Understanding acoustic emission testing reading 2 ndthb vol5 part 2

Understanding Acoustic Emission Testing, AET-Reading II, Part 2 NDTHB-Ed3 Vol.5

My Pre-exam ASNT Self Study Notes 10th September 2015

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Aerospace Applications

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Aerospace Applications

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Aerospace Applications

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The Magical Book of Neutron Radiography

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ASNT Certification Guide NDT Level III / PdM Level III AE - Acoustic Emission Testing Length: 4 hours Questions: 135 1 Principles and Theory • Characteristics of acoustic emission testing • Materials and deformation • Sources of acoustic emission • Wave propagation • Attenuation • Kaiser and Felicity effects, and Felicity ratio • Terminology (refer to acoustic emission glossary, ASTM 1316)

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2 Equipment and Materials • Transducing processes • Sensors • Sensor attachments • Sensor utilization • Simulated acoustic emission sources • Cables

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• Signal conditioning • Signal detection • Signal processing • Source location • Advanced signal processing • Acoustic emission test systems • Accessory materials • Factors affecting test equipment selection

3 Techniques • Equipment calibration and set up for test • Establishing loading procedures • Precautions against noise • Special test procedures • Data displays

4 Interpretation and Evaluation • Data interpretation • Data evaluation • Reports

5 Procedures 6 Safety and Health 7 Applications • Laboratory studies (materialcharacterization) • Structural applications

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Reference Catalog Number NDT Handbook, Second Edition: Volume 5, Acoustic Emission Testing 130 Acoustic Emission: Techniques and Applications 752

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Fion Zhang at Shanghai 10th September 2015 To all my dearest Teachers Happy Teacher Day! 敬爱的老师们,节日快乐!

CharlieChong/ Chong/ Fion Zhang Charlie Fion Zhang

Greek Alphabet

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http://greekhouseoffonts.com/

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Chapter 2 PART 4.

Acoustic Emission Transducers and Their Calibration

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PART 4. Acoustic Emission Transducers and Their Calibration 1.4.1 Definition Acoustic emission transducers are used on a test object’s surface to detect dynamic motion resulting from acoustic emission hits and to convert the detected motion into a voltage-versus-time signal. This voltage-versus-time signal is used for all subsequent steps in the acoustic emission method. The electrical signal is strongly influenced by characteristics of the transducer.

Because the test results obtained from signal processing depend so strongly on the electrical signal, the transducer’s characteristics are important to the success and repeatability of acoustic emission testing.

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1.4.2 Transducer Types Basic transduction mechanisms can be used to achieve a transducer’s functions: the detection of surface motion and the subsequent generation of an electrical signal.

Capacitive transducers have been successfully used as acoustic emission transducers for special laboratory tests. Such transducers can have good fidelity, so that the electrical signal very closely follows the actual dynamic surface displacement. However, the typical minimum displacement measured by a capacitive transducer is on the order of 10-10 m (4 Ă— 10-9 in.). Such sensitivity is not enough for actual acoustic emission testing.

Laser interferometers have also been used as acoustic emission transducers for laboratory experiments. However, if this technique is used with a reasonable bandwidth, the technique lacks sufficient sensitivity for acoustic emission testing.

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Piezoelectric Transducers Acoustic emission testing is nearly always performed with transducers that use piezoelectric elements for transduction. The element is usually a special ceramic such as lead zirconate titanate PZT is acoustically coupled to the surface of the test item so that the dynamic surface motion propagates into the piezoelectric element. The dynamic strain in the element produces a voltage-versus-time signal as the transducer output. Because virtually all acoustic emission testing is performed with a piezoelectric transducer, the term acoustic emission transducer is here taken to mean a sensor with a piezoelectric transduction element.

Keywords: ■ Capacitive transducers ■ Laser interferometers ■ Piezoelectric Transducers

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Direction of Sensitivity to Motion Surface motion of a point on a test object may be the result of acoustic emission. Such motion contains a component normal to the surface and two orthogonal components tangential to the surface. Acoustic emission transducers can be designed to respond principally to any component of motion but virtually all commercial acoustic emission transducers are designed to respond to the component normal to the surface of the structure.

Because waves traveling at the longitudinal, shear and rayleigh wave speeds all typically have a component of motion normal to the surface, acoustic emission transducers can often detect the various wave arrivals.

Exam Question? longitudinal, shear and rayleigh wave speeds all typically have a component of motion normal to the surface, thus the AE transducer can be designed to respond principally to normal component.

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Frequency Range The majority of acoustic emission testing is based on the processing of signals with frequency content in the range from30 kHz to about 1 MHz. In special applications, detection of acoustic emission at frequencies below 20kHz or near audio frequencies can improve testing and conventional microphones or accelerometers are sometimes used.

Attenuation of the wave motion increases rapidly with frequency and, for materials with higher attenuation (such asfiber reinforced plastic composites),it is necessary to sense lower frequencies to detect acoustic emission hits.

At higher frequencies, the background noise is lower; for materials with low attenuation, acoustic emission hits tend to be easier to detect at higher frequencies.

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Acoustic emission transducers can be designed to sense a portion of the whole frequency range of interest by choosing the appropriate dimensions of the piezoelectric element. This, along with its high sensitivity, accounts for the popularity of this transduction mechanism. In fact, by proper design of the piezoelectric transducer, motion in the frequency range from 30 kHz to 1 MHz (and more) can be transduced by a single transducer. This special type of transducer has applications (1) in laboratory experiments, (2) in acoustic emission transducer calibration and (3) in any tests where the actual displacement is to be measured with precision and accuracy.

30 kHz

1 MHz

a portion of the whole frequency range of interest by choosing the appropriate dimensions of the piezoelectric element Charlie Chong/ Fion Zhang

Comments: High frequency → High attenuation Low frequency → Lower attenuation High frequency → Lower noise contribution (mechanical & electrical) Acoustic Testing transducer frequency → 30kHz ~ 1MHz Special case audio frequency is used → <20kHz Selective resonance frequency transducer → increase sensitivity

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1.4.3 Transducer Design Figure 10 is a schematic diagram of a typical acoustic emission transducer mounted on a test object. The transducer is attached to the surface of the test object and a thin intervening layer of couplant is usually used. The couplant facilitates the transmission of acoustic waves from the test object to the transducer. The transducer may also be attached with an adhesive bond designed to act as an acoustic couplant. An acoustic emission transducer normally consists of several parts. The active element is a piezoelectric ceramic with electrodes on each face. One electrode is connected to electrical ground and the other is connected to a signal lead. A wear plate protects the active element.

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FIGURE 10. Schematic diagram of a typical acoustic emission transducer mounted on a test object.

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Behind the active element is usually a backing material, often made by curing epoxy containing high density tungsten particles. The backing is usually designed so that acoustic waves easily propagate into it with little reflection back to the active element. In the backing, much of the waveâ&#x20AC;&#x2122;s energy is attenuated by scattering and absorption. The backing also serves to load the active element so that it is less resonant or more broad band (note that in some applications, a resonant transducer is desirable). Less resonance helps the transducer respond more evenly over a somewhat wider range of frequencies.

Comments: Resonant- selective frequency Broadband transducer- backing to absorb resonance.

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The typical acoustic emission transducer also has a case with a connector for signal cable attachment. The case provides an integrated mechanical package for the transducer components and may also serve as a shield to inimize electromagnetic interference.

There are many variations of this typical transducer design, including (1) designs for high temperature applications, (2) transducers with built-in preamplifiers or line drive transformers, (3) transducers with more than one active element and (4) transducers with active elements whose geometry or polarization is specifically shaped.

There are two principal characteristic dimensions associated with the typical acoustic emission transducer: â&#x2013; the piezoelectric element thickness and â&#x2013; the element diameter.

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Typical Acoustic Emission Transducers

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http://wins-ndt.com/bridge/acoustic-emission/

Element Thickness and Sensitivity Control Element thickness controls the frequencies at which the acoustic emission transducer has the highest sensitivity, that is, the highest electrical output for a given input surface velocity. The half-wave resonant frequencies of the transducer define the approximate frequencies where the transducer will have maximum output. These are the frequencies for which t = 0.5λ, 1.5λ, 2.5λ, and so on, where t is time (second) (?) and λ is the wavelength (meter) of the wave in the element. The wavelength can be defined as the sound speed “c” in the piezoelectric element divided by the acoustic frequency “f”.

Comment: t = thickness?

Poisson coupling in the element can lead to radial resonances at other frequencies and can also lead to some sensitivity to in-plane motion. For common piezoelectric materials and acoustic emission test frequencies, active elements are typically several millimeters (0.1 or 0.2 in.) thick. A lead zirconate titanate PZT disk 4 mm (0.16 in.) thick would normally have a first half-wave resonance of about 0.5 MHz.

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Acoustic emission transducers are usually made with backing and with active elements having relatively high internal damping. Because of this design, the variation in sensitivity from resonant to antiresonant (zero output) frequencies is somewhat smoothed out, providing some sensitivity over a significant frequency range.

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Discussion Subject: “Poisson coupling in the element can lead to radial resonances at other frequencies and can also lead to some sensitivity to in-plane motion. “ Discuss: on the above statement.

Hints: Poisson's ratio is the ratio of transverse contraction strain to longitudinal extension strain in the direction of stretching force. Tensile deformation is considered positive and compressive deformation is considered negative. The definition of Poisson's ratio contains a minus sign so that normal materials have a positive ratio.

Poisson's ratio, also called Poisson ratio or the Poisson coefficient, or coefficient de Poisson, is usually represented as a lower case Greek nu, ʋ. ʋ = - ε trans / ε longitudinal Strain ε is defined in elementary form as the change in length divided by the original length. ε = ∆L/L.

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Active Element Diameter The other principal characteristic of an acoustic emission transducer is the active element diameter. Transducers have been designed with element diameters as small as 1 mm (0.04 in.). Larger diameters are more common.

The element diameter defines the area over which the transducer averages surface motion.

For waves resulting in uniform motion under the transducer (as is the case for a longitudinal wave propagating in a direction perpendicular to the surface), the diameter of the transducer element has little or no effect. (?)

However, for waves traveling along the surface, the element diameter strongly influences the transducer sensitivity as a function of wave frequency.

If the displaced surface of the test object is a spatial sine wave, then there are occasions when one or more full wavelengths (in the object item) will match the diameter of the transducer element. When this occurs, the transducer averages the positive and negative motions to give zero output. This so called aperture effect has been carefully measured and theoretically modeled. (?)

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For transducers larger than the wavelengths of interest in the test object, the sensitivity will vary with the properties of the test material, depending strongly on frequency and on the direction of wave propagation. Transducer sensitivity is also influenced somewhat by the nonplanar nature of the wave front.

Because of these complications, it is recommended that the transducer diameter be as small as other constraints allow. For example, when testing steel, a 3 mm (0.12 in.) diameter transducer works reasonably well below 0.5 MHz. Comment: Disadvantages of large transducer are the sensitivity; â&#x2013; Vary with properties of test materials â&#x2013; Strong dependency on frequency and direction of wave propagation â&#x2013; Affected some what by non-planar nature of wave front

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Because of these complications, it

is recommended that the transducer diameter be as small as other constraints allow. For example, when testing steel, a 3 mm (0.12 in.) diameter transducer works reasonably well below 0.5 MHz.45

ASNT NDT Level III

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Special Acoustic Emission Transducers and Transducer Mounts Acoustic emission transducers are designed for various frequency ranges and are commercially available in a range of sizes with various piezoelectric materials. In addition, transducers or transducer mounts are available for special classes of applications as described below.

■ Severe Environments. Some acoustic emission transducers are designed for high temperatures and other harsh environments. Transducers are available in which all components are chosen and assembled for temperatures up to 550°C (1020°F). Transducers for use in harsh environments are fully encapsulated and are available with integral cable.

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Integral Preamplifiers. Some transducer models combine the transducer and preamplifier functions into one package. These may be miniaturized to the same size as conventional transducers. Transducers with integral preamplifiers have the following advantages: â&#x2013;

(1) reduced (combined) cost, (2) faster test setup, (3) compatibility with permanent installation for some industrial applications and (4) lower noise levels (less sensitivity to electromagnetic interference).

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â&#x2013; Differential Transducers. Differential transducers may be constructed with two or more active elements (or special electrode design) and a positive signal lead, a negative signal lead and a ground lead. The active elements are connected in parallel so that the transducer is less sensitive to electromagnetic interference. Generally, differential transducers are also relatively insensitive to longitudinal aves arriving at normal incidence to the transducer face. The sensitivity of some models may heavily depend on the direction of propagation in the plane of the surface. Differential transducers are designed for use with differential preamplifiers rather than the singleended preamplifiers normally used with conventional transducers.

Keywords: - Two or more active elements - Connect in parallel - Generally, differential transducers are also relatively insensitive to longitudinal aves arriving at normal incidence to the transducer face. (?)

- Separate preamplifiers

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â&#x2013; Acoustic Waveguides. An acoustic waveguide is a special transducer mount that provides a thermal and mechanical distance between the transducer and the test object. A waveguide is typically a metal rod with one end designed for acoustic coupling with the test object. The other end is constructed to accommodate the mounting of an acoustic emission transducer. Waveguides are used for applications in which an acoustic emission transducer cannot be in direct contact with the test object because of (1) temperature conditions or (2) limited access to the objectâ&#x20AC;&#x2122;s surface.

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1.4.4 Couplants and Bonds For an acoustic emission transducer, the purpose of a couplant is to provide a good acoustic path from the test material to the transducer. Without a couplant or a very large transducer hold-down force, only a few random spots of the material-totransducer interface will be in good contact and little energy will arrive at the transducer.

For sensing normal motion, virtually any fluid (oil, water, glycerin) will act as a good couplant and the transducer output can often be thirty times higher than without couplant. Note though that in some applications there are stringent chemical compatibility requirements between the couplant and the test object.

A transducer hold-down force of several newtons (N) is normally used to ensure good contact and to minimize couplant thickness. For sensing tangential motion, a suitable couplant is more difficult to find because most liquids will not transmit shear forces. Some high viscosity liquids such as certain epoxy resins are reasonably efficient for sensing tangential motion.

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An adhesive bond between the transducer and test surface serves to mechanically fix the transducer as well as to provide coupling. Most bonds efficiently transmit both normal and tangential motion. Depending on the application, bonds are sometimes inappropriate. If for example the test surface deforms significantly because of test loads or if there is differential thermal expansion between the surface, bond or transducer, then the bond or the transducer may break and the coupling is lost. A standard has been written for transducer mounting.

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Typical AET Set-up

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http://www.mdpi.com/1424-8220/13/5/6365

1.4.5 Temperature Effects on Acoustic Emission Transducers There can be a strong relation between temperature and the piezoelectric characteristics of the active element in an acoustic emission transducer.

Some of these effects are important to acoustic emission transducers in testing at elevated or changing temperatures.

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Effect of Curie Temperature Typically, there is a temperature for piezoelectric ceramics at which the properties of the ceramic change permanently and the ceramic element no longer exhibits piezoelectricity. This temperature is known as the “curie temperature” and is the point at which a material moves from ferroelectric to paraelectric phase.

Piezoelectric ceramic elements have been used successfully within 50°C (122°F) of their curie temperature.

The curie temperature of lead zirconate titanate ceramics is 300 to 400°C (572 to 752°F) depending on the type of lead zirconate titanate. Other piezoelectric materials have lower curie temperatures, barium titanate at 120°C (258°F) and higher for lithium niobate at 1210°C (2210°F).

Testing limitations are therefore encountered in environments where static elevated temperatures cause the loss of piezoelectricity in the transducer’s active elements. In addition, failure may occur in other transducer components not designed for high temperature applications.

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Effect of Curie Temperature on Transducers  Applicable within (below) 50°C (122°F) of their curie temperature.  Lead zirconate titanate ceramics is 300 to 400°C (572 to 752°F)  barium titanate at 120°C (258°F)  lithium niobate at 1210°C (2210°F)

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Ferro-electricity is a property of certain materials that have a spontaneous electric polarization that can be reversed by the application of an external electric field.[1][2] The term is used in analogy to ferromagnetism, in which a material exhibits a permanent magnetic moment. Ferromagnetism was already known when ferroelectricity was discovered in 1920 in Rochelle salt

by Valasek.[3] Thus, the prefix ferro, meaning iron, was used to describe the property despite the fact that most ferroelectric materials do not contain iron.

Para-electricity is the ability of many materials (specifically ceramics) to become polarized under an applied electric field. Unlike ferroelectricity, this can happen even if there is no permanent electric dipole that exists in the material, and removal of the fields results in the polarization in the material returning to zero. The mechanisms that cause paraelectric behaviour are the distortion of individual ions (displacement of the electron cloud from the nucleus) and polarization of molecules or combinations of ions or defects.

Paraelectricity can occur in crystal phases where electric dipoles are unaligned and thus have the potential to align in an external electric field and weaken it.

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https://en.wikipedia.org/wiki/Dielectric#Paraelectricity

Effect of Fluctuating Temperature Special problems are encountered when transducers are placed in environments with widely changing temperatures. Piezoelectric ceramic active elements have small domains in which the electrical polarization is in one direction. Temperature changes can cause some of these domains to flip, resulting in a spurious electrical signal that is not easily distinguished from the signal produced by an acoustic emission hit in the test object. In a lead zirconate titanate element, a temperature change of 100°C (212°F) can cause an appreciable number of these domain flips. Ceramic elements should be allowed to reach thermal equilibrium before data are taken at differing temperatures.

■ If acoustic emission testing must be done during large temperature changes, then single-crystal piezoelectric materials such as quartz are recommended.

■ Acoustic waveguides may also be used to buffer the transducer from large temperature changes.

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Magnetic Domain/ Spin

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http://wps.prenhall.com/wps/media/objects/3311/3390683/blb0607.html

Keypoints:  If acoustic emission testing must be done during large temperature changes, then single-crystal piezoelectric materials such as quartz are recommended.  Acoustic waveguides may also be used to buffer the transducer from large temperature changes.  Use lithium niobate with Curie temperature at 1210°C (2210°F) for extreme temperature application

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1.4.6 Transducer Calibration Terminology of Transducer Calibration Calibration. The calibration of a transducer is the measurement of its voltage output into an established electrical load for a given mechanical input. The subject of what should be the mechanical input is discussed below.

Calibration results may be expressed either as a frequency response or as an impulse response.

Keywords: Frequency response, impulse response â&#x2013; Test Block. A transducer is attached to the surface of a solid object either for measuring hits in the object or for calibration of the transducer. In this discussion, that solid object is called the test block.

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â&#x2013; Displacement. Displacement is the dynamic particle motion of a point in or on the test block. Displacement is a function of time and three position variables. Here, the word velocity or acceleration could replace displacement.

Normal displacement is displacement perpendicular to the face of a transducer or displacement of the surface of a test block perpendicular to that surface. Tangential displacement is displacement in any direction perpendicular to the direction of normal displacement.

Tangential displacement Normal displacement

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Principles of Transducer Calibration If acoustic emission results are to be quantitative, then it is necessary to have a means for measuring the performance of a transducer. Techniques of doing this have been the subject of much discussion. Because there are many types of transducers in use and because they may be called on to detect waves of different kinds in different materials, it is not possible to have a universal calibration procedure. A transducer calibration, appropriately applied to the signal recorded from a transducer, should provide a record of the displacement of a point on the surface of the object being examined by the transducer. There are several fundamental problems encountered during calibration, as listed below.

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1. The displacement of a point on the surface of a test block is a three dimensional vector but the output of the transducer is a scalar.

2. Displacement is altered by the presence of the transducer. (damping effect?)

3. The face of the transducer covers an area on the surface of the test block and displacement is a function not only of time but of the position within this area.

Because of these problems, transducer calibration is not feasible without making some simplifying assumptions. Various calibration approaches have been taken and they all make implicit 不直接言明的 assumptions. scalar

3D-Vector

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â&#x2013; Calibration Assumptions. Regarding the vectoral nature of displacement (problem 1), it is usually assumed that the transducer is sensitive only to normal displacement. Naturally, errors will be introduced if the transducer is sensitive to tangential displacement. Calibrations for other directions of sensitivity are useful but are not routine.

The loading effect that the transducer has on the surface motion of a test block (problem 2) is significant but is not subject to any simple analysis. In general, the test block may be considered as having a mechanical impedance (source impedance) at the location of the transducer. The transducer also has a mechanical impedance at its face (load impedance). Interaction between the source and load impedances determines the displacement of the transducer face but both of these impedances are likely to be complex functions of frequency and no technique exists for measuring them.

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For calibration purposes, the usual solution to this problem is to define the input to the transducer as the unloaded (free) displacement of the test block with no transducer attached. The calibration is then practical because it is the displacement of the test block (and not the interactive effects) that are of interest. The function of the calibration scheme is to determine what the displacement of the surface of the test block would be in the absence of the transducer. It must be noted, however, that when a transducer is attached to different test blocks having different mechanical impedances, it will have different calibrations. Calibrations are transferable only when the test block impedances are the same.

Comments: â&#x2013; Material impedance â&#x2013; Mechanical impedance at transducer interface â&#x2013; Face/ contact impedance of the transducer

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For several calibration procedures, the test block approximates a semiinfinite half space of steel. Steel was chosen because it was expected that acoustic emission transducers would be used more on steel than on any other material.

The large size of the test block makes the mechanical impedance at its surface a property of the material only, and not of its dimensions within the usual acoustic emission working frequency range. It is demonstrated below that test blocks made of different materials produce significantly different calibrations of the same transducer.

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Experiments have been done to determine how much effect the material of the test block has on calibration results. A commercial ultrasonic transducer and a conical transducer were calibrated on a steel block and then subjected to surface pulse waveforms in aluminum, glass and methyl methacrylate plastic.

The surface pulse waveforms were generated by a pencil break apparatus having the provision for measuring the force. For each material, the surface pulse waveform was calculated at the transducer location, and modified by deconvolution to remove the source characteristics. The results are shown in Figs. 11 and 12. Analysis of the conical transducer has been carried out and the results are shown in Fig. 13. Because the blocks were smaller than optimal, these data are approximate.

The order of magnitude of the effect is clear and in the case of the conical transducer there is reasonable agreement between the theory and the experiment.

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FIGURE 11. Approximate calibrations of a conical transducer.

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FIGURE 12. Approximate calibrations of a transducer done on blocks of four different materials. A pencil graphite break was the source for all except the steel block calibration.

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FIGURE 13. Calculated sensitivity of the conical transducer in Fig. 11 on the same four materials; calculations are based on the theory for the transducer

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FIGURE 14. Straight line waves incident on a circular transducer.

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The finite size of the transducerâ&#x20AC;&#x2122;s face (problem 3) is often ignored. This is equivalent to (1) assuming that the diameter of the face is small compared to all wavelengths of interest in the test block or (2) assuming that all motion is in phase over the face. The latter assumption is only true in the case of plane waves impinging on the transducer from a direction perpendicular to its face.

In general, a transducer responds to a weighted average of the displacement over its face. This averaging or aperture effect may be considered a property of the transducer and grouped with all transducer properties in calibration.

However, it must be observed that, as a consequence, the aperture effect and therefore the calibration will differ depending on the type and speed of the wave motion in the test block.

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â&#x2013; Aperture Effect and Calibration. The aperture effect for an acoustic emission transducer may be described as follows. Neglecting interactive effects between the test block and the transducer, the response of the transducer may be written as follows: (7)

where r (x,y) is the local sensitivity of the transducer face, S is the region (square meter) of the surface contacted by the transducer, A is the area (square meter) of region S and u(x,y,t) is the displacement (meter) of the surface.

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The x,y plane is the surface of the test block. As a special case, assume a straight line wave front incident on a circular transducer having radius a (meter) and uniform sensitivity r (x,y) = 1, over its face (see Fig. 14). Assume a wave of the form:

(8)

where k is Ď&#x2030;â&#x2C6;&#x2122;c-1 and c is the Rayleigh wave speed (meter per second).

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The transducer response (Eq. 7) then becomes:

(9)

which reduces to:

(10)

where J is the first order bessel function.

Figure 15 shows this calculated bessel function response compared to the calibration of an experimental capacitive circular disk transducer.

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FIGURE 15. Results of the calculation of Eq. 10 compared with experimental results from a capacitive disk transducer. Source-to-receiver distance d = 0.1 m (4 in.); transducer radius a = 10 mm (0.4 in.); surface pulse is generated by a capillary break on a steel block.

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â&#x2013; Surface Calibration. Most calibration systems use a configuration in which the transducer under test and the source are both located on the same plane surface of the test block. The result is known as a surface calibration or rayleigh calibration, so called because most of the propagating energy at the transducer is traveling at the rayleigh speed. In this case, the transducerâ&#x20AC;&#x2122;s calibration is strongly influenced by the aperture effect.

surface calibration or rayleigh calibration

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Aperture Effect In plate objects such as vessel walls, it was shown that Rayleigh waves or Lamb waves are dominant . As for these wave modes transducer sensitivity is subject to the aperture effect. Figure 4 shows the mechanism of the effect, where the crests and troughs of the incident Rayleigh or lamb waves cancel out each other within the transducer aperture.

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ASTM STP1353 Acoustic Emission: Standards and Technology Update

â&#x2013; Through-Pulse Calibration. Other calibration systems use a configuration in which the transducer under test and the source are coaxially located on opposite parallel faces of the test block. All wave motion is in phase across the face of the transducer (except for a negligible curvature of the wave fronts at the transducer) and the calibration is essentially free of any aperture effect. The result is a through-pulse calibration or P wave calibration.

Note that because of the axial symmetry of the through-pulse calibration, only normal displacement exists at the location of the transducer under test.

P wave calibration

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Step Function Force Calibration The basis for the step function force calibration is that known, well characterized displacements can be generated on a plane surface of a test block. A step function force applied to a point on one surface of the test block initiates an elastic disturbance that travels through the block. The transducer under test is located either on the same surface (surface calibration) or on the opposite surface at the epicenter of the source (through-pulse calibration).

Given the step function source, the free displacement of the test block at the location of the transducer can be calculated by elasticity theory in both cases.

The calculated block displacement function is the transfer function (mechanical transfer admittance, when expressed in the frequency domain) or the Greenâ&#x20AC;&#x2122;s function for the block. The free displacement of the test block surface can also be measured using a capacitive transducer with a known absolute sensitivity. It is essential to the calibration that the calculated displacement and capacitive transducer measurement agree.

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The calibration facility at the National Institute of Standards and Technology has used a cylindrical steel test block 0.9 m (36 in.) in diameter by 0.43 m (17 in.) long with optically polished end faces. The step function force is made by breaking a glass capillary (see Fig. 16). In the case of surface calibration, free normal displacement of the surface is measured by a capacitive sensor at a location symmetrical to that of the transducer under test with respect to the source location. The displacement is redundantly determined by elasticity theory from a measurement of the force at which the capillary broke. Source and receiver are 0.1 m (4 in.) apart.

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FIGURE 16. Schematic diagram of the surface pulse apparatus.

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For through-pulse calibration, the free normal displacement is determined only by the elasticity theory calculation. Both calibrations are absolute: the results are in output volts per meter of displacement of the (free) block surface. Following the initiation of the step function force, an interval of time exists during which the displacement at the location of the transducer under test is as predicted by the elastic theory for the semiinfinite solid (in the case of the surface calibration) or for the infinite plate (in the case of the throughulse calibration). However, as soon as any reflections arrive from the cylindrical surface of the block, the displacement deviates from the theory.

The dimensions of the block are large enough to allow 100 Îźs of working time between the first arrival at the transducer and the arrival of the first reflection. For most transducers, the 100 Îźs window is long enough to capture most of the information in the output transient waveform in the frequency range of 100 kHz to 1 MHz.

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The transient time waveform from the transducer and that from the capacitive transducer are captured by transient recorders and the information is subsequently processed to produce either a frequency response or an impulse response for the transducer under test. The frequency response contains both the magnitude and the phase information. It is generally assumed that a transducer has only normal sensitivity because of its axial symmetry (an assumption that may not always be justified). Calibration by the surface pulse technique for a transducer having significant sensitivity to tangential displacement will be in error because the surface pulse from the step force contains a tangential component approximately as large as the normal component. It could, however, be calibrated for the normal component of sensitivity by the through-pulse technique because no tangential displacement exists at the location of the transducer under test in the throughulse configuration. It could also be calibrated (assuming no aperture effect exists) by averaging two surface calibrations with the transducer rotated 3.14 rad (180 deg) axially between calibrations. By combining through-pulse and surface calibration results judiciously, more information can be gained about the magnitudes of all three components of sensitivity.

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The certified frequency range of the calibration is 100 kHz to 1 MHz with information that is less accurate provided down to 10 kHz. The low end is limited by the fact that the 100 ms time window limits the certainty of information about frequency content below 100 kHz. The expected low frequency errors depend on how well damped the transducer under test is.

For transducers whose impulse response function damps to a negligible value within 100 ms, the valid range of the calibration could be extended lower than 100 kHz. The high frequency limitation of 1 MHz is determined by the fact that frequency content of the test pulse becomes weak above 1 MHz and electronic front end noise becomes predominant at higher frequencies.

This method of calibration is covered in published standards such as those published elsewhere in this volume.

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Reciprocity Calibration Reciprocity applies to a category of passive electromechanical transducers that have two important characteristics: (1) they are purely electrostatic or purely electromagnetic in nature and (2) they are reversible (can be used as either a source or a receiver of mechanical energy). This category includes all known commercial acoustic emission transducers without preamplifiers. For such a transducer, reciprocity relates its source response and its receiver response in a specific way. If two exactly identical transducers are used, one as a source and one as a receiver, both coupled to a common medium, and if the transfer function or Greenâ&#x20AC;&#x2122;s function of the medium from the source location to the receiver location is known, then from purely electrical measurements of driving current in the source and output voltage at the receiver, the response functions of the transducers can be determined absolutely.

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With nonidentical transducers, three such measurements (using each of the three possible pairs of transducers) provides enough information to determine all of the response functions of the transducers absolutely. The primary advantage of the reciprocity calibration technique is that it avoids the necessity of measuring or producing a known mechanical displacement or force. All of the basic measurements made during the calibration are electrical.

It is important to note, however, that the mechanical transfer function or Greenâ&#x20AC;&#x2122;s function for the transmission of signals from the source location to the receiver location

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must be known. This function is equivalent to the reciprocity parameter and is the frequency domain representation of the elasticity theory solution mentioned in the discussion on the step function force calibration. The application of reciprocity techniques to the calibration of microphones,61-66 hydrophones67 and accelerometers68 is well established. The reciprocity technique was proposed in 1976 for the calibration of acoustic emission transducers coupled to a solid and was subsequently implemented by one steel producer as a commercial service.52,69,70 One steel producerâ&#x20AC;&#x2122;s calibration facility has used a cylindrical steel test block 1.1 m (44 in.) in diameter by 0.76 m (30 in. ) long to perform rayleigh calibration (analogous to surface calibration) and P wave calibration (analogous to through-pulse calibration). In the rayleigh calibration, the transducers are separated by 0.2 m (8 in.) on the same surface of the block; for the P wave calibration, the transducers are on opposite faces on epicenter.

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The technique uses essentially continuous wave measurements but the signals are gated to eliminate reflections from the block walls. For a set of three transducers, the three electrical voltage transfer functions and the electrical impedances of all transducers are measured. From these data, receiving response (in volts of output per meter per second of input) and source response (meters per second of output per volt of input) are calculated for the range of 100 kHz to 1 MHz. This source response applies to any point on the steel surface located 0.2 m (8 in.) from the source.

The same assumptions about direction of sensitivity that were mentioned under step function force calibration apply to all transducers in a reciprocity calibration. A violation of the assumption by any of the three transducers would contaminate the results. The aperture effect also applies to ll transducers in a calibration and the considerations for mechanical loading of the test block are the same as for the step force calibration. A diffuse fieldeciprocity calibration has also been introduced. A broad band ultrasonic transducer and two resonant acoustic emission transducers were coupled to an aluminum block with all its corners sawed off at different angles to produce a diffuse field.

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Method of Reciprocity Calibration Figure 5 shows the fundamental aspects of the method of reciprocity alibration. Three reversible transducers 1, 2, and 3 are prepared, and three independent transmission/reception pairs are configurated through a transfer medium.

The magnitudes of the transmission signal current and reception signal voltage, lij and Eij, respectively, are measured on each pair, where the subscript ij corresponds to transducer i for transmission and j for reception.

If the reciprocity parameter H, which is dependent not on the transducer design but on the mode of elastic waves, constants of medium, and definition of sensitivity, is given, absolute sensitivity is determined by purely electrical measurements.

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ASTM STP1353 Acoustic Emission: Standards and Technology Update

FIG. 5- Three transmission/reception pairs for reciprocity calibration.

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ASTM STP1353 Acoustic Emission: Standards and Technology Update

Secondary Calibration The secondary calibration of an acoustic emission transducer is performed on a system and with a technique that has logical links to the primary calibration system and technique. It provides data of the same type as a primary calibration but the data may be more limited. For example, there may be no phase information or a narrower range of frequencies or the calibration may be applicable to a different material. Because of the logical link between the techniques, the data may be compared if the source-to-transducer geometries are taken into account. Standards have been published describing secondary calibration. Transducer suppliers usually provide data on the sensitivity of acoustic emission transducers over a range of frequencies. In some cases, these data are developed on a system very similar to a primary calibration facility and can be compared with primary calibration data. More often, the supplied data provide relative response rather than absolute response; such information cannot be logically linked to a primary calibration. It is useful for comparing the response of similar transducers or for checking for changes in transducer response. This information is frequently based on a different physical unit (often pressure) than primary and secondary calibrations(displacement or velocity).

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The development of secondary calibration techniques is an area of ongoing research. A secondary technique must offer compromises between system complexity and accurate transducer characterization. There are listed below some tools for further developing secondary calibration procedures.

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High Fidelity Transducers.

Transducers that accurately measure surface motion with high sensitivity are helpful when developing calibration procedures. Such transducers may be used as transfer standards because of their stability and their uniform sensitivity over the frequency range of interest. There are transducers having flat frequency response over the range of 10 kHz to 1 MHz or higher. One such transducer (developed at the National Institute of Standards and Technology) has a small conical element backed up by a large brass block.

Figure 17 shows the voltage-versus-time output of the conical transducer mounted on a large steel plate and responding to the displacement caused by breaking a glass capillary. The transducerâ&#x20AC;&#x2122;s output is compared with the theoretically predicted displacement of a point on a plate, a displacement caused by a point step function input. The favorable comparison indicates that the transducer accurately measures transient displacement.

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FIGURE 17. Conical transducer’s output (lower curve) from a glass capillary breaking on a large steel plate compared to the output of a computer program’s calculation (upper curve) of the Green’s function of the steel plate.

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Computer Programming. A second tool useful for secondary calibration is also demonstrated by Fig. 17. A computer program provides a theoretical prediction of surface motion for various (1) plate materials, (2) simulated acoustic emission sources and (3) source-to-transducer geometries. A mechanical input of known force and time history (such as a breaking glass capillary event51 or a pencil graphite break source54) is used and the source is modeled with the computer program. The predicted displacement time history can be used to determine the sensitivity of a transducer as a function of frequency. Procedures for checking the response of acoustic emission transducers are relatively simple and can be used to check transducers for degradation or to identify transducers that have similar performance. These procedures are discussed in detail elsewhere. They are not capable of providing transducer calibration or of ensuring transferability of data sets between different groups.

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Additional Acoustic Emission Transducer Information A great deal of practical information on transducers is available in the literature. Additional detailed information is available for acoustic emission transducers and their characterization. Much information about ultrasonic test transducers is also valuable for acoustic emission transducers.

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https://www.yumpu.com/en/browse/user/charliechong Charlie Chong/ Fion Zhang

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