Introduction to eddy current part 2 of 2

Eddy Current Testing -An Introduction Part-2 of 2 2014-November My ASNT Level III Pre-Exam Preparatory Self Study Notes

Charlie Chong/ Fion Zhang

Fion Zhang 2014/November

http://meilishouxihu.blog.163.com/

Shanghai 上海 Charlie Chong/ Fion Zhang

看不了”YouTube”学习视频. 我是一个聪明的梯子, 用来干什么你懂的

Charlie Chong/ Fion Zhang

Movie Time

ď Ž http://www.giniko.com/watch.php?id=216 Charlie Chong/ Fion Zhang

Introduction to Eddy Current Theory – 涡流原理

 https://www.youtube.com/watch?v=djFvnFy3rJc Charlie Chong/ Fion Zhang

Eddy Current Math – 涡流公式

 https://www.youtube.com/watch?v=V-IW6cFIt9E Charlie Chong/ Fion Zhang

Standard depth penetration – 标准渗透率

 https://www.youtube.com/watch?v=G2Yh7tZpKbo Charlie Chong/ Fion Zhang

Eddy Current Curve – 涡流曲线

 https://www.youtube.com/watch?v=Bojm5F_4ay4 Charlie Chong/ Fion Zhang

Introduction to Eddy Current Machine – 涡流设备

 https://www.youtube.com/watch?v=S34yt8-zgns Charlie Chong/ Fion Zhang

Conductivity Measurement - 电导率测量

 https://www.youtube.com/watch?v=pvTUomSYEt8 Charlie Chong/ Fion Zhang

Crack Detections - 裂缝探测

 https://www.youtube.com/watch?v=1YUSn___VxQ Charlie Chong/ Fion Zhang

4.0 Probes (Coils) 4.1 Impedance Matching Eddy current testing requires us to determine the components of the impedance of the detecting coil or the potential difference across it. Most applications require the determination only of changes in impedance, which can be measured with a high degree of sensitivity using an AC bridge. The principles of operation of the most commonly used eddy current instruments are based on Maxwell's inductance bridge, in which the components of the impedance of the detecting coil, commonly called a probe, are compared with known variable impedances connected in series and forming the balancing arm of the bridge. Refer back to Bridges.

Charlie Chong/ Fion Zhang

Maxwell inductance bridge

Charlie Chong/ Fion Zhang

The input to the bridge is an AC oscillator, often variable in both frequency and amplitude. The detector arm takes the form of either a meter or a storage cathode-ray oscilloscope, a phase-sensitive detector, a rectifier to provide a steady indication, and usually an attenuator to confine the output indication within a convenient range. Storage facilities are necessary in the oscilloscope in order to retain the signal from the detector for reference during scanning with the probe.

The highest sensitivity of detection is achieved by properly matching the impedance of the probe to the impedance of the measuring instrument.

Thus, with a bridge circuit that is initially balanced, a subsequent but usually small variation in the impedance of the probe upsets the balance, and a potential difference appears across the detector arm of the bridge.

Question: Is it the potential difference appears in the CRT?

Charlie Chong/ Fion Zhang

Although the Maxwell inductance bridge forms the basis of most eddy current instruments, there are several reasons why it cannot be used in its simplest form (i.e. Hague, 1934), including the creation of stray capacitances, such as those formed by the leads and leakages to earth. These unwanted impedances can be eliminated by earthing devices and the addition of suitable impedances to produce one or more wide-band frequency (i.e. low Q) resonance circuits. Instruments having a wide frequency range (i.e. from 1 kHz to 2 MHz) may possess around five of these bands to cover the range.

The value of the impedance of the probe is therefore an important consideration in achieving proper matching and, as a result, it may be necessary to change the probe when switching from one frequency band to another.

Charlie Chong/ Fion Zhang

“Q” Values for frequency (applicable to mechanical sonic or electrical)

The word “Q” does not have any impact on the quality elements in a specific testing method (UT, ET etc.), it is simply a term used to describes the bandwidth of a frequency in questioned.

Charlie Chong/ Fion Zhang

http://community.calrec.com/q-in-60-seconds/

“Q� Values for frequency (applicable to mechanical sonic or electrical)

Charlie Chong/ Fion Zhang

http://www.eeweb.com/blog/rodney_green_2/a-history-of-hf-radio-receivers-part-2

4.2 Coil (Probe) Design The most important feature in eddy current testing is the way in which the eddy currents are induced and detected in the material under test. This depends on the design of the probe. As discussed in the previous pages, probes can contain one or more coils, a core and shielding. All have an important effect on the probe, but the coil requires the most design consideration.

A coil consists of a length of wire wound in a helical manner around the length of a former. The main purpose of the former is to provide a sufficient amount of rigidity in the coil to prevent distortion. Formers used for coils with diameters greater than a few millimeters (i.e. encircling and pancake coils), generally take the form of tubes or rings made from dielectric materials. Small-diameter coils are usually wound directly onto a solid former.

Charlie Chong/ Fion Zhang

The region inside the former is called the core, which can consist of either a solid material or just air. When the core is air or a nonconductive material, the probe is often referred to as an air-core probe. Some coils are wound around a ferrite core which concentrates the coil's magnetic field into a smaller area. These coils are referred to as "loaded" coils.

The wire used in an eddy current probe is typically made from copper or other nonferrous metal to avoid magnetic hysteresis effects. The winding usually has more than one layer so as to increase the value of inductance for a given length of coil. The higher the inductance (L) of a coil, at a given frequency, the greater the sensitivity of eddy current testing.

Keywords: Air core Loaded core Magnetic hysteresis effects

Charlie Chong/ Fion Zhang

Magnetic hysteresis effects

Charlie Chong/ Fion Zhang

It is essential that the current through the coil is as low as possible. Too high a current may produce: ď Ž a rise in temperature, hence an expansion of the coil, which increases the value of L. ď Ž magnetic hysteresis, which is small but detectable when a ferrite core is used.

The simplest type of probe is the single-coil probe, which is in widespread use. The following applet may be used to calculate the effect of the inner and outer diameters, length, number of turns and wire diameter of a simple probe design on the probe's self inductance. Dimensional units are in millimeters.

Charlie Chong/ Fion Zhang

Eddy current inspection

Charlie Chong/ Fion Zhang

https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/ProbesCoilDesign/Popups/applet1/applet1.htm

A more precise value of L is given by:

ro is the mean radius of the coil. rc is the radius of the core. l is the length of the coil. n is the number of turns. Âľr is the relative magnetic permeability of the core. Âľo is the permeability of free space (i.e. 4 pi x 10-7 H/m).

K is a dimensionless constant characteristic of the length and the external and internal radii.

Charlie Chong/ Fion Zhang

4.3 Probes - Mode of Operation Eddy current probes are available in a large variety of shapes and sizes. In fact, one of the major advantages of eddy current inspection is that probes can be custom designed for a wide variety of applications. Eddy current probes are classified by the configuration and mode of operation of the test coils. The configuration of the probe generally refers to the way the coil or coils are packaged to best "couple" to the test area of interest. An example of different configurations of probes would be bobbin probes, which are inserted into a piece of pipe to inspect from the inside out, versus encircling probes, in which the coil or coils encircle the pipe to inspect from the outside in. The mode of operation refers to the way the coil or coils are wired and interface with the test equipment.

The mode of operation of a probe generally falls into one of four categories: (1) absolute, (2) differential, (3) reflection and (4) hybrid.

Each of these classifications will be discussed in more detail below.

Charlie Chong/ Fion Zhang

http://www.vegastel.eu/index.php/en/sukuriniu-sroviu-metodas-en/eddy-current-probes/693-eddy-current-probe-selection-information

Keywords: Configurations of probes (1) Bobbin probes, (2) Encircling probes, Mode of operation (1) absolute, (2) differential, (3) reflection and (4) hybrid.

Charlie Chong/ Fion Zhang

4.3.1 Absolute Probes Absolute probes generally have a single test coil that is used to generate the eddy currents and sense changes in the eddy current field. As discussed in the physics section, AC is passed through the coil and this sets up an expanding and collapsing magnetic field in and around the coil. When the probe is positioned next to a conductive material, the changing magnetic field generates eddy currents within the material.

The generation of the eddy currents take energy from the coil and this appears as an increase in the electrical resistance of the coil. The eddy currents generate their own magnetic field that opposes the magnetic field of the coil and this changes the inductive reactance of the coil.

By measuring the absolute change in impedance of the test coil, much information can be gained about the test material.

Charlie Chong/ Fion Zhang

Absolute coils can be used for flaw detection, conductivity measurements, liftoff measurements and thickness measurements. They are widely used due to their versatility. Since absolute probes are sensitive to things such as conductivity, permeability liftoff and temperature, steps must be taken to minimize these variables when they are not important to the inspection being performed. It is very common for commercially available absolute probes to have a fixed "air loaded" reference coil that compensates for ambient temperature variations.

Charlie Chong/ Fion Zhang

Absolute Probes (Single-Coil Probes) The earliest form of eddy current instruments operated with a single-coil probe that was wound to a specific value frequency. Many newer models of eddy current instruments have kept this circuitry as a popular option for users while also incorporating more sophisticated functions. When these probes are used, a balance coil is also required which may be set from within the eddy current instrument or is commonly found within the probe housing, the cable connector or in a separate adapter. A problem can arise when the probe inductance value is not close enough to the value of the balance coil causing the instrument not to balance correctly. The result is poor performance (noisy or insensitive) or no response at all (signal saturation).

Balancing coil

Charlie Chong/ Fion Zhang

The Principle: The generation of the eddy currents take energy from the coil and this appears as an increase in the electrical resistance of the coil. The eddy currents generate their own magnetic field that opposes the magnetic field of the coil and this changes the inductive reactance of the coil.

Variations: The change in inductive reactance could be increasing or decreasing depending on the magnetic permeability of material.

Commons: Irrespective of magnetic permeability, the resistance always increase.

Charlie Chong/ Fion Zhang

4.3.2 Differential Probes Differential probes have two active coils usually wound in opposition, although they could be wound in addition with similar results. When the two coils are over a flaw-free area of test sample, there is no differential signal developed between the coils since they are both inspecting identical material.

However, when one coil is over a defect and the other is over good material, a differential signal is produced. They have the advantage of being very sensitive to defects yet relatively insensitive to slowly varying properties such as gradual dimensional or temperature variations. Probe wobble signals (摆动 信号) are also reduced with this probe type. There are also disadvantages to using differential probes. Most notably, the signals may be difficult to interpret.

For example, if a flaw is longer than the spacing between the two coils, only the leading and trailing edges will be detected due to signal cancellation when both coils sense the flaw equally.

Charlie Chong/ Fion Zhang

Differential Probes – with two coils wound in different directions

Charlie Chong/ Fion Zhang

Differential (Bridge Type) Probes In this configuration the probe coils are located in an electrical "bridge" (see fig. below). The instrument balances the bridge and any change in balance is displayed as a signal. In this arrangement, the same coil produces the eddy currents and detects the impedance changes caused by the defects (or any other variables). Almost all instruments are able to operate with this type of coil arrangement.

Charlie Chong/ Fion Zhang

4.3.3 Reflection Probes Reflection probes have two coils similar to a differential probe, but one coil is used to excite the eddy currents and the other is used to sense changes in the test material. Probes of this arrangement are often referred to as driver/pickup probes. The advantage of reflection probes is that the driver and pickup coils can be separately optimized for their intended purpose. The driver coil can be made so as to produce a strong and uniform flux field in the vicinity of the pickup coil, while the pickup coil can be made very small so that it will be sensitive to very small defects.

Charlie Chong/ Fion Zhang

Reflection Probes

Charlie Chong/ Fion Zhang

Reflection Type Probe These probes are also known as send-receive or driver-pickup. In this configuration, the eddy currents are produced by a coil connected to the instrument's oscillator (driver). The signals received back in the probe are detected by separate coils called pickups (see Fig. 3 and Fig. 4). All new impedance plane instruments and also many older models are able to operate in both differential (bridge) and reflection modes.

Charlie Chong/ Fion Zhang

4.3.4 Hybrid Probes An example of a hybrid probe is the split D, differential probe shown below.

This probe has a driver coil that surrounds two D shaped sensing coils. It operates in the reflection mode but additionally, its sensing coils operate in the differential mode. This type of probe is very sensitive to surface cracks.

Another example of a hybrid probe is one that uses a conventional coil to generate eddy currents in the material but then uses a different type of sensor to detect changes on the surface and within the test material. An example of a hybrid probe is one that uses a Hall effect sensor to detect changes in the magnetic flux leaking from the test surface. Hybrid probes are usually specially designed for a specific inspection application.

Charlie Chong/ Fion Zhang

4.3.5 Differential (Bridge) or Reflection?

This is a common question asked by those involved in trying to select the best probe for an inspection. The answer is "It depends." Let us consider both systems.

Gain: Reflection probes will give a higher gain, particularly if they are "tuned" to a specific frequency, but normally the difference is on average about 6 dB. It is true that this doubles the signal, but if you consider that the instruments are able to give this increase of gain easily, it is not so important.

Nevertheless, in critical applications this increase is very welcomed.

Frequency range: Reflection probes do not need to balance the driver to the pickup coils. This means that they will give a wider frequency range. As long as the driver produces eddy currents, the pickup will detect them and some signal will be displayed. This may not provide good information at certain frequencies, but the probe is still working!

Charlie Chong/ Fion Zhang

Bridge type probes used to give a limited frequency span in the older instruments, as these had to balance an electrical bridge using its other arms (X and R controls). In modern instruments, the bridge is normally formed with fixed precision resistors, or a fixed transformer inside it. The signals detected in this manner are electronically processed without any "mechanical" adjustments, and this means a greater ability to balance over a wider frequency range.

Drift: Probe drift is mostly caused by temperature change in the coils. This may be caused by varying ambient temperature, or the heat produced by the oscillator current, or both.

There are design parameters that can be optimized to reduce drift, such as wire diameter and ferrite selection, but reflection probes are normally a good choice to avoid this problem even more.

Charlie Chong/ Fion Zhang

In a reflection probe, the driver current does not flow through the pickup coils; in fact, the magnetic field received back from the specimen is normally much smaller and, consequently, the current flowing in the pickups is also reduced.

Most probe types (pencil, spot, ring, bolt hole, etc.) can be made as bridge or reflection. Keep in mind that a reflection probe is almost invariably more difficult to manufacture and therefore more expensive.

Charlie Chong/ Fion Zhang

4.3.6 Absolute, Differential (Bridge) and Reflection Probes This is an area where some confusion exists. Many users have called a probe "differential" when the signal displayed gives an up and down movement or a figure 8 type signal. This is caused by the two coils sensing the defect in sequence. When both sensing coils are on the probe surface, they compensate for lift-off and as a result no line is visible (see Fig. 5).

Fig. 5 Charlie Chong/ Fion Zhang

In contrast, an absolute or bridge display is produced by a single sensing coil (see Fig. 1 through Fig. 4), giving a single, upward movement with a near horizontal lift-off line. Others have called a probe "differential" simply when the coils were connected differentially such as in a bridge circuit. The problem with this definition is that probes can be connected differentially in a reflection system as well as when using two pickups (such as most scanner-driven bolt hole probes). In this case, the two pickup coils are positioned close to one another and contained within a driver coil (see Fig. 6).

The best way out of this confusion is often to specify the probe as absolute, bridge, reflection, bridge differential or reflection differential as needed. It makes more sense to qualify the description according to the displayed signal, since this is what really matters and not many people are concerned as to how the coils are connected internally.

Fig. 6 Charlie Chong/ Fion Zhang

4.3.7 Shielded and Unshielded Probes Probes are normally available in both shielded and unshielded versions; however, there is an increasing demand for the shielded variety.

Shielding restricts the magnetic field produced by the coils to the physical size of the probe. A shield can be made of various materials, but the most common are: ferrite (like a ceramic made of iron oxides), Mu-metal, and mild steel. Ferrite make the best shielding because they provide an easy path for the magnetic field but has poor conductivity. This means that there is little eddy current loss in the shield itself. Mild steel has more losses but is widely used for spot probes and ring probes due to its ease of machining when ferrite is not available in certain sizes or shapes. Mu-metal is sometimes for pencil probes as it is available in thin sheet; however, it is less effective than ferrite.

Note: Mu-metal (高导磁合金) is a nickel-iron alloy, composed of approximately 77% nickel, 16% iron, 5% copper and 2% chromium or molybdenum, that is notable for its high magnetic permeability.

Charlie Chong/ Fion Zhang

http://en.wikipedia.org/wiki/Mu-metal

Shielding has several advantages: first, it allows the probe to be used near geometry changes, such as edges, without giving false indications; next, it allows the probe to touch ferrous fastener heads with minimal interference; last, it allows the detection of smaller defects due to the stronger magnetic field concentrated in a smaller area.

On the other hand, unshielded probes allow somewhat deeper penetration due to the larger magnetic field. They are also slightly more tolerant to lift-off.

Unshielded probes are recommended for the inspection of ferrous materials (steel) for surface cracks, and in particular with meter instruments. The reason for this is that the meter response is too slow to allow the signal from a shielded probe to be displayed at normal scanning speeds due to the smaller sensitive area.

Charlie Chong/ Fion Zhang

4.3.8 Adapters To connect a probe with a connector different from the type used on the instrument, it is necessary to use an adapter. An adapter consists of two different connectors joined and wired to match the inputs and outputs as necessary. It is normally housed in a short body that can be positioned at the instrument's input. Sometimes, it is also possible to have a "cable adapter," which is made to match a connector located at the probe body. Depending on the instrument's wiring, it may be possible to have a single adapter for both bridge and reflection probes. In other cases, it is necessary to have two separate adapters or use a switchable type.

Charlie Chong/ Fion Zhang

http://www.vegastel.eu/index.php/en/sukuriniu-sroviu-metodas-en/eddy-current-probes/693-eddy-current-probe-selection-information

4.4 Probes - Configurations As mentioned on the previous page, eddy current probes are classified by the configuration and mode of operation of the test coils. The configuration of the probe generally refers to the way the coil or coils are packaged to best "couple" to the test area of interest. Some of the common classifications of probes based on their configuration include surface probes, bolt hole probes, inside diameter (ID) probes, and outside diameter (OD) probes.

Charlie Chong/ Fion Zhang

Eddy current probes

Charlie Chong/ Fion Zhang

Eddy current inspection display

Charlie Chong/ Fion Zhang

http://www.ibgndt.com/eddyliner-s-eddy-current-testers-hardness-case-depth-structure.php

Eddy current inspection system

http://idea-ndt.en.alibaba.com/product/488266329-212374104/Automatic_ERW_pipes_eddy_current_and_ultrasonic_testing_systems_and_equipments.html

Charlie Chong/ Fion Zhang

4.4.1 Surface Probes Surface probes are usually designed to be handheld and are intended to be used in contact with the test surface. Surface probes generally consist of a coil of very fine wire encased in a protective housing. The size of the coil and shape of the housing are determined by the intended use of the probe.

Most of the coils are wound so that the axis of the coil is perpendicular to the test surface. This coil configuration is sometimes referred to as a pancake coil and is good for detecting surface discontinuities that are oriented perpendicular to the test surface. Discontinuities, such as delaminations, that are in a parallel plane to the test surface will likely go undetected with this coil configuration.

Charlie Chong/ Fion Zhang

Wide surface coils are used when scanning large areas for relatively large defects. They sample a relatively large area and allow for deeper penetration.

Since they do sample a large area, they are often used for conductivity tests to get more of a bulk material measurement. However, their large sampling area limits their ability to detect small discontinuities.

Pencil probes have a small surface coil that is encased in a long slender housing to permit inspection in restricted spaces. They are available with a straight shaft or with a bent shaft, which facilitates easier handling and use in applications such as the inspection of small diameter bores. Pencil probes are prone to wobble due to their small base and sleeves are sometimes used to provide a wider base.

Keywords: Wide surface- deeper penetration Narrow probe – detect smaller discontinuities Narrow probe – prone to wobble

Charlie Chong/ Fion Zhang

Surface Probe

Charlie Chong/ Fion Zhang

http://advantech.my/Products%20-%20ET.htm

Surface Probe

Charlie Chong/ Fion Zhang

Surface Probe

Charlie Chong/ Fion Zhang

Surface Probe

Charlie Chong/ Fion Zhang

4.4.2 Bolt Hole Probes Bolt hole probes are a special type of surface probe that is designed to be used with a bolt hole scanner. They have a surface coil that is mounted inside a housing that matches the diameter of the hole being inspected. The probe is inserted in the hole and the scanner rotates the probe within the hole.

Charlie Chong/ Fion Zhang

Bolt Hole Probes

Charlie Chong/ Fion Zhang

http://www.phtool.com/pages/eddy.asp

Bolt Hole Probes

Charlie Chong/ Fion Zhang

http://www.olympus-ims.com/en/applications/eddy-current-probes-guide/

Birring NDT Series, Eddy Current Testing # 5, Inspection of Fastener Holes using a Rotary Probe

ď Ž https://www.youtube.com/watch?v=X4yqOLUYrBs

Charlie Chong/ Fion Zhang

4.4.3 ID or Bobbin Probes ID probes, which are also referred to as Bobbin probes or feed-through probes, are inserted into hollow products, such as pipes, to inspect from the inside out. The ID probes have a housing that keep the probe centered in the product and the coil(s) orientation somewhat constant relative to the test surface. The coils are most commonly wound around the circumference of the probe so that the probe inspects an area around the entire circumference of the test object at one time.

Charlie Chong/ Fion Zhang

Configuration: Bobbin Probe

Charlie Chong/ Fion Zhang

Configuration: Bobbin Probe

Charlie Chong/ Fion Zhang

4.4.4 OD or Encircling Coils OD probes are often called encircling coils. They are similar to ID probes except that the coil(s) encircle the material to inspect from the outside in. OD probes are commonly used to inspect solid products, such as bars.

Charlie Chong/ Fion Zhang

Configuration: Encircling probes

Charlie Chong/ Fion Zhang

Configuration: Encircling probes

Charlie Chong/ Fion Zhang

Configuration: Encircling probes

Charlie Chong/ Fion Zhang

Eddy current inspection: Calibration

Charlie Chong/ Fion Zhang

Eddy current inspection: Calibration

Charlie Chong/ Fion Zhang

Eddy current inspection

Charlie Chong/ Fion Zhang

Eddy Current testing – Encircling configuration



http://www.tudou.com/programs/view/html5embed.action?type=0&code=tCK2R4PRoGk&lcode=&resourceId=30911220_06_05_99" allowtransparency=

Charlie Chong/ Fion Zhang

4.5 Probes - Shielding & Loading 4.5.1 Why Shielding?

One of the challenges of performing an eddy current inspection is getting sufficient eddy current field strength in the region of interest within the material. Another challenge is keeping the field away from non-relevant features of the test component. The impedance change caused by nonrelevant features can complicate the interpretation of the signal. Probe shielding and loading are sometimes used to limit the spread and concentrate the magnetic field of the coil. Of course, if the magnetic field is concentrated near the coil, the eddy currents will also be concentrated in this area.

Keywords:  Non-relevant indication  Probe shielding  Field is concentrated  Eddy current also is concentrated

Charlie Chong/ Fion Zhang

4.5.2 Probe Shielding Probe shielding is used to prevent or reduce the interaction of the probe's magnetic field with nonrelevant features in close proximity of the probe.

Shielding could be used to reduce edge effects when testing near dimensional transitions such as a step or an edge. Shielding could also be used to reduce the effects of conductive or magnetic fasteners in the region of testing.

Eddy current probes are most often shielded using magnetic shielding or eddy current shielding. Magnetically shielded probes have their coil surrounded by a ring of ferrite or other material with high permeability and low conductivity. The ferrite creates an area of low magnetic reluctance and the probe's magnetic field is concentrated in this area rather than spreading beyond the shielding. This concentrates the magnetic field into a tighter area around the coil.

Charlie Chong/ Fion Zhang

Eddy current shielding uses a ring of highly conductive but nonmagnetic material, usually copper, to surround the coil. The portion of the coil's magnetic field that cuts across the shielding will generate eddy currents in the shielding material rather than in the non-relevant features outside of the shielded area. The higher the frequency of the current used to drive the probe, the more effective the shielding will be due to the skin effect in the shielding material.

Question: “The portion of the coil's magnetic field that cuts across the shielding will generate eddy currents in the shielding material rather than in the non-relevant features outside of the shielded area.�

How does the above provide shielding effect?

Charlie Chong/ Fion Zhang

Keywords:  reduce the interaction non-relevant features  reduce edge effects with step or an edge.  reduce the effects of conductive or magnetic fasteners nearby.  Shielded using magnetic shielding or eddy current shielding.  Magnetically shielded probes surrounded the coil by a ring of ferrite or other material with high magnetic permeability and low conductivity.  Eddy current shielding uses a ring of highly conductive but nonmagnetic material, usually copper, to surround the coil.

Charlie Chong/ Fion Zhang

Discussion Topic 1: What is plus point for not shielding?

Topic 2: discuss this sentence “The higher the frequency of the current used to drive the probe, the more effective the shielding will be due to the skin effect in the shielding material.�

Charlie Chong/ Fion Zhang

4.5.3 Probe Loading with Ferrite Cores Sometimes coils are wound around a ferrite core. Since ferrite is ferromagnetic, the magnetic flux produced by the coil prefers to travel through the ferrite as opposed to the air. Therefore, the ferrite core concentrates the magnetic field near the center of the probe. This, in turn, concentrates the eddy currents near the center of the probe. Probes with ferrite cores tend to be more sensitive than air core probes and less affected by probe wobble and lift-off.

Charlie Chong/ Fion Zhang

5.0 Procedure Issues 5.1 Reference Standards In eddy current testing, the use of reference standards in setting up the equipment is particularly important since signals are affected by many different variables and slight changes in equipment setup can drastically alter the appearance of a signal. As with most other NDT methods, the most useful information is obtained when comparing the results from an unknown object to results from a similar object with well characterized features and defects. In almost all cases, eddy current inspection procedures require the equipment to be configured using reference standards.

For crack detection, corrosion thinning and other material damage, reference standards are used to setup the equipment to produce a recognizable signal or set of signals from a defect or set of defects. In many cases, the appearance of a test signal can be related to the appearance of a signal from a known defect on the reference standard to estimate the size of a defect in the test component. Signals that vary significantly from the responses produced by the reference standard must be further investigated to the determine the source of the signal.

Charlie Chong/ Fion Zhang

The reference standards

The reference Standards

Charlie Chong/ Fion Zhang

The reference standards

Charlie Chong/ Fion Zhang

The reference standards

Charlie Chong/ Fion Zhang

The reference standards

Charlie Chong/ Fion Zhang

The reference standards

Charlie Chong/ Fion Zhang

The reference standards

Charlie Chong/ Fion Zhang

The reference standards

Charlie Chong/ Fion Zhang

The reference standards

Charlie Chong/ Fion Zhang

The reference standards

Charlie Chong/ Fion Zhang

The reference standards

Charlie Chong/ Fion Zhang

The reference standards

Charlie Chong/ Fion Zhang

The reference standards

Charlie Chong/ Fion Zhang

http://radio.rphf.spbstu.ru/a263/eddy.htm

The reference standard should be of the same material as the test article. If this is not possible or practical, it should be of material that has the same electrical conductivity and magnetic permeability. Component features (material thickness, geometry, etc.) should be the same in the reference standard as those in the test region of interest. If the reference standard is the type with intentional defects, these defects should be as representative of actual defects in the test component as possible. The closer the reference standard is to the actual test component, the better. However, since cracks and corrosion damage are often difficult and costly to produce, artificial defects are commonly used. Narrow notches produced with electron discharge machining (EDM) and saw cuts are commonly used to represent cracks, and drilled holes are often used to simulate corrosion pitting.

Charlie Chong/ Fion Zhang

Common eddy current reference standards include:      

Conductivity standards.

Flat plate discontinuity standards.

Flat plate metal thinning standards (step or tapered wedges).

Tube discontinuity standards.

Tube metal thinning standards.

Hole (with and without fastener) discontinuity standards.

Charlie Chong/ Fion Zhang

5.2 Signal Filtering 5.2.1 Signal filtering is often used in eddy current testing to eliminate unwanted frequencies from the receiver signal. While the correct filter settings can significantly improve the visibility of a defect signal, incorrect settings can distort the signal presentation and even eliminate the defect signal completely.

Therefore, it is important to understand the concept of signal filtering.

Charlie Chong/ Fion Zhang

Filtering is applied to the received signal and, therefore, is not directly related to the probe drive frequency. This is most easily understood when picturing a time versus signal amplitude display. With this display mode, it is easy to see that the signal shape is dependent on the time or duration that the probe coil is sensing something. For example, if a surface probe is placed on the surface of conductor and rocked back and forth, it will produce a wave like signal. When the probe is rocked fast, the signal will have a higher frequency than when the probe is rocked slowly back and forth.

The signal does not need a wavelike appearance to have frequency content and most eddy current signals will be composed of a large number of frequencies. Consider a probe that senses a notch for 1/60th of a second. In a period of one second the probe could (in theory) go over the notch 60 times, resulting in the notch signal having a frequency of 60 Hz. But, imposed on this same signal, could be the signal resulting from probe wobble, electronic noise, a conductivity shift and other factors which occur at different frequencies.

Charlie Chong/ Fion Zhang

Signal filtering

Unfiltered signal with low frequency variation and high frequency noise

Low frequency variation High pass filter employed to remove low frequency variation

High frequency noise

Low pass filter employed to remove high frequency noise

Filter signal Charlie Chong/ Fion Zhang

5.2.2 Filters Effects The two standard filters found in most impedance plane display instruments are the ‘High Pass Filter’ (HPF) and ‘Low Pass Filter’ (LPF). Some instruments also have a‘Band Pass Filter’ (BPF), which is a combination high and low pass filter. Filters are adjusted in Hertz (Hz).

The HPF allows high frequencies to pass and filters out the low frequencies.

The HPF is basically filtering out changes in the signal that occur over a significant period of time.

The LPF allows low frequency to pass and filters out the high frequency. In other words, all portions of the signal that change rapidly (have a high slope) are filtered, such as electronic noise.

Keywords: ■ HPF ■ LPF ■ BPF

Charlie Chong/ Fion Zhang

In the image above, the gradual (low frequency) changes were first filtered out with a HPF and then high frequency electronic noise was filtered with a LPF to leave a clearly visible flaw indication. It should also be noted that since flaw indication signals are comprised of multiple frequencies, both filters have a tendency to reduce the indication signal strength. Additionally, scan speed must be controlled when using filters. Scan over a flaw too slow and the HPF might filter out the flaw indication. Scan over the flaw too fast and the LPF might eliminate the flaw indication.

Keywords: ď Ž Scan over a flaw too slow and the HPF might filter out the flaw indication. ď Ž Scan over the flaw too fast and the LPF might eliminate the flaw indication.

Charlie Chong/ Fion Zhang

Signal Reduction

Unfiltered signal with low frequency variation and high frequency noise

Low frequency variation

Original Signal

High pass filter employed to remove low frequency variation

Filtered Signal Low pass filter employed to remove high frequency noise

Charlie Chong/ Fion Zhang

5.2.3 Filter Settings If the spectrum of the signal frequency and the signal amplitude or attenuation are plotted, the filter responses can be illustrated in graphical form. The image to the right shows the response of a LPF of 20Hz and a HPF of 40Hz. The LPF allows only the frequencies in yellow to pass and the HPF only allow those frequencies in the blue area to pass. Therefore, it can be seen that with these settings there are no frequencies that pass (i.e. the frequencies passed by the LPF are filtered out by the HPF and visa versa).

Charlie Chong/ Fion Zhang

Eddy current inspection

Rejected by LPF

Charlie Chong/ Fion Zhang

Rejected by HPF

Charlie Chong/ Fion Zhang

To create a window of acceptance for the signals, the filters need to overlap.

In the image to the right, the LPF has been adjusted to 60Hz and the HPF to 10Hz. The area shown in gray is where the two frequencies overlap and the signal is passed. A signal of 30Hz will get through at full amplitude, while a signal of 15Hz will be attenuated by approximately 50%. All frequencies above or below the gray area (the pass band) will be rejected by one of the two filters.

Overlap areaAccepted signal

Charlie Chong/ Fion Zhang

5.2.4 Use of Filters The main function of the LPF is to remove high frequency interference noise.

This noise can come from a variety of sources including the instrumentation and/or the probe itself. The noise appears as an unstable dot that produces jagged lines on the display as seen in the signal from a surface notch shown in the left image below. Lowering the LPF frequency will remove more of the higher frequencies from the signal and produce a cleaner signal as shown in the center image below. When using a LPF, it should be set to the highest frequency that produces a usable signal. To reduce noise in large surface or ring probes, it may be necessary to use a very low LPF setting (down to 10Hz). The lower the LPF setting, the slower the scanning speed must be and the more closely it must be controlled. The image on the right below shows a signal that has been clipped due to using a scan speed too fast for the selected HPF setting.

Charlie Chong/ Fion Zhang

Signal Filtering

Unfiltered signals

signal that has been clipped due to using a scan speed too fast for the selected HPF setting

Lowering the LPF frequency will remove more of the higher frequencies from the signal and produce a cleaner signal as shown in the center image Charlie Chong/ Fion Zhang

The HPF is used to eliminate low frequencies which are produced by slow changes, such as conductivity shift within a material, varying distance to an edge while scanning parallel to it, or out-of-round holes in fastener hole inspection. The HPF is useful when performing automated or semiautomatic scans to keep the signal from wandering too far from the null (balance) point.

The most common application for the HPF is the inspection of fastener holes using a rotating scanner. As the scanner rotates at a constant RPM, the HPF can be adjusted to achieve the desired effect.

Charlie Chong/ Fion Zhang

Use of the HPF when scanning manually is not recommended, as keeping a constant scanning speed is difficult, and the signal deforms and amplitude decreases. The size of a signal decreases as the scan speed decreases and a flaw indication can be eliminated completely if the scan is not done with sufficient speed. In the images below, it can be seen that a typical response from a surface notch in aluminum without HPF (left image) looks considerably different when the HPF is activated (right image). With the HPF, looping signals with a positive and similar negative deflection are produced on the impedance plane.

Charlie Chong/ Fion Zhang

The use of a minimal HPF setting (1 or 2 Hz) may be used when manually scanning, provided the operator can largely control the scan speed and becomes familiar with the indication signal changes as scan speed is varied slightly. An good example of such an application would be the manual scan of the radius of a wheel that is rotated by hand, but the speed of rotation can be kept relatively constant.

Charlie Chong/ Fion Zhang

6.0 Applications 6.1 Surface Breaking Cracks Eddy current equipment can be used for a variety of applications such as the detection of cracks (discontinuities), measurement of metal thickness, detection of metal thinning due to corrosion and erosion, determination of coating thickness, and the measurement of electrical conductivity and magnetic permeability. Eddy current inspection is an excellent method for detecting surface and near surface defects when the probable defect location and orientation is well known.

Charlie Chong/ Fion Zhang

Surface breaking cracks

Charlie Chong/ Fion Zhang

Surface breaking cracks

Charlie Chong/ Fion Zhang

Surface breaking cracks

Charlie Chong/ Fion Zhang

Surface breaking cracks

Charlie Chong/ Fion Zhang

Surface breaking cracks

Charlie Chong/ Fion Zhang

Surface breaking cracks

Charlie Chong/ Fion Zhang

Surface breaking cracks

Charlie Chong/ Fion Zhang

http://www.assda.asn.au/component/rsblog/category/13?start=20

Defects such as cracks are detected when they disrupt the path of eddy currents and weaken their strength.

The images to the right show an eddy current surface probe on the surface of a conductive component. The strength of the eddy currents under the coil of the probe ins indicated by color. In the lower image, there is a flaw under the right side of the coil and it can be see that the eddy currents are weaker in this area. weaker

Charlie Chong/ Fion Zhang

Of course, factors such as the type of material, surface finish and condition of the material, the design of the probe, and many other factors can affect the sensitivity of the inspection. Successful detection of surface breaking and near surface cracks requires: 1. A knowledge of probable defect type, position, and orientation.

2. Selection of the proper probe. The probe should fit the geometry of the part and the coil must produce eddy currents that will be disrupted by the flaw.

3. Selection of a reasonable probe drive frequency. For surface flaws, the frequency should be as high as possible for maximum resolution and high sensitivity. For subsurface flaws, lower frequencies are necessary to get the required depth of penetration and this results in less sensitivity.

Ferromagnetic or highly conductive materials require the use of an even lower frequency to arrive at some level of penetration.

4. Setup or reference specimens of similar material to the component being inspected and with features that are representative of the defect or condition being inspected for.

Charlie Chong/ Fion Zhang

6.1.1 Selection of probe frequency: Selection of a reasonable probe drive frequency.  For surface flaws, the frequency should be as high as possible for maximum resolution and high sensitivity.  For subsurface flaws, lower frequencies are necessary to get the required depth of penetration and this results in less sensitivity.  Ferromagnetic or highly conductive materials require the use of an even lower frequency to arrive at some level of penetration.

Charlie Chong/ Fion Zhang

The basic steps in performing an inspection with a surface probe are the following: 1. Select and setup the instrument and probe.

2. Select a frequency to produce the desired depth of penetration.

3. Adjust the instrument to obtain an easily recognizable defect response using a calibration standard or setup specimen.

4. Place the inspection probe (coil) on the component surface and null the instrument.

5. Scan the probe over part of the surface in a pattern that will provide complete coverage of the area being inspected. Care must be taken to maintain the same probe-to-surface orientation as probe wobble can affect interpretation of the signal. In some cases, fixtures to help maintain orientation or automated scanners may be required.

6. Monitor the signal for a local change in impedance (R, XL) that will occur as the probe moves over a discontinuity.

Charlie Chong/ Fion Zhang

 Resistance is essentially friction against the motion of electrons. It is present in all conductors to some extent (except superconductors!), most notably in resistors.

When alternating current goes through a resistance, a voltage drop is produced that is in-phase with the current. Resistance is mathematically symbolized by the letter “R” and is measured in the unit of ohms (Ω).  Reactance is essentially inertia against the motion of electrons. It is present anywhere electric or magnetic fields are developed in proportion to applied voltage or current, respectively; but most notably in capacitors and inductors. When alternating current goes through a pure reactance, a voltage drop is produced that is 90o out of phase with the current. Reactance is mathematically symbolized by the letter “X” and is measured in the unit of ohms (Ω).  Impedance is a comprehensive expression of any and all forms of opposition to electron flow, including both resistance and reactance. It is present in all circuits, and in all components. When alternating current goes through an impedance, a voltage drop is produced that is somewhere between 0o and 90o out of phase with the current. Impedance is mathematically symbolized by the letter “Z” and is measured in the unit of ohms (Ω).

Charlie Chong/ Fion Zhang

http://www.allaboutcircuits.com/vol_2/chpt_5/1.html

Reactance phasor diagram (Impedance plane respond)

Charlie Chong/ Fion Zhang

Surface probe testing – Crack

Charlie Chong/ Fion Zhang

The applet below depicts a simple eddy current probe near the surface of a calibration specimen. Move the probe over the surface of the specimen and compare the signal responses from a surface breaking crack with the signals from the calibration notches. The inspection can be made at a couple of different frequencies to get a feel for the effect that frequency has on sensitivity in this application.

Keywords: ■ Surface breaking crack ■ Calibration notches ■ Effect of frequency on sensitivity

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Graphics/Flash/surfaceBreakingCracks.swf

Eddy Current Crack Detection – 涡流裂缝检测

 www.youtube.com/embed/1YUSn___VxQ?feature=player_detailpage  https://www.youtube.com/watch?v=1YUSn___VxQ Charlie Chong/ Fion Zhang

Eddy Current Crack Testing by Criterion NDT– 涡流裂缝检测

 www.youtube.com/embed/9A5fQtOwnzw Charlie Chong/ Fion Zhang

6.2 Surface Crack Detection Using Sliding Probes Many commercial aircraft applications involve the use of multiple fasteners to connect the multi-layer skins. Because of the fatigue stress that is caused by the typical application of any commercial aircraft, fatigue cracks can be induced in the vicinity of the fastener holes. In order to inspect the fastener holes in an adequate amount of time, sliding probes are an efficient method of inspection. Sliding probes have been named so because they move over fasteners in a sliding motion. There are two types of sliding probes, fixed and adjustable, which are usually operated in the reflection mode. This means that the eddy currents are induced by the driver coil and detected by a separate receiving coil (Mode: reflection coils).

Charlie Chong/ Fion Zhang

Sliding probes are one of the fastest methods to inspect large numbers of fastener holes. They are capable of detecting surface and subsurface discontinuities, but they can only detect defects in one direction. The probes are marked with a detection line to indicate the direction of inspection. In order to make a complete inspection there must be two scans that are orthogonal (90 degrees) to each other.

Charlie Chong/ Fion Zhang

Aircraft applications

Charlie Chong/ Fion Zhang

Aircraft applications

Charlie Chong/ Fion Zhang

6.2.1 Probe Types (configuration- surface probe) (i) Fixed Sliding Probes These probes are generally used for thinner material compared to the adjustable probes. Maximum penetration is about 1/8 inch (3mm). Fixed sliding probes are particularly well suited for finding longitudinal surface or subsurface cracks such as those found in lap joints. Typical frequency range is from 100 Hz to 100 kHz.

Charlie Chong/ Fion Zhang

(ii) Adjustable Sliding Probes These probes are well suited for finding subsurface cracks in thick multi-layer structures, like wing skins. Maximum penetration is about 3/4 inch (19mm).

The frequency range for adjustable sliding probes is from 100 Hz to 40 kHz. Adjustable probes, as the name implies, are adjustable with the use of spacers, which will change the penetration capabilities. The spacer thickness between the coils is normally adjusted for the best detection. For tangential scans or 90 degree scanning with an offset from the center, a thinner spacer is often used.

The spacer thickness range can vary from “0� (no spacer) for inspections close to the surface and small fastener heads to a maximum of about 0.3 inch for deep penetration with large heads in the bigger probe types. A wider spacer will give more tolerance to probe deviation as the sensitive area becomes wider but the instrument will require more gain. Sliding probes usually penetrate thicker materials compared to the donut probes.

Charlie Chong/ Fion Zhang

6.2.2 Reference Standards Reference/calibration standards for setup of sliding probes typically consist of three or four aluminum plates that are fastened together within a lap joint type configuration. EDM notches or naturally/artificially- induced cracks are located in the second or third layer of the standard.

Reference standards used should be manufactured from the same material type, alloy, material thickness, and chemical composition that will be found on the aircraft component to be inspected. Sizes and tolerances of flaws introduced in the standards are usually regulated by inspection specifications.

Charlie Chong/ Fion Zhang

Reference Standards

Charlie Chong/ Fion Zhang

Reference Standards

Charlie Chong/ Fion Zhang

http://www.phtool.com/asntpics.htm

6.2.3 Inspection Variables 6.2.3.1 Liftoff Signal Adjustment Liftoff is normally adjusted to be relatively horizontal. The term "relatively horizontal" is used here because the liftoff signal often appears a curved line rather than a straight line. Sometimes liftoff can be a sharp curve and may need to be adjusted to run slightly upwards before moving downwards. See Figures 1 and 2.

Dotted line Ideal adjustment

The best liftoff adjustment

Charlie Chong/ Fion Zhang

Lift-off Signal Adjustment before testing

 www.youtube.com/embed/1YUSn___VxQ?feature=player_detailpage  https://www.youtube.com/watch?v=1YUSn___VxQ Charlie Chong/ Fion Zhang

6.2.3.2 Scan Patterns A typical scan is centralized over the fastener head and moves along the axis of the fastener holes. This scan is generally used to detect cracks positioned along the axis of the fastener holes. For detecting cracks located transverse or 90 degrees from the axis of the fastener holes, a scan that is 90 degrees from the axis of the fastener holes is recommended.

Charlie Chong/ Fion Zhang

Scan Patterns with differential probes Differential coils have the attraction of built-in lift-off compensation. This has made them useful for many applications. The older types of coils had no ferrite shield and they were built just by placing two coils side-by-side (Figure 3). Later types added individual shields (Figure 4), but the greatest improvement to the sensitivity was achieved when both coils were placed within a common shield (Figure 5). Differential type probes are mostly used in small sizes for surface crack detection only.

Figure 3

Charlie Chong/ Fion Zhang

Figure 4

Figure 5

http://www.olympus-ims.com/en/applications/eddy-current-probes-guide

/

In a probe of this type both coils are wound in opposition. Consequently, signals that affect both simultaneously will cancel out (such as lift- off).

Normally the air point and the working point will be close, but some difference is present due to small coil variations.

Charlie Chong/ Fion Zhang

Normal scan direction is as shown (Figure 6), giving the typical display presentation. The double indication is, in fact helpful, as it doubles the size of the defect in the screen (Figure 7).

Figure 6

Trailing signal

Figure 7

Charlie Chong/ Fion Zhang

Leading signal

Sometimes it is necessary to scan in the same direction as the cracks (Figure 8). This is permissible and the result will be similar for a very short defect. A larger defect affecting both coils will tend to cancel out because they are in opposition, but in practice there are enough differences in angle and depth for this not to happen totally. In any case, the ends of the crack will show normally.

Figure 6

Charlie Chong/ Fion Zhang

6.2.3.3 Signal Interpretation When the probe moves over a fastener hole with a crack, the indication changes and typically will create a larger vertical movement. The vertical amplitude of the loop depends on the crack length, with longer cracks giving higher indications.

If the crack is in the far side of the fastener, as the probe moves over it, the dot will follow the fastener line first but will move upwards (clockwise) as it goes over the crack. If the crack is in the near side, it will be found first and the dot will move along the crack level before coming down to the fastener level. If two cracks on opposite sides of the fastener hole are present, the dot will move upwards to the height by the first crack length and then come back to the fastener line and balance point. If the second crack is longer than the first one, the dot will move even higher and complete the loop (clockwise) before going down to the balance point. See Figures 3 and 4.

Charlie Chong/ Fion Zhang

Signal Interpretation

Charlie Chong/ Fion Zhang

6.2.3.4 Probe Scan Deviation Most probes are designed to give a narrow indication for a good fastener hole so that the loops from the cracks are more noticeable. Some probes and structures can give wider indications and a similar result can be obtained if the probe is not straight when it approaches the fastener. It is important to keep the probe centralized over the fastener heads. Doing this will give you a maximum indication for the fastener and a crack.

If the probe deviates from the center line, the crack indication will move along the loop that we saw in Figure 5 and is now present in Figure 6. The crack indication is at "a" when the probe is centralized and moves toward "b" as it deviates in one direction, or "c" as it deviates in the opposite direction. Point "b" gives an important indication even if it loses a small amount of amplitude it has gained in phase, giving a better separation angle. This is because we deviated to the side where the crack is located.

Charlie Chong/ Fion Zhang

Probe Scan Deviation

Charlie Chong/ Fion Zhang

6.2.3.5 Crack Angle Deviation A reduction in the crack indication occurs when the crack is at an angle to the probe scan direction. This happens if the crack is not completely at 90 degrees to the normal probe scan or changes direction as it grows. Both the fixed and adjustable sliding probes are capable of detecting cracks up to about 30 degrees off angle. See Figures 8 and 9.

Charlie Chong/ Fion Zhang

Crack Angle Deviation

ϴ

Charlie Chong/ Fion Zhang

6.2.4 Electrical Contact When inspecting fasteners that have just been installed or reference standards that have intimate contact with the aluminum skin plate, it is not unusual to obtain a smaller than normal indication. In some extreme cases, the fastener indication may disappear almost completely. This is due to the good electrical contact between the fastener and the skin. This condition allows the eddy currents to circulate without encountering a boundary, and therefore, no obstacle or barrier. Because of this effect, it is recommended to paint the holes before fastener installation.

Charlie Chong/ Fion Zhang

Discussion

Topic: Reasons on the different in phase angles θ for different notch depths

θ2

θ3 θ1

Charlie Chong/ Fion Zhang

Discussion Topic: Why the impedance change on traversing toward the crack and leaving the cracking does not match perfectly on top of each others?

θ2

θ3 θ1

Charlie Chong/ Fion Zhang

6.3 Tube Inspection Eddy current inspection is often used to detect corrosion, erosion, cracking and other changes in tubing. Heat exchangers and steam generators, which are used in power plants, have thousands of tubes that must be prevented from leaking. This is especially important in nuclear power plants where reused, contaminated water must be prevented from mixing with fresh water that will be returned to the environment. The contaminated water flows on one side of the tube (inside or outside) and the fresh water flows on the other side.

The heat is transferred from the contaminated water to the fresh water and the fresh water is then returned back to is source, which is usually a lake or river. It is very important to keep the two water sources from mixing, so power plants are periodically shutdown so the tubes and other equipment can be inspected and repaired. The eddy current test method and the related remote field testing method provide high-speed inspection techniques for these applications.

Charlie Chong/ Fion Zhang

A technique that is often used involves feeding a differential bobbin probe into the individual tube of the heat exchanger. With the differential probe, no signal will be seen on the eddy current instrument as long as no metal thinning is present. When metal thinning is present, a loop will be seen on the impedance plane as one coil of the differential probe passes over the flawed area and a second loop will be produced when the second coil passes over the damage. When the corrosion is on the outside surface of the tube, the depth of corrosion is indicated by a shift in the phase lag. The size of the indication provides an indication of the total extent of the corrosion damage.

A tube inspection using a bobbin probe is simulated below. Click the "null" button and then drag either the absolute or the differential probe through the tube. Note the different signal responses provided by the two probes. Also note that the absolute probe is much more sensitive to dings and the build up of magnetite on the outside of the tube than the differential probe is.

Charlie Chong/ Fion Zhang

Tube Inspection

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Graphics/Flash/DifferentialvsAbsoluteAnim.swf

Tube Inspection

Charlie Chong/ Fion Zhang

http://www.nde.com/ect.htm

Tube Inspection

Charlie Chong/ Fion Zhang

http://www.titanmf.com/photo-gallery/heat-exchangers/

6.4 Conductivity Measurements One of the uses of eddy current instruments is for the measurement of electrical conductivity. The value of the electrical conductivity of a metal depends on several factors, such as its chemical composition and the stress state of its crystalline structure. Therefore, electrical conductivity information can be used for sorting metals, checking for proper heat treatment, and inspecting for heat damage.

Applications:  sorting metals,  checking for proper heat treatment,  inspecting for heat damage.

Charlie Chong/ Fion Zhang

Heat Damage

Charlie Chong/ Fion Zhang

Heat Treatment

Charlie Chong/ Fion Zhang

The technique usually involves nulling an absolute probe in air and placing the probe in contact with the sample surface. For nonmagnetic materials, the change in impedance of the coil can be correlated directly to the conductivity of the material.

The technique can be used to easily sort magnetic materials from nonmagnetic materials but it is difficult to separate the conductivity effects from the magnetic permeability effects, so conductivity measurements are limited to nonmagnetic materials. It is important to control factors that can affect the results such as the inspection temperature and the part geometry.

Conductivity changes with temperature so measurements should be made at a constant temperature and adjustments made for temperature variations when necessary.

The thickness of the specimen should generally be greater than three standard depths of penetration. This is so the eddy currents at the back surface of the sample are sufficiently weaker than the variations in the specimen thickness that are not seen in the measurements.

Charlie Chong/ Fion Zhang

Heat Treatment Discuss on: The technique can be used to easily sort magnetic materials from nonmagnetic materials but it is difficult to separate the conductivity effects from the magnetic permeability effects, so conductivity measurements are limited to nonmagnetic materials

Charlie Chong/ Fion Zhang

Generally large pancake type, surface probes are used to get a value for a relatively large sample area. The instrument is usually setup such that a ferromagnetic material produces a response that is nearly vertical. Then, all conductive but nonmagnetic materials will produce a trace that moves down and to the right as the probe is moved toward the surface. Think back to the discussion on the impedance plane and these type of responses make sense.

Remember that inductive reactance changes are plotted along the y-axis and resistance changes are plotted in the x-axis. Since ferromagnetic materials will concentrate the magnetic field produced by a coil, the inductive reactance of the coil will increase. The effects on the signal from the magnetic permeability overshadow the effects from conductivity since they are so much stronger.

Charlie Chong/ Fion Zhang

Reactance Phasor Diagram

Least conductive material

As the conductivity of the materials being tested increases, the resistance losses will be less and the inductive reactance changes will be greater.

Most conductive material

Charlie Chong/ Fion Zhang

Comments on: As the conductivity of the materials being tested increases, the resistance losses will be less and the inductive reactance changes will be greater.

Note: the underlined statement may not be true universally. (?)

Charlie Chong/ Fion Zhang

When the probe is brought near a conductive but nonmagnetic material, the coil's inductive reactance goes down since the magnetic field from the eddy currents opposes the magnetic field of the coil. The resistance in the coil increases since it takes some of the coil's energy to generate the eddy currents and this appears as additional resistance in the circuit. As the conductivity of the materials being tested increases, the resistance losses will be less and the inductive reactance changes will be greater.

Therefore, the signals will be come more vertical as the conductivity increases, as shown in the image above.

Charlie Chong/ Fion Zhang

Reactance due to conductivity As the conductivity of the materials being tested increases, the resistance losses will be less and the inductive reactance changes will be greater. (see the brown dotted lines)

Charlie Chong/ Fion Zhang

To sort materials using an impedance plane device, the signal from the unknown sample must be compared to a signal from a variety of reference standards. However, there are devices available that can be calibrated to produce a value for electrical conductivity which can then be compared to published values of electrical conductivity in MS/m or percent IACS (International Annealed Copper Standard). Please be aware that the conductivity of a particular material can vary significantly with slight variations in the chemical composition and, thus, a conductivity range is generally provided for a material. The conductivity range for one material may overlap with the range of a second material of interest, so conductivity alone can not always be used to sort materials. The electrical conductivity values for a variety of materials can be found in the material properties reference tables.

Charlie Chong/ Fion Zhang

The following applet is based on codes for nonferrous materials written by Back Blitz from his book, "Electrical and Magnetic Methods of Nondestructive Testing", 2nd ed., Chapman & Hill (1997). The applet demonstrates how an impedance plane eddy current instrument can be used for the sorting of materials.

Charlie Chong/ Fion Zhang

https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Applications/Popups/applet2/applet2.htm

Discuss on the plane impedance diagram

Lead

Aluminum Copper

Quoted from text: “As the conductivity of the materials being tested increases, the resistance losses will be less and the inductive reactance changes will be greater.�

Charlie Chong/ Fion Zhang

Quoted from text: “As the conductivity of the materials being tested increases, the resistance losses will be less and the inductive reactance changes will be greater.�

Discussion: With increase conductivity the resistance component of reactant was decrease. However the inductive reactance component was not increase as compared with that of Aluminum (X1>X2).

X2

Charlie Chong/ Fion Zhang

X1

Discussion on : Material Conductivity, standard penetration and its effect on resistive reactance.

The standard depth of penetration (δ) is the depth where eddy current density drops to 1/e (37%) of its value measured at the surface. This depth of penetration is affected by the operating frequency (ƒ), and conductivity (µ) and permeability (σ) of the material to inspect. This is what we call the skin depth effect.

Charlie Chong/ Fion Zhang

6.5 Heat Treatment Verification 6.5.1 Conductivity Measurements for the Verification of Heat Treatment With some materials, such as solution heat treatable aluminum alloys, conductivity measurements are often made verifying that parts and materials have received the proper heat treatment. High purity aluminum is soft and ductile, and gains strength and hardness with the addition of alloying elements. A few such aluminum alloys are the 2000 series (2014, 2024, etc.), 6000 series (6061, 6063, etc.), and 7000 series (7050, 7075, etc.). The 2xxx series aluminum alloys have copper, the 6xxx series have magnesium, and the 7xxx have zinc as their major alloying elements.

Charlie Chong/ Fion Zhang

Heat treatment of aluminum alloys is accomplished in two phases - solution heat treatment and then aging. In the solution heat treatment step, the alloys are heated to an elevated temperature to dissolve the alloying elements into solution. The metal is then rapidly cooled or quenched to “freeze� the atoms of the alloying elements in the lattice structure of the aluminum. This distorts and stresses the structure, making electron movement more difficult, thereby decreasing the electrical conductivity. In this condition, the alloys are still relatively soft but start to gain strength as the alloying elements begin to precipitate out of solution to form extremely small particles that impede the movement of dislocations within the material.

Charlie Chong/ Fion Zhang

The formation of the precipitates can be controlled for many alloys by heating and holding the material at an elevated temperature for a period of time (artificial aging). As the alloying elements precipitate out of solid solution, the conductivity of the material gradually increases. By controlling the amount of precipitated particles within the aluminum, the properties can be controlled to produce peak strength or some combinations of strength and corrosion resistance. Sometimes, the material must be annealed or put into the softest, most ductile condition possible in order to perform forming operations.

Annealing allows all of the alloying elements to precipitate out of solution to form a coarse, widely spaced precipitate. The electrical conductivity is greatest when the material is in the annealed condition.

Keywords: Annealed condition: ↑Electrical conductivity

Charlie Chong/ Fion Zhang

Since solution heat-treated and aged materials are stronger, components can be made using less material. A lighter or more compact design is often of great importance to the designer and well worth the cost of the heat treating process. However, think of the consequences that could arise if a component that was supposed to be solution heat-treated and aged somehow left the manufacturing facility and was put into service un-heat-treated or annealed.

This is a real possibility since heat-treated aluminum parts look exactly like unheat-treated parts. Consider 2024 aluminum as an example. Select tensile properties and its electrical conductivity for various heat treatment conditions are given in the following table.

Charlie Chong/ Fion Zhang

Properties for Alclad 2024 Aluminum Heat Treatment Condition

Ultimate Strength

Yield Strength

Electrical Conductivity

Annealed (O)

26 ksi (180 MPa)

11 ksi (75 MPa)

50 % IACS

Solution Heat 64 ksi (440 MPa) Treated and Naturally Aged (T42)

42 ksi (290 MPa)

30 % IACS

Solution Heat Treated, Cold worked and Artificially Aged (T861)

66 ksi (455 MPa)

30 % IACS

70 ksi (485 MPa)

IACS: The International Annealed Copper Standard

Charlie Chong/ Fion Zhang

It can be seen that the yield strength for the material is 42 kilopounds/square inch (ksi) (290 MPa) in the solution heat-treated and naturally aged condition (T42 condition). The yield strength can be increased to 66 ksi (455 MPa) when cold worked and artificially aged (T861 condition). But in the annealed condition, the yield strength is reduced to 11 ksi (75 MPa). If an annealed part were accidentally used where a part in the T42 or T861 was intended, it would likely fail prematurely. However, a quick check of the conductivity using an eddy current instrument of all parts prior to shipping would prevent this from occurring.

Charlie Chong/ Fion Zhang

Eddy current inspection

Charlie Chong/ Fion Zhang

6.6 Thickness Measurements 6.6.1 Thickness Measurements of Thin Material Eddy current techniques can be used to perform a number of dimensional measurements. The ability to make rapid measurements without the need for couplant or, in some cases even surface contact, makes eddy current techniques very useful. The type of measurements that can be made include:  thickness of thin metal sheet and foil, and of metallic coatings on metallic and nonmetallic substrate,  cross-sectional dimensions of cylindrical tubes and rods,  thickness of nonmetallic coatings on metallic substrates.

Charlie Chong/ Fion Zhang

Thickness Measurements

Charlie Chong/ Fion Zhang

6.6.2 Corrosion Thinning of Aircraft Skins One application where the eddy current technique is commonly used to measure material thickness is in the detection and characterization of corrosion damage on the skins of aircraft. Eddy current techniques can be used to do spot checks or scanners can be used to inspect small areas. Eddy current inspection has an advantage over ultrasound in this application because no mechanical coupling is required to get the energy into the structure. Therefore, in multi-layered areas of the structure like lap splices, eddy current can often determine if corrosion thinning is present in buried layers.

Charlie Chong/ Fion Zhang

Eddy current inspection has an advantage over radiography for this application because only single sided access is required to perform the inspection. To get a piece of film on the back side of the aircraft skin might require removing interior furnishings, panels, and insulation which could be very costly. Advanced eddy current techniques are being developed that can determine thickness changes down to about three percent of the skin thickness.

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Corrosion thinning is present in buried layers.

Charlie Chong/ Fion Zhang

6.6.3 Thickness Measurement of Thin Conductive Sheet, Strip and Foil Eddy current techniques are used to measure the thickness of hot sheet, strip and foil in rolling mills, and to measure the amount of metal thinning that has occurred over time due to corrosion on fuselage skins of aircraft. On the impedance plane, thickness variations exhibit the same type of eddy current signal response as a subsurface defect, except that the signal represents a void of infinite size and depth. The phase rotation pattern is the same, but the signal amplitude is greater. In the applet, the lift-off curves for different areas of the taper wedge can be produced by nulling the probe in air and touching it to the surface at various locations of the tapered wedge. If a line is drawn between the end points of the lift-off curves, a comma shaped curve is produced. As illustrated in the second applet, this comma shaped curve is the path that is traced on the screen when the probe is scanned down the length of the tapered wedge so that the entire range of thickness values are measured.

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Graphics/Flash/thinningMeasurement1.swf

Charlie Chong/ Fion Zhang

https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Graphics/Flash/thinningMeasurement2.swf

When making this measurement, it is important to keep in mind that the depth of penetration of the eddy currents must cover the entire range of thicknesses being measured. Typically, a frequency is selected that produces about one standard depth of penetration at the maximum thickness. Unfortunately, at lower frequencies, which are often needed to get the necessary penetration, the probe impedance is more sensitive to changes in electrical conductivity.

Thus, the effects of electrical conductivity cannot be phased out and it is important to verify that any variations of conductivity over the region of interest are at a sufficiently low level.

Keywords: ď Ž Typically, a frequency is selected that produces about one standard depth of penetration at the maximum thickness. ď Ž at lower frequencies, which are often needed to get the necessary penetration, the probe impedance is more sensitive to changes in electrical conductivity.

Charlie Chong/ Fion Zhang

6.6.4 Measurement of Cross-sectional Dimensions of Cylindrical Tubes and Rods Dimensions of cylindrical tubes and rods can be measured with either OD coils or internal axial coils, whichever is appropriate. The relationship between change in impedance and change in diameter is fairly constant, except at very low frequencies. However, the advantages of operating at a higher normalized frequency are twofold. First, the contribution of any conductivity change to the impedance of the coil becomes less important and it can easily be phased out. Second, there is an increase in measurement sensitivity resulting from the higher value of the inductive component of the impedance. Because of the large phase difference between the impedance vectors corresponding to changes in fill-factor and conductivity (and defect size), simultaneous testing for dimensions, conductivity, and defects can be carried out.

Keywords: â– Impedance vectors â– Fill-factor and conductivity (and defect size)

Charlie Chong/ Fion Zhang

Typical applications include measuring eccentricities of the diameters of tubes and rods and the thickness of tube walls. Long tubes are often tested by passing them at a constant speed through encircling coils (generally differential) and providing a close fit to achieve as high a fill-factor as possible.

An important application of tube-wall thickness measurement is the detection and assessment of corrosion, both external and internal. Internal probes must be used when the external surface is not accessible, such as when testing pipes that are buried or supported by brackets. Success has been achieved in measuring thickness variations in ferromagnetic metal pipes with the remote field technique.

Keywords: â– Fill-factor â– Remote field technique.

Charlie Chong/ Fion Zhang

6.6.5 Thickness Measurement of Thin Conductive Layers It is also possible to measure the thickness of a thin layer of metal on a metallic substrate, provided the two metals have widely differing electrical conductivities (i.e. silver on lead where s= 67 and 10 MS/m, respectively). A frequency must be selected such that there is complete eddy current penetration of the layer, but not of the substrate itself. The method has also been used successfully for measuring thickness of very thin protective coatings of ferromagnetic metals (i.e. chromium and nickel) on nonferromagnetic metal bases.

Depending on the required degree of penetration, measurements can be made using a single-coil probe or a transformer probe, preferably reflection type. Small-diameter probe coils are usually preferred since they can provide very high sensitivity and minimize effects related to property or thickness variations in the underlying base metal when used in combination with suitably high test frequencies. The goal is to confine the magnetizing field, and the resulting eddy current distribution, to just beyond the thin coating layer and to minimize the field within the base metals.

Charlie Chong/ Fion Zhang

6.7 Thickness of Coatings 6.7.1 Thickness Measurements of Non-conducting Coatings on Conductive Materials The thickness of nonmetallic coatings on metal substrates can be determined simply from the effect of liftoff on impedance. This method has widespread use for measuring thickness of paint and plastic coatings. The coating serves as a spacer between the probe and the conductive surface. As the distance between the probe and the conductive base metal increases, the eddy current field strength decreases because less of the probe's magnetic field can interact with the base metal. Thicknesses between 0.5 and 25 Âľm can be measured to an accuracy between 10% for lower values and 4% for higher values. Contributions to impedance changes due to conductivity variations should be phased out, unless it is known that conductivity variations are negligible, as normally found at higher frequencies.

Keywords: conductivity variations are negligible, as normally found at higher frequencies.

Charlie Chong/ Fion Zhang

Fairly precise measurements can be made with a standard eddy current flaw detector and a calibration specimen. The probe is nulled in air and the direction of the lift-off signal is established. The location of the signal is marked on the screen as the probe is placed on the calibration specimen in areas of decreasing coating thickness. When the probe is placed on the test surface, the position of the signal will move from the air null position to a point that can be correlated to the calibration markings.

Charlie Chong/ Fion Zhang

Specialized eddy current coating thickness detectors are also available and are often pocket-sized with the probe resembling a small pencil. They are usually operated by a small battery and provide a digital read-out in the appropriate units. Calibration adjustments, some of which are laid down by standards such as BS EN 2360 (1995) and ASTM B 244 and E 376, may be assisted by the use of an inbuilt microprocessor.

Charlie Chong/ Fion Zhang

Eddy current thickness gage

Charlie Chong/ Fion Zhang

7.0 Advanced Techniques 7.1 Scanning Eddy current data can be collected using automated scanning systems to improve the quality of the measurements and to construct images of scanned areas. The most common type of scanning is line scanning where an automated system is used to push the probe at a fixed speed. Line scan systems are often used when performing tube inspections or aircraft engine blade slot inspections, where scanning in one dimension is needed. The data is usually presented as a strip chart recording. The advantage of using a linear scanning system is that the probe is moved at a constant speed, so indications on the strip chart can be correlated to a position on the part being scanned. As with all automated scanning systems, operator variables, such as wobble of the probe, are reduced.

Keywords: ■ Line scan system ■ Strip chart recording ■ Wobble of probe Charlie Chong/ Fion Zhang

Two-dimensional scanning systems are used to scan a two-dimensional area.

This could be a scanning system that scans over a relatively flat area in a X-Y raster mode, or it could be a bolt hole inspection system that rotates the probe as it is moved into the hole. The data is typically displayed as a falsecolor plot of signal strength or phase angle shift as a function of position, just like an ultrasonic C-scan presentation. Shown below is a portable scanning system that is designed to work on the skins of aircraft fuselage and wing sections.

Charlie Chong/ Fion Zhang

Eddy current Line Scanner

Charlie Chong/ Fion Zhang

/

http://www.inspectech.ca/products/OnLine_Eddy_Current

Listed below are some automated scanning advantages: 1. minimizes changes in liftoff or fill factor resulting from probe wobble, uneven surfaces, and eccentricity of tubes caused by faulty manufacture or damage, 2. accurate indexing, 3. Repeatability, 4. high resolution mapping.

Charlie Chong/ Fion Zhang

7.2 Multiple Frequency Techniques Multiple frequency eddy current techniques simply involve collecting data at several different frequencies and then comparing the data or mixing the data in some way.

Why the need for multiple frequencies? - Some background information. The impedance of an eddy current probe may be affected by the following factors:  variations in operating frequency,  variations in electrical conductivity and the magnetic permeability of a object or structure, caused by structural changes such as grain structure, work hardening, heat treatment, etc.,  changes in liftoff or fill factor resulting from probe wobble, uneven surfaces, and eccentricity of tubes caused by faulty manufacture or damage,  the presence of surface defects such as cracks, and subsurface defects such as voids and nonmetallic inclusions,  dimensional changes, for example, thinning of tube walls due to corrosion, deposition of metal deposits or sludge, and the effects of denting,  the presence of supports, walls, and brackets,  the presence of discontinuities such as edges.

Charlie Chong/ Fion Zhang

Several of these factors are often present simultaneously. In the simple case where interest is confined to detecting defects or other abrupt changes in geometry, a differential probe can be used to eliminate unwanted factors, providing they vary in a gradual manner. For example, variations in electrical conductivity and tube thinning affect both coils of a differential probe simultaneously. However, if unwanted parameters that occur abruptly are affecting the measurements, they can sometimes be negated by mixing signals collected at several frequencies.

An example of where a multi-frequency eddy current inspection is used is in heat exchanger tube inspections. Heat exchanger assemblies are often a collection of tubing that have support brackets on the outside. When attempting to inspect the full wall thickness of the tubing, the signal from the mounting bracket is often troublesome. By collecting a signal at the frequency necessary to inspect the full thickness of the tube and subtracting a second signal collected at a lower frequency (which will be more sensitive to the bracket but less sensitive to features in the tubing), the effects of the bracket can be reduced.

Charlie Chong/ Fion Zhang

Discussion Subject: Discuss and reasoning on the following sentences; “When attempting to inspect the full wall thickness of the tubing, the signal from the mounting bracket is often troublesome. By collecting a signal at the frequency necessary to inspect the full thickness of the tube and subtracting a second signal collected at a lower frequency (which will be more sensitive to the bracket but less sensitive to features in the tubing), the effects of the bracket can be reduced.�

Charlie Chong/ Fion Zhang

Heat Exchanger Tube Bundles

Charlie Chong/ Fion Zhang

There are a number of commercially available multi-frequency eddy current instruments. Most operate at only two frequencies at a time but some units can collect data at up to four frequencies simultaneously. Multi-frequency measurements can also be made using an impedance analyzer but this equipment is generally not suitable for field measurements. A typical impedance analyzer system is shown below. The interest in pulsed eddy current instruments is largely due to their ability to, in essence, perform multifrequency measurements very quickly and easily.

Charlie Chong/ Fion Zhang

7.3 Swept Frequency Swept frequency eddy current techniques involve collecting eddy current data at a wide range of frequencies. This usually involves the use of a specialized piece of equipment such as an impedance analyzer, which can be configured to automatically make measurements over a range of frequencies. The swept-frequency technique can be implemented with commercial equipment but it is a difficult and time-consuming measurement. The advantage of a swept frequency measurement is that depth information can be obtained since eddy current depth of penetration varies as a function of frequency.

Swept frequency measurements are useful in applications such as measuring the thickness of conductive coatings on conductive base metal, differentiating between flaws in surface coatings and flaws in the base metal and differentiating between flaws in various layers of built-up structure. An example application would be the lap splice of a commercial aircraft. Swept frequency measurements would make it possible to tell if cracking was occurring on the outer skin, the inner skin or a double layer. Below is an example of the type of data that can be obtained from swept-frequency measurements.

Charlie Chong/ Fion Zhang

Data from swept-frequency measurements on two heats of material.

2.2 MHz

Charlie Chong/ Fion Zhang

It can be seen that in the etched condition, the material labeled "good" exhibits a much different signal response than the material labeled "bad." It can also be seen that a frequency of around 2.2 MHz provides the largest separation in the curves.

Therefore, this frequency should be used if a single frequency is used to sort the parts made from the two metals.

7.4 Pulsed Eddy Current Inspection Conventional eddy current inspection techniques use sinusoidal alternating electrical current of a particular frequency to excite the probe. The pulsed eddy current technique uses a step function voltage to excite the probe. The advantage of using a step function voltage is that it contains a continuum of frequencies. As a result, the electromagnetic response to several different frequencies can be measured with just a single step. Since the depth of penetration is dependent on the frequency of excitation, information from a range of depths can be obtained all at once. If measurements are made in the time domain (that is by looking at signal strength as a function of time), indications produced by flaws or other features near the inspection coil will be seen first and more distant features will be seen later in time.

Charlie Chong/ Fion Zhang

To improve the strength and ease interpretation of the signal, a reference signal is usually collected, to which all other signals are compared (just like nulling the probe in conventional eddy current inspection). Flaws, conductivity, and dimensional changes produce a change in the signal and a difference between the reference signal and the measurement signal that is displayed.

The distance of the flaw and other features relative to the probe will cause the signal to shift in time. Therefore, time gating techniques (like in ultrasonic inspection) can be used to gain information about the depth of a feature of interest.

Charlie Chong/ Fion Zhang

7.5 Background on Pulsed Eddy Current (adapted from Blitz, 1997) The use of pulsed eddy currents has long been considered for testing metals (Libby, 1971) and it has been applied to operations in specialized areas, such as in the nuclear energy industry, where testing equipment is often constructed to order. However, significant progress in this direction has taken place only recently after appropriate advances in technology (Krzwosz et al.

1985; Sather, 1981; Waidelich, 1981; Wittig and Thomas 1981), but at the time of writing, commercial equipment was not yet available. The method has the potential advantages of greater penetration, the ability to locate discontinuities from time-of-flight determinations, and a ready means of multifrequency measurement. At present, it does not generally have the precision of the conventional methods. The apparatus is somewhat complicated in design and not readily usable by the average operator who is experienced with the conventional eddy current equipment. Its main successes are in the testing of thin metal tubes and sheets, as well as metal cladding for measuring thickness and for the location and sizing of internal defects.

Charlie Chong/ Fion Zhang

When comparing the pulsed method with the conventional eddy current technique, the conventional technique must be regarded as a continuous wave method for which propagation takes place at a single frequency or, more correctly, over a very narrow frequency bandwidth. With pulse methods, the frequencies are excited over a wide band, the extent of which varies inversely with the pulse length; this allows multi-frequency operation. As found with ultrasonic testing, the total amount of energy dissipated within a given period of time is considerably less for pulsed waves than for continuous waves having the same intensity. For example, with pulses containing only one or two wavelengths and generated 1000 times per second, the energy produced is only about 0.002 of that for continuous waves having the same amplitude. Thus, considerably higher input voltages can be applied to the exciting coil for pulsed operation than for continuous wave operation.

Charlie Chong/ Fion Zhang

Pulsed waves can reasonably be expected to allow penetration of measurable currents through a metal sample to a depth of about 10 times the standard penetration depth, provided a suitable probe is used (i.e. a shielded ferrite-cored coil, see section 5.3). Therefore, penetration is possible through a 2 mm thick plate at frequencies of 1-3 kHz for non-ferromagnetic metals having corresponding electrical conductivities ranging from 60 down to 20MS/m. However, with an unmagnetized steel plate 2 mm thick, where sigma = 5 MS/m and Âľr = 100, the maximum frequency for throughpenetration is only 100 Hz. Pulsed eddy currents may be generated by a thyratron connected in series with the exciting coil through a capacitor (e.g. Waidelich, 1981). A direct voltage, on the order of 1200 V, slowly charges the capacitance and when the thyratron conducts there is an abrupt discharge through the coil in which freedamped harmonic oscillations occur. This is repeated periodically (i.e. at 1 kHz), so as to propagate the eddy current pulses through the metal.

Charlie Chong/ Fion Zhang

The currents are detected by a receiving probe located either adjacent to or on the opposite side of the metal sample from the exciting probe when access is possible. The range of propagated frequencies depends on the logarithmic decrement of the exciting circuit, and because the speed of the waves is a function of frequency, dispersion takes place and the pulse changes in shape as it progresses through the metal. As one would expect, the height of the peak and its time delay can be related to the thickness of the metal. Waidelich reports a maximum penetration of 90 mm for aluminum sheet and 10 mm for steel. For 6 mm thick sheets, the peak value of the received pulse voltage was 13 V for aluminum but only 20 mV for steel. Krzwosz et al. (1985) has shown how pulses that result from the presence of internal simulated defects produce broadening with an increase in depth.

Charlie Chong/ Fion Zhang

The frequency content of the pulses depends on their lengths, and in the extreme, contains continuous spectra ranging from less than 100 Hz to 1 or 2 kHz. By performing a Fourier transformation, the pulse obtained by the receiving probe can be displayed in the form of the variation of amplitude (or phase) with frequency. By sampling different delay times within a pulse, different parts of the spectrum can be evaluated (Sather, 1981). If both amplitude and phase are measured, two parameters (i.e. presence of defects, variations in tube thickness, and changes in fill-factor or liftoff) can be evaluated for each frequency selected in the same way as with the multifrequency method, although, at present, with a lower degree of precision.

Dodd et al.(1988) has designed and developed a pulsed magnetic saturation method for the eddy current testing of ferromagnetic metals. The DC field pulses are generated by passing a high-current pulse through an electromagnet so as to produce saturation in the metal object; the pulse length is made equal to the thickness of the object, thus ensuring complete eddy current penetration where feasible. The DC pulse, on the order of 1 ms duration, simultaneously produces an eddy current pulse, which is detected by a probe; the output of the probe is characteristic of the material being tested.

Charlie Chong/ Fion Zhang

This technique has the advantage of producing high magnetic peak powers with low average powers, thus keeping any heating of the test sample down to an acceptable level. It has been applied successfully to the internal testing of the walls of steel steam generator tubes, and tubes of diameter 10.9 mm and wall thickness 5 mm have been examined with peak powers of 500 kW. Small defects close to the external surfaces can be detected, and by taking advantage of the multi-frequency properties of pulsed eddy currents, their indications can be resolved from those that originate from other characteristics of the tubes.

More recent work on the use of pulsed eddy currents has been reported by Gibbs and Campbell (1991), who inspected cracks under fasteners in aluminum aircraft structures. Here, a Hall element was used as a receiver.

Radial position, approximate depth, and relative size of defects hidden under fastener heads could be determined in countersunk areas for defect depths of up to 7 mm for nonferrous fasteners and 14 mm for ferrous fasteners.

Charlie Chong/ Fion Zhang

Lebrun et al. (1975) reported the detection of deep cracks in ferromagnetic samples using an emission coil excited by square pulses of high intensity and employing highly sensitive magneto-resistive sensors to measure the resultant magnetic fields. Defects of 1 mm x 1 mm could be detected at a depth of 5 mm and 3 mm x 4 mm at a depth of 20 mm.

Charlie Chong/ Fion Zhang

7.6 Remote Field Sensing Eddy current testing for external defects in tubes where external access is not possible (e.g. buried pipelines), is conducted using internal probes. When testing thick-walled ferromagnetic metal pipes with conventional internal probes, very low frequencies (e.g. 30 Hz for a steel pipe 10 mm thick) are necessary to achieve the through-penetration of the eddy currents. This situation produces a very low sensitivity of flaw detection. The degree of penetration can, in principle, be increased by the application of a saturation magnetic field. However, because of the large volume of metal present, a large saturation unit carrying a heavy direct current may be required to produce an adequate saturating field.

Charlie Chong/ Fion Zhang

The difficulties encountered in the internal testing of ferromagnetic tubes can be greatly alleviated with the use of the remote field eddy current method. This method provides measurable through penetration of the walls at three times the maximum frequency possible with the conventional direct field method. This technique was introduced by Schmidt in 1958. Although it has been used by the petroleum industry for detecting corrosion in their installations since the early 1960s, it has only recently evoked general interest. This interest is largely because the method is highly sensitive to variations in wall thickness, but relatively insensitive to fill-factor changes. The method has the added advantage of allowing equal sensitivities of detection at both the inner and outer surfaces of a ferromagnetic tube. It cannot, however, differentiate between signals from these respective surfaces.

For more information on Remote Field Testing can be found in the Next separate section.

Charlie Chong/ Fion Zhang

8.0 Remote Field Testing (RFT) 8.1 Remote Field Testing or "RFT" is one of several electromagnetic testing methods commonly employed in the field of nondestructive testing. Other electromagnetic inspection methods include magnetic flux leakage MFLT, conventional eddy current ET and alternating current field measurement ACFM testing. Remote field testing is associated with eddy current testing and the term "Remote Field Eddy Current Testing" is often used when describing remote field testing. However, there are several major differences between eddy current testing and remote field testing which will be noted in this section.

Keywords: ■ MFLT ■ ET ■ ACFM ■ RFT

Charlie Chong/ Fion Zhang

Corroded Pipe

Charlie Chong/ Fion Zhang

RFT is primarily used to inspect ferromagnetic tubing since conventional eddy current techniques have difficulty inspecting the full thickness of the tube wall due to the strong skin effect in ferromagnetic materials. For example, using conventional eddy current bobbin probes to inspect a steel pipe 10 mm thick (such as what might be found in heat exchangers) would require frequencies around 30 Hz to achieve the adequate I.D. to O.D. penetration through the tube wall. The use of such a low frequency results in a very low sensitivity of flaw detection. The degree of penetration can, in principle, be increased by the use of partial saturation eddy current probes, magnetically biased probes, and pulsed saturation probes. However, because of the large volume of metal present as well as potential permeability variations within the product, these specialized eddy current probes are still limited in their inspection capabilities.

Charlie Chong/ Fion Zhang

The difficulties encountered in the testing of ferromagnetic tubes can be greatly alleviated with the use of the remote field testing method. The RFT method has the advantage of allowing nearly equal sensitivities of detection at both the inner and outer surfaces of a ferromagnetic tube. The method is highly sensitive to variations in wall thickness and tends to be less sensitive to fill-factor changes between the coil and tube. RFT can be used to inspect any conducting tubular product, but it is generally considered to be less sensitive than conventional eddy current techniques when inspecting non-ferromagnetic materials.

Keywords: Ferromagnetic material Fill factor

Charlie Chong/ Fion Zhang

8.2 RFT Theory of Operation A probe consisting of an exciter coil and one or more detectors is pulled through the tube. The exciter coil and the detector coil(s) are rigidly fixed at an axial distance of two tube diameters or more between them. The exciter coil is driven with a relatively low frequency sinusoidal current to produce a magnetic field.

Exciting coils

This changing magnetic field induces strong circumferential eddy currents which extend axially, as well as radially in the tube wall.

Charlie Chong/ Fion Zhang

Exciting coils

Concentration of eddy current

These eddy currents, in turn, produce their own magnetic field, which opposes the magnetic field from the exciter coil. Due to resistance in the tube wall and imperfect inductive coupling, the magnetic field from the eddy currents does not fully counterbalance the magnetic exciting field. However, since the eddy current field is more spread out than the exciter field, the magnetic field from the eddy currents extends farther along the tube axis. The interaction between the two fields is fairly complex but the simple fact is that the exciter field is dominant near the exciter coil and the eddy current field becomes dominant at some distance away from the exciter coil.

Charlie Chong/ Fion Zhang

They are placed at a distance where they are unaffected by the magnetic field from the exciter coil but can still adequately measure the field strength from the secondary magnetic field.

Receiving coils

Magnetic field due to eddy current

Charlie Chong/ Fion Zhang

Exciting coils

Attenuated magnetic field due to exciting coils

The receiving coils are positioned at a distance where the magnetic field from the eddy currents is dominant. In other words, they are placed at a distance where they are unaffected by the magnetic field from the exciter coil but can still adequately measure the field strength from the secondary magnetic field.

Electromagnetic induction occurs as the changing magnetic field cuts across the pick-up coil array. By monitoring the consistency of the voltage induced in the pick-up coils one can monitor changes in the test specimen. The strength of the magnetic field at this distance from the excitation coil is fairly weak but it is sensitive to changes in the pipe wall from the I.D. to the O.D.

Charlie Chong/ Fion Zhang

8.3 The RFT Zones

Exciting coils

Charlie Chong/ Fion Zhang

8.3.1 Direct Couple Zone The region where the magnetic field from the exciter coil is interacting with the tube wall to produce a concentrated field of eddy currents is called the direct field or direct coupled zone. This zone does not contribute a great deal of useful data to the RFT inspection due to problems with rather high noise levels due to the intense varying magnetic field from the excitation coil.

Charlie Chong/ Fion Zhang

8.3.2 Transition Zone The region just outside the direct couple zone is known as the transition zone.

In this zone there is a great deal of interaction between the magnet flux from the exciter coil and the flux induced by the eddy currents. As can be seen in the graph, the interaction of the two opposing fields is strongest near the ID of the tube and fairly subtle at the OD of the tube. The "resultant" field strength (the magnetic field at the sum of the two fields) in this region tends to change abruptly on the ID due to the interaction of the fields with differing directional characteristics of the two fields.

The receiver coil's signal phase, with respect to the exciter coil, as a function of distance between the two coils is also shown in the graph. When the two coils are directly coupled and there is no interference from a secondary field, their currents are in phase as seen at location zero. In the transition zone, it can be seen that the phase swiftly shifts, indicating the location where the magnetic field from the eddy currents becomes dominate and the start of the remote field.

Charlie Chong/ Fion Zhang

8.3.3 Remote Field Zone The remote field zone is the region in which direct coupling between the exciter coil and the receiver coil(s) is negligible. Coupling takes place indirectly through the generation of eddy currents and their resulting magnetic field. The remote field zone starts to occur at approximately two tube diameters away from the exciter coil. The amplitude of the field strength on the OD actually exceeds that of the ID after an axial distance of approximately 1.65 tube diameters. Therefore, RFT is sensitive to changes in material that occur at the outside diameter as well as the inside diameter of the tube.

Keywords: ď ŽThe remote field zone starts to occur at approximately two tube diameters away from the exciter coil. ď ŽThe amplitude of the field strength on the OD actually exceeds that of the ID after an axial distance of approximately 1.65 tube diameters.

Charlie Chong/ Fion Zhang

8.4 RFT Probes Probes for inspection of pipe and tubing are typically of the bobbin (ID) variety.

These probes use either a single or dual excitation coil to develop an electromagnetic field through the pipe or tube. The excitation coils are driven by alternating current. The sensing coil or coils are located a few tube diameters away in the remote field zone. Probes can be used in differential or absolute modes for detection of general discontinuities, pitting, and variations from the I.D. in ferromagnetic tubing. To insure maximum sensitivity, each probe is specifically designed for the inside diameter, composition, and the wall thickness of a particular tube.

Charlie Chong/ Fion Zhang

8.5 RFT Instrumentation Instruments used for RFT inspection are often dual use eddy current / RFT instruments employing multi-frequency technology. The excitation current from these instruments is passed on to the probe that contains an exciter coil, sometimes referred to as the driver coil. The receiving coil voltage is typically in the microvolt range, so an amplifier is required to boost the signal strength.

Certain systems will incorporate a probe excitation method known as multiplexing. This utilizes an extreme high speed switching method that excites the probe at more than one frequency in sequence. Another method of coil excitation that may be used is simultaneous injection. In this coil stimulation technique, the exciter coil is excited with multiple frequencies at the same time while incorporating filter schemes that subtract aspects of the acquired data. The instrument monitors the pickup coils and passes the data to the display section of the instrument. Some systems are capable of recording the data to some type of storage device for later review.

Charlie Chong/ Fion Zhang

RFT Instrumentation

Charlie Chong/ Fion Zhang

8.6 RFT Signal Interpretation The signals obtained with RFT are very similar to those obtained with conventional eddy current testing. When all the proper conditions are met, changes in the phase of the receiver signal with respect to the phase of the exciter voltage are directly proportional to the sum of the wall thickness within the inspection area. Localized changes in wall thickness result in phase and amplitude changes. These changes can be indicative of defects such as cracks, corrosion pitting or corrosion/erosion thinning.

Charlie Chong/ Fion Zhang

8.7 RFT Reference Standards Reference standards for the RFT inspection of tubular products come in many variations. In order to produce reliable and consistent test results, the material used for manufacturing calibration standards must closely match the physical and chemical properties of the inspection specimen. Some of the important properties that must be considered include conductivity, permeability and alloy content. In addition, tube dimensions including I.D., O.D. and wall thickness must also be controlled.

The type of damage mechanisms that are expected to be encountered must also be carfully considered when developing or selecting a reference standard. In order to get accurate quantitative data, artificial discontinuity conditions are typically machined into the standards that will closely match those conditions that may be found in the tubing bundle.

Charlie Chong/ Fion Zhang

RFT Reference Standards

Charlie Chong/ Fion Zhang

Charlie Chong/ Fion Zhang

Good Luck!

Charlie Chong/ Fion Zhang

Good Luck!

Charlie Chong/ Fion Zhang