a) concentrates the magnetic field of the transmitting coil (dark) to the core of the receiving coil (bright) and the sensitivity of the probe is 100%. When cascading the coils along a probe line the transmitted field is distributed to two neighbours (Fig. 8b) although only one of them is active in every time slot. The flux through the passive neighbour remains unused. This way the usable flux and the sensitivity reduce to one half. When arranging two shifted lines the flux through the active element diminishes to one quarter and in a complete two-dimensional array to one sixth. This fragmentation of the flux reduces the effect of flux enhancement by ferrite cores. This drawback of rod cores can be avoided by pot cores but here spatial resolution will be lower due to a larger diameter of these cores. Alternatively, air coils could perform close to that of rod cores and are worth further investigation. a)

b)

Fig. 7. Bimodal sensors, their Point Spread Function and cascading to an array, a) pitchcatch probe, b) single core probe (parametric or transformer)

Fig. 8. Fragmentation of the magnetic flux to a) one, b) two, c) four and d) six neighbours Concerning the quality of imaging, the spatial resolution plays an important role. In the single core mode every coil corresponds to one probe and spatial resolution is that of the core density. Along a probe line the Point Spread Functions are superimposing as it is shown in Fig. 230

9 above. The part below explains that in the pitchcatch mode the spatial resolution along a probe line is twice as high.

Fig. 9. Spatial resolution in a) single mode, b) pitch-catch mode 2 PROBE LINES Two probe lines have been manufactured, each consisting of 32 and 64 rod core coils, respectively. The speed of electronic field movement ranges from 0.3 to 3 m/s according to the inspection requirements. To visualize a certain area of the workpiece the sensor line must be guided over the surface. When handled manually, a measuring wheel connected to the sensor picks up the distance. In automatic inspection, a robot can guide the sensor with constant velocity from 6 to 48 mm/s. Fig. 10 displays the probe lines and Fig. 11 shows their components. The eddy current instrument is reduced to a multiplexer and an AD/DA-converter. All necessary electronics is united in the probe housing. A simple USB cable connects the probe line to a notebook. All other components and functions are addressed to the software. Fig. 12 presents the first result of detecting hidden holes in an aluminium plate simulating hidden pores in aluminium casts. The underlying varies from 0.1 to 0.5 mm. The adjustment of the eddy current signals is chosen in a common way. The lift-off signal is turned horizontally to the left in the XY-plane. All holes can be visualized in a gray scale or false colour image. For a more detailed evaluation of the hidden defects the depth of underlying can be colour coded. This depth is represented by the phase angle of the defect signal. Fig. 13 shows how this phase angle can be converted into a

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colour with clear association to severity of the defect. The closer a pore comes to the surface, the higher the probability of opening this pore during machining. The left part of Fig. 13 displays three signals of 1 mm holes simulating hidden pores. The closest underlying provides the leftmost signal, while the farthest gives the rightmost and with increasing underlying the signal amplitude diminishes. Pores of the same diameter provide decreasing amplitudes and clockwise turning signals with increasing underlying. For a suitable colour coding [23] proposed to code the phase shift and the signal amplitude by the colour and its saturation, respectively. The maximum saturation should be gained with diminishing amplitudes. a)

b)

For further assessment of the probe line, an aluminium sheet was engraved with letters and signs simulating defects of different size and orientation (Fig. 14a). The first number in every line indicates the height of the font in millimetres. By moving the line sensor over this reference sheet its “eyesight test” is obtained. a)

b)

c)

Fig. 13. Indication of three hidden holes simulating hidden pores, a) indication in the XYplane, b) translation of angle into a colour, c) the resultant image brings up the closest defect in red (dark), the farthest in green (bright) a)

b)

Fig. 10. a) Array probe 32, 1.5 mm ferrite rod cores, pitch-catch mode, coil centre distance 3 mm, total track width 45 mm, probe centre distance 1.5 mm, b) array probe 64, 1.05 mm ferrite rod cores, pitch-catch mode, coil centre distance 2 mm, total track width 61 mm, probe centre distance ca. 1 mm Fig. 14. a) Engraved aluminium sheet as reference. The first number in the line indicates the font size in millimetre, b) array probe 64 with measuring wheel on the sheet Fig. 11. Hardware components of the probe lines a)

b)

Fig. 12. Visualization of hidden defects in aluminium, a) moving field sensor, b) y-component image of the plate

When the engraving is turned up (Fig. 15a) the array probe 32 ends up with a 5 mm font. The Fig. 15b brings up the performance when “looking” through a sound 1 mm aluminium sheet on the engraving. The signal-to-noiseratio is lower but big enough for reading the font down to 7 mm font. Figs. 15c and d display the readability with a turned down engraving. The largest underlying is reached in Fig. 15d with an additional coverage of a sound 1 mm sheet. Here, the signal-to-noise ratio is reduced significantly but the 7 mm font may be read anyway.

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Summarizing this investigation, all parts of the letters and signs can be read independently of their orientation despite the anisotropy of the probe elements of the array.

Fig. 18 shows the eddy current images of two aluminium engine blocks. The outer shape can be recognized as well as 5 drilling holes. In addition, in every casting pore-like and loop-like anomalies can be seen.

Fig. 18. Visualization of hidden anomalies in two aluminium engine block

Fig. 15. Eyesight test of the probe array 32 Fig. 16 compares the spatial resolution of the probe arrays 32 and 64. With the array 64 most signs of the smallest font size 3 can be read. a)

Fig. 19 summarizes the results of aluminium squeeze castings. The bright spots in the indicated areas correspond to open and hidden defects. The other objects are drilling holes and outer shape.

b)

Fig. 16. Comparison of spatial resolution of the probe arrays, a) 32 and b) 64

Fig. 19. Open and hidden anomalies in aluminium squeeze castings

Fig. 17 displays a real aluminium cast with hidden anomalies. The photograph in Fig. 17a does not show any anomalies. The eddy current images recorded by the probe array 32 bring up patterns of local conductivity variations as points and a closed loop. The phase image indicates different underlying. After 1 mm rework the looplike pattern becomes visible.

For curved surfaces eddy current array probes may also be fitted. Fig. 20 gives some ideas for convex objects like pipes, rods or rails.

Fig. 20. Potential applications of probe arrays for curved surfaces 3 FLAT ARRAYS Fig. 17. Visualization of hidden anomalies in aluminium cast: a) photograph of the area of interest, b) electromagnetic amplitude and c) phase signature of this area, d) photograph after 1 mm rework brings up the anomaly visually 232

The principle of multiplexed coils may be extended to 2D-arrays. Fig. 21 presents a flat array with a sensitive area of 38 x 44 mm. The array electronics contains the 256-channel eddy current instrument and communicates with a Windows-

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Notebook via USB. The array need not be moved during imaging. The sensors may work in two different modes. In half-transmission mode each coil corresponds to its neighbours (705 sensors), in single core mode each coil works separately (256 sensors). In summary 961 sensors are available. Fig. 23. Photograph of the array and results of hidden corrosion in an aluminium sheet For increasing the signal-to-noise-ratio the software can integrate over a selectable number of subsequent images. The images may be stored for quality documentation or defect growth analysis. Based on the magnitude and the angle of the defect signal in the complex measurement plane defect classification becomes possible. Fig. 21. Flat array consisting of 16 x 16 coils working as 961 sensors covering an area of 38 x 44 mm. Dimension of the array housing: W × D × H = 60 × 60 × 82 mm The aluminium sheet with flat bottom holes was scanned by this array. Fig. 22 shows the results. Although the resolution is a little lower than that of the probe line all reference defects can be recognised. The build-up time is 0.64 seconds providing an image refresh rate of 1.56 Hz. a)

4 FREE-FORM ARRAY Although not flexible, the coil arrangement may be adapted to the inspection surface by moulding techniques. Once moulded and cured, the array forms a solid body easy to handle. As an example a spherical surface was transferred to the array. Fig. 24 gives an idea of the array and the aluminium reference. a)

c)

b)

Fig. 22. Visualization of hidden flat bottom holes in an aluminium sheet (conductivity 21 MS/m) using the 2D-sensor array, a) sketch of the sheet, b) electromagnetic images of 4 sections each covering 38 × 44 mm Fig. 23 presents a photograph of the array and gives an example of hidden corrosion detection. Here, the single core mode was selected giving a clear image of the shape of the corroded area. Again, the angle of the signals corresponds to the underlying of the defect.

b)

Fig. 24. Visualization of hidden holes in an aluminium hemisphere using a free-form sensor array, a) sketch of the array, b) sketch of the reference, c) electromagnetic fingerprints of the defect area Further potential applications are shown in Fig. 25. On the left side a corner is inspected covered by a dedicated array. On the right side, an array covers a part of a turbine blade.

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Fig. 25. Potential applications of free-form arrays on complex surfaces 5 MICRO-SCANNING When put on the engraved aluminium sheet (Fig. 26) the array provides images shown in the four right frames of Fig. 27. The individual sensors can be recognized in the image. Only the 10 mm font and partially the 7 mm font can be read.

the distance between the single sensors and accumulate a weighted resultant image. When moved by 0.5 mm steps the resultant image is much clearer and less noisy than every one of the original images. The right frame in Fig. 27 gives the evidence of the increase in readability down to the 5 mm font. The dark halo of every sign is the result of the Mexican hat Point Spread Function. When the engraving is turned down and covered by a sound aluminium sheet a single frame does not provide readable results. The existence of the engraving can hardly be recognized. The micro-scanning enhances the readability so that most of the 7 mm letters can be read. They are mirrored (Fig. 28). a)

b)

Fig. 28. Engraving on the bottom of the second layer, a) Sequence of 4 eddy current images in pitch-catch mode gathered with minimal shift of the array by 0.5 mm, b) Weighted images following the principles of micro-scanning

Fig. 26. The flat array on the engraved aluminium sheet for testing the spatial resolution a)

b)

The potential of micro-scanning is demonstrated in Fig. 29 using the single core mode. Here, the distance of individual sensors is 3 mm and the engraving cannot be read directly. Simply moving the array in horizontal direction over the engraving, the micro-scanning easily enables recognition of the 7 mm letters of the second line. a)

Fig. 27. Images of the flat array in pitch-catch mode on the engraved aluminium sheet, a) Sequence of 4 eddy current images gathered with minimal shift of the array by 0.5 mm, b) Weighted images following the principles of micro-scanning The idea of micro-scanning consists of moving the array by steps, much lower than 234

b)

Fig. 29. Single core mode of the flat array, a) Sequence of 4 eddy current images in pitch-catch mode gathered with minimal shift of the array by 0.5 mm, b) Weighted images following the principles of micro-scanning merely in horizontal direction

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For this micro-scanning the displacement of the array should be measured. Among the possible solutions mechanical systems like wheels or optical systems like sensors known from computer mice are found. These sensors calculate the displacement using two or more subsequent images of the substrate. Known techniques like differential image processing and 2D-correlation algorithms are implemented in a special processor inside the mouse. This method requires a difference in the treated images. When missing this difference the sensor is blind. This effect is known from computer mice when moved on smooth and unstructured surfaces. This principle of relative displacement measurement can also be accomplished by the eddy current probe array. Again, a structured area of inspection is needed. This “structure” may even be provided by defects. The array visualizes the movement of the defect and calculates the displacement of the array in x- and y-coordinates. When the object is free from defects or any other structure seen by eddy current method, the displacement cannot be picked up. For some applications, this restriction can be accepted. If only defect documentation is needed, the position of the array is not required. For complete visualization of the inspection area the position of the array should always be known. For this application the array is equipped with a laser optical mouse sensor nearly working on all surfaces. Fig. 30 explains how an array can fill the whole area of inspection without any gaps.

Fig. 30. Covering the area of inspection by a probe array track by track. An optical mouse sensor records the relative position of the array and helps to stitch the track images The micro-scanning has another advantage even in this situation. When scanning large areas,

the array may be moved fast. If there is any indication the sensor is moved slower and the image quality automatically will enhance for good visualization and documentation. In this way, an optimum between inspection speed and image quality is found. 6 CONCLUSION The moving electromagnetic field sensor visualizes open and hidden defects saving all advantages of eddy current inspection. Compared with single probe scanners the handling becomes much easier and cheaper. The eddy current hardware is reduced to a minimum and addresses most signal treatment to the software. In this way, the complete electronics is enclosed in the array’s housing and is easily connected to the notebook via USB cable. 7 ACKNOWLEDGEMENTS The presented work is partially supported by the “Bundesministerium für Bildung und Forschung” in the project group AL-CAST. 8 REFERENCES [1] Mook, G., Hesse, O., Uchanin, V. (2006). Deep penetrating eddy currents and probes. Proc. 9th ECNDT, Berlin, Tu.3.6.2. [2] Yashan, A., Bisle, W., Meier, T. (2006). Inspection of hidden defects in metal-metal joints of aircraft structures using eddy current technique with GMR sensor array. Proc. 9th ECNDT, Berlin, Tu.4.4.4. [3] Vacher, F., Gilles-Pascaud, C., Decitre, J.M., Fermon, C., Pannetier, M., Cattiaux, G. (2006). Non destructive testing with GMR magnetic sensor arrays. Proc. 9th ECNDT, Berlin, Tu.4.4.2. [4] Thomas, H.-M., Weigelt, G. (1991). Application of eddy current technique for depth assessment of corrosion in aluminium structures. DECHEMA - Final report 11L084. (In German) [5] Gramz, M., Stepinski, T. (1994). Eddy current imaging array sensors and flaw reconstruction. Research in Nondestructive Evaluation, vol. 5, p. 157-174.

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[6] Mook, G. (1998). Imaging eddy current inspection. 27. Proc. of Krajowa Konferencja Badan Nieniszczacych, Miedzyzdroje, p. 197-205. [7] Feist, W.D., Mook, G., Hinken, J.H., Simonin, J., Wrobel, H. (2005). Electromagnetic detection and characterization of tungsten carbide inclusions in non-ferromagnetic alloys. Advanced Engineering Materials, vol. 7, no. 9, p. 841-846. [8] Mook, G., Pohl, J., Michel, F. (2003), Nondestructive characterization of smart CFRP structures. Smart Mater. Struct., vol. 12, p. 997-1004. [9] Scholz, A. (1990). Probe mat - new approach for surface inspection by eddy currents. Proc. DGZfP Annl. Conf., Trier, p. 218-222. [10] Pelletier, E., Grenier, M., Chahbaz, A., Bourgelas, T. (2005). Array eddy current for fatigue crack detection of aircraft skin structures. Proc. Vth International Workshop, Advances in Signal Processing for Non Destructive Evaluation of Materials, Québec City. [11] Sullivan, S.P., Cecco, V.S., Obrutsky, L.S., Lakhan, J.R., Park, A.H. (1998). Validating eddy current array probes for inspecting steam generator tubes. ndt.net, vol. 3, no. 1. [12] Lafontaine, G., Samson, R. (2000). Eddy Current array probes for faster, better and cheaper inspections. ndt.net, vol. 5, no. 10. [13] Sollier, T., Talvard, M., Aïd, M. (2000). Use of EC sensor arrays on thin films. Upda, S.S. et al. (eds.), Electromagnetic Nondestructive Evaluation (IV), IOS Press. [14] Gilles-Pascaud, C., Lorecki, B., Pierantoni, M. (2004). Eddy current array probe development for non-destructive testing. 16th World Conf. on NDT, Montreal. [15] Zilberstein, V., Goldfine, N., Washabaugh, A., Weiss, V., Grundy, D. (2004). The use of fatigue monitoring MWM-arrays in production of NDI-Standards with real fatigue cracks for reliability studies. 16th World Conf. on NDT, Montreal.

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[16] Perez, L., Dolabdjian, C., Waché, C.W., Butin, L. (2004). Advance in magnetoresistance magnetometer performances applied in eddy current sensor arrays, 16th World Conf. on NDT, Montreal. [17] Joubert, P.-Y., Le Diraison, Y., Pinassaud, J., Satie, L. (2006). Eddy current imager for the detection of buried flaws in large metallic structures, Proc. 9th ECNDT, Berlin, Tu.3.6.1. [18] Decitre, J.-M., Premel, D., Mangenet, G., Juliac, E., Feist, W.D. (2006). Flexible EC array probe for the inspection of complex parts developed within the European VERDICT project. Proc. 9th ECNDT, Berlin, Tu.4.4.3. [19] Meilland, P. (2006). Novel multiplexed eddy-current array for surface crack detection on rough steel surface. Proc. 9th ECNDT, Berlin, Tu.4.8.1. [20] Grimberg, R., Savin, A., Leitoiu, S., Bruma, A., Steigmann, R., Udpa, L., Udpa, S. (2007). Automated eddy current data analysis. 4th Int. Conf. on NDT, Hellenic Society for NDT, Chania. [21] Reimche, W., Duhm, R., Zwoch, S., Bernard, M., Bach, F.-W. (2006). Development and qualification of a process-oriented nondestructive test method for weld joints to operate with remote field eddy current technique, Proc. 9th ECNDT, Berlin, Fr.1.7.2. [22] Mook, G., Michel, F., Simonin, J., Krüger, M., Luther, M. (2008). Subsurface imaging using moving electromagnetic fields and surface acoustic waves. Busse, G., v. Hemelrijck, D., Solodov, I., Anastasopoulus, A. (eds.), Emerging Technologies in NonDestructive Testing, Taylor & Francis, London, p. 275 -280. [23] Mook, G., Michel, F., Simonin, J. (2008). Eddy current moving field sensors - potential in industry. Proc. DGZfP Annl. Conf., St. Gallen, Mo.3.B.1. (In German)

Mook, G. - Michel, F. - Simonin, J.


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, 237-244 DOI:10.5545/sv-jme.2010.174

Paper received: 28.02.2008 Paper accepted: 29.10.2009

Study of Weak Electric Current Emissions on Cement Mortar under Uniaxial Compressional Mechanical Stress up to the Vicinity of Fracture Kyriazopoulos, A. - Stavrakas, I. - Anastasiadis, C. - Triantis, D. Antonios Kyriazopoulos* - Ilias Stavrakas - Cimon Anastasiadis - Dimos Triantis Laboratory of Electric Properties of Materials, Department of Electronics, Technological Educational Institution of Athens, Greece

An experimental technique that deals with the detection of weak electric signals emitted during the application of temporal uniaxial stress on solid materials has been applied on cement mortar samples. These electric signals are met in the literature as Pressure Stimulated Currents (PSC). Two different stress techniques were applied: uniaxial compressional stress at a) a low and b) at a high rate. Both qualitative and quantitative characteristics of the PSC are correlated with the mechanical state of the samples with respect to crack creation and propagation in the bulk of the material and consequently with the stages of composite damage. ©2011 Journal of Mechanical Engineering. All rights reserved. Keywords: cement mortar, electric current emissions, PSC, uniaxial stress, microcracks 0 INTRODUCTION It has been experimentally verified that mechanical stress application on geo-material samples is accompanied by the production of weak electric variations. Several laboratory experiments have been conducted to study the behaviour of geo-material samples under stress and have showed electromagnetic activity as well as electric current emissions. More precisely, experiments have been conducted on rock specimens suggesting that electric signals are produced by the piezoelectric effect due to presence of quartz [1] and [2], electrokinetic effect due to water movement [3] and [4], point defects [5] and [6], emission of electrons [7] and [8], moving charged dislocations (MCD) [9] to [11]. Laboratory experiments to detect and record weak transient current produced when brittle materials like marble and amphibolite are subjected to a temporal stress variation leading to a catastrophic process up to fracture have recently been realised [11] to [16]. The above mentioned electric currents are met under the term Pressure Stimulated Currents (PSC) and the corresponding experimental technique is known as Pressure Stimulated Currents technique. In the present work for the very first time the PSC technique is applied on cement

mortar samples and the results after systematic recordings are presented here. In this work, various experimental techniques showing up PSC were applied on cement mortar samples and PSC measurements were systematically recorded. Electric emissions in cement mortar under low compressional loading (less than 30% of compressional strength) have also been observed by other researchers [17] and were attributed to various mechanisms including crack opening. Additionally, electric current emissions have been recently reported on hardened cement paste and were attributed to the Piezoelectric effect [18]. A satisfactory interpretation of electric signal emission during the deformation of brittle materials after stress application is attributed to mechanisms of crack generation and propagation as well as to the moving charged dislocations by a number of researchers [9], [19] and [20]. According to the MCD model in an ionic or composite amorphous structure there will be an excess or absence of a line of ions along the dislocation line, with the consequence that the dislocation be charged. In thermal equilibrium, dislocation lines are surrounded by the DebyeHueckel charge cloud and will be electrically neutral [21]. In dynamic processes when dislocations move faster than the Debye-Hueckel cloud can follow, neutrality can no longer be

*Corr. Author’s Address: Laboratory of Electric Properties of Materials, Department of Electronics, Technological Educational Institution of Athens, Athens, 12210, Greece, akyriazo@teiath.gr

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maintained. It has been proved that when a brittle material is uniaxially compressed with a time varying stress σ, the Pressure Stimulated Current is proportional to the strain rate [11]: 1 dσ (1) dε I∝ , ∝⋅ ⋅ Ε dt dt where dε/dt is the compressional strain rate, dσ/dt is the stress rate and Ε is the modulus of elasticity. E is constant in the linear-elastic behaviour range while it varies in the non-linear deformation range and in the localized failure crack zone that follows, as a result of microcrack formation. As can be seen, Eq. (1) which comes from the MCD model is a tool giving qualitative rather than quantitative information.

This stepwise stress technique hereafter called Step Stress Technique (SST) is suitable to expose PSC in both the linear and non-linear ranges of the material deformation. Typical values of stress rate σ vary between 1.5 and 5 MPa/s and are always greater than those of the LSRT technique.

1 DESCRIPTION OF THE EXPERIMENTAL TECHNIQUES Two groups of PSC recording experiments are presented and they refer to the case when the samples suffer a uniaxial compressional stress. In the first group, the uniaxially applied stress σ is increasing linearly at a slow rate described by: σ = a · t, (2) where a is the stress rate, the values of which do not usually exceed 500 kPa/s and the ordinary values are around 100 kPa/s. At t = tf = σmax / a, where σmax is the ultimate compressional stress, the sample fails. This experimental technique henceforth will be referred to as Low Stress Rate Technique (LSRT). In the second group, while the sample is in a state of constant uniaxial stress σk, an abrupt stepwise stress increase of short duration Δt is applied so that the uniaxial stress increases by Δσ = σk + 1 – σk , where σk + 1 is the new state after the application of the stress increment (see Fig. 1). It must be noted that the new stress state σk + 1 remains constant until a following stress increment is applied. The aforementioned temporal variation of stress σ, as recorded during this experimental procedure can be described in a good approximation by Eq. 3.

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 σ k = constant for t < tk  σ (t ) =  σ k + b(t − tk ) for tk < t < tk +1 , σ = constant for t > t k +1  k +1

(3)

Fig. 1. Schematic representation of uniaxial stress σ variation with respect to time as it was recorded experimentally using the Step Stress Technique

Fig. 2. Schematic representation of the SST and LSRT experimental techniques Fig. 2 shows the schematic representation of the SST and LSRT experimental techniques. A pair of electrodes was attached to the sample in a direction perpendicular to the axis of the applied stress and the PSC measurements were achieved using a sensitive programmable electrometer (Keithley 6514) and all data were stored in a computer hard disk through a GPIB interface. The stressing system comprised an uniaxial hydraulic load machine (Enerpac–RC106) that applied compressional stress to the sample.

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The experiment was conducted in a Faraday shield to prevent electric noise. Between each sample and the stressing system thin Teflon plates were placed in the direction of stress, in order to provide electrical insulation. The electrodes were attached to the cement mortar sample, using conductive paste.

relative compressional stress acquires values greater than 0.7 approximately, the material is driven to a range of non-linear deformation and eventually into the localized failure zone.

2 SAMPLES Cement mortar samples were used, composed of Portland type cement OPC (Ordinary Portland Cement), sand and water at a ratio 1:3:0.5 respectively. The samples were utilized 3 months after their preparation in order to age properly and achieve an approximate 95% of their maximum strength. The maximum diameter of the sand grains of the composition was 2 mm. Its density was 2.2 gr/cm3 and its porosity was approximately 8%. Table 1 incorporates the characteristics of the samples including their geometric shapes, the fracture limits and the applied stress or load rates for each experimental technique. Fig. 3 shows a representative relative compressional stress ( σˆ ) - strain (ε) curve of the used samples. The relative compressional stress value is given as σˆ = σ / σmax, where σmax is the ultimate compressional stress. It is evident that it can be characterized by a linear material behavior at least up to stresses of 70% ( σˆ = 0.7) approximately of the ultimate compressional stress. In this range the material is characterized by a linear-elastic behavior and an elasticity modulus Ε. The stress σ on the material is related to the deformation ε (i.e. strain) according to the following linear law: σ = Ε0 · ε , (4) where Ε0 is the elasticity modulus of the undamaged material and according to the experimental data presented in the stress - strain curve Ε0 = 13 GPa approximately. When the

Fig. 3. Stress–strain diagram of the used cement mortar samples; the stress axis corresponds to the relative compressive stress 3 EXPERIMENTAL RESULTS AND DISCUSSION Representative PSC recordings from a set of experiments using both of the referred techniques (LSRT and SST) were conducted. The PSC recordings from the respective experiments are characterized by a systematic repeatability if all experimental requirements are carefully satisfied. In the first type of experiments (LSRT), the samples were loaded at a constant positive uniaxial stress rate 0.18 MPa/s. Fig. 4 depicts the recordings of the PSC emitted from cement mortar along with the relative compressional stress. As long as the sample is stressed uniaxially at low stress values corresponding to relative

Table 1. Characteristics of the samples, and parameters of the applied experimental techniques Sample Code CM01 CM02

Dimensions [mm] 50 x 50 x 70 50 x 50 x 70

Fracture limit [MPa] 59 52

Experimental technique LSRT SST

Stress rate [MPa/s] 0.18 ± 0.01 5.0 ± 0.1

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compressional stress values lower than 0.65 approximately, the PSC values are very small. According to what Eq. (1) indicates and given that in the LSRT technique (dσ / dt = const.) no PSC appearance should be expected in a range of relative compressional stress less than 0.65. The fact that a very weak but distinguishable PSC is recorded, must be related with the activation of some microcracks which might start from the interfaces between grains and cement especially in regions of the material near the compressed surfaces of the sample.

of localized charges is expected as a consequence of this. On the other hand, it has been observed on cement mortar samples that no measurable compressional stress – induced cracks have been formed before stress approaching 70% of the ultimate stress had been reached [22]. As can be seen in Fig. 4 the recorded PSC is a weak systematically increasing transient current. Although the creation and propagation of a microcrack is a random phenomenon in the catastrophic process, there is nevertheless, always a first microcrack which develops to a main – mother – crack defining a dominant orientation. Consequently, there must be a dominant current component whose orientation is related with the mother crack orientation. Evidently, the measured PSC corresponds to one or more of the current components which are definitely different from zero as has been proved experimentally.

Fig. 4. The behaviour of PSC with respect to linearly increasing stress (Low Stress Rate Technique); in the inset graph the PSC is shown on a logarithmic axis for σˆ > 0.6 When the relative compressional stress upon the sample becomes greater than 0.65 approximately, then, a very intense exponential increase of the PSC values is observed which is directly related with the fact that the material has been driven into the non linear deformation range (see Fig. 3). In this range the elasticity modulus gradually decreases and a PSC emission is also expected by Eq. (1). The PSC values increase rapidly and continuously up to the failure limit. In this range microcracks occur leading to the appearance of fresh surfaces due to bond breaking and lattice destruction. The appearance 240

Fig. 5. a) Presentation of the stepwise stress steps, b) the corresponding PSC recordings with respect to time If the PSC values are correlated with the relative compressional stress values σˆ in the range (zone nonlinear deformation) (0.7 < σˆ < 0.85) then, as can be seen in the inset diagram of Fig. 4, they are described by an exponential law of the form: I =⋅ I O exp (α ⋅ σˆ ) , (5)

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where IO is a current constant and the characteristic exponent α describing the magnitude of the PSC increase in the referred range, (after fitting) has a value α ≈ 24. The deviations of PSC values from the exponential law are evident when the relative compressional stress becomes greater than the value ( σˆ ≈ 0.85), a value corresponding to a transition zone signaling the onset of unstable crack growth and the material enters a localized failure crack zone. In this range the PSC values keep on increasing intensely and as the ultimate strength point is reached, the PSC gets to a maximum. During the second type of experiment (SST), four sequential abrupt stress increases at a constant rate (b = 5 MPa/s approximately) were performed (see Fig. 5a). The temporal variation of the PSC during the above procedure is depicted in the diagram of Fig. 5b. On application of each abrupt stepwise uniaxial compressional stress, an equally abrupt current spike appears, having a peak at a value PSCpeak, as soon as the stress gets to the final state σk + 1. It should be noted that the appearance of the PSC is due to the abrupt application of stress even at low stress levels. The abrupt stress increase during the stepwise stress procedure produces an inhomogeneous stress field within the non-homogeneous structure of the material under compression. When the local stress exceeds local strength, then, a microcrack occurs. During a microcrack opening two fresh surfaces are produced in the bulk of the material since the bonds are broken and the lattice is destroyed. These new surfaces are responsible for the charge separation phenomenon. Instantaneous charges are produced due to the destruction of the lattice and weak electric currents flow in order to get the distorted equilibrium state to a new stable equilibrium state. Thus, the PSCpeak can be interpreted. During microcrack creation and the consequent charge appearence, a time-varying microcurrent appears around the microcrack. The recorded PSC is the superposition of such microcurrents. Its magnitude gets to a maximum when the local concentration of microcracks gets also to a maximum. Particularly, when two or more cracks meet a hard aggregate particle like sand grain, they merge leading to the generation and propagation of a mother crack so that instead of a single

crack advancing, a whole family of cracks could gradually be formed, and this requires larger forces and uses more energy.

Fig. 6. The two relaxation mechanisms of the fourth step are characterized by two different slopes Accordingly, after the appearance of PSCpeak, relaxations of a complex exponential decrease law to background level follow. Fig. 6 depicts the PSC recording of the fourth abrupt uniaxial compressional stress step, on a logarithmic current axis with respect to time. It is evident that after the appearance of the PSCpeak, two relaxation processes occur. Fitting PSC values as a function of time indicates that an exponential relaxation law should exist, with an initially short relaxation time τ1 followed by a fairly longer τ2. Such a relaxation can be empirically described by the relation in Eq. (6).

I(t) = A1 ⋅ exp  - t  + A2 ⋅ exp  - t   τ1   τ2 

(6)

where A1 and A2 are constants. Results from other experiments conducted on materials like marble and amphibolite, are verified qualitatively [15] and [20]. After the PSC becomes maximum and taking into account that the stress remains constant, the microcrack production rate decreases rapidly and consequently the PSC decreases with a short relaxation time τ1. This decrease does not continue at the same rate because another mechanism may keep the PSC for a long time, so

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that the PSC relaxation takes place with a longer time constant τ2. A probable cause is the continuing material strain, even at a very low rate, although stress is unchanged. The new microcracks that go on appearing produce new microcurrents and result in conserving PSC at relatively high values that do not permit a direct relaxation to noise level.

after the abrupt stepwise uniaxial compressional stress procedure. Such an increase is directly related with the continuously increasing PSCpeak value (Fig. 7), and with the values of the relaxation time constants τ2 and τ2 , which, as it becomes evident from Table 2, keep on increasing with the value of the final state σˆ k +1 .

σˆ k +1

σˆ k +1

Fig. 7. Correlation of the PSCpeak values with respect to the final stress level σˆ k +1

Fig. 8. Correlation of the totally emitted electric charge with respect to final stress level σˆ k +1

The PSCpeak value of each PSC following the application of an abrupt stepwise uniaxial compressional stress depends on the final state σk + 1. In Table 2, PSCpeak values can be read with respect to the initial σk and the final σk + 1 states of each stress step, as well as with the respective relative compressional stress states σˆ k and σˆ k +1 . Fig. 7 depicts graphically the PSCpeak values with respect to the corresponding values of the final state σˆ k +1 of each step. The final states σˆ k +1 of steps 3 and 4 correspond to stress values which have driven the material into the non-linear deformation range ( σˆ k +1 > 0.7). The PSCpeak values are evidently greater than the initial. A similar behaviour has been observed in laboratory experiments using the SST technique on marble samples [12]. Table 2 shows the calculated values of the totally emitted electric charge Q during the four sequential abrupt stress increases as well as the values of the relaxation time constants τ1 and τ2. An intense increase of the electric charge can be observed (Fig. 8), as long as the value of the final state σˆ k +1 in which the material will relax

4 CONCLUSIONS

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PSC laboratory experiments were conducted on hardened cement mortar samples. It was found that the used samples exhibit a familiar behaviour, which has similarities to those of rocks like marble and amphibolite. In the present work, the experimental results can be summarised as follows: From the microphysical point of view it is noted that in the non-linear deformation range micro structural changes occur within the samples depending on the stress magnitude. They constitute the dominant form of all heterogeneities that determine the process of eventual failure. In particular, in the cement mortar there is a transition zone between the aggregate and the hydrated cement paste which constitutes a region of relative weakness containing a number of microcracks even before loading, during the shrinkage state. The increasing number of microcracks at the lateral edges of shear cracks reaches a minimum critical distance with respect to each other and begins to merge.

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Table 2. Values of the parameters of the SST technique (b ≈ 5 MPa/s) step 1 2 3 4

σk [MPa] 5.5 14.5 23.0 33.5

σk+1 [MPa] 14.5 23.0 33.5 42.0

σˆ k

σˆ k +1

0.11 0.30 0.48 0.70

0.30 0.48 0.70 0.88

The above are directly correlated with the emission of weak electric currents in all of the used techniques of mechanical stress. Both qualitative and quantitative characteristics of the Pressure Stimulated Currents may show the stress range to which the sample has been subjected. Summarizing, PSC measurements can provide a prediction of the stress state of the material relative to the crack openings and in general to the stages of composite damage.

PSCpeak [pA] 70.5 77.1 98.0 137.5

[6]

[7] [8]

5 ACKNOWLEDGEMENTS The authors wish to thank Prof. Z. Agioutantis, Director of the Rock Mechanics Laboratory of the Technical University of Crete for providing the equipment to draw the stressstrain diagram of the cement mortar samples. 6 REFERENCES [1] Nitsan, U. (1977). Electromagnetic emission accompanying fracture of quartz-hearing rocks. Geophysical Research Letters, vol. 4, p. 333-337. [2] Ogawa, T., Oike, K., Mirura, T. (1985). Electromagnetic radiations from rocks. J. Geophys. Res., vol. 90, p. 6245-6249. [3] Ishido, T., Mizutani, H. (1981). Experimental and theoretical basis of electrokinetic phenomena in rock-water systems and its applications to geophysics. J. Geophys. Res., vol. 86, p. 1763-1775. [4] Yoshida, S., Clint, O.C., Sammonds, P.R. (1998). Electric potential changes prior to shear fracture in dry and saturated rocks. Geophys. Res. Lettters, vol. 25, p. 15771580. [5] Varotsos, P., Alexopoulos, K. (1986). Thermodynamics of point defects and

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τ1 [s] 5.6 7.4 9.1 13.1

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Q [nC] 0.95 1.38 2.25 5.34

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paste. Cement and Concrete Composites, vol. 26, p. 717-720. Tzanis, A., Vallianatos, F. (2002). A physical model of electrical earthquake precursors due to crack propagation and the motion of charged edge dislocations. Hayakawa, M., Molchanov, O.A. (eds.), Seismo Electromagnetics: Lithosphere-AtmosphereIonosphere Coupling, TERRAPUB, Tokyo, p. 117-130. Tzanis, A., Vallianatos, F., Gruszow, S. (2000). Identification and discrimination of transient electrical earthquake precursors: Fact, fiction and some possibilities. Phys. Earth Planet Int., vol. 121, p. 223-248. Whitworth, R.W. (1975). Charged dislocations in ionic crystals. Advances in Physics, vol. 24, p. 203-304. Pomeroy, C.D. (1980). Physics in cement and concrete technology. Physics Education, vol. 15, p. 171-176. Kyriazis, P., Anastasiadis, C., Triantis, D., Vallianatos, F. (2006). Wavelet analysis on Pressure Stimulated Currents emitted by marble samples. Natural Hazards and Earth System Sciences, vol. 6, p. 889-894.

Kyriazopoulos, A. - Stavrakas, I. - Anastasiadis, C. - Triantis, D.


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, 245-256 DOI:10.5545/sv-jme.2010.175

Paper received: 22.08.2009 Paper accepted: 30.06.2010

Focus Variation – a Robust Technology for High Resolution Optical 3D Surface Metrology Danzl, R. - Helmli, F. - Scherer, S. Reinhard Danzl* - Franz Helmli - Stefan Scherer Alicona, Grambach, Austria

This article describes and evaluates the focus variation method, an optical 3D measurement technique. The goal is to analyse the performance of the method on a series of typical measurement tasks including roughness measurements, form and wear measurements. First, a comparison of roughness measurements between the proposed method and a tactile device on a newly developed roughness standard is made. Results show that both systems deliver Ra values that are comparable to each other with differences of a few nanometers. Afterwards form measurements are performed on a calibration standard with hemi-spherical calottes, showing a repeatability of sphere measurements < 100 nm. Finally, two typical engineering applications are provided. The first is wear measurement of cutting tools, the second the inspection and classification of welding spots. Both applications demonstrate the ability of the method to measure steep surface flanks up to 80° and surfaces with difficult reflectance behaviour. ©2011 Journal of Mechanical Engineering. All rights reserved. Keywords: metrology, 3D, optical, focus variation, measurement, roughness, form, accuracy, comparison, tactile 0 INTRODUCTION The 3D measurement of technical surfaces is a crucial part in checking and controlling the properties and the function of materials or engineering parts. Traditionally, 3D measurements have been performed merely by tactile devices, which can be divided into two main categories. Among the first are contact stylus systems for the measurement of small scale surface features such as surface roughness. These systems typically operate with a stylus tip, which is traced along a profile over the specimen surface in order to deliver roughness parameters such as Ra, Rq, and Rz. Among the second category are (micro) coordinate measurement machines (CMMs) where a stylus tip, usually a synthetic ruby ball, is moved to (few) different positions on the specimen in order to measure large scale features such as different form parameters (e.g. a sphere radius, the cylinder diameter, etc.). A good overview of surface metrology systems in general and tactile devices in particular can be found in [1]. Tactile systems have a long tradition in surface measurement and are well understood and accepted in science and industry. Moreover, a lot of international standards

on tactile systems exist, which describe the basis structure of a tactile system [2] and standards how to calibrate it [3]. Nevertheless, optical measurement devices have become increasingly popular in the last decade as described by Jiang [4] in his historical overview of surface metrology. This is above all due to their ability to perform area based measurements which are a prerequisite for many powerful surface texture parameters [5]. Although tactile systems nowadays are also able to perform area based measurements, such measurements usually last very long. Apart from the measurement time, there is a range of additional advantages of optical instruments. Firstly, they operate in a noncontact way and therefore, do not damage the surface. Secondly, they usually do not require as much maintenance as a tactile instrument since there are typically none or only very few parts that have to be regularly replaced. Moreover, they do not suffer from several limitations of tactile systems such as a “smoothing effect” of surface profiles due to the radius of the contact stylus tip. In the field of optical measurement many technologies have become increasingly popular recently. Among them are methods based on white light interferometry, phase shifting

*Corr. Author’s Address: R&D Team, Alicona, Teslastraße 8, 8074 Grambach, Austria, reinhard.danzl@alicona.com

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interferometry, confocal microscopy, chromatic probe microscopy, structured light techniques, atomic force microscopy and scanning electron microscopy [6]. However, also many optical techniques have their limitations when measuring certain surfaces. Optical techniques that have been typically used, such as white light interferometry are very good for measuring smooth surfaces, but have restrictions in terms of complex geometry measurement, large Z heights, high slope angles and high aspect ratio measurements. White light interferometry for example, has been found to produce erroneous results for roughness measurements of periodic standards with Ra values between 50 and 300 nm [7]. Another report shows jumps or spikes of half the mean wavelength, which is reported more frequently as the surface gradient increases or when there is a step discontinuity [8]. Other typical limitations of optical instruments are summarized in [1], where one of the most important ones is the limitation of the maximum measurable flank angle in relation to the numerical aperture of the used objective. For objectives with low numerical aperture light can only be gathered by an instrument if the surface topography gradients are sufficiently small, otherwise no information can be obtained. Another limitation is the lateral resolution which is typically limited by the wavelength of visible light (> 400 nm). Here, we present and evaluate the technology focus variation (FV), a rather new technique which exploits the small depth of focus of an optical system with vertical scanning to provide topographical and colour information from the variation of focus. In contrast to other optical techniques, two issues should be especially adressed. First, the method is not limited to coaxial illumination or other special illumination techniques, which allows to overcome some limitations with respect to the maximum measurable slope angle. Second, the technology delivers true colour information for each measurement point. This article will evaluate the performance of the technology with respect to the measurement of small scale surface roughness, the measurement of form and the measurement of steep surface flanks. There are many studies on the evaluation of optical surface instruments. Typically, such an 246

evaluation is based on the measurement of special parameters of a surface (e.g. roughness, form elements) which are compared to measurements performed by a certified measurement institute. The difficulty hereby is that most reference measurements are performed by tactile devices and that the used standards are especially designed for tactile devices, which makes a comparison difficult. This is e.g. addressed by Dietsch [9] who has compared two tactile and two optical devices (WLI and confocal) using roughness standards and step height artefacts. He concludes that the results do not always correlate which may be among others due to the fact that the standards are not suitable for optical measurements. There are several publications where the focus variation method has been evaluated. In [10] an instrument based on focus variation and other instruments have been used for form measurements on cylindrical parts of a micro contour artefact. The measurements of the FV instrument lead to radius deviations < 200 to 300 nm on radius measurements of the cylindrical elements. In [11], a comparison of FV has been made using a random roughness standard leading to roughness measurement results of the system that lay within the uncertainty range of the tactile measurement. Apart from evaluations with calibration standards several articles have been published on the use of focus variation instruments for special applications. In [12] for example a FV instrument has been used for the measurement of dental erosion and has been compared to tactile measurements. Both technologies showed similar trends and focus variation has been found to be a suitable tool for the proposed task without the disadvantage of the tactile device making visible scratch marks. Another study [13] compares focus variation to traditional indirect measurement methods for the roughness of paper, which are based on measuring air flows. The results show that a discrimination of papers with different roughness is also possible with the FV method and not only by traditional indirect methods. In this article an extension to previous studies is given by providing both, evaluations on different calibration targets and on typical

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engineering examples as shown in the next section. 1 EVALUATION OF FOCUS VARIATION First, a description of the focus variation principle and the focus variation instrument that is used for the evaluations in this section is given. Afterwards several evaluations of the instrument are provided; two of them based on special calibration standards. The first application is the roughness measurement of newly developed roughness standards with sinusoidal profiles (Section 1.2). Hereby, the focus is particularly on the comparison of the results between the device and a traditional tactile system. In the second application (Section 1.3) the focus is on form measurements, in particular the measurement of hemi-spherical calottes that are part of a special calibration target. The remaining two sections show two typical applications. The first is the wear measurement of milling-cutters (Section 1.4) where, in particular, the possibility to measure steep flanks is evaluated. Finally, a typical engineering example is provided, that is the measurement and inspection of welding spots (Section 1.5), which is challenging due to the difficult reflectance characteristics of welding spots.

strong into each direction. In case of specular reflections, the light is scattered mainly into one direction. All rays emerging from the specimen and hitting the objective lens are bundled in the optics and gathered by a light sensitive sensor behind the beam splitting mirror. Due to the small depth of field of the optics only small regions of the object are sharply imaged. To perform a complete detection of the surface with full depth of field, the precision optic is moved vertically along the optical axis, while continuously capturing data from the surface. This means that each region of the object is sharply focused. Algorithms convert the acquired sensor data into 3D information and a true colour image with full depth of field. This is achieved by analyzing the variation of focus along the vertical axis.

1.1 3D Measurement with Focus Variation Focus variation [14] combines the small depth of focus of an optical system with vertical scanning to provide topographical and colour information from the variation of focus. In the following, the operating principle is described based on a schematic system shown in Fig. 1. The main component of the system is a precision optic containing various lens systems that can be equipped with different objectives, allowing measurements with different resolution. With a beam splitting mirror, light emerging from a white light source is inserted into the optical path of the system and focused onto the specimen via the objective. Depending on the topography of the specimen, the light is reflected into several directions as soon as it hits the specimen via the objective. If the topography shows diffuse reflective properties, the light is reflected equally

1. array detector, 2. lenses, 3. white light source, 4. beam splitter, 5. objective, 6. specimen, 7. vertical scan, 8. focus curve, 9. light beam, 10. analyzer, 11. polarizer, 12. ring light Fig. 1. Schematic diagram of a typical measurement device based on focus variation In this article the focus variation instrument InfiniteFocus is evaluated. Its vertical resolution depends on the chosen objective and can be as low as 10 nm. The vertical scan range depends on the working distance of the objective and ranges from 3.2 to 22 mm. The x-y range is determined by the used objective and typically ranges from 0.14 × 0.1 to 5 × 4 mm for a single measurement.

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By using special algorithms and a motorized x-y stage the x-y range can be exceeded up to 100 x 100 mm and more. In contrast to many other optical techniques that are limited to coaxial illumination, the maximum measurable slope angle is not only dependent on the numerical aperture of the objective. However, many different illumination sources (such as a ring light), which allow the measurement of slope angles exceeding 80° are possible. Since the technique is very flexible in terms of using light, most limitations when measuring surfaces with strongly varying reflection properties within the same field of view can be avoided. In addition to the scanned height data, focus variation delivers a colour image with full depth of field which is registered to the 3D points. This provides an optical colour image which eases measurements as far as the identification and localization of measurement fields or distinctive surface features are concerned. Since the described technique relies on analyzing the variation of focus, it is only applicable to surfaces where the focus varies sufficiently during the vertical scanning process. Surfaces not fulfilling this requirement such as transparent specimen or components with only a small local roughness are hardly measurable. Typically, focus variation delivers repeatable measurement results for surfaces with a local Ra of 10 nm at a cut-off wavelength Lc of 2 µm. 1.2 Roughness Measurement on a Newly Developed Roughness Standard The measurement of surface roughness is one of the most common and important ways to judge the quality of a technical surface. In order to verify whether a metrology device is able to measure certain types of roughness accurately, various roughness standards with calibrated roughness values are available. Below, the process of performing a roughness measurement comparison between the focus variation instrument InfiniteFocus and a tactile device on a newly developed roughness standard is described. After a review of the main problems that may occur when making such a comparison, 248

the roughness standard and the results of the comparison are described. The main problem of contact stylus instruments in the context of roughness measurement is that the form of the contact stylus tip has a smoothing effect on the surface profile and can therefore influence the measurement result. Another error source of contact stylus instruments is that the stylus tip may modify the surface if the material is not hard enough (Fig. 3b). Sometimes the stylus tip is not traced along a straight line but may be deflected, e.g. when composite materials consisting of very smooth and very hard components are measured.

a)

b) Fig. 2. Periodic roughness standard used for the comparison between a focus variation instrument and a tactile system, a) photograph of the standard, b) schematic height profile of the roughness standard showing the sinusoidal structure, the nano-roughness and the meaning of the parameters Sm and Pt The main problem of optical instruments is that most existing roughness standards are rather smooth and can hardly be measured with several optical instruments. Therefore, a new roughness standard which contains a certain amount of nanoscale roughness and which is well-measurable with optical devices, has been developed. The roughness standard used for the comparison is a precision roughness specimen [15] with a regular periodic sinusoidal profile with a nominal peak-to-peak spacing Sm = 50 µm, a peak-to-valley height Pt = 1.5 µm and a resulting nominal Ra = 0.5 µm.

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a)

b)

c)

d)

e)

f) Fig. 3. 3D measurement of a periodic roughness standard (Ra = 500 nm) with focus variation, a) sharp color image of the roughness standard provided by focus variation, b) detailed view of a sharp colour image showË?sing the horizontal trace of a contact stylus instrument that has been used for reference roughness measurements, c) 3D measurement of the newly developed roughness standard with superimposed nano-roughness, d) 3D measurement of a conventional roughness standard without nanoroughness, e) height map of the roughness standard with profile path and f) surface profile obtained from the profile path in e) Starting from the precision diamondturned master specimen, various electroformed nickel replicas, which are all faithful copies of the original and of each other, were produced.

The sloping flanks between the peaks and valleys of the sinusoidal roughness profile, however, are smooth and shiny; and in order to introduce some nano-roughness onto these surfaces, the nickel

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Fig. 4. Visualisation of the roughness measurements by focus variation and the tactile instrument showing the mean Ra values and the standard deviation Ď&#x192; of the measurements (all values are in nm) specimens were etched with a dilute acid solution for varying lengths of time. The roughness standard is shown in Fig. 2a whereas Fig. 2b shows a schematic height profile of the standard and the meaning of the parameters Sm and Pt. The latter diagram illustrates both the overall sinusoidal shape formed by the machining process, and also the superimposed random nano-roughness, which is the result of the acid etch. Note that the nanoroughness is relatively small in comparison to the sinusoidal roughness, so that the overall Ra values measured on the etched surface will be very close (within ~1%) to those measured on the unetched sinusoidal surface. Table 1. Comparison of the measurements performed by the tactile instrument and by the device based on focus variation

# Measurements Mean Ra Std Ra

Tactile instrument 30 503.5 nm 4.95 nm

Focus variation 25 501.32 nm 0.93 nm

A 3D measurement of the standard is provided in Fig. 3c together with a sharp measured image in Fig. 3a. In order to calculate the roughness of the surface, a surface profile has been extracted along a horizontal profile path as shown in Fig. 3e. The measured surface profile is visualized in Fig. 3f showing the regular sinusoidal shape of the surface. The need for the superimposed nano-roughness is visualized in 250

Fig. 3d where the measurement of a traditional roughness standard without nano-roughness is shown. The measurement is not complete in comparison to those of the newly developed standard in Fig. 3c, since the traditional standard does not have enough small scale structures that can be exploited by the focus variation method. In order to compare the roughness measurements of the tactile and the optical system, the following procedure has been used. First, the roughness standard has been measured by the tactile instrument at 30 different positions arranged in three straight rows of ten each. This coverage of the whole measuring area serves as a check upon the uniformity of the roughness values from place to place. For the FV device only a single measurement position has been used, where roughness measurements have been repeated 25 times. This allowed a comparison of the measurement results to those of the tactile system and to calculate the repeatability of the measurements of the optical system. The results of this measurement row are graphically visualized in Fig. 4, showing the different measurement results as well as a Gaussian distribution curve depending on the repeatability of the measurements. It should be noted that the measurements of the tactile instrument have been performed at 30 different positions, so that the standard deviation contains the variability of the measurement device and the variability of the roughness standard. In contrast to this, the measurements by focus variation have all been performed at the same position. As a

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result, the standard deviation of the focus variation instrument only contains the variability of the measurement device. Overall, both measurements are very similar to each other with a difference in the mean Ra value of ~3 nm showing the possibility of the optical device to perform roughness measurements that are comparable to traditional tactile devices. In addition to profile based roughness measurements, the FV system is equipped with an area-based roughness module that allows a calculation of roughness measurements conform to a draft of an ISO standard on area-based roughness measurement [5]. In comparison to traditional profile based roughness measurement this allows a calculation of a much larger range of different surface texture

parameters including amplitude parameters, volume parameters or the fractal dimension of the surface. The advantage of area based roughness measurement is that the results usually get more representative and repeatable due to a larger amount of data used for calculation. This module also allows the subtraction of different forms (spherical, cylindrical), two-dimensional Gaussian filtering and the filtration of measurement points with bad quality. 1.3 Form Measurement of Hemi-Spherical Calottes Below, an evaluation of the proposed instrument with respect to form measurements is provided. The form of hemi-spherical calottes on

Fig. 5. Measurements on a calibration standard with hemi-spherical calotte, a) the PTB calibration standard, b) 3D dataset of one side of the calibration standard measured by focus variation, c) measured sharp true colour image with profile path and d) extracted height profile

a)

b) Fig. 6. Form measurements by focus variation, a) height profile of one calotte with fitted circle, b) absolute difference height map between measured and fitted sphere Focus Variation â&#x20AC;&#x201C; a Robust Technology for High Resolution Optical 3D Surface Metrology

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a calibration standard [16] developed by the PTB, the German national metrology institute has been measured. This calibration standard has the form of a cube with dimensions 10 x 10 x 10 mm with 25 hemi-spherical calottes with a nominal radius of 400 µm on three faces (Fig. 5a). Each side of the standard has a dimension of 10 × 10 mm, which can only be measured with devices that allow sufficiently large measurement areas. Secondly, the standard has to be measured with sufficient vertical resolution and accuracy to provide reliable data for the sphere fitting process. Thirdly, the device has to be able to measure even steep surface flanks since the calottes consist of surface patches with angles up to 90°. Below, measurements of this standard using the focus variation instrument described in Section 1.1. are provided In Fig. 5 the measurement results are demonstrated for one side of the calibration standard. In Fig. 5b a 3D dataset, which covers all 25 calottes on one side of the cube is shown. In Fig. 5c the measured sharp colour image is provided. Into this image a 2D profile path was drawn along which a surface profile was extracted as shown in Fig. 5d. Fig. 6a provides a detailed surface profile where a circle was fitted into the measured points. The fitted circle and the measured points show good correspondence even at the steep flanks. In Fig. 6b a difference height dataset that shows the absolute differences between measured points and a sphere fitted in the

least-squares sense, is provided. The deviations lie in a range between 0 µm (dark grey-values) and 3 µm (bright grey-values). In order to evaluate the repeatability of the system the radii of a sphere was measured 30 times in a row. The standard deviation sigma of the measurements is ~15 nm, which is rather small considering the sphere radius of 400 µm. The standard deviation can be converted into a confidence interval [mean – 2·σ, mean + 2·σ] which is ~60 nm and covers about 95% of all measurement results. The mean radius of the measurements is 402.594 µm. More information on the measurement of these samples can be found in [17]. 1.4 Wear Measurement of Cutting Tools In order to judge the quality of cutting tools, it is necessary to measure their geometry and wear during their use in the industrial process. This allows taking measures to improve the quality and durability of the tools as well as to increase the machining speed. Below, such measurements using the presented focus variation instrument are demonstrated. The wear on corners of a milling cutter (Fig. 7) was measured. First 3D datasets of the corner (circle in Fig. 7) had been measured before and after usage. Afterwards, the difference between the two 3D datasets which contains

Fig. 7. 3D datasets of a milling cutter measured by focus variation; the parts that have been investigated in detail are marked with circles, a) 3D dataset with superimposed true color image, b) 3D dataset where each greyscale represents a different height 252

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the worn material, was calculated. In order to assure that the difference is calculated from corresponding surface regions, the two 3D datasets were registered to each other before difference calculation. In Fig. 8a a 3D dataset of the original corner is provided, whereas Fig. 8b contains a 3D dataset of the used corner. Both 3D datasets were overlaid with the true color image measured by the focus variation instrument. This allows a classification into original regions (dark) and worn regions (bright). After the two 3D datasets were registered to each other, a difference height dataset was calculated (Fig. 8c) which allows the quantification of the worn volume (~601400 µm³). Another possibility to measure the amount of the worn volume is to extract height profiles of the original and the worn part and to overlay them (Fig. 8d) in a single diagram. This allows a good visualization of regions where much and regions where little material was removed. Additional measurements of milling cutters and cutting edges are provided by Danzl [18].

a)

1.5 Inspection of Welding Spots Laser beam welding is commonly used because of high-strength welding assembly and the small weld seam and the high qualitative weld seams without brittle occurrence. Checking and evaluating the classification of welding spots into good and bad parts during the production, saves expenses and time consuming rework. The combination of high resolution measurement data with the accordingly measured true color information is an important requirement for the classification. Due to this fact the localization and the topographic acquisition of high temperature oxidation can be realized. Below, several results of welding spot measurements by focus variation are presented. There are many different criteria that are used for a classification into good and bad welding spots. Typically, the discrimination of good and bad welding spots is performed by the following procedure: 1. Measure a 3D dataset of the welding spot. 2. Extract a surface profile of the welding spot. b)

c)

d) Fig. 8. Wear measurement of a corner of a milling cutter with focus variation, a) unused cutting edge, b) used cutting edge, c) difference volume of used and unused cutting edge, d) profiles of the used and unused cutting edge Focus Variation – a Robust Technology for High Resolution Optical 3D Surface Metrology

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a)

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Fig. 9. Comparison of good a-c) and bad welding spots d-e), a-b): visualization of the measured 3D dataset, c-d): depth image with position of profile path, e-f) extracted profile; the bad welding spot has a very steep transition between the pin and the ground plate, whereas the good welding spot has a more smooth transition 254

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3. Extract discriminative parameters from the surface profile, the 3D dataset and the color information. 4. Decide on the quality of the welding spot on the basis of the extracted parameters. The fourth step is typically performed by a classificator that has been trained by a series of labeled welding spots whose status (Ok, Not Ok) is known. In Fig. 9 3D datasets of a good and bad welding spot are presented. The good welding spot in Fig. 9a has a smooth transition between the metal pin and the ground plate, whereas this transition is very abrupt for the bad welding spot in Fig. 9b. This is also visible in surface profiles in Figs. 9e and f that were extracted along horizontal paths in Figs. 9c and d. A possibility for the discrimination of these two welding spots is to analyze the height histogram of the extracted surface profiles, the area and the position of the welding spot. 2 CONCLUSIONS This article contains an evaluation of a focus variation instrument with respect to different measurement tasks including form, roughness and wear measurements. Roughness measurements on a newly developed roughness standard have provided very similar results of the focus variation instrument to traditional tactile devices. The Ra differences are in the range of a few nanometers. Comparisons of measurements with traditional roughness standards show that the new standard is measurable much better due to its superimposed nano-roughness. Measurements on hemispherical calottes have shown an evaluation of the repeatability of the proposed system with respect to sphere measurements that were in the range of < 100 nm. The evaluation of the system on two engineering applications shows that the system is able to measure steep surface flanks, which has been reported to be difficult for a series of other 3D measurement technologies [7]. This is the case for the milling cutter whose wear during the use in the industrial process could be quantified by means of 3D registration of 3D measurements. On the other hand, this is true for welding spot inspections which have a very irregular shape with steep flanks and difficult reflective behaviour.

3 REFERENCES [1] Leach, R.K. (2009). Fundamental Principles of Engineering Nanometrology, William Andrew, Oxford. [2] ISO 3274 (1996). Geometrical Product Specifications (GPS) – Surface texture: Profile method – Nominal characteristics of contact (stylus) instruments, International Organization of Standardization. [3] ISO 5436-1 (2000). Geometrical Product Specifications (GPS) — Surface texture: Profile method; Measurement standards — Part 1: Material measures, International Organization of Standardization. [4] Jiang, X., Scott, P.J., Whitehouse, D.J., Blunt, L. (2007). Paradigm shifts in surface metrology; Part II; The current shift. Proceedings of the Royal Society, vol. 463, no. 2085, p. 2071-2099. [5] ISO/DIS 25178-2. Geometrical product specifications (GPS) – Surface texture: Areal – Part 2: Terms, definitions and surface texture parameters. International Organization of Standardization. [6] ISO 25178-6 (2010). Geometrical product specifications (GPS) – Surface texture: Areal – Part 6: Classification of methods for measuring surface texture. International Organization of Standardization. [7] Vorburger, T.V., Rhee, H.G., Renegar, T.B., Song, J.F., Zheng, A. (2007). Comparison of optical and stylus methods for measurement of surface texture. International Journal of Advanced Manufacturing Technology, vol. 33, no. 1-2, p. 110-118. [8] Gao, F., Leach, R.K., Petzing, J., Coupland, J.M. (2008). Surface Measurement errors using commercial scanning white light interferometers. Measurement Science and Technology, vol. 19, no. 1.. [9] Dietsch, M., König, N., Schmitt, R., Seewig, J. (2007). Sind taktile und optische Rauheitsmessungen vergleichbar?. VDI Berichte, vol. 1996, p. 187-206. (in German) [10] Neugebauer, M., Neuschaefer-Rube, U. (2005). A new micro artefact for testing of optical and tactile sensors. Proc. of the 5th Euspen International Conference and 7th annual general meeting of the European

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society for precision engineering and nanotechnology, Montpellier, p. 201-204. Danzl, R., Helmli, F., Scherer, S. (2006). Comparison of roughness measurements between a contact stylus instrument and an optical measurement device based on a colour focus sensor. Proc. of the Nanotechnology Conference, Boston, vol. 3, p. 284-287. Ren, Y.-F., Zhao, Q., Malmstrom, H., Barnes, V., Xu, T. (2009). Assessing fluoride treatment and resistance of dental enamel to soft drink erosion in vitro: applications of focus variation 3D scanning microscopy and stylus profilometry. Journal of Dentistry, vol. 37, no. 3, p. 167-176. Lechthaler, M., Bauer, W. (2006). Rauigkeit und Topografie – ein Vergleich unterschiedlicher Messverfahren. Wochenblatt für Papierfabrikation, vol. 21, no. 21, p. 1227-1234 Scherer, S. (2007). Focus-Variation for optical 3D measurement in the micro- and nano-range. Bauer, N. (ed.) Handbuch zur

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Industriellen Bildverarbeitung, Fraunhofer IRB Verlag, p. 198-210. Rubert, P. (2008). Report on the results of an intercomparison, using stylus-type and non-contacting surface measurement instruments. Proc. XII. Int. Colloquium on Surfaces, Chemnitz. Neugebauer, M., Hilpert, U., Bartscher, M., Gerwien, N., Kunz, S., Neumann, F., Goebbels, J., Weidemann, G. (2007). Ein geometrisches Normal zur Prüfung von Röntgen-Mikrocomputertomographiemess systemen. Technisches Messen, vol. 74, no. 11, p. 565-571. (In German) Danzl, R., Helmli, F., Scherer, S. (2007). Automatic measurement of calibration standards with arrays of hemi-spherical calottes. Proc. 11th Int. Conf. on Metrology and Properties of Engineering Surfaces, Huddersfield, p. 41-46. Danzl, R., Helmli, F. (2008). Geometry and wear measurement of cutting tools. Int. Conf. on High Performance Cutting, Dublin, vol. 1, p. 111-118.

Danzl, R. - Helmli, F. - Scherer, S.


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, 257-266 DOI:10.5545/sv-jme.2010.176

Paper received: 04.08.2008 Paper accepted: 24.12.2008

Surface Defect Detection on Power Transmission Belts Using Laser Profilometry Bračun, D. - Perdan, B. - Diaci, J. Drago Bračun1,* - Boštjan Perdan2 - Janez Diaci1 1 University of Ljubljana, Faculty of Mechanical Engineering, Slovenia 2 Veyance Technologies Europe, d.o.o., Slovenia

Quality control in production of the power transmission belts currently relies mostly on visual inspection by skilled workers. They primary inspect belt geometry for defects like small bumps, dents and unformed teeth. Despite the controller experience, the result depends on the person’s mood and general condition. To avoid subjective inspection, an experimental system for automated inspection of the belt geometry is developed. It operates on the basis of the laser triangulation system, capable of acquiring a cloud of points in 3D space representing a complete belt surface. By processing the acquired data cloud, most typical belt defects can be identified and assessed. We demonstrate two different methods of data processing. The first one imitates the established “manual” procedure, where the individual tooth profile is compared to a template specified by a technical documentation. The second method uses a novel approach based on the deviation map. That enables automated analyses of the complete tooth surface (not only profile), identification of 4 typical surface defects and their pass/fail quality assessment. We found shape of the surface defects sufficiently recognizable in the acquired data cloud, which means that point measurement accuracy of the developed laser triangulation system is sufficient. We demonstrate identification of the typical surface defects. We found that further work is needed to develop pass/fail criteria of the quality assessment to comply with the requirements of the industry. ©2011 Journal of Mechanical Engineering. All rights reserved. Keywords: three-dimensional shape measurement, laser based active triangulation, laser profilometry, power transmission belts, surface defects, defect detection 0 INTRODUCTION Efficient integration of all activities from design to manufacture often requires good knowledge of the product properties and management of the manufacturing process. Detection of flaws in the product often results in a decrease in product rejection, higher manufacturing efficiency and improved quality [1]. Assuring quality control is essential for every manufacturing process. Since 100% control is often too expensive, statistical process control (SPC) is recommended. In this case not all of the products are controlled directly, but the process is controlled on the basis of statistical samples. The most common way to procure SPC is the control chart. Many types of control charts have been developed (Shewhart, Cusum, Exponential moving average are presented elsewhere, e.g. in [2]). Their economic design is key to the successful control and minimum quality cost [3].

In the production of the power transmission belts quality control currently relies mostly on visual inspection by skilled workers. They visually inspect the surface of each belt (100% control by a naked eye). Some surface defects are also found by examining the teeth with a hand. Tooth profile of the sample belts is inspected by means of a profile projector and assessed with the help of a template. Although this is done by skilled workers, it is subjective because it depends on the person’s mood and general condition. To rule out subjectivity and allow detection of smaller defects that are difficult to detect by visual inspection alone, a quick and reliable automatic or semiautomatic measuring system is required. The reliability of a belt drive depends on belt geometry, construction and the materials it is made of. No matter how good the materials are, their supreme quality is diminished by poor belt geometry if it does not mesh properly into the sprocket. Proper tooth geometry will develop less interference between belt and sprockets

*Corr. Author’s Address: University of Ljubljana, Faculty of Mechanical Engineering, Askerceva cesta 6, 1000 Ljubljana, Slovenia, drago.bracun@fs.uni-lj.si

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during meshing and hence better load distribution which can affect belt life by causing tooth root cracking failures [4]. Improper meshing will lead to increased belt temperature, higher tooth wear and eventually to a belt failure. In addition to belt life, meshing will also influence transmission error, noise and vibration [5]. In [6], the author distinguishes between several sources of noise, the most important being impact sound made by meshing of the belt and pulleys and the sound made by transverse vibration of the belt that too is influenced by meshing. Belt geometry is defined by the tooling; it also depends on various cure process parameters and is subject to external influences that are sometimes difficult to control (mold damage, dirt on the mold, etc.). All these influences can cause different belt surface defects that need to be detected during the production process. A wide range of non-destructive testing (NDT) techniques can be used when assuring quality control in the manufacturing process. Fast developing fields of laser technologies supported by advanced signal processing and high-speed computers accelerated the development of several novel non-contact NDT methods that are based on the interaction between the laser pulse and solids or liquids. By analysing the propagation of laser-induced ultrasonic and/or acoustic waves, properties of solids [7], liquids [8] or boundary regions [9] can be investigated. Non-contact measurement of a three-dimensional (3D) body shape gained acceleration with advancing vision systems [10]. In terms of speed and accuracy of measurement, laser triangulation systems exhibit several advantages and have been applied in several industrial applications such as resistance spot welding [11], weld shape inspection [12] and quality assessment of die-castings [13]. Advantages of laser triangulation systems directed us to applying this method to quality control in belt manufacturing process. The main idea is to have measurement system integrated into the belt production line to carry out 100% control of belts by on-line measurement of belt geometry and its assessment. This paper presents an experimental system for automated control of belt geometry. Its use is demonstrated on belt samples with different characteristic surface defects that 258

are encountered in the production process. A defect detection method is also described. 1 Experimental system Experimental set-up is shown in Fig. 1. It consists of a belt drive and a laser profilometer module that (1) measures 3D shape of the belt using the principle of laser triangulation. The later consists of a digital camera and laser line projector. A laser beam, formed into a narrow laser plane, illuminates the belt surface. A bright line (2) visible on the belt surface is acquired by a camera placed at a specific triangulation angle with respect to the direction of illumination. This arrangement enhances surface topography and enables determination of 3D shape [10]. The result of one measurement is a profile representing cross-section of the laser plane and the illuminated surface of the belt. In the reported set-up laser profilometer has a digital camera with a monochrome 640 x 480 pixel CCD sensor and IEEE-1394 output. The optics was adjusted to the measuring range of 25 mm along the laser line and 20 mm perpendicular to the measured surface. One measured profile consists of 640 measured points. The profile measurement resolution was 0.04 mm (the distance between measured points in one profile). The 3D coordinates (X, Y, Z) of the measured points are calculated by a triangulation model described in [14]. This model assumes a pinhole model of the camera, and treats the focal length of the camera optics separately for direction xc being parallel and yc perpendicular to the measured profile (subscript c denotes camera coordinate system). It is also capable of compensating for systematic measurement errors in optical triangulation originating from the camera optics and light sheet curvature. The set-up was calibrated using a planar checkerboard and verified by measuring the shape of the reference calibration body (v-groved plate) similar to how it is described in [13]. We found measurement precision of a particular point to be 0.035 mm (2Ď&#x192;). In order to acquire 3D shape of a particular belt tooth, its surface is scanned (with profilometer module in vertical position) by rotating the laser plane in small angular steps (approx. 8x10-3 degrees). When the plane settles in a new position

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a)

b)

Fig.1. Experimental set-up, a) photo of the system: (1) laser profilometer module, (2) laser plane, (3) belt, (4) pulleys, (5) movable cart; b) schematic of the experimental system

a)

b)

c)

Fig. 2. Typical surface defects, a) poor tooth formation, b) bumps on the surface, c) open splice

a new shape profile is acquired. By repeating this procedure, acquired profiles can be set as little as 0.05 mm apart (scanning resolution). This scanning procedure is carried out with frequency of 80 profiles per second. Measurement of one tooth (belt width 20 mm) takes about 5 seconds and is composed of approximately 400 profiles. For the initial measurements, maximum resolution was used to achieve good surface characterization, however for the inspection purposes a compromise will have to be made

between the speed of operation and the smallest defect we still need to detect and characterize. Experience shows that a resolution of 0.2 mm should suffice. To measure the next tooth, belt is translated along its length for one tooth pitch length, by using a belt drive to move next tooth into the measuring field. The procedure of tooth scanning and belt translation is then repeated for all the teeth. Measurement of a complete belt with 100 teeth takes approximately 10 minutes.

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The belt drive is basically a two pulley drive, where driver pulley is rotated by a micro-stepping motor, while the driven pulley is mounted on a moveable cart, which moves along a linear guide to accommodate different belt lengths. Belts from 675 mm up to 2300 mm of length can be measured with the current construction. Constant tension force is provided by means of a weight suspended on a steel wire that is connected to the moveable chart. The belt is running on its back over the pulleys and over the belt support, which prevents belt vibrations and is the base for thickness assessment. In its present form the system is not yet ready for online assessment of belts in the production as it was designed for testing purposes and laboratory use only. To implement surface defect inspection procedure into the production, a faster performing laser profile module is required. Since the impact on the production process needs to be minimal, it will have to be integrated into the existing belt length measuring machine that is quite similar in design to our belt drive. 2 experiment A measuring system can be used to inspect both multi rib and timing belts, however within scope of this paper we will only focus on timing belts, where tooth geometry plays an important role in load transmission. Belt defects can occur both during production and operation. In this case, interest lies primarily in identifying various surface defects that occur in production in order to use the surface defect detection procedure as a part of the quality assurance system. Surface defects that occur on teeth can generally be classified into 4 different categories: poor tooth formation, dented teeth, bumps on the surface and so called open splice defect; a few typical defects are shown in Fig. 2 and their cross section (profiles) depicted by drawings in Fig. 3. These show a comparison of the measured profile with the ideal shape defined by the mold. Profiles with 5 and 10% shrinkage in vertical direction are also added that are used to assess the profile and represent the tolerance range within which the tooth profile should be. Poor tooth formation is shown in Fig. 2a (note 2nd tooth from the left side marked with arrows); a cross section of an extreme example is 260

shown in Fig. 3a. This defect is caused by locally insufficient rubber gum flow or pressure to force the fabric against the mold surface during the cure process. Such a defect is usually quite obvious and can be easily spotted by visual inspection or by going over the teeth with a hand. Since the teeth do not have the required shape to properly transfer the load to the sprocket, such a belt can jump teeth and is not acceptable. A similar defect is “rounded teeth” depicted in Fig. 3b. These are difficult to spot by visual inspection alone because deviation is much less obvious and an experienced eye is required. In the case of doubt, the profile is checked using an optical projector and a template. This defect is also not acceptable as the possibility of belt jumping teeth is increased, especially at high loads. Dented teeth are caused by dirt that can get between the teeth on the mold and prevent the cover fabric to conform to the surface of the mold (see Fig. 3c). Dents are usually smaller in size and easily detected by visual inspection. They do not harm the performance of the belt as they do not affect the whole tooth width and are mostly of cosmetic nature. Nevertheless, they are not acceptable as they put a bad image to the quality of the product. Bumps on the surface are the opposite of dented teeth and are the consequence of mold damage (dents, etc.). An example is shown in Fig. 2b (marked with two ellipses) and depicted in Fig. 3d. These too are small in size but easy to spot. As there is excess of material on the tooth, this can cause problems with tooth meshing into the sprocket and are not acceptable. They will usually occur in the land area between the teeth as the mold (negative shape) is most likely to get damaged on the top of the teeth. Fig. 2c shows an example of open splice defect where rubber gum penetrated between the mold and the facing fabric (marked with arrows); this can only happen with the end belt and is recognized by a very smooth tooth surface. This defect will only have a small effect on the performance as there is a possibility of uneven local load distribution if the rubber would wear sooner than the fabric, but is not acceptable because of visual appearance of the belt. The described experimental set-up is used for measuring 3D shape of belts. Two typical 3D measurements are shown in Fig. 4 that shows screen captured images of measured points

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a)

b)

c)

d)

Fig. 3. Typical surface defectsâ&#x20AC;&#x2122; cross sections, a) poor tooth formation, b) rounded teeth, c) dented teeth, d) bumps on the surface

a)

b)

Fig. 4. Typical measurement of 3D shape of the tooth surface, a) poor tooth formation, b) bumps on the surface

rendered in OpenGL. Left image shows a tooth with poor tooth formation defect where we can observe a substantial cross section reduction as we move towards the middle of the tooth. In the right image a tooth with two bumps in the land area is

shown (note two light spots). These are outlined with ellipses for clarity. Lighter spots mean that measured points are above the reference surface, which means that defects are protruding the surface.

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3 DEFECT DETECTION METHODS In this article two methods for characterization of surface defects are examined. The first method imitates the established “manual” method that utilizes profile projector, where actual belt profile is compared to a template based on a technical drawing with 5 and 10% shrinkage added. The profile is considered acceptable if it falls within this range. With this method only one profile of the belt at a time is measured. Top diagram in Fig. 5 shows a comparison of a belt profile measured in the middle of the belt (an example of dented teeth) and a template based on the technical drawing. Deviation diagram below shows absolute difference between the two profiles in vertical direction. Shaded areas represent the range within which the profile shape is considered acceptable. Only tooth tip and land area between the teeth are checked because deviation is calculated in a vertical direction, which produces inaccurate results for the tooth flanks. These are also the most likely to have a defect. Belt profile is defined by the negative shape of the mold, which is more likely to get damaged on the top of the teeth, producing a bump in belt’s land area; and since dirt will usually accumulate between the mold’s teeth, dents are formed on the top of belt’s teeth. In the case of tooth tip the outline of shaded area represents the allowed 10% shrinkage in vertical direction. The same approach cannot be used for the land area (allowed shrinkage would equal 0), so the range is defined with a constant that equals maximum allowed tip deviation divided by 4. In land area there are stricter requirements as there is usually an excess of material (a bump) that can interfere with the sprocket during operation which is not acceptable. A lack of material in tooth tip will have much less of an effect on belt performance. Such a method is very strict because one point out of range is enough for the product to be classified as unacceptable. With this approach do Not all the information about the defect in question can be acquired in this way. This is because only one profile of the tooth in the particular position is measured and the surface defect can easily be missed or measured aside its maximal deviation. 262

If this method is used for automatic defect evaluation without any supervision a certain defect could be considered acceptable even though in reality it is not. For example, all values in the deviation diagram (see Fig. 5) could be within the allowed range but would otherwise be changing rapidly, which is not acceptable. For automatic detection a more selective method is required capable of identifying and evaluating various defects. The second method is an upgrade of the first one, since the complete 3D shape of the tooth and not only one profile at the specific position on the tooth is analyzed. Our goal in developing the second method was derivation of characteristic parameters (numerical values), which could be used for an automatic defect evaluation and classification and would lead to similar conclusions that today an operator makes using a template and their common sense.

Fig. 5. Comparison of measured profile (thick line) a) with a template (thin line), b) deviation diagram The main idea of the second method for an automatic detection of surface defects is based on a comparison of measured 3D tooth shape (MS) to a reference (or ideal) tooth shape (RS). The result of this comparison is a deviation map (DM) that shows the differences between the two surfaces (see Fig. 6). The surface of DM shows the difference in each point where “x” axis points in a direction across the teeth and “y” axis in a direction along the teeth. The difference between the measured and reference value in each point (axis “z”) perpendicular to the plane, defined by axes “x” and “y”, is represented by a colour scale on the right side of DM in the range from -1 to +1 mm. The RS would normally be defined

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by a technical drawing of the belt in question. However, in this case, RS is simply generated from the profile obtained as an average of several profiles measured on a good tooth (without defects). This is primarily intended for initial testing, while further RS will be based on the technical documentation. When calculating DM, the coordinate system is oriented so that the differences between MS and RS are expressed only in vertical “z” axis as ΔZ(X,Y), where X and Y are discrete values representing the coordinates of each point in DM (for clarity written without any indices). Dark areas of the DM show deviations from RS in a negative direction (in surface) and lighter areas in a positive direction (out of surface). With the example on the left (a) it is clear that it is poor tooth formation because in the middle of the tooth the whole profile is below the RS. Many slanted stripes across the tooth that represent the cover fabric texture can also be noticed. On the right (b) there is DM of the tooth with two bumps, which are observed as two lighter spots located in the land area of the tooth. They are above the RS (positive direction) and hence lighter colour. Surface defect identification and tooth quality assessment are done by further analysis of DM. By processing the DM, like calculating deviations volume, area, average height and maximum height, surface defects can be identified and evaluated as acceptable or not-acceptable. The use of the proposed method on two typical surface defect examples shown in Fig˝s. 3 and 4 is demonstrated. DM was used to calculate defect volume above and below the RS:

Va = ∆X ∆Y Vb = ∆X ∆Y

∑ ∑ [ ∆Z ( X , Y )] x

∆Z ( X ,Y ) > 0

y

y

Aa =

∑ ∑ [ ∆X ∆Y ]

∆Z ( X ,Y ) > 0

, (3)

Ab =

∑ ∑ [ ∆X ∆Y ]

∆Z ( X ,Y ) < 0

, (4)

∆Z ( X ,Y ) < 0

, (2)

where Va is defect volume above RS; DZ(X,Y) > 0, Vb is defect volume below RS; DZ(X,Y) < 0, DZ(X,Y) is difference between MS and RS, DX is distance between points in DM in x direction, DY is distance between points in DM in y direction. Subscript DZ(X,Y) > 0 or < 0 means that only those DZ(X,Y) are summed, which are greater or less than zero. For each side a cumulative defect area is calculated:

x

y

x

y

where Aa is cumulative defect area above RS; DZ(X,Y) > 0, Ab is cumulative defect volume below RS; DZ(X,Y) < 0. Average height is calculated as:

Ha =

Hb =

∑ ∑ [ ∆Z ( X , Y )] x

∆Z ( X ,Y ) > 0

y

na

∑ ∑ [ ∆Z ( X , Y )] x

∆Z ( X ,Y ) < 0

y

nb

, (5)

, (6)

where Ha is the average defect height above RS; DZ(X,Y) > 0, Hb is the average defect height below RS; DZ(X,Y) < 0, na is the number of points above RS, nb is the number of points below RS. Maximum defect height and depth is found as:

H a,max = max ( ∆Z ( X , Y ) > 0 ) , (7)

H a,min = min ( ∆Z ( X , Y ) < 0 ) , (8)

where Ha,max is maximum defect height above RS; DZ(X,Y) > 0, Hb,max is maximum defect depth below RS; DZ(X,Y) < 0. The final value is total tooth area defined as:

∑ ∑ [ ∆Z ( X , Y )] x

, (1)

A=

∑ ∑ ∆X ∆Y , (9) x

y

where A is the total tooth area. These 9 values are then used to calculate characteristic parameters that define the type of defect and are used for defect classification: Relative volume share of positive defect (above the RS) compared to total defect volume: VR = Va / (Va + Vb); then the side with higher volume share is further analyzed. Relative share of cumulative defect area compared to total area (to evaluate the defect size): AR = Aa / A if Va > Vb, else AR = Ab / A.

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Ratio of maximum and average defect height to evaluate its distribution, where small ratio gives good confidence in the method: RH = Ha,max / Ha if Va > Vb, else RH = Hb,max / Hb. Average defect height/depth to further evaluate the defect: HA = Ha if Va > Vb, else HA = Hb. Table 1. Characteristic parameter values for the two given examples Example VR AR RH HA [mm]

1 6% 40% 1.84 0.74

2 87% 22% 2.77 0.20

Parameters are used in an elimination process that brings us to a single possible defect. For the given examples, these are listed in Table 1 and the use of elimination process is demonstrated.

a)

Example 1: In the first case the relative volume share VR of the positive defect (above RS) is very small, so there must be a lack of material and possible defects are poor tooth formation or dented teeth. The stress is put on the negative defect (below RS) and its cumulative surface area AR is calculate The share of 40% is quite large, which means that the defect is poor tooth formation (or rounded teeth defect, which falls under the same category). The small ratio of maximum and the average defect depth (RH = 1.84) means that defect distribution is quite even, which confirms the previous conclusion. It is also quite substantial with an average depth HA of 0.74 mm and this rules out the possibility of rounded teeth defect. Determined defect is poor tooth formation. Example 2: In this case positive defect volume share VR is quite large, which means there is an excess

b)

Fig. 6. DM of teeth shown in Fig. 4, a) poor tooth formation, b) bumps on the surface

Fig. 7. Calculated tooth height variation across the belt 264

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of material and it can be concluded that there is a bump on the surface. This time the emphasis is on the positive defect and once again its cumulative area share AR is calculated and it amounts to 22% and confirms the initial conclusion, because defects caused by mold damage are usually small. The ratio of the maximum and average defect height RH is a bit larger this time probably because the defect surface is isn’t parallel with the RS. The average defect height HA of 0.2 mm is quite small but still relatively large if compared with the average depth of negative defect which is 0.03 mm. The above confirms that the defect is a bump on the surface caused by mold damage. To demonstrate the use of the system on the whole belt a chart showing calculated, tooth height is shown in Fig. 7. Various parameters can be presented this way and tooth height was selected as it is easy to picture as a physical quantity and can serve as a parameter indicating teeth with possible defects which can be then analyzed in more detail. It can be seen from the chart that on a belt without defects, tooth height variation will normally be quite small (less than +/- 0.1 mm). 4 CONCLUSIONS We have developed an experimental system for automated control of the belt geometry. A three-dimensional shape of the belt is acquired as a data cloud of points measured by a laser triangulation based measurement system. The acquired data can be further processed by two alternate methods. The first one imitates the current ‘manual’ procedure, where belt profiles are compared to the template defined by technical specifications. Quality assessment is based on 5 or 10% shrinkage control limits. The second method uses a novel approach based on the deviation map that enables automated identification of defects on complete teeth (not only one profile) and quality assessment of defects. In this paper, the focus has been pimarily on quality assessment of timing belts. Timing belts with the most typical surface defects have been analyzed. The results show that the shape of the surface defects is sufficiently recognizable in the acquired data cloud. This means that point measurement accuracy of the developed

laser triangulation system is sufficient, i.e. the measurement system has the capability to measure the surface with the precision required for surface defect detection. Typical defects were classified into 4 categories: poor tooth formation, dented teeth, bumps on the surface and the open splice defect. This classification was found useful for the development of the defect characterization and detection methods. Two defect characterization methods were examined. Both were based on the comparison of a measured 3D shape to a reference (or ideal) shape. The difference between the two shapes (called the deviation map) was used to calculate characteristic parameters, which were used for defect characterization. We found that by setting suitable limits to the characteristic parameters and their combinations it is possible to perform pass/fail classification of belts, accepting belts with insignificant (‘cosmetical’) irregularities and rejecting belts with significant surface defects. Further work is needed to develop the pass/fail criteria to comply with the requirements of the industry. The presented approach opens up the possibility of developing algorithms capable of automatic defect identification and classification, which would help the quality control personnel to identify the defect causes and sources. 5 REFERENCES [1] Grum, J. (2003). Material and product quality assurance by application of nondestructive testing methods in automated manufacturing systems. 8th ECNDT Barcelona 2002 – technical papers. Novice Društva za neporušitvene preiskave, vol. 11, no. 2, p. 53-62. [2] Zupančič, R., Sluga, A. (2007). Statistical process control: empirical comparison of control chart methods. Ventil, vol. 13, no. 1, p. 30-36. [3] Zupančič, R., Sluga, A. (2008). Economic design of control charts. Strojniški vestnik Journal of mechanical engineering, vol. 54, no. 12, p. 855-865. [4] Dalgarno, K.W. (1998). Power transmission belt performance and failure. Rubber Chemistry and Technology, vol. 71, no. 3, p. 619-636.

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[5] Tokoro, H., Nakamura, M, Siguira, N., Tani, H., Yamamoto, K.-I., Shuku, T. (1998). Analysis of high frequency noise in engine timing belt. JSAE Review, vol. 19, no. 1, p. 33-38. [6] Koyama, T., Marshek, K.M. (1988). Toothed Belt drives - past, present and future. Mechanism and Machine Theory, vol. 23, no. 3, p. 227-241. [7] Hrovatin, R., Možina, J. (2000). Nondestructive and non-contact materials evaluation by means of the optodynamic method. Insight, vol. 42, no. 12, p. 801-804. [8] Horvat, D., Možina, J. (2000). Optodynamic measurement of ultrasound propagation in liquids. Insight, vol. 42, no. 12, p. 792-795. [9] Gregorčič, P., Petkovšek, R., Možina, J. (2007). Investigation of a cavitation bubble between a rigid boundary and a free surface. Journal of applied physics, vol. 102, no. 9.

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[10] Girod, B., (2002). Principles of 3D image analysis and synthesis, Kluver, Dordrecht. [11] Bračun, D., Polajnar, I., Diaci, J. (2006). Indentation shape parameters as indicators of spot weld quality. International Journal of Materials and Product Technology, vol. 27, no. 3-4. [12] Jezeršek, M., Polajnar, I., Diaci, J. (2007). Feasibility study of in-process weld quality control by means of scanning laser profilometry. Optical Measurement Systems for Industrial Inspection, no. 5, p. 18-22. [13] Bračun, D., Gruden, V., Možina, J. (2008). A method for surface quality assessment of die-castings based on laser triangulation. Measurement Science & Technology, vol. 19, no. 4, p. 1-8. [14] Bračun, D., Jezeršek, M., Diaci, J. (2006). Triangulation model taking into account light sheet curvature. Measurement Science & Technology, vol. 17, no. 8, p. 2191-2196.

Bračun, D. - Perdan, B. - Diaci, J.


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, 267-278 DOI:10.5545/sv-jme.2010.181

Paper received: 22.07.2010 Paper accepted: 23.09.2010

Monitoring Gas Metal Arc Welding Process by Using Audible Sound Signal Horvat, J. - Prezelj, J. - Polajnar, I. - Čudina, M. Jožef Horvat1 - Jurij Prezelj2 - Ivan Polajnar2 - Mirko Čudina2,* 1 University of Ljubljana, College of Health Studies, Slovenia 2 University of Ljubljana, Faculty of Mechanical Engineering, Slovenia

The most frequently used arc welding process is gas metal arc welding (GMAW). Different methods are in use for monitoring the quality of a welding process. In this paper sound generated during the GMAW process is used for assessing and monitoring of the welding process and for prediction of welding process stability and quality. Theoretical and experimental analyses of the acoustic signals have shown that there are two main noise-generating mechanisms; the first is arc extinction and arc ignition having impulse character, the second is the arc itself acting as an ionization sound source. A new algorithm based on the measured welding current was established for the calculation of emitted sound during the welding process. The algorithm was verified for different welding condition, different welding materials and different specimen. The comparisons have shown that the calculated values are in good agreement with the measured values of sound signal. ©2011 Journal of Mechanical Engineering. All rights reserved. Keywords: process monitoring, welding condition, sound emission, optimization process, weld quality 0 INTRODUCTION The gas metal arc welding (GMAW) process is widely used because it is highly productive and cost effective. The process is suitable for robotization and mechanization. To maintain and direct the welding arc, an experienced welder uses their senses, especially eyes and ears to combine visual and audible information [1]. Unfortunately, during the GMAW process there are more high intensity side effects, such as heat, light and noise. At the same welding parameters audible A-weighted sound pressure level can rise above the daily permissible level of the welder’s ear, 80 dB. Measurements have shown that equivalent A-weighted sound pressure level can even exceed 100 dB at the welder ear. On the other hand, the noise can be used for monitoring the quality of the welding process as well as for checking for anomalies in the welding process. The significance of sound in monitoring the arc welding processes has been known for a long time, but relatively few studies have been published in which sound waves are regarded as a source of information for monitoring the welding process. Erdmann-Jesnitzer [2] and Jolly [3] published the first study on acoustic waves

generated during the GMAW process. They found that sound waves are synchronized with short-circuiting and also discovered that the pressure of produced sound increases with the arc length and welding current. Much scientific work has been performed to verify the suitability of different arc signals for on-line monitoring. On-line quality control in automated welding operations is an important factor contributing to higher productivity, lower costs and greater reliability of the welded components. Arata [4] performed important measurements and concluded that the sound traveling into the sample and into the surrounding air has an influence on the welding process by affecting the behavior of molten pool. Sound waves generated by the arc excite the molten pool and cause its movement. Sound affects the welding process and its quality. Some attempts to use acoustic signals for on line monitoring of submerged arc welding were presented by Mayer, [5]. Rostek used computeraided acoustic pattern recognition to test the monitoring capabilities of acoustic signals, [6]. He observed the effect of operating parameters, such as voltage supply, feeding rate of consumable wire electrodes, stand off of contact pipe and flow rate of shielding gas on an arithmetic average of all frequencies amplitude and squared center

*Corr. Author’s Address: University of Ljubljana, Faculty of Mechanical Engineering, Askerceva cesta 6, 1000 Ljubljana, Slovenia, mirko.cudina@fs.uni-lj.si

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of noise spectra gravity. He stated that within a wide range of the observed parameters there is a direct connection with the emitted noise. He also discovered that an unsuitable flow rate of shielding gas and its distribution, welding material dirtiness and non-uniformity of joint edge configuration have an effect on emitted noise. In 1994 a system for real-time analysis of weld quality in an arc welding process was patented [7]. The acoustic signals generated during the welding process are sampled and digitalized. The digitalized signals are transformed into frequency domain and then divided into a plurality of frequency bands. The average power for each frequency band is calculated and used as input to an artificial neural network for analysis of weld quality. A monitoring method using different statistic parameters to evaluate welding process stability was developed at the Faculty of Mechanical Engineering in Ljubljana [8] to [11]. Some authors believe that surrounding noise, which is not a result of the welding process, obstructs analysis of measured signals and might be considered as one of the most important obstacles for the acoustic monitoring technique, [4], [6] and [12]. From the available literature it can be seen that in the gas shielded arc-welding process, which most important parameters that have an influence on the emitted sound, are arc stability, metal transfer and oscillations of the molten pool. However, a deeper understanding of the relation between physical processes and the generation of sound is needed to overcome obstacles of pure statistical approach in the use of acoustic signals for on line monitoring. The present study is focused on establishing a theoretical and experimental base to implement acoustic monitoring of the welding processes in industrial environment. By a detailed analysis of each part of the acoustic signal, we tried to find the correlated sound source in time and space. Only investigations of audible sound (between 20 and 20000 Hz) generated during GMAW in short circuiting mode are discussed in this paper. 1 SOUND GENERATED DURING GAS METAL ARC WELDING PROCESS In the GMAW process an electric arc is established between consumable wire electrodes and a melted zone on the welding material. Both 268

are shielded by gas in the form of active CO2 or gas mixtures containing Ar, CO2, etc. The power source usually has a constant static voltage characteristic of U = f(I) = const. At constant adding velocity of consumable electrode the result is a stabile arc length. This phenomenon is known as the principle of internal self-regulation, [13] and [14]. Different phases of material transfer during the GMAW process, in short circuit mode and in dependence on the arc formation are also depicted in this works. The corresponding welding current is presented in Fig. 1. At points 1 to 3 in Fig. 1 there is no arc, whereas at points 4 to 6 an arc is formed. In points 1 to 3 and towards point 4 a consumable electrode is in contact with the molten pool; this period of short circuit is depicted as a welding current peak. As accompanying phenomena during the GMAW process, more different sound sources appear simultaneously and/or successively. They have different characteristics and they differ in time and form of appearance. With regards to time, there are two characteristic time intervals of noise generation. The first is the short circuit that ends with arc ignition (see points 1 to 4 in Fig. 1) and the second is the oscillations of the burning arc that ends with arc extinction (points 4 to 6 and 1 in Fig. 1). Emitted sound can be classified in the two main groups. One appears in the form of sound impulse and the other appears in the form of smallchangeable broadband or so-called “turbulent” noise. The term is used from machinery noise. The sound impulse is connected with the rapid current changes and has two origins. The first origin is the result of short-circuiting between the electrode and welding material, and arc extinction thereof, which is accompanied by partial spraying of the molten drop when it strikes the molten pool (see point 1 in Fig. 1). The second origin of sound impulse is the result of tearing off the molten drop from the welding electrode and sudden arc ignition (spark ignition) which causes a rapid increase in temperature and expansion of shielding gas around the arc, causing a strong pressure pulsation in the surrounding air, (see time 4 in Fig. 1). The “turbulent” noise is the result of many processes and mechanisms that generate sound or have influence on sound wave propagation. Among them the oscillation of the arc, the

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Fig. 1. Description of different phases of material transfer during GMAW in short circuit electrode and the molten pool, as well as racking of the materials due to inner tension relaxation are the most important. Others can be neglected, either due to their small contribution to the acoustic signal or due to their appearance out of the audible frequency range. In industrial environment, a background noise caused by secondary sources and reverberation (parasitic) noise cased by reflection are presented. In this case, the sound impulse as well as the turbulent noise are additionally influenced and contaminated, and even overlapped by background noise. To check the effect of background noise, the sound pressure levels with and without a welding process have to be measured. If the difference between them is higher than 10 dB(A), the effect of the background noise is negligible. On the other hand, if background noise is equal or higher than the common level of noise, then the noise due to welding process is negligible and measurement of the welding noise is not valid. In special cases the background noise can be extracted by filters. The presented measurements have shown that the impulse noise generated during the arc ignition (point 4 in Fig. 1) is the most dominant noise source during the welding process. In principle, sound impulse involves acoustic transient phenomena. The core of the ignition spark is heated up to 24,000 K, and expansion speed of the shock wave is over 1000 m/s. As a consequence of rapid increase of temperature, a strong pressure oscillation appears which causes the sound impulse.

Some authors tried to find a correlation between the emitted sound pressure and the arc volume, while other studies have revealed that amplitude of sound pressure p(t) [Pa] is correlated to the arc voltage supply (U), [16], or to the electrical power (UI) supplied to the arc, [17] and [18]:

p (t ) = C1

d [U (t ) ⋅ I (t )] , (1) dt

where C1 is the factor of proportionality, which, for a spherical wave propagation, depends on geometrical factor α, on adiabatic expansion coefficient κ, and on speed of sound in the arc c [m/s] by [18]:

C1 = α

κ −1 . (2) c2

The geometrical factor α depends on microphone position relative to the arc r (see Fig. 2): α = 1 / r [m-1]. The speed of sound in the arc c is changing during the process and represents a non-linearity. In the case of the three phase arc furnace, the current in each phase must be compared with the wave form of the total sound emitted by the furnace. Such a comparison reveals which of the three arcs needs to be adjusted and therefore, which electrode has to be lowered or raised in the furnace [16]. The voltage supply U(t) in Eq. (1) is defined by electrical circuit. The electrical circuit of the welding appliance (power supply, cable, consumable wire, arc and welding material) consists of the basic elements: resistance R [Ω],

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inductance L [H] and capacitance C [F]. A suitable equation for describing the electrical circuit can be obtained by using the Ohm’s and Kirchoff’s law: U (t ) = RI (t ) + L

Ra I (t ) dI (t ) + , (3) dt 1 + jωCa Ra

where I(t) is time dependent the electric current [A], U is the equivalent open-circuit voltage supply [V] usually of constant value, Ra and Ca are the resistance and capacitance of the arc ω is the angular frequency [s-1]. Since the arc is responsible for noise generation can be expressed the arc voltage Ua explicitly, similarly to Choi [19]:

U a = Ra I = U − U R + U L = U − RI − L I . (4)

By substituting Eq. (4) into Eq. (1) and after making time derivation the next equation follows: p(t ) = C1

• d    U − RI − L I ) I   =  dt  

• ••   • •  = C1 U I -2R I I -L  I + I I   ,   

(5)

where I is derivation of the current [A·s-1] and I is second derivation of the current [A·s-2]. Eq. (5) is a transfer function between the welding current (I) as an input into the welding process and emitted sound pressure p(t) as an output from the welding process. The transfer function,

which depends on the voltage supply and welding current, the current first and second derivation, allows us to compare the welding current with the generated sound pressure. 2 MEASUREMENT SETUP Experiments were performed using an experimental set-up shown in Fig. 2. Standard industrial welding equipment was used in the experiments. As power supply an ISKRA E-450 with constant static voltage characteristics was used, the consumable wire was electrode VAC 60 (φ = 1.2 mm with 1.5% Mn, 0.9% Si and 0.1% C), and the shielding gas mixture was CORGON (82% Ar and 18% CO2, with flow rate Q = 9 l/min). Measurements were performed in the short circuit mode of operation, as it is widely used in industrial environment. Two different materials were used in the experiment, heated steel (with 0.4% C, 0.65% Mn, 0.045% Smax and 0,045% Pmax) with a thickness of the specimen 6 mm and structural steel (with 0.1% C, 0.45% Mn, 0.03 Smax and 0.03% Pmax) with a thickness of the specimen 12 mm. The welding current ranged from 120 to 135 A and the speed of welding was 40 mm/s, whereas the voltage supply was approximately constant (U1 = 21 V). An A/D converter with a sampling rate of 48 kHz per channel and with 16-bit data resolution was used for data acquisition. Twenty seconds of the welding process were recorded for each

Fig. 2. Experimental set-up of the GMAW process 270

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single environment, and stored on hard disk. The welding current was measured via a shunt resistor. A half-inch Bruel&Kjear, type 4134 condenser microphone was fixed to the welding head on a distance of L = 0.35 m from the arc in order to maintain constant distance from the welding process. This distance is approximately equal to the distance of the welder’s head from the arc, which means that the measured signal of noise can also be used for assessing the effect of welding noise on the welder. 3 EXPERIMENTAL RESULTS AND DISCUSSION In Fig. 3 the measured welding current (a) and sound pressure signal (b) during the GMAW process with regards to time are presented. The welding current signal consists of peaks, which are connected with short circuits. The sound signal consists of short impulses separated by long-term “turbulent” noise; see also Fig. 4. There are two different sound impulses and they are in correlation with the welding current peak. The first appears at the moment of short circuit arc extinction (smaller inexpressive one) and the other appears at the moment of arc ignition (the most powerful one). This means that greater sudden changes of the current cause higher level of sound impulse.

Analyses of the measured results have shown that amplitudes of the sound impulses generated during short circuit arc extinction depend on the sorts of shielding gas and structure of welded material used. Alternatively, the amplitude of the sound impulses generated during arc ignition also depends on the energy input during the welding current peak. The first impulse (smaller one) can be neglected in comparison to the second larger one. The larger impulse, occurring at the end of the welding current peak, can thus be assigned to the corresponding welding current peak, see Figs. 3 and 4. From sound signal analyses it follows that between two sound impulse peaks due to arc extinction and arc ignition, under the welding current peak, the sound signal consists of just low-level background noise (denoted by background noise in Fig. 4). The sound signal between two successive impulses, which is between two welding current peaks, consists of turbulent noise, generated mostly by pulsation of the shielding gas in the arc (denoted by “turbulent” noise in Fig. 4). It has much higher level than the background noise. The audible sound after welding during cooling is of the turbulent character, but of a much lower level than during the welding process and therefore not important for the welder and the total emitted noise. This denotes that the welding sound is generated and dominated by the arc itself, as already stated by many authors, [16] and [18] The

a)

b)

Fig. 3. a) measured welding current, b) acoustic signal during GMAW process for heatted steel Monitoring Gas Metal Arc Welding Process by Using Audible Sound Signal

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turbulent arc noise appears in a high frequency range above 5 kHz, see Fig. 5. In Fig. 5 spectra of four successive sound impulses are presented. Each of them corresponds to the particular welding current peak. The third successive impulse has a much lower level of sound then the others, which means that, in this case, the welding process was incomplete (defective). However, the sound pulse after it (fourth in Fig. 5) has a much more pronounced level, which means that the energy from the incompletely developed welding process is

transferred to the following one. The higher level of turbulent arc noise is a consequence of lagging behind energy from the previous pulse and dissipation of energy (see framed part of the spectrogram in Fig. 5). Sound spectra amplitude of impulses can thus be used for on-line control of some parameters, as for example, volume of the molten material and therefore for control of welding process stability and quality [9]. From Fig. 5 it can be seen that the frequency spectra of the turbulent arc noise is pronounced above 5 and 7 kHz, respectively,

Fig. 4. Three successive welding current peaks with corresponding sound impulses for heated steel

Fig. 5. Spectra cascade (waterfall) of four successive sound impulses with arc turbulent noise for heated steel 272

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whereas the frequency spectra of the sound impulses prevails above 3 and 5 kHz, respectively, with some pronounced peaks in the frequency range between 8 and 16 kHz. In order to check the relative importance of the turbulent arc noise in comparison with the impulse noise the sound pressure level Lp [dB] was used. For this purpose the following equation was used:

Lp = 20 log p / po , (6)

where p is the RMS value of sound pressure [Pa] and po is the reference sound pressure, 2路10-5 Pa. For assessment of human perception of noise A-weighted sound pressure level [dB(A)] is commonly used. Detailed analyses have shown that the total level of welding noise is 91.5

dB(A) and that of turbulent noise alone is 81.0 dB(A). The impulse noise is up to 10 dB(A) and higher than the turbulent arc noise, wich means that the contribution of the turbulent arc noise is negligible. This means that the impulse noise can represent the total emitted noise, [20] to [22]. 4 NUMERICAL ANALYSIS Eq. (5) was used for the calculation of the emitted sound pressure generated by the GMAW process. It is based on the measured welding current data only. Values of the constants in Eq. (5), resistance R, inductance L and capacitance C, were determined by comparison of the calculated and measured values of the sound pressure using the least mean square (LMS) method. The value

Fig. 6. a) Measured acoustic signal of the stable welding process and b) calculated acoustic signal from measured welding current using Eq. (5) for heated steel

Fig. 7. a) Measured acoustic signal of the welding process initialization and b) calculated acoustic signal from the measured welding current using Eq. (5) for structural steel Monitoring Gas Metal Arc Welding Process by Using Audible Sound Signal

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of the constants (resistance R, inductance L and capacitance C) are changing over a longer time scale during stabile welding, due to instability in the welding process, but detailed analyses have shown that these values (R, L, C), as well as the constant of proportionality C1 in Eq. (5), can be treated as constant values for the calculation of impulse sound. Using the average value of the constants, the emitted sound pressure was calculated and compared with the measured acoustic signal. Figs. 6 and 7 show the comparison between calculated and measured acoustic signals. Agreement between calculated and measured results is especially good for the arc burning process in the period between two successive noise impulses; whereas the calculated value of the sound impulses is higher than the measured one, although the sound impulses appear at the same positions as the measured signal, see Figs.

6 and 7. Some of the results have already been published by [23]: Detailed analyses have shown that the values of resistance R, inductance L and capacitance C in Eq. (5) can be treated as constant values for calculation of turbulent and impulse sound, whereas the constant of proportionality C1 is not the same for turbulent arc noise and for the sound impulse. The value of the constant C1 is smaller for the impulse noise. The reason for this is sound non-linearity at the time of sound impulse. The coefficient C1 represents, thus, a measure of non-linearity. Using a smaller value of the constant C1 in Eq. (5), which is appropriate for the calculation of the sound impulse, reduces the level of the calculated turbulent noise. A comparison between the measured and calculated values by using of Eq. (5) for four successive sound impulses is presented in Fig. 8 for heated steel. Agreement between the calculated values

Fig. 8. Comparison between measured values of four successive sound impulses from Fig. 3b (thin curve) and Calculated values from welding current using Eq. (5) (thick curve) for heated steel 274

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Fig. 9. Comparison between measured values of four successive noise impulses from Fig. 11 (thin curve) and calculated values from welding current using Eq. (5) (thick curve) for structural steel (thick curve) and measured values (thin curve) is quite good. The small differences between the measured and calculated noise after the occurrence of impulses can also be assigned to the effect of reverberation noise from the surrounding room walls. The greater differences between the measured and calculated values for arc extinction can be neglected since the sound pressure of the impulse due to arc extinction is much lower than the noise of the arc ignition, see scale in Fig. 8 and impulse at the arc extinction in Fig. 4. Similar results were obtained for structural steel, see Fig. 9. Since the total A-weighted sound pressure level of impulses is far more than 10 dB(A) higher then the corresponding total level of the turbulent arc noise and even higher than the A-weighted total sound pressure level including the impulse and turbulent arc noise (see also Fig. 5) it can be said that the level of impulse noise also represents the total emitted noise of the welding process. The

proposed algorithm, Eq. (5), with constant C1 for calculation of impulse noise can, thus, be used for calculation of the total noise level generated by the GMAW process. It can also be used for online monitoring and control of stability and quality of the welding process. Fig. 10 shows an inclined plate specimen of heated steel with a defect in the welding process and Fig. 11 shows a flat specimen of structural steel with a defect in the welding process in the enlarged scale, which is noticed in the welding current and sound pressure records. The inclination of the plate specimen in Fig. 10 causes change in the torch-to-work distance as welding progresses (in this case, the distance becomes increasingly smaller) and consequently, change in the process parameters, which can result in the observed defect. Detailed analyses have shown that the observed defect is not the result of the inclination (which is relatively small, less then 2.5%), but the result of anomalies in the welding process, possibly due to dirtiness or

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erratic wire feed. Similar defects have also been observed on a flat plate specimen. The variation in the welding current corresponded to sound

pressure records. Fig. 11 shows the point where the defect occurred (enlarged scale). The sound pressure record can also be used to calculate the

Table 1: Characteristic noise levels at measuring parameters: current I = 180 A, voltage U = 21 V and feeding rate of consumable wire electrode v = 11 m/min. Mic. distance L in Fig. 2 [m] 1.5 1 0.3

Peak noise LApk(MaxP) [dB(A)] 127 127 128

Equivalent sound pressure level, LAeq [dB(A)] 89 93 98

Impulse noise LAim [dB(A)] 91 95 100

Fig. 10. Specimen of heated steel with detail of the welding defect

Fig. 11. Enlargement of defect in welding process with the records of welding current and corresponding sound pressure for structural steel 276

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total noise level and then to assess the effect of the welding noise on the worker. The main advantage of the proposed algorithm, based on the current welding data, is that the calculated values are not corrupted by background and reverberation noise as is in the case of measuring noise in an industrial environment. Determination of welding noise using the proposed algorithm is thus more exact, robust and user-friendly. Some measurements made in a metal (industrial) environment, without important secondary sources, have shown that A-weighted sound pressure level can reach and even exceed 100 dB at the welder’s ear, see Table 1. In Table 1, characteristic measured parameters of noise (peak noise, equivalent noise and impulse noise) at welding of construction steel IPE-400 profiles, measured by a precise sound pressure analyzer, B&K, Type 2260, are presented. This means that the noise can affect the worker’s health since daily A-weighted equivalent sound pressure level from approximately 65 to 85 dB causes an acoustic trauma, whereas the sound pressure level above 85 dB, and especially above 90 dB, can cause hearing damage. 5 CONCLUSIONS During the GMAW process noise emission can rise above the welder daily permissible A-weighted sound pressure level of 80 dB(A). Therefore, theoretical and experimental analyses of the sound signal were performed, examining both time and frequency, in order to find the sound origin in time and space. Analyses have shown that the sound spectra of the welding process are broadband with pronounced peaks at higher frequencies (above 5 kHz). There are two mechanisms that generate the overall noise during the welding process: impulse noise and the so-called “turbulent” noise. The impulse noise has its origin in short circuit arc extinction and arc ignition, and the turbulent noise has different origins; the most important are: oscillation of the arc, the electrode and the molten pool, as well as racking of the material due to inner tension relaxation. The impulse noise is far more than 10 dB higher then the turbulent noise and thus represents the dominant noise generating

mechanism during the welding process. A new algorithm for calculation of sound generated during welding process was established. The algorithm represents a transfer function between the welding current and emitted sound pressure. The algorithm was verified on different welded materials (structural steel and heated steel). Comparisons have shown that the calculated values are in good agreement with the measured results. The main advantage of the proposed algorithm based on the current welding data, is that the calculated values are not influenced or corrupted by background and reverberation noise as it is in a case of noise measurement in industrial environment. Therefore, it is also suitable for online monitoring and control of welding process stability and quality. 6 REFERENCES [1] Saini, D., Floyd, S. (1998). An investigation of gas metal arc welding sound signature for on-line quality control. Welding Journal, vol. 77, no. 4, p. 172s -179s. [2] Choi, J.H., Lee, J.Y., Yoo, C.D. (2001). Simulation of dynamic behaviour in a GMAW System. Welding Journal, vol. 80, no. 10, p. 239s-245s. [3] Erdmann-Jesnitzer, F., Feustel, E., Rehfeldt, D. (1967). Akustische Untersuchungen am Schweislichtbogen. Schw. und Schn., vol. 19, no. 3, p. 95-100. [4] Jolly, W.D. (1969). Acoustic emission exposes cracks during welding. Welding Journal, vol. 48, no. 1, p. 21-27. [5] Arata, Y. (1979). Investigation on welding arc sound. Report 1, IIW Doc.S.G.212-451-79. [6] Mayer, J.L. (1987). Application of acoustic emission to in process monitoring of submerged arc welding. IIW Doc V-WG329-87. [7] Rostek, W. (1990). Investigations on the connection between the welding process and airborne noise emission in gas shielded metal arc welding. Schw. und Schn., vol. 42, no. 6, p. E96–E97. [8] Grad, L., Kralj, V. (1996). On line monitoring of arc welding process using acoustic signals. Proc. of the 13th Conference BIAM ‘96, Zagreb.

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[9] Morris, R.A., Tate, R.C., Matteson, M.A. (1994). Weld Acoustic Monitor, US Patent, No. 5,306,893. [10] Ravnik, F., Grum, J. (2009). Sound emitted at boundary layer during steel quenching. Strojniški vestnik - Journal of Mechanical Engineering, vol. 55, no. 3, p. 199-211. [11] Kim, J.S., Eagar, T.W. (1993). Metal transfer in pulsed current gas metal arc welding, Welding Journal, vol. 72, no. 7, p. 279-284. [12] Ogukbiyi, B., Nixon, J., Richardson, I., Blackman S. (1999). Monitoring indices for assessing pulsed gas metal arc welding process. Science and Technology of Welding and Joining, vol. 4, no. 4, p. 209-213. [13] Morita, T., Ogawa, Y., Sumitomo, T. (1995). Analysis of acoustics signals on welding and cutting, Materials Engineering, ASME, vol. III. [14] Polajnar, I., Prezelj, J., Čudina, M. (2006). Comparison of sound pressure level generated in an authomatized MIG/MAG and resistance spot welding, Varilna tehnika, vol. 56 no. 2, p. 61-69. (in Slovene) [15] Dadgar, H., Pilorget, A., Fitaire, M. (1977). Acoustic noise excited by electric arc. IEEE International Conference on Plasma Science: Conference record - Abstracts, R. PI., p. 117. [16] Mansoor, A.M., Huisson J.P. (1999). Acoustics identification of GMAW process. Special publication of the 9th international

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conference on computer technology in welding, p. 312-323. Drouet, M., Nadeau, F. (1982). Acoustic measurement of the arc voltage applicable to arc welding and arc furnaces. Journal of Physics E: Scientific Instruments, vol. 15, no. 3, p. 268 - 269. Grad, L., Prezelj, J., Polajnar, I., Grum, J. (2001). Welding process assessment by analyzing on line measured acoustic signals. Proceedings of the 6th International Conference of the Slovenian Society for NonDestructive Testing, Portorož, p. 185-189. Čudina, M., Prezelj, J., Polajnar, I. (2008). Use of audible sound for on-line monitoring of gas metal arc welding process. Metallurgy, vol. 47, no. 2, p. 81-85. Manz, A.F. (1981). Welding arc sound. Welding Journal, vol. 60, no. 5, p. 23-27. Polajnar, I., Prezelj, J., Mišina, N., Čudina, M. (2007). Noise on the working place of a welder. Sigurnost, vol. 49, no. 2, p. 113-124. (In Croatian) Horvat, J., Polajnar, I., Čudina, M., Dahmane, R. (2007). Ergonomic stresses of welders. Strojarstvo, vol. 49, no. 5, p. 377382. Grum, J., Gorkič, A., Kejžar, R., Polajnar I. (2007). Influence of the type of workpiece adjustment and energy input on the quality of a RPW. Int. j. mater. prod. technol., vol. 29, no. 1/2/3/4, p. 272-296.

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REFERENCES A reference list must be included using the following information as a guide. Only cited text references are included. Each reference is referred to in the text by a number enclosed in a square bracket (i.e., [3] or [2] to [6] for more references). No reference to the author is necessary. References must be numbered and ordered according to where they are first mentioned in the paper, not alphabetically. All references must be complete and accurate. All non-English or. non-German titles must be translated into English with the added note (in language) at the end of reference. Examples follow. Journal Papers: Surname 1, Initials, Surname 2, Initials (year). Title. Journal, volume, number, pages. [1] Zadnik, Ž., Karakašič, M., Kljajin, M., Duhovnik, J. (2009). Function and functionality in the conceptual design process. Strojniški vestnik Journal of Mechanical Engineering, vol. 55, no. 7-8, p. 455-471. Journal titles should not be abbreviated. Note that journal title is set in italics. Books: Surname 1, Initials, Surname 2, Initials (year). Title. Publisher, place of publication. [2] Groover, M.P. (2007). Fundamentals of Modern Manufacturing. John Wiley & Sons, Hoboken. Note that the title of the book is italicized. Chapters in Books: Surname 1, Initials, Surname 2, Initials (year). Chapter title. Editor(s) of book, book title. Publisher, place of publication, pages. [3] Carbone, G., Ceccarelli, M. (2005). Legged robotic systems. Kordić, V., Lazinica, A., Merdan, M. (Eds.), Cutting Edge Robotics. Pro literatur Verlag, Mammendorf, p. 553-576. Proceedings Papers: Surname 1, Initials, Surname 2, Initials (year). Paper title. Proceedings title, pages. [4] Štefanić, N., Martinčević-Mikić, S., Tošanović, N. (2009). Applied Lean System in Process Industry. MOTSP 2009 Conference Proceedings, p. 422427. Standards: Standard-Code (year). Title. Organisation. Place.

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[5] ISO/DIS 16000-6.2:2002. Indoor Air – Part 6: Determination of Volatile Organic Compounds in Indoor and Chamber Air by Active Sampling on TENAX TA Sorbent, Thermal Desorption and Gas Chromatography using MSD/FID. International Organization for Standardization. Geneva. www pages: Surname, Initials or Company name. Title, from http:// address, date of access. [6] Rockwell Automation. Arena, from http://www. arenasimulation.com, accessed on 2009-09-07.

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3 Vsebina

Vsebina Strojniški vestnik - Journal of Mechanical Engineering letnik 57, (2011), številka 3 Ljubljana, marec 2011 ISSN 0039-2480 Izhaja mesečno

Uvodnik Povzetki člankov Igor Solodov, Daniel Döring, Gerd Busse: Nove priložnosti za neporušne preiskave z uporabo nelinearnih interakcij med elastičnimi valovi in napakami Philipp Menner, Henry Gerhard, Gerd Busse: Lockin-Interferometrija: princip in uporaba pri neporušnih preiskavah Theodoros Hasiotis, Efstratios Badogiannis, Nicolaos Georgios Tsouvalis: Uporaba tehnik ultrazvočnih C-skenov za odkrivanje napak v kompozitnih laminatih Raimond Grimberg: Elektromagnetne neporušne preiskave: sedanjost in prihodnost Bernd Wolter, Gerd Dobmann, Christian Boller: Nadzor in upravljanje procesov na osnovi neporušnih preiskav Gerhard Mook, Fritz Michel, Jouri Simonin: Elektromagnetno slikanje s polji merilnih glav Antonios Kyriazopoulos, Ilias Stavrakas, Cimon Anastasiadis Dimos Triantis: Študija emisije šibkih električnih tokov v cementni malti med enoosnim mehanskim tlačnim obremenjevanjem do bližine točke loma Reinhard Danzl, Franz Helmli, Stefan Scherer: Spreminjanje fokusa – robustna tehnologija za visokoločljivostno optično merjenje 3D-površin Drago Bračun, Boštjan Perdan, Janez Diaci: Odkrivanje površinskih napak na pogonskih jermenih z uporabo laserske profilometrije Jožef Horvat, Jurij Prezelj, Ivan Polajnar, Mirko Čudina: Nadzor procesa MIG varjenja s pomočjo signala slišnega zvoka

SI 37

SI 39 SI 40 SI 41 SI 42 SI 43 SI 44 SI 45 SI 46 SI 47 SI 48

Navodila avtorjem

SI 49

Osebne vesti Doktorati, magisteriji, specializacije in diplome Prof. dr. Viktor Prosenc - 90 letnik

SI 51 SI 53


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 37

Uvodnik

Tematska številka: Neporušno testiranje in preiskava materialov

Preiskava materialov je izredno pomembna naloga v sodobni industrijski proizvodnji, ko je potrebno zagotoviti strojne dele ali dele orodij z želenimi lastnostmi in brez napak. Za današnje stanje proizvodnje je značilna vse močnejša računalniška podpora, kar zagotavlja sprotno sledenje kakovosti proizvodov in tudi statistično obdelavo podatkov o kakovosti proizvodnje za daljše obdobje. V sklop preiskave materialov sodijo porušne in v zadnjih nekaj desetletjih tudi neporušne metode testiranja. Neporušno testiranje materialov in konstrukcij zato postaja vse pomembnejša dejavnost, tako v proizvodnji različnih delov in konstrukcij, kot tudi za njihovo testiranje med obratovanjem. Zelo prepričljiva je uporaba neporušnih metod pri periodičnem testiranju javnih transportnih sredstev v letalskem in železniškem prometu ter neporušno testiranje termoenergetskih objektov in še posebej objektov v jedrskih elektrarnah. Pogosto so periodični pregledi predpisani že s strani proizvajalca opreme, ki predpiše metodo testiranja podprto z ustreznimi aparati in pripomočki. Današnje zahteve pri zagotavljanju kakovosti izdelkov so, da je njihov nadzor prirejen nivoju proizvodnje. Avtomatizirana proizvodnja zahteva čim popolnejši nadzor nad kakovostjo materiala, ki prihaja v izdelovalni proces in čim popolnejši nadzor nad stanjem materiala med obdelavo, kar pomeni proizvodnjo brez motenj in proizvodnjo visoko kakovostnih izdelkov. Zato se je potrebno zavedati, da uvajanje avtomatiziranih in računalniško podprtih proizvodnih sistemov zahteva tudi avtomatizirano testiranje materialov, kot tudi posameznih vrst napak v materialih zahteva izbiro ustrezne metode preizkušanja. Torej, če razpoznamo odstopanje od dane lastnosti materiala in/ali ugotovimo prisotnost napake v njem, sledi končna odločitev o izmetu obdelovanca. Po odkritju napake je potrebno ugotoviti kako nevarna je ta napaka glede na postavljene kriterije o sprejetju ali zavržbi strojnega dela. Danes so že poznane številne naprave za avtomatizirano razpoznavanje in ocenjevanje defektov v materialu z računalniško podporo na osnovi penetrantskih, ultrazvočnih, elektro-magnetnih

in radiografskih preiskav. Takšni sistemi nudijo številne prednosti v primerjavi s klasičnimi načini preiskave materialov brez porušitve in sicer: znatno povečano hitrost preizkušanja, izključen je subjektivni vpliv operaterja pri ocenjevanju lastnosti materiala ali velikosti oziroma nevarnosti prisotne napake v njem in dajejo zanesljiv vpogled v kakovost izdelka. V zadnji dekadi pa se intenzivno razvijajo in uveljavljajo tudi številne neporušne metode za preiskavo materialov predvsem zaradi napredka v razvoju različnih vrst senzorjev in ustrezne računalniške podpore z vizualizacijo stanja materiala oziroma napak v materialu. Na razvoj neporušnih metod za preiskavo materialov danes vpliva predvsem avtomatizacija proizvodnje, ki mora zagotavljati proizvodnjo izdelkov brez izmeta. Da poteka proces avtomatizacije proizvodnje usklajeno z razvojem in uvajanjem neporušnih preiskav materialov, so prispevali sinergetski učinki razvoja senzorske tehnike, elektronike, mikroprocesorske tehnike in računalniške tehnologije. Za posamezne načine preiskave materialov brez porušitve so pomembne osnovne karakteristike naprave, ki omogoča odkrivanje posameznih lastnosti materiala, kot tudi različnih vrst napak z ustrezno vizualizacijo in registracijo. Članki zbrani v tej posebni številki so bili predstavljeni na jubilejni 10. mednarodni konferenci o neporušnih preiskavah, ki je potekala od 1. do 3. septembra 2009 v Ljubljani. Ob tej priliki je bilo predstavljenih 60 prispevkov izmed katerih je znanstveni odbor konference izbral le 15 prispevkov, ki so prestali ponovno recenzijsko presojo in pred nami so članki v razširjeni in dopolnjeni obliki. V tej številki je predstavljenih deset prispevkov, preostalih pet prispevkov pa je bilo uvrščenih v redne številke naše revije z naslovom Journal of Mechanical Engineering, let. 56, št. 9 in 10, 2010 ter let. 57, št. 2, 2011, ki pa so dostopni na spletni strani revije (http://www.svjme.eu). Janez Grum, gostujoči urednik SI 37


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 39

Prejeto: 22.08.2009 Sprejeto: 04.03.2010

Nove priložnosti za neporušne preiskave z uporabo nelinearnih interakcij med elastičnimi valovi in napakami Igor Solodov - Daniel Döring - Gerd Busse* Institut za tehnologijo polimerov, Oddelek za neporušne preiskave, Univerza v Stuttgartu, Nemčija Hitra rast uporabe novih visokotehnoloških materialov pri aplikacijah, ki so kritične glede varnosti, prinaša tudi stroge zahteve glede zanesljivosti proizvodnje, zagotavljanja kakovosti industrijskih izdelkov in nadzora stanja obstoječih komponent. To je tudi povod za razvoj nove generacije metod neporušnih preiskav, ki so znatno bolj občutljive na strukturo materiala, začetne mikronapake in napredujoče poškodbe. Konvencionalni instrumenti za ultrazvočne neporušne preiskave običajno delujejo na eni sami frekvenci in delujejo po principu zaznavanja sprememb amplitude in faze vhodnega signala zaradi odbojev na napakah. Takšna pomanjkljiva količina informacij je lahko sprejemljiva, dokler so interakcije med valovi in napakami linearne. Nelinearni pristop k neporušnim preiskavam (NNDT) obravnava nelinearne odzive na napake, ki so povezani s spremembami frekvence vhodnega signala. Te spektralne spremembe povzroča visoka stopnja nelinearnosti napak na mikro- in makroravni. Nepoškodovani deli materiala vibrirajo linearno, v izhodnem spektru torej ni frekvenčnih sprememb. Pri metodi NNDT se majhna razpoka (ki je za linearno ultrazvočno defektoskopijo nevidna) torej obnaša kot aktiven vir oddajanja novih frekvenčnih komponent in ne kot pasivni povzročitelj odboja valov pri konvencionalnih ultrazvočnih preiskavah. NNDT je zato edinstven instrument za selektivno lokalizacijo in slikanje nelinearnih napak. Med njimi so številni razredi kontaktnih napak, ki obsegajo vse od dislokacij (na nanoskali) do utrujenostnih (mikro) razpok in makroskopskih prekinitev v spojih. Mikrokontaktne (nelinearne) razpoke so običajno samo predhodnice večjih poškodb, zato je metoda NNDT primerna za zgodnje prepoznavanje degradacije materiala in “napovedovanje” verjetnega loma. V pričujočem članku so analizirani nelinearni spektri ravninskih napak (delaminacija, razpoke, udarci itd.) in predstavljeni rezultati NNDT, pridobljeni z metodami nelinearne laserske vibrometrije (NLV) in nelinearne emisije z zrakom kot prenosnim medijem (NACE). Metoda NLV zaznava nelinearne vibracije napak z občutljivim laserskim interferometrom. Vzbujalni sistem vsebuje piezoelektrične pretvornike, ki delujejo pri 20 in 40 kHz. Po dvodimenzionalnem skenu in hitri Fourierjevi transformaciji prejetega signala se pridobijo C-skeni pregledovanega območja za vsako spektralno črto znotraj frekvenčnega pasu 1 MHz. V nasprotju s primerljivo optično metodo, kjer se analizira svetloba, odbita od preizkušanca, se metoda NACE-NDT zanaša na nelinearno akustično oddajanje iz napak, ki se prenaša po zraku. Neposredne optične meritve so pokazale, da takšno nelinearno emisijo oddajajo samo napake in da emisija kaže značilne vzorce usmerjenosti. Visokofrekvenčni (300 – 400 kHz) ultrazvočni pretvorniki z zrakom kot prenosnim medijem se uporabljajo za skeniranje preizkušanca, ki ga vzbujajo nizkofrekvenčni (od 20 do 40 kHz) upogibni valovi. V pasovni širini pretvornika se sprejme nekaj višjih harmonikov (od 10 do 20), ki se uporabijo za odkrivanje nelinearnih napak. Eksperimenti so pokazali, da se metoda NACE dobro obnese pri različnih gradbenih materialih s hrapavimi površinami in grobimi napakami v komponentah. Predstavljenih bo več študij primerov, ki demonstrirajo uporabnost metod NACE in NLV za NNDE in selektivno snemanje napak v različnih materialih. Med posebno uspešnimi primeri so tudi visokotehnološki in gradbeni materiali: nedotaknjen in poškodovan les, udarne poškodbe in delaminacija v plastiki, ojačeni z vlakni, utrujenostne mikrorazpoke v kovinah in delaminacija v kovinskih laminatih, ojačenih z vlakni. Rezultati kažejo, da ima metoda NNDT velik potencial za vrednotenje kakovosti materialov in komponent v avtomobilski industriji, letalski industriji in gradbeništvu. © 2011 Strojniški vestnik. Vse pravice pridržane. Ključne besede: nelinearne neporušne preiskave, selektivno snemanje napak, nelinearna emisija z zrakom kot prenosnim medijem, nelinearna laserska vibrometrij *Naslov avtorja za dopisovanje: Institut za tehnologijo polimerov, Oddelek za neporušne preiskave (IKT-ZFP), Univerza v Stuttgartu, Pfaffenwaldring 32, 70569 Stuttgart, Nemčija

SI 39


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3 SI 40

Prejeto: 22.08.2009 Sprejeto: 04.03.2010

Lockin-Interferometrija: princip in uporaba pri neporušnih preiskavah Philipp Menner - Henry Gerhard - Gerd Busse* Institut za tehnologijo polimerov, Oddelek za neporušne preiskave, Univerza v Stuttgartu, Nemčija Metode, ki uporabljajo interferenčne proge, kot sta interferometrija z vzorcem interferenčnih prog (ESPI, tudi elektrooptična holografija, TV-holografija) in interferometrija z zamikom vzorcev prog (shearografija), spremljajo mehansko obnašanje predmetov pod obremenitvijo brez dotika in v polnem polju, zato so pomembno orodje za neporušne preiskave. Te metode pa ne morejo razlikovati napak na različnih globinah in zaznavajo tudi deformacije celotnega preizkušanca, ki pa lahko napake prikrijejo. Odstranitev ali zmanjšanje vpliva naštetih omejitev bi lahko povečalo sprejemljivost teh metod. Naš pristop uporablja periodično osvetljevanje predmeta s svetlobo modulirane intenzitete, ki se absorbira na površini, ustvarja toploto in posledično modulira termične raztezke. Sočasno se zajemajo posnetki prog s faznim zamikom (bodisi po postopku ESPI ali z interferometrijo z zamikom vzorcev prog), ki se takoj obdelajo v enoti za grafično obdelavo in dajo sklad interferenčnih posnetkov. Po razčlenitvi vsakega interferenčnega posnetka se z diskretno Fourierjevo transformacijo pri frekvenci vzbujanja izloči časovno odvisna vsebina vsake slikovne točke. Na ta način je možno določiti lokalno amplitudo in fazo moduliranega odgovora pri dani frekvenci. Princip vrednotenja signala sklada posnetkov je dobro znan iz optično vzbujane Lockin-termografije in ga je bilo mogoče uspešno prenesti tudi na metodo ESPI in na shearografijo. Posnetek amplitude kaže lokalno velikost učinka modulacije, posnetek faze pa prikazuje lokalni časovni zamik med vzbujanjem in odgovorom. Deformacija celotnega vzorca poteka enakomerno in zato daje konstanten signal na posnetku faze. Napake so jasno vidne na konstantnem ozadju, kar pomeni, da jih je možno razločevati na faznem posnetku (če je vzorec konstantne debeline). Z izvedbo več meritev pri različnih frekvencah modulacije je možno razlikovati napake na različnih globinah, saj se faza spreminja samo na območjih z drugačno debelino. Z vrednotenjem ne le enega ampak več sto posnetkov se močno izboljša razmerje med signalom in šumom. Velikost sklada posnetkov je omejena s pomnilnikom računalnika. Različne programske arhitekture tako lahko povečajo število posnetkov in še dodatno izboljšajo predstavljeno tehniko. Nova metoda je najbolj primerna za polimerne in kompozitne strukture, ki imajo nizko toplotno difuzivnost in velik koeficient temperaturnega raztezka. Čeprav je bil na ESPI najprej prenesen princip Lockin, ki je znan iz optično vzbujane Lockin-termografije, je uporabnost v industrijski praksi verjetnejša za Lockinshearografijo, ki je bolj odporna proti vibracijam. Prihodnje delo bo usmerjeno v kvantitativno ugotavljanje globine napak in v preizkušanje drugih metod vzbujanja, kot sta indukcijsko segrevanje in modulirano spreminjanje tlaka. © 2011 Strojniški vestnik. Vse pravice pridržane. Ključne besede: Lockin, ESPI, shearografija, interferometrija s programi, neporušne preiskave, globinsko profiliranje

SI 40

*Naslov avtorja za dopisovanje: Institut za tehnologijo polimerov, Oddelek za neporušne preiskave (IKT-ZFP), Univerza v Stuttgartu, Pfaffenwaldring 32, 70569 Stuttgart, Nemčija


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 41

Prejeto: 28.05.2009 Sprejeto: 08.07.2009

Uporaba tehnik ultrazvočnih C-skenov za odkrivanje napak v kompozitnih laminatih Theodoros Hasiotis – Efstratios Badogiannis* – Nicolaos Georgios Tsouvalis Laboratorij za tehnologije v ladjedelništvu, Nacionalna tehnična univerza v Atenah, Grčija V študiji so bile razvite in uporabljene praktične tehnike merjenja odmeva ultrazvočnih impulzov za neporušne preiskave dveh tipičnih kompozitnih laminatnih materialov, ki se uporabljata v ladjedelništvu, pri čemer so rezultati prikazani v obliki c-skenov. Preiskovana materiala sta bila napreden karbonski/ epoksi sistem, sestavljen iz enosmernih ogljikovih vlaken in epoksi smole, ter tipičen sistem steklo/poliester za gradnjo plovil, sestavljen iz roving tkanine in poliesterske smole. Oba materiala sta bila proizvedena po dveh metodah (ročno polaganje in vakuumska infuzija). V preizkusnih ploščah je bilo vključenih več umetnih napak, ki so se razlikovale po obliki, velikosti in položaju v prerezu, namenjene pa so bile simulaciji delaminacije in drugih napak pri izdelavi. Testne plošče so bile pregledane z ultrazvočnim sistemom ULTRAPAC II, programsko opremo ULTRAWIN in tipičnimi preiskovalnimi tehnikami z merjenjem odziva na impulz (pregledovanje od sloja do sloja, pregled celotne širine itd.). Glavni cilj je bil odkriti oz. zaznati umetne napake in izmeriti njihovo velikost. Zbrani podatki so morali biti zanesljivi (RF-podatki, C-skeni, podatki analize grozdov), da bo metodologija uporabna tudi kot referenčna praksa za preiskave na večji skali. Preiskovanje večjih konstrukcij, izdelanih iz enakih materialov in po enakem postopku, bo tako lahko postalo standardna praksa pri kontroli kakovosti, pri raziskavah in drugje. Med raziskavo so bili uporabljeni ustrezni postopki programskih prilagoditev in strategije preiskovanja kot je DAC (krivulja korekture amplitud glede na razdaljo) za premagovanje težav pri kompozitnih materialih (dušenje, sipanje itd.), ki še dodatno izpopolnjujejo in optimizirajo postopek skeniranja. Plod teh naporov so učinkoviti C-skeni preizkušancev CFRP, ki omogočajo natančno določanje položaja in oblike umetnih napak. Kar se tiče velikosti napak, jo uporabljena metoda pri preizkušancih CFRP precenjuje. Velikost in oblika napak pri preizkušancih GFRP nasprotno nista bila dobro definirani in ocenjevanje velikosti sploh ni bilo možno. Rezultati kažejo, da je za ultrazvočno pregledovanje materiala GFRP potreben drug senzor, ki napake prikazuje mnogo bolje. Končno je bila pri obeh vrstah preizkušancev jasno zaznana orientacija vlaken, natančno pa je bila izmerjena tudi debelina preizkušancev. Uporabljena oprema in postopki so bili precej bolj učinkoviti pri preizkušancih CFRP kot pri preizkušancih GFRP. Vzrok za to je bila velika debelina testnih plošč in uporaba roving tkanine pri preizkušancih GFRP. V tem primeru bi uporaba pretvornika z nižjo frekvenco (na primer 3-3,5 MHz) izboljšala zmogljivost zaznavanja uporabljenega postopka na račun ločljivosti (prepoznavanje oblik in dimenzij). Iz navedenega sledi, da povečevanje debeline predstavlja omejitev. Članek predlaga integrirano metodologijo za ultrazvočno c-skeniranje kompozitnih laminatov in obravnava omejitve, ki so posledica narave materialov. Iz praktičnega vidika je pomembno, da je predlagan podroben postopek, ki uporablja že znane tehnike. Pomen tega dela za znanost je velik, ker raziskovalcem omogoča uporabo kombinacije metod in tehnik za preiskovanje takšnih materialov, ter za vrednotenje njihove funkcionalnosti in ne le postopka proizvodnje. © 2011 Strojniški vestnik. Vse pravice pridržane. Ključne besede: kompoziti, ultrazvočni pregled, tehnike c-scan, napake, delaminacija, zaznavanje napak

Naslov avtorja za dopisovanje: Laboratorij za tehnologije v ladjedelništvu, Nacionalna tehnična univerza v Atenah Heroon Polytechniou 9, GR-157 73 Zografos, Atene, Grčija, nautheo@yahoo.gr

SI 41


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 42

Prejeto: 22.08.2009 Sprejeto: 04.03.2010

Elektromagnetne neporušne preiskave: sedanjost in prihodnost Raimond Grimberg Nacionalni institut za raziskave in razvoj v tehnični fiziki, Iasi, Romunija Elektromagnetne neporušne preiskave so se pojavile pred več kot 125 leti in so se iz “umetnosti” do danes razvile v priznano inženirsko disciplino. Podlaga za ta razvoj je bilo izpopolnjevanje teorije ustreznih pretvornikov in pripadajoče merilne elektronike na osnovi Maxwellovih enačb. Disciplina je spektakularen preboj doživela z razvojem računalništva. V članku je predstavljen pregled teoretičnih principov, ki se začne z Maxwellovimi enačbami, definicijo standardne globine penetracije in razvojem vektorske valovne enačbe za električno polje v znanem delu in v delu prevodnega preizkušanca, kjer se nahaja napaka. Ta enačba je fundamentalna enačba neporušnih preiskav z vrtinčnimi tokovi in je načeloma rešljiva na dva načina: analitično in numerično. Med analitičnimi postopki bosta predstavljena Greenova funkcija in metoda integrala prostornine vira. Rezultat je Freedholmova integralska enačba drugega reda. Enačba nima natančne rešitve, zato je bil uporabljen postopek diskretizacije. Ena od najpogosteje uporabljenih metod je metoda momentov, ki integralsko enačbo pretvori v sistem algebrajskih enačb. Diskretne celice morajo biti dovolj majhne, da je mogoče privzeti, da je polje v celotni celici mreže enako vrednosti polja v središču celice. V tem primeru je neznanka sistema algebrajskih enačb električna prevodnost v celicah mreže. Med numeričnimi metodami je predstavljena metoda končnih elementov. Aproksimacija zveznih problemov z metodo končnih elementov je razmeroma enostavna. Postopek vključuje naslednje korake: diskretizacija, aproksimacija, iskanje minimalne napake, rešitev algebrajskega sistema enačb (linearnih in nelinearnih) ter naknadna obdelava podatkov. Z analitičnimi in numeričnimi postopki lahko napovedujemo odziv pretvornika na vrtinčne tokove na napake. To je problem napovedovanja. Obraten problem je vrednotenje značilnosti preizkušanca z napakami kot fizikalnega sistema na osnovi poznavanja elektromagnetnega polja v različnih točkah obdelovanca ter odziva naprav za merjenje vrtinčnih tokov v istih točkah. Pretvorniki vrtinčnih tokov morajo za pridobivanje informacij o napaki (geometrija, položaj itd.) odigrati dve vlogi: inducirati morajo vrtinčne tokove v preiskovanem električno prevodnem materialu in poudariti morajo spremembe tokov zaradi degradacije materiala. V članku so predstavljene glavne vrste pretvornikov vrtinčnih tokov in nekatere novosti kot so senzorska polja, vrteče se ploske tuljave in pretvorniki vrtinčnih tokov z vrtečim se magnetnim poljem. Največji vpliv na razvoj instrumentov je najverjetneje imel prihod mikroprocesorjev v sedemdesetih letih prejšnjega stoletja in razpoložljivost cenenih analogno-digitalnih pretvornikov v zadnjih letih. Ni težko zaključiti, da bo spektakularen razvoj računskih in pomnilniških zmogljivosti tudi vnaprej vplival na to disciplino. Zelo verjetno je, da bo ta razvoj vplival tudi na način interpretacije podatkov, pridobljenih pri kontroli. Predpostavimo lahko, da bodo algoritmi za invertiranje postali sestavni del funkcij instrumentarija in da bo ocenjene profile napak celo možno uporabiti za izračun njihovega vpliva na strukturno integriteto preizkušanih komponent opreme. Preiskave z vrtinčnimi tokovi so zahvaljujoč novim materialom in njihovim številnim aplikacijam postale zelo dinamična domena, ki se intenzivno razvija. © 2011 Strojniški vestnik. Vse pravice pridržane. Ključne besede: elektromagnetne neporušne preiskave, teorija, problem napovedovanja, obraten problem, pretvorniki vrtinčnih tokov, razvoj računskih in pomnilniških zmogljivosti, pretvorba signala

SI 42

*Naslov avtorja za dopisovanje: Nacionalni institut za raziskave in razvoj v tehnični fiziki, Iasi 47 D.Mangeron Blvd., Iasi, 70050, Romunija, grimberg@phys-iasi.ro


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 43

Prejeto: 22.08.2009 Sprejeto: 08.04.2010

Nadzor in upravljanje procesov na osnovi neporušnih preiskav Bernd Wolter* - Gerd Dobmann - Christian Boller Fraunhoferjev institut za neporušne preiskave (IZFP), Nemčija Namen: Cilj avtomatizacije je bil nekdaj predvsem povečanje produktivnosti in zmanjšanje stroškov. Do danes pa se je pomen avtomatizacije razširil tudi na druga področja. Visokoavtomatizirani procesi v povezavi z visoko ravnjo (avtomatiziranega) nadzora in upravljanja zagotavljajo visoko kakovost na nespremenljivi ravni. Neprekinjen nadzor procesov in kakovosti z neporušnimi preiskavami se je medtem uveljavil kot postopek za zgodnjo diagnozo nesprejemljivih procesnih stanj, sledila pa sta mu upravljanje s povratno zanko in optimizacija na osnovi neporušnih preiskav. Razvoj integracije neporušnih preiskav v procese je zato pomembna znanstvena naloga. Novosti morajo izpolnjevati zahteve današnjega industrijskega proizvodnega okolja glede integrabilnosti, avtomatizacije, hitrosti, zanesljivosti in dobičkonosnosti. Proizvodnja in obdelava pločevine, postopki brizganja s plinom in postopki hladnega spajanja so le nekateri primeri uspešne integracije neporušnih preiskav v procese. Metodologija: Pristop k izpolnjevanju zahtev po nadzoru in upravljanju procesov z integracijo neporušnih preiskav vključuje izbiro metodologij, ki so dovolj hitre za delo v realnem času (gre torej pretežno za brezdotično merjenje preizkušancev) in dajejo več kot eno kontrolno veličino s potencialom za karakterizacijo kakovosti, namen pa je izboljšanje zanesljivosti kontrole z raznoterimi in tudi redundantnimi informacijami. Rezultati: Predstavljeni rezultati osvetljujejo prednosti neprekinjenega on-line nadzora ustreznih parametrov kakovosti za upravljanje kakovosti s povratno zanko, ki bi ga sicer lahko izvajali samo s porušnimi preiskavami. Novosti: Vsi predstavljeni primeri so izvirni in nadgrajujejo trenutno stanje tehnike z novo kakovostjo za procesne aplikacije. © 2011 Strojniški vestnik. Vse pravice pridržane. Ključne besede: avtomatizacija, mehanske lastnosti materialov, mikromagnetna metoda, mikrovalovi, neporušne preiskave (NDT), integracija NDT v procese, upravljanje procesov, nadzor procesov

*Naslov avtorja za dopisovanje: Fraunhoferjev institut za neporušne preiskave (IZFP), Campus E3 1, D-66123 Saarbruecken, Nemčija, bernd.wolter@izfp.fraunhofer.de

SI 43


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 44

Prejeto: 22.08.2009 Sprejeto: 19.03.2010

Elektromagnetno slikanje s polji merilnih glav Gerhard Mook* - Fritz Michel - Jouri Simonin Institut za materiale in tehnologijo spajanja, Univerza Otto von Guericke v Magdeburgu, POB 4120, 39016 Magdeburg, Nemčija Namen članka je predstavitev osnov, merilnih glav in tehnologije neporušnih preiskav napak v materialih s polji glav za merjenje vrtinčnih tokov. Težava pri elektromagnetnih metodah kot je tehnika vrtinčnih tokov je v tem, da ne dajejo slike materiala, ampak le ustvarjajo lokalne signale v merilni ravnini, ki jih je težko interpretirati. Poskusi ustvarjanja posnetkov, ki bi bili primerljivi z rentgenskimi slikami, so večinoma zasnovani na dragih in zamudnih tehnikah mehanskega skeniranja površin. Pristop te raziskave je uporaba linij oz. polj merilnih glav namesto posameznih skenirnih merilnih glav. Mehansko skenirno gibanje merilne glave zamenja elektronsko premikanje polja z multipleksiranjem merilnih glav. V nasprotju z drugimi poskusi morajo biti polja primerna za delo z nizkimi frekvencami, ki lahko penetrirajo pod površino in dajejo dobro lateralno ločljivost. Ovrednotene so različne merilne glave z ozirom na njihovo zmogljivost snemanja, sposobnost ločevanja med različnimi napakami, možnost sestavljanja v polja, enostavnost izdelave in elektronsko združljivost. Izbrane merilne glave so bile tudi razmeščene v linije in polja za prve praktične eksperimente. Elektronika je bila združena v ohišje, ki je bilo priključeno neposredno na vmesnik USB prenosnega računalnika. Z merilno opremo so bili ustvarjeni posnetki referenčnih delov in vzorcev iz proizvodnje, ki dajejo vpogled pod površino aluminijastih ulitkov. Te slike so osnova za iskanje metode za karakterizacijo napak pod površino. Vzbujalno-detektorske glave dajejo najboljše rezultate za polja glav za merjenje vrtinčnih tokov. Funkcija točkovnega raztrosa kot merilo kakovosti posnetkov merilne glave je podobna sombreru. Ta oblika je podobna kot pri optičnih sistemih in je enostavna za interpretacijo. Razen tega je penetracija pri tej vrsti glav razmeroma globoka in odkrije tudi skrite napake kot so enomilimetrske pore v globini 0,5 mm. Prostorska ločljivost je odvisna od premera in razdalje med glavami. Z enomilimetrskimi feritnimi jedri in razmikom 2 mm je možno doseči ločljivost približno 1 mm. Vzbujalno-detektorske glave vsebujejo eno samo tuljavo. Elektronsko uravnoteženje kompenzira individualne spremembe električnih lastnosti merilnih glav. Na ta način je možno uporabljati tudi cenene industrijske merilne glave. Za enakomeren razmik med glavami polja je bilo uporabljeno plastično satovje. Razen ravnih polj je bilo postavljeno tudi polje v obliki polkrogle. Posnetke pregledovanih materialov, ki jih dajejo ta polja, je možno enostavno interpretirati. Očrtane so možnosti za industrijsko uporabo. Novost so nizkofrekvenčna polja glav za merjenje vrtinčnih tokov z visoko ločljivostjo, ki elektronsko premikajo elektromagnetno polje po preizkušanem materialu. Eno- in dvodimenzionalna polja na osnovi enostavnih vzbujalno-detektorskih glav omogočajo ne le vizualizacijo površinskih napak, temveč tudi odkrivanje skritih napak pod površino električno prevodnih materialov. Obe vrsti polj je možno razmestiti tudi na ukrivljene površine. Strojna oprema za merjenje vrtinčnih tokov je zmanjšana na minimum, obdelavo signalov pa izvaja programska oprema. © 2011 Strojniški vestnik. Vse pravice pridržane. Ključne besede: digitalni instrument za merjenje vrtinčnih tokov, snemanje z vrtinčnimi tokovi, polja merilnih glav, nizkofrekvenčna aplikacija

SI 44

*Naslov avtorja ta dopisovanje: Institut za materiale in tehnologijo spajanja, Univerza Otto von Guericke v Magdeburgu, POB 4120, 39016 Magdeburg, Nemčija, mook@ovgu.de


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 45

Prejem: 28.02.2008 Sprejem: 29.10.2009

Študija emisije šibkih električnih tokov v cementni malti med enoosnim mehanskim tlačnim obremenjevanjem do bližine točke loma Antonios Kyriazopoulos* - Ilias Stavrakas - Cimon Anastasiadis - Dimos Triantis Laboratorij za električne lastnosti materialov, Oddelek za elektroniko, Tehniški izobraževalni institut v Atenah, Grčija

Eksperimentalna tehnika merjenja šibkih električnih signalov, ki se emitirajo med začasnim enoosnim obremenjevanjem trdnih snovi, je bila uporabljena na vzorcih cementne malte. Ti električni signali so v literaturi poimenovani kot tlačno stimulirani tokovi (PSC). Dokazano je, da je tlačno stimuliran tok med enoosnim stiskanjem krhkega materiala s časovno spremenljivo napetostjo σ sorazmeren s hitrostjo deformacij: I ∝ d ε ∝ ⋅ 1 ⋅ dσ , (En. 1), kjer je dε / dt hitrost tlačnih deformacij, dσ / dt odvod dt

Ε dt

napetosti in E elastični modul. Predstavljeni sta dve različni skupini eksperimentov PSC, kjer so vzorci podvrženi enoosnim tlačnim napetostim. Napetost σ v prvi skupini se povečuje linearno in z majhno hitrostjo a po naslednji enačbi: σ = a · t. Ta eksperimentalna tehnika se imenuje tehnika počasnih sprememb napetosti (LSRT). V drugi skupini se med konstantno enoosno obremenitvijo σk uvede nenadno koračno povečanje napetosti kratkega trajanja Δt, pri čemer se enoosna napetost poveča za Δσ = σk+1 – σk, kjer je σk+1 novo stanje po povečanju napetosti. Novo napetostno stanje σk+1 ostane konstantno do naslednjega povečanja napetosti. Ta tehnika se imenuje tehnika za t < tk  σ k = konstanta koračne napetosti (SST) in je primerna za  σ ( t ) =  σ k + b ( t − tk ) za tk < t < tk +1 . razkrivanje tokov PSC tako pri linearnih σ = konstanta za t > tk +1  k +1 kot pri nelinearnih deformacijah materiala. Uporabljeni so bili primerno starani vzorci cementne malte, sestavljeni iz portland cementa, peska in vode. Ko relativna tlačna obremenitev vzorca preseže pribl. vrednost 0,65, opazimo intenzivno eksponentno povečanje PSC ob vstopu v območje nelinearnih deformacij. Elastični modul se zmanjša in emisijo PSC napoveduje En. (1). Tokovi PSC so v korelaciji z relativno tlačno napetostjo σˆ v območju nelinearnih deformacij (0,7 < σˆ < 0,85) in Sl. 1: PSC pri LSRT. Manjši Sl. 2: a) Prikaz koračnih po manjšem diagramu iz Sl. 1 jih je možno grafikon kaže PSC na logaritmični obremenitev (SST), b) Pripadajoči opisati z eksponentnim zakonom oblike: ˆ osi za σ > 0.6 tokovi PSC v odvisnosti od časa I = I0 · exp(α· σˆ ), kjer je I0 tokovna konstanta in α značilni eksponent. V drugem eksperimentu (SST) so bili izvedeni štirje zaporedni nenadni napetostni skoki (Sl. 2a). Časovno spreminjanje tokov PSC med tem postopkom prikazuje diagram na Sl. 2b. Vsaki nenadni koračni spremembi napetosti ustreza nenadna tokovna konica, z vrhom pri PSCpeak v končnem stanju σk+1. Ko lokalna napetost preseže lokalno trdnost, nastane mikrorazpoka in stečejo šibki električni tokovi, ko se vzpostavlja novo ravnotežno stanje. Na ta način je možna interpretacija vrednosti PSCpeak. Tako kvalitativne kot kvantitativne značilnosti tokov PSC so povezane z mehanskim stanjem vzorcev, natančneje z nastankom in širjenjem razpok v materialu. Posledično so tokovi PSC uporabni za napovedovanje stopnje sestavljenih poškodb. © 2011 Strojniški vestnik. Vse pravice pridržane. Ključne besede: cementna malta, emisije šibkih električnih tokov, PSC, med enoosno tlačno obremenjevanje, mikrorazpoke *Naslov avtorja za dopisovanje: Laboratorij za električne lastnosti materialov, Oddelek za elektroniko, Tehniški izobraževalni institut v Atenah, Atene 12210, Grčija, akyriazo@teiath.gr

SI 45


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 46

Prejeto: 22.08.2009 Sprejeto: 30.06.2010

Spreminjanje fokusa – robustna tehnologija za visokoločljivostno optično merjenje 3D-površin Reinhard Danzl* - Franz Helmli - Stefan Scherer Alicona, Grambach, Avstrija 3D-meritve tehničnih površin so pomemben del preverjanja in kontrole lastnosti in funkcije materialov in tehničnih izdelkov. Takšne meritve so se tradicionalno izvajale z dotikalnimi napravami, v zadnjem desetletju pa postajajo vse bolj priljubljene tudi optične naprave. V članku je opisana in ovrednotena metoda spreminjanja fokusa kot tridimenzionalna optična merilna metoda. Cilj je analiza zmogljivost metode pri vrsti tipičnih merilnih nalog. Nasprotno kot pri dosedanjih primerjavah članek ni usmerjen samo na posamezne merilne aplikacije, ampak podaja primerjave meritev hrapavosti, oblike in obrabe ter tipične tehnične aplikacije. Tako dotikalne kot optične naprave imajo svoje prednosti in slabosti. Čeprav imajo dotikalne naprave dolgoletno tradicijo pri merjenju površin in so dobro znane in sprejete tako v znanosti kot v industriji, so prav tako obremenjene z različnimi težavami, kot so npr. potreba po redni menjavi konic tipal, učinki glajenja zaradi geometrije tipal in dolgi merilni časi pri merjenju površin. Optične naprave lahko po drugi strani hitro premerijo večje površine, ne da bi se jih morale dotikati in jih pri tem poškodovati. Obstajajo pa tudi optične tehnike, ki imajo težave pri merjenju strmih bokov (npr. interferometrija z belo svetlobo) ali pri merjenju površin z določenimi frekvencami. Instrument za spreminjanje fokusa, ki je bil uporabljen pri tej študiji, je InfiniteFocus. Gre za napravo, ki izkorišča majhno globino ostrenja optičnega sistema z vertikalnim skeniranjem, podatke o topografiji in barvi pa pridobiva s spreminjanjem fokusa. Za razliko od drugih optičnih tehnik ima dve glavni prednosti. Metoda prvič ni omejena na koaksialno osvetlitev ali druge posebne tehnike osvetlitve, kar odpravlja nekatere omejitve glede največjega naklona, ki ga je še mogoče meriti. Drugič pa ta tehnologija daje pravo informacijo o barvi za vsako merilno točko. Začenjamo s primerjavo merjenja hrapavosti na novem etalonu za hrapavost s predlagano metodo in z dotikalno napravo. Rezultati kažejo, da sistema dajeta vrednosti Ra, ki se med seboj razlikujejo le za par nanometrov. Primerjava z meritvami tradicionalnih etalonov za hrapavost, ki so zasnovani za dotikalne naprave, pokaže, da je novi etalon zaradi naložene nanohrapavosti možno premeriti veliko bolje. Nato so bile opravljene meritve oblike na etalonu za umerjanje s kalotami v obliki polkrogle, ki so pokazale ponovljivost pri meritvah krogle, manjšo od 100 nm. Vrednotenje sistema pri dveh tehničnih aplikacijah je pokazalo, da lahko sistem meri tudi strme boke. Tak primer je rezkar, katerega obrabo zaradi uporabe v industrijskem procesu je možno kvantificirati s 3D-registracijo 3D-meritev. Sistem je primeren tudi za kontrolo točkovnih zvarnih spojev, ki imajo zelo nepravilno obliko s strmimi boki in težavne odbojne lastnosti površine. © 2011 Strojniški vestnik. Vse pravice pridržane. Ključne besede: 3D, optično, spreminjanje fokusa, meritve, hrapavost, oblika, natančnost, dotikalno

SI 46

*Naslov avtorja za dopisovanje: R&D Skupina, Alicona, Teslastraße 8, 8074 Grambach, Avstrija, reinhard.danzl@alicona.com


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 47

Prejeto: 04.08.2008 Sprejeto: 24.12.2008

Odkrivanje površinskih napak na pogonskih jermenih z uporabo laserske profilometrije Drago Bračun1,* - Boštjan Perdan2 - Janez Diaci1 1 Univerza v Ljubljani, Fakulteta za strojništvo, Slovenija 2 Veyance Technologies Europe, d.o.o., Slovenija Nadzor kakovosti pri proizvodnji pogonskih jermenov danes v glavnem vključuje vizualno kontrolo, ki jo izvajajo izkušeni delavci. Delavci pregledujejo predvsem geometrijo jermenov glede napak kot so manjše izbokline, vdrtine in neizoblikovani zobje. Kljub izkušnjam kontrolorjev pa so rezultati odvisni od počutja in splošnega stanja ljudi. Da bi se izognili subjektivni kontroli, je bil razvit eksperimentalni sistem za avtomatizirano kontrolo geometrije jermenov. Sistem vključuje jermenski pogon, merilno napravo in računalnik za obdelavo podatkov. Merilna naprava deluje po principu laserske triangulacije. Laserski žarek, ki je oblikovan v ozko svetlobno ravnino, osvetli površino jermena. Svetlobno progo, ki je vidna na površini jermena, posname kamera, ki je postavljena pod kotom glede na smer osvetlitve. Takšna postavitev razkrije topografijo površine in omogoča ugotavljanje prereza jermena. Rezultat posamezne meritve je profil, ki predstavlja prerez laserske ravnine in osvetljene površine jermena. Popolna tridimenzionalna oblika zoba je izmerjena s skeniranjem. Merilna ločljivost sistema je 0,04 mm in površinske napake so jasno razvidne v izmerjenih podatkih. Z obdelavo pridobljenega oblaka točk je možno identificirati in ovrednotiti najpogostejše napake na jermenih. Predstavljeni sta dve različni metodi za obdelavo podatkov. Prva posnema dobro uveljavljeni »ročni« postopek, kjer se profil posameznih zob primerja s predlogo iz tehnične dokumentacije. Vrednotenje kakovosti uporablja 5 ali 10-odstotne kontrolne meje zaradi skrčkov. Profil velja za sprejemljivega, če pade v to območje. S tem pristopom pa ni mogoče pridobiti vseh informacij o napaki, ki jo kontroliramo. Merimo namreč samo en profil zoba v določenem položaju in pri tem zlahka zgrešimo površinsko napako oziroma je ne izmerimo tam, kjer je odstopanje največje. Za samodejno zaznavanje je potrebna bolj selektivna metoda, ki je sposobna identifikacije in vrednotenja različnih napak. Druga metoda za samodejno zaznavanje površinskih napak uporablja nov pristop na osnovi karte odstopanj. Izmerjena tridimenzionalna oblika zoba se primerja z referenčno (oz. idealno) obliko zoba. Rezultat primerjave je karta odstopanj, ki kaže razlike med dvema površinama. Karta odstopanj se analizira z izračunom prostornine, površine, povprečja in največje višine odstopanj. Z določitvijo ustreznih mej za te karakteristične parametre in njihove kombinacije je možno razvrščanje jermenov na ustrezne in škartne, pri čemer sprejmemo jermene z nesignifikantnimi (kozmetičnimi) nepravilnostmi in zavržemo jermene s signifikantnimi površinskimi napakami. Dodatno je bil razvit tudi postopek vrednotenja sprejemljivosti/nesprejemljivosti za štiri tipične površinske napake. Predstavljena je identifikacija tipičnih površinskih napak. Avtorji ugotavljajo, da je potrebno dodatno delo na razvoju kriterijev sprejemljivosti v sklopu vrednotenja kakovosti, ki bodo ustrezali zahtevam industrije. © 2011 Strojniški vestnik. Vse pravice pridržane. Ključne besede: pogonski jermen, površinske napake, vrednotenje kakovosti, merjenje tridimenzionalnih oblik, laserska triangulacija

*Naslov avtorja za dopisovanje: Univerza v Ljubljani, Fakulteta za strojništvo, Askerceva cesta 6, 1000 Ljubljana, Slovenija, drago.bracun@fs.uni-lj.si

SI 47


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 48

Prejeto: 04.08.2008 Sprejeto: 24.12.2008

Nadzor procesa MIG varjenja s pomočjo signala slišnega zvoka Jožef Horvat1 - Jurij Prezelj2 - Ivan Polajnar2 - Mirko Čudina2,* 1 Univerza v Ljubljani, Zdravstvena fakulteta, Slovenija 2 Univerza v Ljubljani, Fakulteta za strojništvo, Slovenija MIG varjenje je najpogosteje uporabljeni postopek obločnega varjenja, saj je primerno za robotizacijo in mehanizacijo, s tem pa je tudi visoko produktivno in stroškovno učinkovito. V mnogo primerih je najpomembnejša kakovost zvara. Za nadzor kakovosti procesa varjenja se uporabljajo različne metode, od izkušenj varilca (s pomočjo vida in sluha) do rentgenskih žarkov in akustične emisije ultrazvoka. V prispevku je opisana uporaba slišnega zvoka (od 20 do 20.000 Hz), ki nastaja med procesom MIG varjenja, za vrednotenje in nadzor varilskega procesa ter za napovedovanje stabilnosti in kakovosti varilskega procesa. Zvok, ki nastaja med procesom varjenja, se spremlja v časovni in frekvenčni domeni. Teoretične in eksperimentalne analize akustičnih signalov so pokazale, da obstajata dva glavna mehanizma ustvarjanja hrupa: prvi je ugašanje in vžiganje obloka z impulznim značajem, drugi pa je sam oblok, ki deluje kot ionizacijski zvočni vir s širokopasovnim oz. t.i. “turbulentnim” zvokom. Sočasno sta bila merjena tudi raven emitiranega zvočnega tlaka in tok varjenja med procesom MIG varjenja. Rezultati so pokazali, da se zvočni impulzi ujemajo z vrhovi varilnega toka. Podrobne analize so pokazale, da je impulzni hrup močnejši od turbulentnega hrupa obloka tudi za 10 dB(A) ali več, zato je bila raven impulznega hrupa uporabljena za vrednotenje celotne ravni hrupa pri MIG varjenju. Eksperimenti so pokazali, da je frekvenčni spekter turbulentnega hrupa obloka poudarjen nad frekvencama 5 oz. 7 kHz, pri čemer frekvenčni spekter zvočnih impulzov prevladuje nad 3 oz. 5 kHz. Amplitudo zvočnega spektra impulzov je zato možno uporabiti za sprotno upravljanje količine taline in s tem za upravljanje stabilnosti in kakovosti varilnega procesa. Poleg tega je bil razvit tudi nov algoritem za izračunavanje emitiranega zvoka med procesom MIG varjenja na osnovi izmerjenega varilnega toka. Algoritem je bil verificiran za različne pogoje varjenja (različni tokovi in varilni izvori), različne materiale varjencev in različne oblike preizkušancev. Uporabljeni sta bili dve vrsti materialov: toplotno obdelano in konstrukcijsko jeklo, z ravno in poševno obliko plošče različnih debelin. Primerjave so pokazale, da se izračunane vrednosti dobro ujemajo z izmerjenimi vrednostmi zvočnega signala. Dodatna prednost algoritma je tudi v tem, da ga je mogoče uporabiti za sprotni nadzor in upravljanje stabilnosti in kakovosti procesa varjenja. Analize so pokazale, da se spremembe varilnega toka ujemajo s posnetki zvočnega tlaka. Posnetek zvočnega tlaka je zato možno uporabiti za izračun celotne ravni hrupa in za vrednotenje vpliva hrupa med varjenjem na varilca. Glavna prednost predlaganega algoritma na osnovi obstoječih podatkov varjenja je v tem, da izračunanih vrednosti ne kvarijo zvoki iz ozadja in odmevi, ki se običajno pojavljajo pri merjenju hrupa v industrijskem okolju. Merjenje hrupa pri varjenju s predlaganim algoritmom je zato bolj natančno, robustno in uporabniku prijazno. Vrednotenje uporabnosti algoritma za druge materiale je še v teku. © 2011 Strojniški vestnik. Vse pravice pridržane. Ključne besede: nadzor procesa, pogoji varjenja, emisija zvoka, postopek optimizacije, kakovost zvara

SI 48

*Naslov avtorja za dopisovanje: Univerza v Ljubljani, Fakulteta za strojništvo, Askerceva cesta 6, 1000 Ljubljana, Slovenija, mirko.cudina@fs.uni-lj.si


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 49-50 Navodila avtorjem

Navodila avtorjem Članke pošljite na naslov: Strojniški vestnik Journal of Mechanical Engineering Aškerčeva 6, 1000 Ljubljana, Slovenija Tel.: 00386 1 4771 137 Faks: 00386 1 2518 567 E-mail: info@sv-jme.eu strojniski.vestnik@fs.uni-lj.si Članki morajo biti napisani v angleškem jeziku. Strani morajo biti zaporedno označene. Prispevki so lahko dolgi največ 10 strani. Daljši članki so lahko v objavo sprejeti iz posebnih razlogov, katere morate navesti v spremnem dopisu. Kratki članki naj ne bodo daljši od štirih strani. Navodila so v celoti na voljo v rubriki “Informacija za avtorje” na spletni strani revije: http://en.sv-jme.eu/ Prosimo vas, da članku priložite spremno pismo, ki naj vsebuje: 1. naslov članka, seznam avtorjev ter podatke avtorjev; 2. opredelitev članka v eno izmed tipologij; izvirni znanstveni (1.01), pregledni znanstveni (1.02) ali kratki znanstveni članek (1.03); 3. opredelitev, da članek ni objavljen oziroma poslan v presojo za objavo drugam; 4. zaželeno je, da avtorji v spremnem pismu opredelijo ključni doprinos članka; 5. predlog dveh potencialnih recenzentov, ter kontaktne podatke recenzentov. Navedete lahko tudi razloge, zaradi katerih ne želite, da bi določen recenzent recenziral vaš članek. OBLIKA ČLANKA Članek naj bo napisan v naslednji obliki: Naslov, ki primerno opisuje vsebino članka. Povzetek, ki naj bo skrajšana oblika članka in naj ne presega 250 besed. Povzetek mora vsebovati osnove, jedro in cilje raziskave, uporabljeno metodologijo dela, povzetek rezultatov in osnovne sklepe. - Uvod, v katerem naj bo pregled novejšega stanja in zadostne informacije za razumevanje ter pregled rezultatov dela, predstavljenih v članku. - Teorija. - -

-

Eksperimentalni del, ki naj vsebuje podatke o postavitvi preskusa in metode, uporabljene pri pridobitvi rezultatov. - Rezultati, ki naj bodo jasno prikazani, po potrebi v obliki slik in preglednic. - Razprava, v kateri naj bodo prikazane povezave in posplošitve, uporabljene za pridobitev rezultatov. Prikazana naj bo tudi pomembnost rezultatov in primerjava s poprej objavljenimi deli. (Zaradi narave posameznih raziskav so lahko rezultati in razprava, za jasnost in preprostejše bralčevo razumevanje, združeni v eno poglavje.) - Sklepi, v katerih naj bo prikazan en ali več sklepov, ki izhajajo iz rezultatov in razprave. - Literatura, ki mora biti v besedilu oštevilčena zaporedno in označena z oglatimi oklepaji [1] ter na koncu članka zbrana v seznamu literature. Enote - uporabljajte standardne SI simbole in okrajšave. Simboli za fizične veličine naj bodo v ležečem tisku (npr. v, T, n itd.). Simboli za enote, ki vsebujejo črke, naj bodo v navadnem tisku (npr. ms1, K, min, mm itd.) Okrajšave naj bodo, ko se prvič pojavijo v besedilu, izpisane v celoti, npr. časovno spremenljiva geometrija (ČSG). Pomen simbolov in pripadajočih enot mora biti vedno razložen ali naveden v posebni tabeli na koncu članka pred referencami. Slike morajo biti zaporedno oštevilčene in označene, v besedilu in podnaslovu, kot sl. 1, sl. 2 itn. Posnete naj bodo v ločljivosti, primerni za tisk, v kateremkoli od razširjenih formatov, npr. BMP, JPG, GIF. Diagrami in risbe morajo biti pripravljeni v vektorskem formatu, npr. CDR, AI. Vse slike morajo biti pripravljene v črnobeli tehniki, brez obrob okoli slik in na beli podlagi. Ločeno pošljite vse slike v izvirni obliki Pri označevanju osi v diagramih, kadar je le mogoče, uporabite označbe veličin (npr. t, v, m itn.). V diagramih z več krivuljami, mora biti vsaka krivulja označena. Pomen oznake mora biti pojasnjen v podnapisu slike. Tabele naj imajo svoj naslov in naj bodo zaporedno oštevilčene in tudi v besedilu poimenovane kot Tabela 1, Tabela 2 itd.. Poleg fizikalne veličine, npr t (v ležečem tisku), mora biti v oglatih oklepajih navedena tudi enota. V tabelah naj se ne podvajajo podatki, ki se nahajajo v besedilu.

SI 49


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 49-50

Potrditev sodelovanja ali pomoči pri pripravi članka je lahko navedena pred referencami. Navedite vir finančne podpore za raziskavo. REFERENCE Seznam referenc MORA biti vključen v članek, oblikovan pa mora biti v skladu s sledečimi navodili. Navedene reference morajo biti citirane v besedilu. Vsaka navedena referenca je v besedilu oštevilčena s številko v oglatem oklepaju (npr. [3] ali [2] do [6] za več referenc). Sklicevanje na avtorja ni potrebno. Reference morajo biti oštevilčene in razvrščene glede na to, kdaj se prvič pojavijo v članku in ne po abecednem vrstnem redu. Reference morajo biti popolne in točne. Vse neangleške oz. nenemške naslove je potrebno prevesti v angleški jezik z dodano opombo (in Slovene) na koncu Navajamo primere: Članki iz revij: Priimek 1, začetnica imena, priimek 2, začetnica imena (leto). Naslov. Ime revije, letnik, številka, strani. [1] Zadnik, Ž., Karakašič, M., Kljajin, M., Duhovnik, J. (2009). Function and Functionality in the Conceptual Design Process. Strojniški vestnik – Journal of Mechanical Engineering, vol. 55, no. 7-8, p. 455-471. Ime revije ne sme biti okrajšano. Ime revije je zapisano v ležečem tisku. Knjige: Priimek 1, začetnica imena, priimek 2, začetnica imena (leto). Naslov. Izdajatelj, kraj izdaje [2] Groover, M. P. (2007). Fundamentals of Modern Manufacturing. John Wiley & Sons, Hoboken. Ime knjige je zapisano v ležečem tisku. Poglavja iz knjig: Priimek 1, začetnica imena, priimek 2, začetnica imena (leto). Naslov poglavja. Urednik(i) knjige, naslov knjige. Izdajatelj, kraj izdaje, strani. [3] Carbone, G., Ceccarelli, M. (2005). Legged robotic systems. Kordić, V., Lazinica, A., Merdan, M. (Eds.), Cutting Edge Robotics. Pro literatur Verlag, Mammendorf, p. 553-576. Članki s konferenc: Priimek 1, začetnica imena, priimek 2, začetnica imena (leto). Naslov. Naziv konference, strani. [4] Štefanić, N., Martinčević-Mikić, S., Tošanović, N. (2009). Applied Lean System in Process Industry. MOTSP 2009 Conference Proceedings, p. 422-427.

SI 50

Standardi: Standard (leto). Naslov. Ustanova. Kraj. [5] ISO/DIS 16000-6.2:2002. Indoor Air – Part 6: Determination of Volatile Organic Compounds in Indoor and Chamber Air by Active Sampling on TENAX TA Sorbent, Thermal Desorption and Gas Chromatography using MSD/FID. International Organization for Standardization. Geneva. Spletne strani: Priimek, Začetnice imena podjetja. Naslov, z naslova http://naslov, datum dostopa. [6] Rockwell Automation. Arena, from http://www. arenasimulation.com, accessed on 2009-09-27. RAZŠIRJENI POVZETEK Ko je članek sprejet v objavo, avtorji pošljejo razširjeni povzetek na eni strani A4 (približno 3.000 - 3.500 znakov). Navodila za pripravo razširjenega povzetka so objavljeni na spletni strani http://sl.svjme.eu/informacije-za-avtorje/. AVTORSKE PRAVICE Avtorji v uredništvo predložijo članek ob predpostavki, da članek prej ni bil nikjer objavljen, ni v postopku sprejema v objavo drugje in je bil prebran in potrjen s strani vseh avtorjev. Predložitev članka pomeni, da se avtorji avtomatično strinjajo s prenosom avtorskih pravic SV-JME, ko je članek sprejet v objavo. Vsem sprejetim člankom mora biti priloženo soglasje za prenos avtorskih pravic, katerega avtorji pošljejo uredniku. Članek mora biti izvirno delo avtorjev in brez pisnega dovoljenja izdajatelja ne sme biti v katerem koli jeziku objavljeno drugje. Avtorju bo v potrditev poslana zadnja verzija članka. Morebitni popravki morajo biti minimalni in poslani v kratkem času. Zato je pomembno, da so članki že ob predložitvi napisani natančno. Avtorji lahko stanje svojih sprejetih člankov spremljajo na http://en.sv-jme.eu/. PLAČILO OBJAVE Domači avtorji vseh sprejetih prispevkov morajo za objavo plačati prispevek, le v primeru, da članek presega dovoljenih 10 strani oziroma za objavo barvnih strani v članku, in sicer za vsako dodatno stran 20 EUR ter dodatni strošek za barvni tisk, ki znaša 90,00 EUR na stran.


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 51-54 Osebne objave

Magisteriji in diplome

MAGISTRSKA DELA Na Fakulteti za strojništvo Univerze v Ljubljani so z uspehom zagovarjali svoje magistrsko delo: dne 10. februarja 2011 Robert ŽERJAL z naslovom: »Načrtovanje preizkuševališča za meritve vlaknenih filtrov« (mentor: izr. prof. dr. Ivan Bajsić); dne 16. februarja 2011 Gustavo Beulke STRINGARI z naslovom: »Časovno odvisne lastnosti bimodalnega POM z aplikacijo v procesu injekcijskega brizganja prahu (Time-dependent properties of bimodal POM – application in powder injection molding)« (mentor: prof. dr. Igor Emri); dne 16. februarja 2011 Abdulkobi Gafurovich PARSOHONOV z naslovom: »Karakterizacija mehanskih lastnosti viskoelastičnih materialov, izdelanih iz odpadnih pnevmatik (Mechanical characterization of the viscoelastic material made of waste tyres)« (mentor: prof. dr. Igor Emri). SPECIALISTIČNO DELO Na Fakulteti za strojništvo Univerze v Mariboru je z uspehom zagovarjal svoje specialistično delo: dne 18. februarja 2011 Boštjan CAFUTA z naslovom: »Vplivi na kakovost razvoja izdelave orodja za tlačno litje aluminija« (mentor: prof. dr. Andrej Polajnar). DIPLOMIRALI SO Na Fakulteti za strojništvo Univerze v Ljubljani so pridobili naziv univerzitetni diplomirani inženir strojništva: dne 24. februarja 2011: Franc PREPADNIK z naslovom: »Primerjava med obločnim in laserskim reparaturnim varjenjem orodij« (mentor: prof. dr. Janez Tušek); Andrej RIJAVEC z naslovom: »Didaktično hidravlično preizkuševališče« (mentor: doc. dr. Jožef Pezdirnik);

Urban ŽAŽE z naslovom: »Mehko spajkanje nerjavnih avstenitnih jekel« (mentor: prof. dr. Janez Tušek); dne 28. februarja 2011: Klemen POVŠIČ z naslovom: »3D lasersko merjenje oblike prsnega koša med dihanjem« (mentor: prof. dr. Janez Možina, somentor: doc. dr. Matija Jezeršek); Anna REPOVŠ z naslovom: »Vpliv procesnih parametrov na kakovost izdelkov pri mikro-brizganju plastike« (mentor: izr. prof. dr. Peter Butala). * Na Fakulteti za strojništvo Univerze v Mariboru sta pridobila naziv univerzitetni diplomirani inženir strojništva: dne 24. februarja 2011: Marko PIŠEK z naslovom: »Zasnova in razvoj menjalnika motokultivatorja« (mentor: prof. dr. Srečko Glodež); Sašo STANOJEVIĆ z naslovom: »Planiranje proizvodnje mehanske obdelave tlačnih ulitkov« (mentor: prof. dr. Andrej Polajnar, mentor EPF: prof. dr. Duško Uršič). * Na Fakulteti za strojništvo Univerze v Ljubljani so pridobili naziv diplomirani inženir strojništva: dne 9. februarja 2011: Simon ČERV z naslovom: »Odstranjevanje lamel izpod izsekovalnega orodja« (mentor: prof. dr. Janez Kopač); Franjo MAZAJ z naslovom: »Vrednotenje elementov rudniškega podajalnika lokov« (mentor: prof. dr. Marko Nagode); Lovrenc Stanislav PESTOTNIK z naslovom: »Klasifikacija zračnega prostora v Republiki Sloveniji« (mentor: viš. pred. mag. Aleksander Čičerov, somentor: doc. dr. Tadej Kosel); dne 10. februarja 2011: Peter ČEPON z naslovom: »Razvoj orodja za brizganje sifona« (mentor: izr. prof. dr. Zlatko Kampuš); SI 51


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 51-54

Nejc MARKOVIČ z naslovom: »Talne cestne ovire za umirjanje hitrosti vozil« (mentor: doc. dr. Samo Zupan, somentor: prof. dr. Ivan Prebil); Tomaž RIFELJ z naslovom: »Izračun potrebne energije za ogrevanje in hlajenje prostora« (mentor: prof. dr. Vincenc Butala); dne 11. februarja 2011: Jakob GRLICA z naslovom: »Mešalna posoda za uporabo v potencialno eksplozivni atmosferi« (mentor: doc. dr. Boris Jerman); Marja KUNST z naslovom: »Ugotavljanje stanja pogonskih reakcijskih motorjev potniških letal z boreskopiranjem« (mentor: doc. dr. Tomaž Katrašnik); Sašo MUHIČ z naslovom: »Uporaba optičnih merilnih metod in infrardeče termografije za analizo konvektivnega prenosa toplote« (mentor: prof. dr. Iztok Golobič, somentor: izr. prof. dr. Ivan Bajsić); Primož NERAD z naslovom: »Generator pulzirajočega toka kapljevine« (mentor: izr. prof. dr. Ivan Bajsić, somentor: doc. dr. Jože Kutin);

SI 52

Jure ROT z naslovom: »Odvisnost med obrabo in generiranjem toplote v triboloških kontaktih« (mentor: prof. dr. Iztok Golobič, somentor: prof. dr. Mitjan Kalin). * Na Fakulteti za strojništvo Univerze v Mariboru so pridobili naziv diplomirani inženir strojništva: dne 24. februarja 2011: Marjan CEHTL z naslovom: »Avtomatizacija polnilne linije« (mentor: doc. dr. Darko Lovrec, somentor: doc. dr. Samo Ulaga); Leon MARKL z naslovom: »Primerjava sistemov ogrevanja stanovanjske hiše« (mentor: doc. dr. Matjaž Ramšak); Marko PLANK z naslovom: »Analiza vzrokov za notranje napetostne razpoke« (mentor: izr. prof. dr. Ivan Pahole, somentor: dr. Miha Kovačič); Kristjan RAMŠAK z naslovom: »Vzpostavitev sistematičnega vzdrževanja v podjetju« (mentor: doc. dr. Samo Ulaga, somentor: doc. dr. Darko Lovrec).


Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 51-54

Prof. dr. Viktor Prosenc - 90 letnik Dne 12. decembra 2010 je praznoval častitljiv jubilej, 90-letnico prof. Viktor Prosenc, eden od pionirjev varilstva v Sloveniji in pionir poučevanja varilstva na Fakulteti za strojništvo Univerze v Ljubljani (FS UL). Kot visoko cenjen učitelj in uspešen znanstvenik-raziskovalec v Sloveniji in v tujini, je bil dve mandatni obdobji (od 1979 do 1983) dekan FS UL, obenem pa mentor in vzornik več mlajšim učiteljem varilstva v Sloveniji in v Jugoslaviji. Rodil se je v Zagorju, kjer je končal osnovno šolo. Leta 1941 je maturiral na oddelku realke, na takratni I državni gimnaziji v Ljubljani. Zaradi vojne je bil prisiljen prekiniti nadaljnje šolanje in se je kot rudar zaposlil v rudniku Zagorje. Od tu je bil prisilno mobiliziran v nemško vojsko. Konec vojne je dočakal na Norveškem. Po koncu vojne in vrnitvi v domovino se je vpisal na Tehniško fakulteto, Univerze v Ljubljani, na Metalurški odsek, oddelek Montanistike, kjer je leta 1951 tudi diplomiral. Po diplomi je bil, s strani Fakultete za rudarstvo in metalurgijo Tehniške visoke šole v Ljubljani, povabljen za asistenta na Katedro za metalurško strojništvo in bil na tem mestu do leta 1958. Njegova naslednja zaposlitev je bila na novoustanovljenem (1956) Zavodu za varjenje SRS v Ljubljani, kjer je vodil Tehnološki oddelek. Takratna osrednja dejavnost oddelka je bila intenzivno šolanje na t.i. tečajih za inženirje in tehnike. Tečajno šolanje je bilo prvenstveno namenjeno vzgoji strokovnjakov za področje varjenje in usmerjeno v raziskave, razvoj in prenos domačih in tujih znanj s področja varilstva v industrijo in gospodarstvo v tedanji Jugoslaviji. Potekalo je na takratnem Zavodu (danes Institut za varilstvo) v Ljubljani, pa tudi po republiških društvih za varilno tehniko v Beogradu, Novem Sadu, Sarajevu, Sisku, Splitu in Zagrebu. V okviru tega izobraževanja so bila leta 1959 na Zavodu izdana prva obsežna skripta o varilskih tehnologijah. Avtorji skript so bili predavatelji na teh tečajih. Prof. Prosenc je bil avtor treh poglavij: Metalizacija, Metalurgija in metalografija varjenja ter Kriteriji za varivost in klasifikacijo varivostnih preizkusov. Pri svojem raziskovalnem delu se je usmeril predvsem v proučevanje tehnologij varilskih procesov in varivosti. V tej zvezi je bil tudi na tri mesečni specializaciji na Institutu za varjenje v Parizu (Institute de la soudure). Njegov bogat predavateljski opus pa je še obsežnejši. Ob rednem delu na Zavodu,

je dve leti predaval tudi na Tehniški srednji šoli za strojno strok, na varilskem oddelku predmet Tehnologijo kovin. Vse navedeno je bilo usodno za njegovo nadaljnje strokovno, pedagoško in znanstveno raziskovalno delovanje, saj se je tedaj za vedno zapisal varilstvu. Že v šolskem letu 1962/63 je bil na Fakulteti za naravoslovje in tehnologijo Univerze v Ljubljani, na Odseku za montanistiko izvoljen za predavatelja za področje Varjenje. Po izvolitvi za predavatelja za varilske predmete in toplotno obdelavo na FS UL, je tedaj na tej fakulteti bil tudi sodelavec Instituta za strojništvo in predstojnik Tehnološkega odseka. Leta 1966 je bil na FS UL izvoljen za docenta. V tem času je kot soavtor veliko truda vložil v pripravo gradiva s področja preizkušanja kovin, ki je bil izdan v Metalurškem priročniku pri Tehniški založbi Slovenije (1972). Napisal je podpoglavji Varilni preizkusi in Preizkušanje kovin in zlitin. Doktoriral je leta 1975 na Tehniški Univerzi v Hannovru (TU Hannover, Institut für Metallkunde, ZRN), pri mednarodno uveljavljenem strokovnjaku za varilstvo prof. dr. Friedrich Erdmann-Jesnitzer-ju, na področju kristalizacije kovin. Problem nukleacije in kristalizacije je raziskoval po jeklarskem postopku pretaljevanja pod žlindro. Rezultati tega dela so bili zelo odmevni in deležni velikega zanimanja v strokovnih krogih, zlasti v ZR Nemčiji. Leto 1976 je bil izvoljen na FS UL za izrednega profesorja ter leta 1981 za rednega profesorja, obakrat za področji Varjenje in Gradiva. Na matični fakulteti je v okviru PRE za tehnologijo materialov leta 1972 osnoval Laboratorij za varjenje in prvi uvedel laboratorijske vaje iz varjenja in izdelal program vaj za varilske postopke in procese ter pridobil tudi potrebno varilno opremo, ki je bila v veliki meri izposojena. To izposojeno opremo se je med pedagoškim in raziskovalnim delom v Laboratoriju za varjenje testiralo in posodabljalo, obenem pa se je prav s to opremo reševalo številnih tehnološke in varivostne probleme za potrebe domačega gospodarstva. Osnovni predmeti, ki jih je predaval na matični fakulteti, so bili: Tehnika spajanja, Varjenje, Toplotna obdelava kovin in zaščita ter Tehnologija

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Strojniški vestnik - Journal of Mechanical Engineering 57(2011)3, SI 51-54

kovin II. V šolskem letu 1974/75 je prvič uvedel varilsko usmeritev študija strojništva in izdelal program predavanj in vaj za predmet Varilska tehnologija. V šolskem letu 1985/86 je, skupaj s prof. dr. Viljemom Kraljem, uvedel nov predmet Fizikalno-kemijske osnove varilskih procesov. Mnogo let je bil tudi nosilec predmeta Varilni procesi na podiplomskem študiju za področje Avtomatizacija in proizvodna kibernetika. V zadnjih letih dela je predaval še predmet Nauk o kovinah ter v 80. letih spisal skripta Varjenje in Varilska tehnologija. Veliko je predaval na drugih fakultetah,. Med službeno odsotnostjo prof. dr. Inoslava Raka je več let predaval vse varilske predmete na Fakulteti za strojništvo Univerze v Mariboru ter na Univerzah v Mostarju, v Sarajevu in v Zagrebu. Bogato je tudi njegovo znanstvenoraziskovalno in strokovno delovanje. Že kot mlad asistent je sodeloval pri projektiranju metalurških obratov (Železarna Sisak, Tovarna ferozlitin Šibenik, Livarna STT Trbovlje). Deloval je na področju preiskav materialov za Rudarski inštitut v Ljubljani ter za ladijske registre. Na koncu 70-tih in na začetku 80-tih let je v TZ Litostroj razvil tehnologijo varjenja in dodajne materiale za zvarjanje in navarjanje martenzitnih jekel. S tem so bili ustvarjeni pogoji za prehoda izdelovalnih tehnologij iz litih na varjene

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konstrukcije, tudi za hidromehansko opreme iz visokotrdnostne martenzitne litine, za kar je prejel priznanje TZ Litostroj za posebno inovacijo. Kot avtor poglavja Varjenje je sodeloval pri Strojnotehnološkem priročniku, ki ga je v letih od 1978 do 1998 izdajala Tehnološka založba Slovenije. Jubilant je bil predstojnik, predsednik in član več odborov, znanstveno raziskovalnih skupin, svetov in društev, ter za svoje aktivno delo dobil veliko domačih, državnih in mednarodnih priznanj. Tudi po upokojitvi leta 1991 se redno vrača na matično fakulteto. Še ne dolgo nazaj je recenziral rokopise učbenikov s področja varilstva ter do leta 2003 predaval na tečajih s področja. Vse dokler mu aktivnosti ni prekinila bolezen in operacija na kolku, je redno zahajal v naravo in gojil smučarski tek. Ob njegovi 90. letnici je decembra 2010 prejel jubilejno nagrado FS UL. Jubilantu prof. dr. Viktorju Prosencu iskreno čestitamo in mu iz srca želimo še veliko let, čim več zdravja ter sreče in zadovoljstva v krogu svojih najbližjih. prof. dr. Viljem Kralj, izr. prof. dr. Ivan Polajnar


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