1. Eddy CurrentTesting-An IntroductionPart-2 of 22014-NovemberMy ASNT Level IIIPre-ExamPreparatory Self StudyNotesCharlie Chong/ Fion Zhang

2. Fion Zhang2014/Novemberhttp://meilishouxihu.blog.163.com/Charlie Chong/ Fion ZhangShanghai 3. YouTube., Charlie Chong/ Fion Zhang 4. Movie Time http://www.giniko.com/watch.php?id=216Charlie Chong/ Fion Zhang 5. Introduction to Eddy Current Theory https://www.youtube.com/watch?v=djFvnFy3rJcCharlie Chong/ Fion Zhang 6. Eddy Current Math https://www.youtube.com/watch?v=V-IW6cFIt9ECharlie Chong/ Fion Zhang 7. Standard depth penetration https://www.youtube.com/watch?v=G2Yh7tZpKboCharlie Chong/ Fion Zhang 8. Eddy Current Curve https://www.youtube.com/watch?v=Bojm5F_4ay4Charlie Chong/ Fion Zhang 9. Introduction to Eddy Current Machine https://www.youtube.com/watch?v=S34yt8-zgnsCharlie Chong/ Fion Zhang 10. Conductivity Measurement - https://www.youtube.com/watch?v=pvTUomSYEt8Charlie Chong/ Fion Zhang 11. Crack Detections - https://www.youtube.com/watch?v=1YUSn___VxQCharlie Chong/ Fion Zhang 12. 4.0 Probes (Coils)4.1 Impedance MatchingEddy current testing requires us to determine the components of theimpedance of the detecting coil or the potential difference across it. Mostapplications require the determination only of changes in impedance, whichcan be measured with a high degree of sensitivity using an AC bridge. Theprinciples of operation of the most commonly used eddy current instrumentsare based on Maxwell's inductance bridge, in which the components of theimpedance of the detecting coil, commonly called a probe, are compared withknown variable impedances connected in series and forming the balancingarm of the bridge. Refer back to Bridges.Charlie Chong/ Fion Zhang 13. Maxwell inductance bridgeCharlie Chong/ Fion Zhang 14. The input to the bridge is an AC oscillator, often variable in both frequencyand amplitude. The detector arm takes the form of either a meter or a storagecathode-ray oscilloscope, a phase-sensitive detector, a rectifier to provide asteady indication, and usually an attenuator to confine the output indicationwithin a convenient range. Storage facilities are necessary in the oscilloscopein order to retain the signal from the detector for reference during scanningwith the probe.The highest sensitivity of detection is achieved by properly matching theimpedance of the probe to the impedance of the measuring instrument.Thus, with a bridge circuit that is initially balanced, a subsequent but usuallysmall variation in the impedance of the probe upsets the balance, and apotential difference appears across the detector arm of the bridge.Question:Is it the potential difference appears in the CRT?Charlie Chong/ Fion Zhang 15. Although the Maxwell inductance bridge forms the basis of most eddy currentinstruments, there are several reasons why it cannot be used in its simplestform (i.e. Hague, 1934), including the creation of stray capacitances, such asthose formed by the leads and leakages to earth. These unwantedimpedances can be eliminated by earthing devices and the addition ofsuitable impedances to produce one or more wide-band frequency (i.e. low Q)resonance circuits. Instruments having a wide frequency range (i.e. from 1kHz 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 importantconsideration in achieving proper matching and, as a result, it may benecessary to change the probe when switching from one frequency band toanother.Charlie Chong/ Fion Zhang 16. Q Values for frequency (applicable to mechanical sonic or electrical)Charlie Chong/ Fion ZhangThe word Q does nothave any impact on thequality elements in aspecific testing method(UT, ET etc.), it is simply aterm used to describesthe bandwidth of afrequency in questioned.http://community.calrec.com/q-in-60-seconds/ 17. Q Values for frequency (applicable to mechanical sonic or electrical)Charlie Chong/ Fion Zhanghttp://www.eeweb.com/blog/rodney_green_2/a-history-of-hf-radio-receivers-part-2 18. 4.2 Coil (Probe) DesignThe most important feature in eddy current testing is the way in which theeddy currents are induced and detected in the material under test. Thisdepends 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 animportant effect on the probe, but the coil requires the most designconsideration.A coil consists of a length of wire wound in a helical manner around thelength of a former. The main purpose of the former is to provide a sufficientamount of rigidity in the coil to prevent distortion. Formers used for coils withdiameters greater than a few millimeters (i.e. encircling and pancake coils),generally take the form of tubes or rings made from dielectricmaterials. Small-diameter coils are usually wound directly onto a solid former.Charlie Chong/ Fion Zhang 19. The region inside the former is called the core, which can consist of either asolid 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 woundaround a ferrite core which concentrates the coil's magnetic field into asmaller area. These coils are referred to as "loaded" coils.The wire used in an eddy current probe is typically made from copper or othernonferrous metal to avoid magnetic hysteresis effects. The winding usuallyhas more than one layer so as to increase the value of inductance for a givenlength of coil. The higher the inductance (L) of a coil, at a givenfrequency, the greater the sensitivity of eddy current testing.Keywords:Air coreLoaded coreMagnetic hysteresis effectsCharlie Chong/ Fion Zhang 20. Magnetic hysteresis effectsCharlie Chong/ Fion Zhang 21. It is essential that the current through the coil is as low as possible. Too higha current may produce: a rise in temperature, hence an expansion of the coil, which increases thevalue of L. magnetic hysteresis, which is small but detectable when a ferrite core isused.The simplest type of probe is the single-coil probe, which is in widespreaduse. The following applet may be used to calculate the effect of the inner andouter diameters, length, number of turns and wire diameter of a simple probedesign on the probe's self inductance. Dimensional units are in millimeters.Charlie Chong/ Fion Zhang 22. Eddy current inspectionCharlie Chong/ Fion Zhanghttps://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/ProbesCoilDesign/Popups/applet1/applet1.htm 23. 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.r is the relative magnetic permeability of the core.o 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 internalradii.Charlie Chong/ Fion Zhang 24. 4.3 Probes - Mode of OperationEddy current probes are available in a large variety of shapes and sizes. Infact, one of the major advantages of eddy current inspection is that probescan be custom designed for a wide variety of applications. Eddy currentprobes are classified by the configuration and mode of operation of the testcoils. The configuration of the probe generally refers to the way the coil orcoils are packaged to best "couple" to the test area of interest. An example ofdifferent configurations of probes would be bobbin probes, which are insertedinto a piece of pipe to inspect from the inside out, versus encircling probes, inwhich the coil or coils encircle the pipe to inspect from the outside in. Themode of operation refers to the way the coil or coils are wired and interfacewith 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 Zhanghttp://www.vegastel.eu/index.php/en/sukuriniu-sroviu-metodas-en/eddy-current-probes/693-eddy-current-probe-selection-information 25. 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 26. 4.3.1 Absolute ProbesAbsolute probes generally have a single test coil that is used to generate theeddy currents and sense changes in the eddy current field. As discussed inthe physics section, AC is passed through the coil and this sets up anexpanding and collapsing magnetic field in and around the coil. When theprobe is positioned next to a conductive material, the changing magnetic fieldgenerates eddy currents within the material.The generation of the eddy currents take energy from the coil and thisappears as an increase in the electrical resistance of the coil. The eddycurrents generate their own magnetic field that opposes the magneticfield of the coil and this changes the inductive reactance of the coil.By measuring the absolute change in impedance of the test coil, muchinformation can be gained about the test material.Charlie Chong/ Fion Zhang 27. Absolute coils can be used for flaw detection, conductivity measurements,liftoff measurements and thickness measurements. They are widely used dueto their versatility. Since absolute probes are sensitive to things such asconductivity, permeability liftoff and temperature, steps must be taken tominimize these variables when they are not important to the inspection beingperformed. It is very common for commercially available absolute probes tohave a fixed "air loaded" reference coil that compensates for ambienttemperature variations.Charlie Chong/ Fion Zhang 28. Absolute Probes (Single-Coil Probes)The earliest form of eddy current instruments operated with a single-coilprobe that was wound to a specific value frequency. Many newer models ofeddy current instruments have kept this circuitry as a popular option for userswhile also incorporating more sophisticated functions. When these probes areused, a balance coil is also required which may be set from within the eddycurrent instrument or is commonly found within the probe housing, the cableconnector or in a separate adapter. A problem can arise when the probeinductance value is not close enough to the value of the balance coil causingthe instrument not to balance correctly. The result is poor performance (noisyor insensitive) or no response at all (signal saturation).Charlie Chong/ Fion ZhangBalancing coil 29. The Principle:The generation of the eddy currents take energy from the coil and thisappears as an increase in the electrical resistance of the coil. The eddycurrents generate their own magnetic field that opposes the magnetic field ofthe coil and this changes the inductive reactance of the coil.Variations:The change in inductive reactance could be increasing or decreasingdepending on the magnetic permeability of material.Commons:Irrespective of magnetic permeability, the resistance always increase.Charlie Chong/ Fion Zhang 30. 4.3.2 Differential ProbesDifferential probes have two active coils usually wound in opposition,although they could be wound in addition with similar results. When the twocoils are over a flaw-free area of test sample, there is no differential signaldeveloped 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 verysensitive to defects yet relatively insensitive to slowly varying properties suchas gradual dimensional or temperature variations. Probe wobble signals () are also reduced with this probe type. There are also disadvantages tousing 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, onlythe leading and trailing edges will be detected due to signal cancellation whenboth coils sense the flaw equally.Charlie Chong/ Fion Zhang 31. Differential Probes with two coils wound in different directionsCharlie Chong/ Fion Zhang 32. Differential (Bridge Type) ProbesIn this configuration the probe coils are located in an electrical "bridge" (seefig. below). The instrument balances the bridge and any change in balance isdisplayed as a signal. In this arrangement, the same coil produces the eddycurrents and detects the impedance changes caused by the defects (or anyother variables). Almost all instruments are able to operate with this type ofcoil arrangement.Charlie Chong/ Fion Zhang 33. 4.3.3 Reflection ProbesReflection probes have two coils similar to a differential probe, but one coil isused to excite the eddy currents and the other is used to sense changes inthe test material. Probes of this arrangement are often referred to asdriver/pickup probes. The advantage of reflection probes is that the driver andpickup coils can be separately optimized for their intended purpose. Thedriver coil can be made so as to produce a strong and uniform flux field in thevicinity of the pickup coil, while the pickup coil can be made very small so thatit will be sensitive to very small defects.Charlie Chong/ Fion Zhang 34. Reflection ProbesCharlie Chong/ Fion Zhang 35. Reflection Type ProbeThese probes are also known as send-receive or driver-pickup. In thisconfiguration, the eddy currents are produced by a coil connected to theinstrument's oscillator (driver). The signals received back in the probe aredetected by separate coils called pickups (see Fig. 3 and Fig. 4). All newimpedance plane instruments and also many older models are able tooperate in both differential (bridge) and reflection modes.Charlie Chong/ Fion Zhang 36. 4.3.4 Hybrid ProbesAn 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. Itoperates in the reflection mode but additionally, its sensing coils operate inthe 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 togenerate eddy currents in the material but then uses a different type of sensorto detect changes on the surface and within the test material. An example of ahybrid probe is one that uses a Hall effect sensor to detect changes in themagnetic flux leaking from the test surface. Hybrid probes are usuallyspecially designed for a specific inspection application.Charlie Chong/ Fion Zhang 37. 4.3.5 Differential (Bridge) or Reflection?This is a common question asked by those involved in trying to select the bestprobe for an inspection. The answer is "It depends." Let us consider bothsystems.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 instrumentsare 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 thepickup coils. This means that they will give a wider frequency range. As longas the driver produces eddy currents, the pickup will detect them and somesignal will be displayed. This may not provide good information at certainfrequencies, but the probe is still working!Charlie Chong/ Fion Zhang 38. Bridge type probes used to give a limited frequency span in the olderinstruments, as these had to balance an electrical bridge using its other arms(X and R controls). In modern instruments, the bridge is normally formed withfixed precision resistors, or a fixed transformer inside it. The signals detectedin this manner are electronically processed without any "mechanical"adjustments, and this means a greater ability to balance over a widerfrequency range.Drift: Probe drift is mostly caused by temperature change in the coils. Thismay be caused by varyingambient temperature, or the heat produced by the oscillator current, or both.There are design parameters that can be optimized to reduce drift, such aswire diameter and ferrite selection, but reflection probes are normally a goodchoice to avoid this problem even more.Charlie Chong/ Fion Zhang 39. 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 muchsmaller 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 orreflection. Keep in mind that a reflection probe is almost invariably moredifficult to manufacture and therefore more expensive.Charlie Chong/ Fion Zhang 40. 4.3.6 Absolute, Differential (Bridge) and Reflection ProbesThis 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 afigure 8 type signal. This is caused by the two coils sensing the defect insequence. When both sensing coils are on the probe surface, theycompensate for lift-off and as a result no line is visible (see Fig. 5).Charlie Chong/ Fion ZhangFig. 5 41. 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 nearhorizontal lift-off line. Others have called a probe "differential" simply whenthe coils were connected differentially such as in a bridge circuit. The problemwith this definition is that probes can be connected differentially in a reflectionsystem as well as when using two pickups (such as most scanner-driven bolthole probes). In this case, the two pickup coils are positioned close to oneanother and contained within a driver coil (see Fig. 6).Fig. 6Charlie Chong/ Fion ZhangThe best way out of this confusion isoften to specify the probe asabsolute, bridge, reflection, bridgedifferential or reflection differential asneeded. It makes more sense toqualify the description according tothe displayed signal, since this iswhat really matters and not manypeople are concerned as to how thecoils are connected internally. 42. 4.3.7 Shielded and Unshielded ProbesProbes 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 physicalsize of the probe. A shield can be made of various materials, but the mostcommon are: ferrite (like a ceramic made of iron oxides), Mu-metal, and mildsteel. Ferrite make the best shielding because they provide an easy path forthe magnetic field but has poor conductivity. This means that there is littleeddy current loss in the shield itself. Mild steel has more losses but is widelyused for spot probes and ring probes due to its ease of machining whenferrite is not available in certain sizes or shapes. Mu-metal is sometimes forpencil probes as it is available in thin sheet; however, it is less effective thanferrite.Note: Mu-metal () is a nickel-iron alloy, composed ofapproximately 77% nickel, 16% iron, 5% copper and 2% chromium ormolybdenum, that is notable for its high magnetic permeability.Charlie Chong/ Fion Zhanghttp://en.wikipedia.org/wiki/Mu-metal 43. Shielding has several advantages: first, it allows the probe to be used neargeometry changes, such as edges, without giving false indications; next, itallows the probe to touch ferrous fastener heads with minimal interference;last, it allows the detection of smaller defects due to the stronger magneticfield concentrated in a smaller area.On the other hand, unshielded probes allow somewhat deeper penetrationdue 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. Thereason for this is that the meter response is too slow to allow the signal from ashielded probe to be displayed at normal scanning speeds due to the smallersensitive area.Charlie Chong/ Fion Zhang 44. 4.3.8 AdaptersTo connect a probe with a connector different from the type used on theinstrument, it is necessary to use an adapter. An adapter consists of twodifferent connectors joined and wired to match the inputs and outputs asnecessary. It is normally housed in a short body that can be positioned at theinstrument'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 onthe instrument's wiring, it may be possible to have a single adapter for bothbridge and reflection probes. In other cases, it is necessary to have twoseparate adapters or use a switchable type.Charlie Chong/ Fion Zhanghttp://www.vegastel.eu/index.php/en/sukuriniu-sroviu-metodas-en/eddy-current-probes/693-eddy-current-probe-selection-information 45. 4.4 Probes - ConfigurationsAs mentioned on the previous page, eddy current probes are classified by theconfiguration and mode of operation of the test coils. The configuration of theprobe 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 ofprobes based on their configuration include surface probes, bolt hole probes,inside diameter (ID) probes, and outside diameter (OD) probes.Charlie Chong/ Fion Zhang 46. Eddy current probesCharlie Chong/ Fion Zhang 47. Eddy current inspection displayCharlie Chong/ Fion Zhanghttp://www.ibgndt.com/eddyliner-s-eddy-current-testers-hardness-case-depth-structure.php 48. Eddy current inspection systemhttp://idea-ndt.en.alibaba.com/product/488266329-212374104/Automatic_ERW_pipes_eddy_current_and_ultrasonic_testing_systems_and_equipments.htmlCharlie Chong/ Fion Zhang 49. 4.4.1 Surface ProbesSurface probes are usually designed to be handheld and are intended to beused in contact with the test surface. Surface probes generally consist of acoil of very fine wire encased in a protective housing. The size of the coil andshape 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 thetest surface. This coil configuration is sometimes referred to as a pancake coiland is good for detecting surface discontinuities that are orientedperpendicular to the test surface. Discontinuities, such as delaminations, thatare in a parallel plane to the test surface will likely go undetected with this coilconfiguration.Charlie Chong/ Fion Zhang 50. Wide surface coils are used when scanning large areas for relatively largedefects. They sample a relatively large area and allow for deeper penetration.Since they do sample a large area, they are often used for conductivity teststo get more of a bulk material measurement. However, their large samplingarea limits their ability to detect small discontinuities.Pencil probes have a small surface coil that is encased in a long slenderhousing to permit inspection in restricted spaces. They are available with astraight shaft or with a bent shaft, which facilitates easier handling and use inapplications such as the inspection of small diameter bores. Pencil probesare prone to wobble due to their small base and sleeves are sometimes usedto provide a wider base.Keywords:Wide surface- deeper penetrationNarrow probe detect smaller discontinuitiesNarrow probe prone to wobbleCharlie Chong/ Fion Zhang 51. Surface ProbeCharlie Chong/ Fion Zhanghttp://advantech.my/Products%20-%20ET.htm 52. Surface ProbeCharlie Chong/ Fion Zhang 53. Surface ProbeCharlie Chong/ Fion Zhang 54. Surface ProbeCharlie Chong/ Fion Zhang 55. 4.4.2 Bolt Hole ProbesBolt hole probes are a special type of surface probe that is designed to beused with a bolt hole scanner. They have a surface coil that is mounted insidea housing that matches the diameter of the hole being inspected. The probeis inserted in the hole and the scanner rotates the probe within the hole.Charlie Chong/ Fion Zhang 56. Bolt Hole ProbesCharlie Chong/ Fion Zhanghttp://www.phtool.com/pages/eddy.asp 57. Bolt Hole ProbesCharlie Chong/ Fion Zhanghttp://www.olympus-ims.com/en/applications/eddy-current-probes-guide/ 58. Birring NDT Series, Eddy Current Testing # 5,Inspection of Fastener Holes using a Rotary Probe https://www.youtube.com/watch?v=X4yqOLUYrBsCharlie Chong/ Fion Zhang 59. 4.4.3 ID or Bobbin ProbesID probes, which are also referred to as Bobbin probes or feed-throughprobes, are inserted into hollow products, such as pipes, to inspect from theinside out. The ID probes have a housing that keep the probe centered in theproduct and the coil(s) orientation somewhat constant relative to the testsurface. The coils are most commonly wound around the circumference of theprobe so that the probe inspects an area around the entire circumference ofthe test object at one time.Charlie Chong/ Fion Zhang 60. Configuration: Bobbin ProbeCharlie Chong/ Fion Zhang 61. Configuration:Bobbin ProbeCharlie Chong/ Fion Zhang 62. 4.4.4 OD or Encircling CoilsOD probes are often called encircling coils. They are similar to ID probesexcept that the coil(s) encircle the material to inspect from the outside in. ODprobes are commonly used to inspect solid products, such as bars.Charlie Chong/ Fion Zhang 63. Configuration: Encircling probesCharlie Chong/ Fion Zhang 64. Configuration: Encircling probesCharlie Chong/ Fion Zhang 65. Configuration: Encircling probesCharlie Chong/ Fion Zhang 66. Eddy current inspection: CalibrationCharlie Chong/ Fion Zhang 67. Eddy current inspection: CalibrationCharlie Chong/ Fion Zhang 68. Eddy current inspectionCharlie Chong/ Fion Zhang 69. 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 70. 4.5 Probes - Shielding & Loading4.5.1 Why Shielding?One of the challenges of performing an eddy current inspection is gettingsufficient eddy current field strength in the region of interest within thematerial. Another challenge is keeping the field away from non-relevantfeatures of the test component. The impedance change caused by non-relevantfeatures can complicate the interpretation of the signal. Probeshielding and loading are sometimes used to limit the spread and concentratethe magnetic field of the coil. Of course, if the magnetic field is concentratednear the coil, the eddy currents will also be concentrated in this area.Charlie Chong/ Fion ZhangKeywords: Non-relevant indication Probe shielding Field is concentrated Eddy current also is concentrated 71. 4.5.2 Probe ShieldingProbe shielding is used to prevent or reduce the interaction of the probe'smagnetic field with nonrelevant features in close proximity of the probe.Shielding could be used to reduce edge effects when testing neardimensional transitions such as a step or an edge. Shielding could also beused to reduce the effects of conductive or magnetic fasteners in the region oftesting.Eddy current probes are most often shielded using magnetic shielding oreddy current shielding. Magnetically shielded probes have their coilsurrounded by a ring of ferrite or other material with high permeability and lowconductivity. The ferrite creates an area of low magnetic reluctance and theprobe's magnetic field is concentrated in this area rather than spreadingbeyond the shielding. This concentrates the magnetic field into a tighter areaaround the coil.Charlie Chong/ Fion Zhang 72. Eddy current shielding uses a ring of highly conductive but nonmagneticmaterial, usually copper, to surround the coil. The portion of the coil'smagnetic field that cuts across the shielding will generate eddy currents in theshielding material rather than in the non-relevant features outside of theshielded 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 shieldingmaterial.Charlie Chong/ Fion ZhangQuestion:The portion of the coil's magnetic field that cutsacross the shielding will generate eddy currents inthe shielding material rather than in the non-relevantfeatures outside of the shielded area.How does the above provide shielding effect? 73. 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 orother material with high magnetic permeability and low conductivity. Eddy current shielding uses a ring of highly conductive but nonmagneticmaterial, usually copper, to surround the coil.Charlie Chong/ Fion Zhang 74. DiscussionTopic 1: What is plus point for not shielding?Topic 2: discuss this sentenceThe higher the frequency of the current used to drive the probe, the moreeffective the shielding will be due to the skin effect in the shielding material.Charlie Chong/ Fion Zhang 75. 4.5.3 Probe Loading with Ferrite CoresSometimes coils are wound around a ferrite core. Since ferrite isferromagnetic, the magnetic flux produced by the coil prefers to travel throughthe ferrite as opposed to the air. Therefore, the ferrite core concentrates themagnetic field near the center of the probe. This, in turn, concentrates theeddy currents near the center of the probe. Probes with ferrite cores tend tobe more sensitive than air core probes and less affected by probe wobble andlift-off.Charlie Chong/ Fion Zhang 76. 5.0 Procedure Issues5.1 Reference StandardsIn eddy current testing, the use of reference standards in setting up theequipment is particularly important since signals are affected by manydifferent variables and slight changes in equipment setup can drastically alterthe appearance of a signal. As with most other NDT methods, the most usefulinformation is obtained when comparing the results from an unknown objectto results from a similar object with well characterized features and defects. Inalmost all cases, eddy current inspection procedures require the equipment tobe configured using reference standards.For crack detection, corrosion thinning and other material damage, referencestandards are used to setup the equipment to produce a recognizable signalor set of signals from a defect or set of defects. In many cases, theappearance of a test signal can be related to the appearance of a signal froma known defect on the reference standard to estimate the size of a defect inthe test component. Signals that vary significantly from the responsesproduced by the reference standard must be further investigated to thedetermine the source of the signal.Charlie Chong/ Fion Zhang 77. The reference standardsCharlie Chong/ Fion ZhangThe referenceStandards 78. The reference standardsCharlie Chong/ Fion Zhang 79. The reference standardsCharlie Chong/ Fion Zhang 80. The reference standardsCharlie Chong/ Fion Zhang 81. The reference standardsCharlie Chong/ Fion Zhang 82. The reference standardsCharlie Chong/ Fion Zhang 83. The reference standardsCharlie Chong/ Fion Zhang 84. The reference standardsCharlie Chong/ Fion Zhang 85. The reference standardsCharlie Chong/ Fion Zhang 86. The reference standardsCharlie Chong/ Fion Zhang 87. The reference standardsCharlie Chong/ Fion Zhang 88. The reference standardsCharlie Chong/ Fion Zhanghttp://radio.rphf.spbstu.ru/a263/eddy.htm 89. The reference standard should be of the same material as the test article. Ifthis is not possible or practical, it should be of material that has the sameelectrical conductivity and magnetic permeability. Component features(material thickness, geometry, etc.) should be the same in the referencestandard as those in the test region of interest. If the reference standard is thetype with intentional defects, these defects should be as representative ofactual defects in the test component as possible. The closer the referencestandard is to the actual test component, the better. However, since cracksand corrosion damage are often difficult and costly to produce, artificialdefects are commonly used. Narrow notches produced with electrondischarge machining (EDM) and saw cuts are commonly used to representcracks, and drilled holes are often used to simulate corrosion pitting.Charlie Chong/ Fion Zhang 90. 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 91. 5.2 Signal Filtering5.2.1 Signal filtering is often used in eddy current testing to eliminateunwanted frequencies from the receiver signal. While the correct filter settingscan significantly improve the visibility of a defect signal, incorrect settings candistort 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 92. Filtering is applied to the received signal and, therefore, is not directly relatedto the probe drive frequency. This is most easily understood when picturing atime versus signal amplitude display. With this display mode, it is easy to seethat the signal shape is dependent on the time or duration that the probe coilis sensing something. For example, if a surface probe is placed on thesurface of conductor and rocked back and forth, it will produce a wave likesignal. When the probe is rocked fast, the signal will have a higher frequencythan when the probe is rocked slowly back and forth.The signal does not need a wavelike appearance to have frequency contentand most eddy current signals will be composed of a large number offrequencies. Consider a probe that senses a notch for 1/60th of a second. Ina 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 thissame 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 93. Signal filtering Unfiltered signal with lowCharlie Chong/ Fion Zhangfrequency variation andhigh frequency noiseHigh pass filter employedto remove low frequencyvariationLow pass filter employed toremove high frequency noiseFilter signalHigh frequency noiseLow frequency variation 94. 5.2.2 Filters EffectsThe two standard filters found in most impedance plane display instrumentsare the High Pass Filter (HPF) and Low Pass Filter (LPF). Someinstruments also have aBand Pass Filter (BPF), which is a combination highand 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 asignificant period of time.The LPF allows low frequency to pass and filters out the high frequency. Inother words, all portions of the signal that change rapidly (have a high slope)are filtered, such as electronic noise.Keywords: HPF LPF BPFCharlie Chong/ Fion Zhang 95. In the image above, the gradual (low frequency) changes were first filteredout with a HPF and then high frequency electronic noise was filtered with aLPF to leave a clearly visible flaw indication. It should also be noted that sinceflaw indication signals are comprised of multiple frequencies, both filters havea tendency to reduce the indication signal strength. Additionally, scan speedmust be controlled when using filters. Scan over a flaw too slow and the HPFmight filter out the flaw indication. Scan over the flaw too fast and the LPFmight eliminate the flaw indication.Keywords: 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.Charlie Chong/ Fion Zhang 96. Signal Reduction Unfiltered signal with lowCharlie Chong/ Fion Zhangfrequency variation andhigh frequency noiseHigh pass filter employedto remove low frequencyvariationLow pass filter employed toremove high frequency noiseOriginal SignalLow frequency variationFiltered Signal 97. 5.2.3 Filter SettingsIf the spectrum of the signal frequency and the signal amplitude or attenuationare plotted, the filter responses can be illustrated in graphical form. Theimage 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 allowthose frequencies in the blue area to pass. Therefore, it can be seen that withthese settings there are no frequencies that pass (i.e. the frequencies passedby the LPF are filtered out by the HPF and visa versa).Charlie Chong/ Fion Zhang 98. Eddy current inspectionCharlie Chong/ Fion ZhangRejected by LPF 99. Rejected by HPFCharlie Chong/ Fion Zhang 100. 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 to10Hz. The area shown in gray is where the two frequencies overlap and thesignal is passed. A signal of 30Hz will get through at full amplitude, while asignal of 15Hz will be attenuated by approximately 50%. All frequenciesabove or below the gray area (the pass band) will be rejected by one of thetwo filters.Overlap area-Accepted signalCharlie Chong/ Fion Zhang 101. 5.2.4 Use of FiltersThe main function of the LPF is to remove high frequency interference noise.This noise can come from a variety of sources including the instrumentationand/or the probe itself. The noise appears as an unstable dot that producesjagged lines on the display as seen in the signal from a surface notch shownin the left image below. Lowering the LPF frequency will remove more of thehigher frequencies from the signal and produce a cleaner signal as shown inthe center image below. When using a LPF, it should be set to the highestfrequency that produces a usable signal. To reduce noise in large surface orring probes, it may be necessary to use a very low LPF setting (down to10Hz). The lower the LPF setting, the slower the scanning speed must be andthe more closely it must be controlled. The image on the right below shows asignal that has been clipped due to using a scan speed too fast for theselected HPF setting.Charlie Chong/ Fion Zhang 102. Signal FilteringUnfiltered signals selected HPF settingCharlie Chong/ Fion Zhangsignal that has been clipped due tousing a scan speed too fast for theLowering the LPF frequency will removemore of the higher frequencies from thesignal and produce a cleaner signal asshown in the center image 103. The HPF is used to eliminate low frequencies which are produced by slowchanges, such as conductivity shift within a material, varying distance to anedge while scanning parallel to it, or out-of-round holes in fastener holeinspection. The HPF is useful when performing automated or semiautomaticscans 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 holesusing a rotating scanner. As the scanner rotates at a constant RPM, the HPFcan be adjusted to achieve the desired effect.Charlie Chong/ Fion Zhang 104. Use of the HPF when scanning manually is not recommended, askeeping a constant scanning speed is difficult, and the signal deforms andamplitude decreases. The size of a signal decreases as the scan speeddecreases and a flaw indication can be eliminated completely if the scan isnot done with sufficient speed. In the images below, it can be seen that atypical response from a surface notch in aluminum without HPF (left image)looks considerably different when the HPF is activated (right image). With theHPF, looping signals with a positive and similar negative deflection areproduced on the impedance plane.Charlie Chong/ Fion Zhang 105. The use of a minimal HPF setting (1 or 2 Hz) may be used when manuallyscanning, provided the operator can largely control the scan speed andbecomes familiar with the indication signal changes as scan speed is variedslightly. An good example of such an application would be the manual scan ofthe radius of a wheel that is rotated by hand, but the speed of rotation can bekept relatively constant.Charlie Chong/ Fion Zhang 106. 6.0 Applications6.1 Surface Breaking CracksEddy current equipment can be used for a variety of applications such as thedetection of cracks (discontinuities), measurement of metal thickness,detection of metal thinning due to corrosion and erosion, determination ofcoating thickness, and the measurement of electrical conductivity andmagnetic permeability. Eddy current inspection is an excellent method fordetecting surface and near surface defects when the probable defect locationand orientation is well known.Charlie Chong/ Fion Zhang 107. Surface breaking cracksCharlie Chong/ Fion Zhang 108. Surface breaking cracksCharlie Chong/ Fion Zhang 109. Surface breaking cracksCharlie Chong/ Fion Zhang 110. Surface breaking cracksCharlie Chong/ Fion Zhang 111. Surface breaking cracksCharlie Chong/ Fion Zhang 112. Surface breaking cracksCharlie Chong/ Fion Zhang 113. Surface breaking cracksCharlie Chong/ Fion Zhanghttp://www.assda.asn.au/component/rsblog/category/13?start=20 114. Defects such as cracks are detectedwhen they disrupt the path of eddycurrents and weaken their strength.The images to the right show an eddycurrent surface probe on the surface ofa conductive component. The strengthof the eddy currents under the coil ofthe probe ins indicated by color. In thelower image, there is a flaw under theright side of the coil and it can be seethat the eddy currents are weaker inthis area.Charlie Chong/ Fion Zhangweaker 115. Of course, factors such as the type of material, surface finish and condition ofthe material, the design of the probe, and many other factors can affect thesensitivity of the inspection. Successful detection of surface breaking andnear 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 thepart and the coil must produce eddy currents that will be disrupted by theflaw.3. Selection of a reasonable probe drive frequency. For surface flaws, thefrequency should be as high as possible for maximum resolution and highsensitivity. For subsurface flaws, lower frequencies are necessary to getthe required depth of penetration and this results in less sensitivity.Ferromagnetic or highly conductive materials require the use of an evenlower frequency to arrive at some level of penetration.4. Setup or reference specimens of similar material to the component beinginspected and with features that are representative of the defect orcondition being inspected for.Charlie Chong/ Fion Zhang 116. 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 formaximum resolution and high sensitivity. For subsurface flaws, lower frequencies are necessary to get the requireddepth of penetration and this results in less sensitivity. Ferromagnetic or highly conductive materials require the use of an evenlower frequency to arrive at some level of penetration.Charlie Chong/ Fion Zhang 117. The basic steps in performing an inspection with a surface probe are thefollowing: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 responseusing a calibration standard or setup specimen.4. Place the inspection probe (coil) on the component surface and null theinstrument.5. Scan the probe over part of the surface in a pattern that will providecomplete coverage of the area being inspected. Care must be taken tomaintain the same probe-to-surface orientation as probe wobble can affectinterpretation of the signal. In some cases, fixtures to help maintainorientation or automated scanners may be required.6. Monitor the signal for a local change in impedance (R, XL) that will occuras the probe moves over a discontinuity.Charlie Chong/ Fion Zhang 118. Resistance is essentially friction against the motion of electrons. It is present in allconductors to some extent (except superconductors!), most notably in resistors.When alternating current goes through a resistance, a voltage drop is producedthat is in-phase with the current. Resistance is mathematically symbolized by theletter R and is measured in the unit of ohms (). Reactance is essentially inertia against the motion of electrons. It is presentanywhere electric or magnetic fields are developed in proportion to applied voltageor current, respectively; but most notably in capacitors and inductors. Whenalternating current goes through a pure reactance, a voltage drop is produced thatis 90o out of phase with the current. Reactance is mathematically symbolized bythe letter X and is measured in the unit of ohms (). Impedance is a comprehensive expression of any and all forms of opposition toelectron flow, including both resistance and reactance. It is present in all circuits,and in all components. When alternating current goes through an impedance, avoltage drop is produced that is somewhere between 0o and 90o out of phase withthe current. Impedance is mathematically symbolized by the letter Z and ismeasured in the unit of ohms ().Charlie Chong/ Fion Zhanghttp://www.allaboutcircuits.com/vol_2/chpt_5/1.html 119. Reactance phasor diagram (Impedance plane respond)Charlie Chong/ Fion Zhang 120. Surface probe testing CrackCharlie Chong/ Fion Zhang 121. The applet below depicts a simple eddy current probe near the surface of acalibration specimen. Move the probe over the surface of the specimen andcompare the signal responses from a surface breaking crack with the signalsfrom the calibration notches. The inspection can be made at a couple ofdifferent frequencies to get a feel for the effect that frequency has onsensitivity in this application.Keywords: Surface breaking crack Calibration notches Effect of frequency on sensitivityCharlie Chong/ Fion Zhang 122. Charlie Chong/ Fion Zhang https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Graphics/Flash/surfaceBreakingCracks.swf 123. Eddy Current Crack Detection www.youtube.com/embed/1YUSn___VxQ?feature=player_detailpage https://www.youtube.com/watch?v=1YUSn___VxQCharlie Chong/ Fion Zhang 124. Eddy Current Crack Testing by Criterion NDT www.youtube.com/embed/9A5fQtOwnzwCharlie Chong/ Fion Zhang 125. 6.2 Surface Crack Detection Using Sliding ProbesMany commercial aircraft applications involve the use of multiple fasteners toconnect the multi-layer skins. Because of the fatigue stress that is caused bythe typical application of any commercial aircraft, fatigue cracks can beinduced in the vicinity of the fastener holes. In order to inspect the fastenerholes in an adequate amount of time, sliding probes are an efficient method ofinspection. Sliding probes have been named so because they move overfasteners in a sliding motion. There are two types of sliding probes, fixed andadjustable, which are usually operated in the reflection mode. This meansthat the eddy currents are induced by the driver coil and detected by aseparate receiving coil (Mode: reflection coils).Charlie Chong/ Fion Zhang 126. Sliding probes are one of the fastest methods to inspect large numbers offastener holes. They are capable of detecting surface and subsurfacediscontinuities, but they can only detect defects in one direction. The probesare marked with a detection line to indicate the direction of inspection. Inorder to make a complete inspection there must be two scans that areorthogonal (90 degrees) to each other.Charlie Chong/ Fion Zhang 127. Aircraft applicationsCharlie Chong/ Fion Zhang 128. Aircraft applicationsCharlie Chong/ Fion Zhang 129. 6.2.1 Probe Types (configuration- surface probe)(i) Fixed Sliding ProbesThese probes are generally used for thinner material compared to theadjustable probes. Maximum penetration is about 1/8 inch (3mm). Fixedsliding probes are particularly well suited for finding longitudinal surface orsubsurface cracks such as those found in lap joints. Typical frequency rangeis from 100 Hz to 100 kHz.Charlie Chong/ Fion Zhang 130. (ii) Adjustable Sliding ProbesThese probes are well suited for finding subsurface cracks in thick multi-layerstructures, 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 ofspacers, which will change the penetration capabilities. The spacer thicknessbetween the coils is normally adjusted for the best detection. For tangentialscans or 90 degree scanning with an offset from the center, a thinner spaceris often used.The spacer thickness range can vary from 0 (no spacer) for inspectionsclose to the surface and small fastener heads to a maximum of about 0.3 inchfor deep penetration with large heads in the bigger probe types. A widerspacer will give more tolerance to probe deviation as the sensitive areabecomes wider but the instrument will require more gain. Sliding probesusually penetrate thicker materials compared to the donut probes.Charlie Chong/ Fion Zhang 131. 6.2.2 Reference StandardsReference/calibration standards for setup of sliding probes typically consist ofthree or four aluminum plates that are fastened together within a lap joint typeconfiguration. EDM notches or naturally/artificially- induced cracks are locatedin the second or third layer of the standard.Reference standards used should be manufactured from the same materialtype, alloy, material thickness, and chemical composition that will be found onthe aircraft component to be inspected. Sizes and tolerances of flawsintroduced in the standards are usually regulated by inspection specifications.Charlie Chong/ Fion Zhang 132. Reference StandardsCharlie Chong/ Fion Zhang 133. Reference StandardsCharlie Chong/ Fion Zhanghttp://www.phtool.com/asntpics.htm 134. 6.2.3 Inspection Variables6.2.3.1 Liftoff Signal AdjustmentLiftoff is normally adjusted to be relatively horizontal. The term "relativelyhorizontal" is used here because the liftoff signal often appears a curved linerather than a straight line. Sometimes liftoff can be a sharp curve and mayneed to be adjusted to run slightly upwards before moving downwards. SeeFigures 1 and 2.Charlie Chong/ Fion ZhangDotted line IdealadjustmentThe best liftoffadjustment 135. Lift-off Signal Adjustment before testing www.youtube.com/embed/1YUSn___VxQ?feature=player_detailpage https://www.youtube.com/watch?v=1YUSn___VxQCharlie Chong/ Fion Zhang 136. 6.2.3.2 Scan PatternsA typical scan is centralized over the fastener head and moves along the axisof the fastener holes. This scan is generally used to detect cracks positionedalong the axis of the fastener holes. For detecting cracks located transverseor 90 degrees from the axis of the fastener holes, a scan that is 90 degreesfrom the axis of the fastener holes is recommended.Charlie Chong/ Fion Zhang 137. Scan Patterns with differential probesDifferential coils have the attraction of built-in lift-off compensation. This hasmade them useful for many applications. The older types of coils had noferrite shield and they were built just by placing two coils side-by-side (Figure3). Later types added individual shields (Figure 4), but the greatestimprovement to the sensitivity was achieved when both coils were placedwithin a common shield (Figure 5). Differential type probes are mostly used insmall sizes for surface crack detection only.Figure 3 Figure 4 Figure 5Charlie Chong/ Fion Zhanghttp://www.olympus-ims.com/en/applications/eddy-current-probes-guide/ 138. 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 differenceis present due to small coil variations.Charlie Chong/ Fion Zhang 139. Normal scan direction is as shown (Figure 6), giving the typical displaypresentation. The double indication is, in fact helpful, as it doubles the size ofthe defect in the screen (Figure 7).Charlie Chong/ Fion ZhangFigure 7Figure 6LeadingsignalTrailingsignal 140. Sometimes it is necessary to scan in the same direction as the cracks (Figure8). This is permissible and the result will be similar for a very short defect. Alarger defect affecting both coils will tend to cancel out because they are inopposition, but in practice there are enough differences in angle and depth forthis not to happen totally. In any case, the ends of the crack will shownormally.Figure 6Charlie Chong/ Fion Zhang 141. 6.2.3.3 Signal InterpretationWhen the probe moves over a fastener hole with a crack, the indicationchanges and typically will create a larger vertical movement. The verticalamplitude of the loop depends on the crack length, with longer cracks givinghigher indications.If the crack is in the far side of the fastener, as the probe moves over it, thedot will follow the fastener line first but will move upwards (clockwise) as itgoes over the crack. If the crack is in the near side, it will be found first andthe dot will move along the crack level before coming down to the fastenerlevel. If two cracks on opposite sides of the fastener hole are present, the dotwill move upwards to the height by the first crack length and then come backto the fastener line and balance point. If the second crack is longer than thefirst 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 142. Signal InterpretationCharlie Chong/ Fion Zhang 143. 6.2.3.4 Probe Scan DeviationMost probes are designed to give a narrow indication for a good fastener holeso that the loops from the cracks are more noticeable. Some probes andstructures can give wider indications and a similar result can be obtained ifthe probe is not straight when it approaches the fastener. It is important tokeep the probe centralized over the fastener heads. Doing this will give you amaximum indication for the fastener and a crack.If the probe deviates from the center line, the crack indication will move alongthe loop that we saw in Figure 5 and is now present in Figure 6. The crackindication is at "a" when the probe is centralized and moves toward "b" as itdeviates 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 ithas gained in phase, giving a better separation angle. This is because wedeviated to the side where the crack is located.Charlie Chong/ Fion Zhang 144. Probe Scan DeviationCharlie Chong/ Fion Zhang 145. 6.2.3.5 Crack Angle DeviationA reduction in the crack indication occurs when the crack is at an angle to theprobe scan direction. This happens if the crack is not completely at 90degrees to the normal probe scan or changes direction as it grows. Both thefixed and adjustable sliding probes are capable of detecting cracks up toabout 30 degrees off angle. See Figures 8 and 9.Charlie Chong/ Fion Zhang 146. Crack Angle DeviationCharlie Chong/ Fion Zhang 147. 6.2.4 Electrical ContactWhen inspecting fasteners that have just been installed or referencestandards that have intimate contact with the aluminum skin plate, it is notunusual to obtain a smaller than normal indication. In some extreme cases,the fastener indication may disappear almost completely. This is due to thegood electrical contact between the fastener and the skin. This conditionallows the eddy currents to circulate without encountering a boundary, andtherefore, no obstacle or barrier. Because of this effect, it is recommended topaint the holes before fastener installation.Charlie Chong/ Fion Zhang 148. DiscussionTopic: Reasons on the different in phase angles for different notch depthsCharlie Chong/ Fion Zhang123 149. DiscussionTopic: Why the impedance change on traversing toward the crack andleaving the cracking does not match perfectly on top of each others?Charlie Chong/ Fion Zhang123 150. 6.3 Tube InspectionEddy current inspection is often used to detect corrosion, erosion, crackingand other changes in tubing. Heat exchangers and steam generators, whichare used in power plants, have thousands of tubes that must be preventedfrom leaking. This is especially important in nuclear power plants wherereused, contaminated water must be prevented from mixing with fresh waterthat will be returned to the environment. The contaminated water flows on oneside 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 andthe fresh water is then returned back to is source, which is usually a lake orriver. It is very important to keep the two water sources from mixing, so powerplants are periodically shutdown so the tubes and other equipment can beinspected and repaired. The eddy current test method and the related remotefield testing method provide high-speed inspection techniques for theseapplications.Charlie Chong/ Fion Zhang 151. A technique that is often used involves feeding a differential bobbin probe intothe individual tube of the heat exchanger. With the differential probe, nosignal will be seen on the eddy current instrument as long as no metalthinning is present. When metal thinning is present, a loop will be seen on theimpedance plane as one coil of the differential probe passes over the flawedarea and a second loop will be produced when the second coil passes overthe damage. When the corrosion is on the outside surface of the tube, thedepth of corrosion is indicated by a shift in the phase lag. The size of theindication 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 thetube. Note the different signal responses provided by the two probes. Alsonote that the absolute probe is much more sensitive to dings and the build upof magnetite on the outside of the tube than the differential probe is.Charlie Chong/ Fion Zhang 152. Tube InspectionCharlie Chong/ Fion Zhang 153. Charlie Chong/ Fion Zhang https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Graphics/Flash/DifferentialvsAbsoluteAnim.swf 154. Tube InspectionCharlie Chong/ Fion Zhanghttp://www.nde.com/ect.htm 155. Tube InspectionCharlie Chong/ Fion Zhanghttp://www.titanmf.com/photo-gallery/heat-exchangers/ 156. 6.4 Conductivity MeasurementsOne of the uses of eddy current instruments is for the measurement ofelectrical conductivity. The value of the electrical conductivity of a metaldepends on several factors, such as its chemical composition and the stressstate of its crystalline structure. Therefore, electrical conductivity informationcan be used for sorting metals, checking for proper heat treatment, andinspecting for heat damage.Charlie Chong/ Fion ZhangApplications: sorting metals, checking for proper heattreatment, inspecting for heat damage. 157. Heat DamageCharlie Chong/ Fion Zhang 158. Heat TreatmentCharlie Chong/ Fion Zhang 159. The technique usually involves nulling an absolute probe in air and placingthe probe in contact with the sample surface. For nonmagnetic materials, thechange in impedance of the coil can be correlated directly to the conductivityof the material.The technique can be used to easily sort magnetic materials fromnonmagnetic materials but it is difficult to separate the conductivity effectsfrom the magnetic permeability effects, so conductivity measurements arelimited to nonmagnetic materials. It is important to control factors that canaffect the results such as the inspection temperature and the part geometry.Conductivity changes with temperature so measurements should be made ata constant temperature and adjustments made for temperature variationswhen necessary.The thickness of the specimen should generally be greater than threestandard depths of penetration. This is so the eddy currents at the backsurface of the sample are sufficiently weaker than the variations in thespecimen thickness that are not seen in the measurements.Charlie Chong/ Fion Zhang 160. Heat TreatmentDiscuss on:The technique can be used to easily sort magnetic materials fromnonmagnetic materials but it is difficult to separate the conductivity effectsfrom the magnetic permeability effects, so conductivity measurements arelimited to nonmagnetic materialsCharlie Chong/ Fion Zhang 161. Generally large pancake type, surface probes are used to get a value for arelatively large sample area. The instrument is usually setup such that aferromagnetic material produces a response that is nearly vertical. Then, allconductive but nonmagnetic materials will produce a trace that moves downand to the right as the probe is moved toward the surface. Think back to thediscussion on the impedance plane and these type of responses make sense.Remember that inductive reactance changes are plotted along the y-axis andresistance changes are plotted in the x-axis. Since ferromagnetic materialswill concentrate the magnetic field produced by a coil, the inductive reactanceof the coil will increase. The effects on the signal from the magneticpermeability overshadow the effects from conductivity since they are so muchstronger.Charlie Chong/ Fion Zhang 162. Reactance Phasor DiagramCharlie Chong/ Fion ZhangLeast conductive materialMost conductive materialAs the conductivity ofthe materials beingtested increases, theresistance losses willbe less and theinductive reactancechanges will begreater. 163. Comments on: As the conductivity of the materials being tested increases,the resistance losses will be less and the inductive reactance changes willbe greater.Note: the underlined statement may not be true universally. (?)Charlie Chong/ Fion Zhang 164. When the probe is brought near a conductive but nonmagnetic material, thecoil's inductive reactance goes down since the magnetic field from the eddycurrents opposes the magnetic field of the coil. The resistance in the coilincreases since it takes some of the coil's energy to generate the eddycurrents and this appears as additional resistance in the circuit. As theconductivity of the materials being tested increases, the resistancelosses will be less and the inductive reactance changes will be greater.Therefore, the signals will be come more vertical as the conductivityincreases, as shown in the image above.Charlie Chong/ Fion Zhang 165. Reactance due to conductivityAs the conductivity of the materials being tested increases, the resistancelosses will be less and the inductive reactance changes will be greater. (seethe brown dotted lines)Charlie Chong/ Fion Zhang 166. To sort materials using an impedance plane device, the signal from theunknown sample must be compared to a signal from a variety of referencestandards. However, there are devices available that can be calibrated toproduce a value for electrical conductivity which can then be compared topublished values of electrical conductivity in MS/m or percent IACS(International Annealed Copper Standard). Please be aware that theconductivity of a particular material can vary significantly with slight variationsin the chemical composition and, thus, a conductivity range is generallyprovided for a material. The conductivity range for one material may overlapwith the range of a second material of interest, so conductivity alone cannot always be used to sort materials. The electrical conductivity values fora variety of materials can be found in the material properties reference tables.Charlie Chong/ Fion Zhang 167. The following applet is based on codes for nonferrous materials written byBack Blitz from his book, "Electrical and Magnetic Methods of NondestructiveTesting", 2nd ed., Chapman & Hill (1997). The applet demonstrates how animpedance plane eddy current instrument can be used for the sorting ofmaterials.Charlie Chong/ Fion Zhanghttps://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Applications/Popups/applet2/applet2.htm 168. Discuss on the plane impedance diagramCharlie Chong/ Fion ZhangLeadAluminumCopperQuoted from text: As the conductivity of the materials being testedincreases, the resistance losses will be less and the inductive reactancechanges will be greater. 169. Quoted from text: As the conductivity of the materials being tested increases,the resistance losses will be less and the inductive reactance changes will begreater.Discussion: With increase conductivity the resistance component of reactantwas decrease. However the inductive reactance component was not increaseas compared with that of Aluminum (X1>X2).Charlie Chong/ Fion ZhangX2 X1 170. Discussion on : Material Conductivity, standard penetration and itseffect on resistive reactance.The standard depth of penetration () is the depth where eddy current densitydrops to 1/e (37%) of its value measured at the surface. This depth ofpenetration is affected by the operating frequency (), and conductivity ()and permeability () of the material to inspect. This is what we call the skindepth effect.Charlie Chong/ Fion Zhang 171. 6.5 Heat Treatment Verification6.5.1 Conductivity Measurements for the Verification of Heat TreatmentWith some materials, such as solution heat treatable aluminum alloys,conductivity measurements are often made verifying that parts and materialshave received the proper heat treatment. High purity aluminum is soft andductile, and gains strength and hardness with the addition of alloyingelements. A few such aluminum alloys are the 2000 series (2014, 2024, etc.),6000 series (6061, 6063, etc.), and 7000 series (7050, 7075, etc.). The 2xxxseries aluminum alloys have copper, the 6xxx series have magnesium, andthe 7xxx have zinc as their major alloying elements.Charlie Chong/ Fion Zhang 172. Heat treatment of aluminum alloys is accomplished in two phases - solutionheat treatment and then aging. In the solution heat treatment step, the alloysare heated to an elevated temperature to dissolve the alloying elements intosolution. The metal is then rapidly cooled or quenched to freeze the atomsof the alloying elements in the lattice structure of the aluminum. This distortsand stresses the structure, making electron movement more difficult, therebydecreasing the electrical conductivity. In this condition, the alloys are stillrelatively soft but start to gain strength as the alloying elements begin toprecipitate out of solution to form extremely small particles that impede themovement of dislocations within the material.Charlie Chong/ Fion Zhang 173. The formation of the precipitates can be controlled for many alloys by heatingand holding the material at an elevated temperature for a period of time(artificial aging). As the alloying elements precipitate out of solid solution, theconductivity of the material gradually increases. By controlling the amount ofprecipitated particles within the aluminum, the properties can be controlled toproduce peak strength or some combinations of strength and corrosionresistance. 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 toform a coarse, widely spaced precipitate. The electrical conductivity isgreatest when the material is in the annealed condition.Keywords:Annealed condition: Electrical conductivityCharlie Chong/ Fion Zhang 174. Since solution heat-treated and aged materials are stronger, components canbe made using less material. A lighter or more compact design is often ofgreat importance to the designer and well worth the cost of the heat treatingprocess. However, think of the consequences that could arise if a componentthat was supposed to be solution heat-treated and aged somehow left themanufacturing facility and was put into service un-heat-treated or annealed.This is a real possibility since heat-treated aluminum parts look exactly likeunheat-treated parts. Consider 2024 aluminum as an example. Select tensileproperties and its electrical conductivity for various heat treatment conditionsare given in the following table.Charlie Chong/ Fion Zhang 175. Properties for Alclad 2024 AluminumAnnealed (O) 26 ksi (180 MPa) 11 ksi (75 MPa) 50 % IACSSolution Heat 64 ksi (440 MPa) 42 ksi (290 MPa) 30 % IACSTreated andNaturally Aged (T42)Solution Heat 70 ksi (485 MPa) 66 ksi (455 MPa) 30 % IACSTreated, Coldworked andArtificially Aged(T861)Charlie Chong/ Fion ZhangElectricalConductivityHeat Treatment Ultimate Strength Yield StrengthConditionIACS: The International Annealed Copper Standard 176. It can be seen that the yield strength for the material is 42 kilopounds/squareinch (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 annealedcondition, the yield strength is reduced to 11 ksi (75 MPa). If an annealed partwere accidentally used where a part in the T42 or T861 was intended, itwould likely fail prematurely. However, a quick check of the conductivity usingan eddy current instrument of all parts prior to shipping would prevent thisfrom occurring.Charlie Chong/ Fion Zhang 177. Eddy current inspectionCharlie Chong/ Fion Zhang 178. 6.6 Thickness Measurements6.6.1 Thickness Measurements of Thin MaterialEddy current techniques can be used to perform a number of dimensionalmeasurements. The ability to make rapid measurements without the need forcouplant or, in some cases even surface contact, makes eddy currenttechniques very useful. The type of measurements that can be made include: thickness of thin metal sheet and foil, and of metallic coatings on metallicand nonmetallic substrate, cross-sectional dimensions of cylindrical tubes and rods, thickness of nonmetallic coatings on metallic substrates.Charlie Chong/ Fion Zhang 179. Thickness MeasurementsCharlie Chong/ Fion Zhang 180. 6.6.2 Corrosion Thinning of Aircraft SkinsOne application where the eddy current technique is commonly used tomeasure material thickness is in the detection and characterization ofcorrosion damage on the skins of aircraft. Eddy current techniques can beused to do spot checks or scanners can be used to inspect small areas. Eddycurrent inspection has an advantage over ultrasound in this applicationbecause no mechanical coupling is required to get the energy into thestructure. Therefore, in multi-layered areas of the structure like lap splices,eddy current can often determine if corrosion thinning is present in buriedlayers.Charlie Chong/ Fion Zhang 181. Eddy current inspection has an advantage over radiography for thisapplication because only single sided access is required to perform theinspection. To get a piece of film on the back side of the aircraft skin mightrequire removing interior furnishings, panels, and insulation which could bevery costly. Advanced eddy current techniques are being developed that candetermine thickness changes down to about three percent of the skinthickness.Charlie Chong/ Fion Zhang 182. Charlie Chong/ Fion Zhang 183. Corrosion thinning is present in buried layers.Charlie Chong/ Fion Zhang 184. 6.6.3 Thickness Measurement of Thin Conductive Sheet, Strip and FoilEddy current techniques are used to measure the thickness of hot sheet, stripand foil in rolling mills, and to measure the amount of metal thinning that hasoccurred over time due to corrosion on fuselage skins of aircraft. On theimpedance plane, thickness variations exhibit the same type of eddy currentsignal response as a subsurface defect, except that the signal represents avoid of infinite size and depth. The phase rotation pattern is the same, but thesignal amplitude is greater. In the applet, the lift-off curves for different areasof the taper wedge can be produced by nulling the probe in air and touching itto the surface at various locations of the tapered wedge. If a line is drawnbetween the end points of the lift-off curves, a comma shaped curve isproduced. As illustrated in the second applet, this comma shaped curve is thepath that is traced on the screen when the probe is scanned down the lengthof the tapered wedge so that the entire range of thickness values aremeasured.Charlie Chong/ Fion Zhang 185. Charlie Chong/ Fion Zhang https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Graphics/Flash/thinningMeasurement1.swf 186. Charlie Chong/ Fion Zhang https://www.nde-ed.org/EducationResources/CommunityCollege/EddyCurrents/Graphics/Flash/thinningMeasurement2.swf 187. When making this measurement, it is important to keep in mind that the depthof penetration of the eddy currents must cover the entire range of thicknessesbeing measured. Typically, a frequency is selected that produces about onestandard depth of penetration at the maximum thickness. Unfortunately, atlower 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 isimportant to verify that any variations of conductivity over the region ofinterest are at a sufficiently low level.Keywords: Typically, a frequency is selected that produces about one standarddepth of penetration at the maximum thickness. at lower frequencies, which are often needed to get the necessarypenetration, the probe impedance is more sensitive to changes inelectrical conductivity.Charlie Chong/ Fion Zhang 188. 6.6.4 Measurement of Cross-sectional Dimensions of Cylindrical Tubesand RodsDimensions of cylindrical tubes and rods can be measured with either ODcoils or internal axial coils, whichever is appropriate. The relationshipbetween change in impedance and change in diameter is fairly constant,except at very low frequencies. However, the advantages of operating at ahigher normalized frequency are twofold. First, the contribution of anyconductivity change to the impedance of the coil becomes less important andit can easily be phased out. Second, there is an increase in measurementsensitivity resulting from the higher value of the inductive component of theimpedance. Because of the large phase difference between the impedancevectors corresponding to changes in fill-factor and conductivity (and defectsize), simultaneous testing for dimensions, conductivity, and defects can becarried out.Keywords: Impedance vectors Fill-factor and conductivity (and defect size)Charlie Chong/ Fion Zhang 189. Typical applications include measuring eccentricities of the diameters oftubes and rods and the thickness of tube walls. Long tubes are often testedby passing them at a constant speed through encircling coils (generallydifferential) and providing a close fit to achieve as high a fill-factor as possible.An important application of tube-wall thickness measurement is the detectionand assessment of corrosion, both external and internal. Internal probes mustbe used when the external surface is not accessible, such as when testingpipes that are buried or supported by brackets. Success has been achieved inmeasuring thickness variations in ferromagnetic metal pipes with the remotefield technique.Keywords: Fill-factor Remote field technique.Charlie Chong/ Fion Zhang 190. 6.6.5 Thickness Measurement of Thin Conductive LayersIt is also possible to measure the thickness of a thin layer of metal on ametallic substrate, provided the two metals have widely differing electricalconductivities (i.e. silver on lead where s= 67 and 10 MS/m, respectively). Afrequency must be selected such that there is complete eddy currentpenetration of the layer, but not of the substrate itself. The method has alsobeen used successfully for measuring thickness of very thin protectivecoatings of ferromagnetic metals (i.e. chromium and nickel) on non-ferromagneticCharlie Chong/ Fion Zhangmetal bases.Depending on the required degree of penetration, measurements can bemade using a single-coil probe or a transformer probe, preferably reflectiontype. Small-diameter probe coils are usually preferred since they can providevery high sensitivity and minimize effects related to property or thicknessvariations in the underlying base metal when used in combination withsuitably high test frequencies. The goal is to confine the magnetizing field,and the resulting eddy current distribution, to just beyond the thin coatinglayer and to minimize the field within the base metals. 191. 6.7 Thickness of Coatings6.7.1 Thickness Measurements of Non-conducting Coatings onConductive MaterialsThe thickness of nonmetallic coatings on metal substrates can be determinedsimply from the effect of liftoff on impedance. This method has widespreaduse for measuring thickness of paint and plastic coatings. The coating servesas a spacer between the probe and the conductive surface. As the distancebetween the probe and the conductive base metal increases, the eddy currentfield strength decreases because less of the probe's magnetic field caninteract with the base metal. Thicknesses between 0.5 and 25 m can bemeasured to an accuracy between 10% for lower values and 4% for highervalues. Contributions to impedance changes due to conductivity variationsshould be phased out, unless it is known that conductivity variations arenegligible, as normally found at higher frequencies.Keywords:conductivity variations are negligible, as normally found at higher frequencies.Charlie Chong/ Fion Zhang 192. Fairly precise measurements can be made with a standard eddy current flawdetector and a calibration specimen. The probe is nulled in air and thedirection of the lift-off signal is established. The location of the signal ismarked on the screen as the probe is placed on the calibration specimen inareas of decreasing coating thickness. When the probe is placed on the testsurface, the position of the signal will move from the air null position to a pointthat can be correlated to the calibration markings.Charlie Chong/ Fion Zhang 193. Specialized eddy current coating thickness detectors are also available andare often pocket-sized with the probe resembling a small pencil. They areusually operated by a small battery and provide a digital read-out in theappropriate units. Calibration adjustments, some of which are laid down bystandards such as BS EN 2360 (1995) and ASTM B 244 and E 376, may beassisted by the use of an inbuilt microprocessor.Charlie Chong/ Fion Zhang 194. Eddy current thickness gageCharlie Chong/ Fion Zhang 195. 7.0 Advanced Techniques7.1 ScanningEddy current data can be collected using automated scanning systems toimprove the quality of the measurements and to construct images of scannedareas. The most common type of scanning is line scanning where anautomated system is used to push the probe at a fixed speed. Line scansystems are often used when performing tube inspections or aircraft engineblade slot inspections, where scanning in one dimension is needed. The datais usually presented as a strip chart recording. The advantage of using alinear scanning system is that the probe is moved at a constant speed, soindications on the strip chart can be correlated to a position on the part beingscanned. As with all automated scanning systems, operator variables, suchas wobble of the probe, are reduced.Keywords: Line scan system Strip chart recording Wobble of probeCharlie Chong/ Fion Zhang 196. 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-Yraster mode, or it could be a bolt hole inspection system that rotates theprobe as it is moved into the hole. The data is typically displayed as a false-colorplot of signal strength or phase angle shift as a function of position, justlike an ultrasonic C-scan presentation. Shown below is a portable scanningsystem that is designed to work on the skins of aircraft fuselage and wingsections.Charlie Chong/ Fion Zhang 197. Eddy current Line ScannerCharlie Chong/ Fion Zhanghttp://www.inspectech.ca/products/OnLine_Eddy_Current/ 198. 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 manufactureor damage,2. accurate indexing,3. Repeatability,4. high resolution mapping.Charlie Chong/ Fion Zhang 199. 7.2 Multiple Frequency TechniquesMultiple frequency eddy current techniques simply involve collecting data atseveral different frequencies and then comparing the data or mixing the datain some way.Why the need for multiple frequencies? - Some background information. Theimpedance 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 aobject 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 defectssuch 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 200. Several of these factors are often present simultaneously. In the simple casewhere interest is confined to detecting defects or other abrupt changes ingeometry, a differential probe can be used to eliminate unwanted factors,providing they vary in a gradual manner. For example, variations in electricalconductivity and tube thinning affect both coils of a differential probesimultaneously. However, if unwanted parameters that occur abruptly areaffecting the measurements, they can sometimes be negated by mixingsignals collected at several frequencies.An example of where a multi-frequency eddy current inspection is used is inheat exchanger tube inspections. Heat exchanger assemblies are often acollection of tubing that have support brackets on the outside. Whenattempting to inspect the full wall thickness of the tubing, the signal from themounting bracket is often troublesome. By collecting a signal at the frequencynecessary to inspect the full thickness of the tube and subtracting a secondsignal collected at a lower frequency (which will be more sensitive to thebracket but less sensitive to features in the tubing), the effects of the bracketcan be reduced.Charlie Chong/ Fion Zhang 201. DiscussionSubject: Discuss and reasoning on the following sentences;When attempting to inspect the full wall thickness of the tubing, the signalfrom the mounting bracket is often troublesome. By collecting a signal at thefrequency necessary to inspect the full thickness of the tube and subtracting asecond signal collected at a lower frequency (which will be more sensitive tothe bracket but less sensitive to features in the tubing), the effects of thebracket can be reduced.Charlie Chong/ Fion Zhang 202. Heat Exchanger Tube BundlesCharlie Chong/ Fion Zhang 203. There are a number of commercially available multi-frequency eddy currentinstruments. Most operate at only two frequencies at a time but some unitscan collect data at up to four frequencies simultaneously. Multi-frequencymeasurements can also be made using an impedance analyzer but thisequipment is generally not suitable for field measurements. A typicalimpedance analyzer system is shown below. The interest in pulsed eddycurrent instruments is largely due to their ability to, in essence, perform multi-frequencymeasurements very quickly and easily.Charlie Chong/ Fion Zhang 204. 7.3 Swept FrequencySwept frequency eddy current techniques involve collecting eddy current dataat a wide range of frequencies. This usually involves the use of a specializedpiece of equipment such as an impedance analyzer, which can be configuredto automatically make measurements over a range of frequencies. Theswept-frequency technique can be implemented with commercial equipmentbut it is a difficult and time-consuming measurement. The advantage of aswept frequency measurement is that depth information can be obtainedsince eddy current depth of penetration varies as a function of frequency.Swept frequency measurements are useful in applications such as measuringthe thickness of conductive coatings on conductive base metal, differentiatingbetween flaws in surface coatings and flaws in the base metal anddifferentiating between flaws in various layers of built-up structure. Anexample application would be the lap splice of a commercial aircraft. Sweptfrequency measurements would make it possible to tell if cracking wasoccurring on the outer skin, the inner skin or a double layer. Below is anexample of the type of data that can be obtained from swept-frequencymeasurements.Charlie Chong/ Fion Zhang 205. Data from swept-frequency measurements on two heats of material.Charlie Chong/ Fion ZhangIt can be seen that in theetched condition, thematerial labeled "good"exhibits a much differentsignal response than thematerial labeled "bad." Itcan also be seen that afrequency of around 2.2MHz provides the largestseparation in the curves.Therefore, this frequencyshould be used if a singlefrequency is used to sortthe parts made from thetwo metals.2.2 MHz 206. 7.4 Pulsed Eddy Current InspectionConventional eddy current inspection techniques use sinusoidal alternatingelectrical current of a particular frequency to excite the probe. The pulsededdy current technique uses a step function voltage to excite the probe. Theadvantage of using a step function voltage is that it contains a continuum offrequencies. As a result, the electromagnetic response to several differentfrequencies can be measured with just a single step. Since the depth ofpenetration is dependent on the frequency of excitation, information from arange of depths can be obtained all at once. If measurements are made in thetime 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 beseen first and more distant features will be seen later in time.Charlie Chong/ Fion Zhang 207. To improve the strength and ease interpretation of the signal, a referencesignal is usually collected, to which all other signals are compared (just likenulling the probe in conventional eddy current inspection). Flaws, conductivity,and dimensional changes produce a change in the signal and a differencebetween the reference signal and the measurement signal that is displayed.The distance of the flaw and other features relative to the probe will cause thesignal to shift in time. Therefore, time gating techniques (like in ultrasonicinspection) can be used to gain information about the depth of a feature ofinterest.Charlie Chong/ Fion Zhang 208. 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, suchas in the nuclear energy industry, where testing equipment is oftenconstructed to order. However, significant progress in this direction has takenplace only recently after appropriate advances in technology (Krzwosz et al.1985; Sather, 1981; Waidelich, 1981; Wittig and Thomas 1981), but at thetime of writing, commercial equipment was not yet available. The method hasthe potential advantages of greater penetration, the ability to locatediscontinuities from time-of-flight determinations, and a ready means of multi-frequencymeasurement. At present, it does not generally have the precisionof the conventional methods. The apparatus is somewhat complicated indesign and not readily usable by the average operator who is experiencedwith the conventional eddy current equipment. Its main successes are in thetesting of thin metal tubes and sheets, as well as metal cladding formeasuring thickness and for the location and sizing of internal defects.Charlie Chong/ Fion Zhang 209. When comparing the pulsed method with the conventional eddy currenttechnique, the conventional technique must be regarded as a continuouswave 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 variesinversely with the pulse length; this allows multi-frequency operation. Asfound with ultrasonic testing, the total amount of energy dissipated within agiven period of time is considerably less for pulsed waves than for continuouswaves having the same intensity. For example, with pulses containing onlyone or two wavelengths and generated 1000 times per secon