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http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 1, Number 4

INTRODUCTIONEven though eddy current testing is one of the oldest non-destructive evaluation methods, it was not widely understoodand did not reach full, widespread use until the 1980s.[1]Whereas portable ultrasonic instrumentation offeringconsiderable versatility for nondestructive testing (NDT) hasbeen available since the 1960s, comparable eddy current testingequipment was not widely available until the 1980s. In addition,eddy current theory did not become available until the late1970s. Now, excellent tutorial information is available forscientists and engineers without advanced degrees.

History of Eddy Current TestingEddy current testing (Figure 1) has its roots in discoveries that were made in the 1800s. The most fundamentalbreakthrough was the discovery of electromagnetism by HansChristian Orstead in 1820. About a decade later in 1831,Michael Faraday discovered electromagnetic induction*. Then in 1834, Heinrich Lenz developed the principal thatdefines how the electromagnetic properties of a test object arecommunicated back to the test system. And finally, JamesMaxwell, who is famous for his defining equations ofelectromagnetic theory, discovered eddy currents in 1864.

D.E. Hughes was the first to use eddy current testing in1879 to conduct metallurgical sorting tests†. More than a halfcentury later, eddy current testing made a leap forward whenFriedrich Foerster developed and marketed practical eddycurrent testing equipment in the 1940s. His major contributions led to the development of the impedance planedisplay, which greatly aided presentation of test information.In addition, he formulated the Law of Similarity, whichenables eddy current test results to be duplicated under a widevariety of test situations.

An equipment manufacturer, Intercontrolle of France, madethe next major advancement in 1974, when the companydeveloped multi-frequency testing. Driving the device atmultiple frequencies enabled eddy current testing to overcomethe major limitation of having to interpret eddy current signalsfrom a single display. Multi-frequency methods can alsooptimize conflicting test variables such as sensitivity andpenetration. The development of microprocessor-based eddy current instruments in the 1980s greatly enhanced thepotential and user-friendliness of the method, and allowed forthe development of automated eddy current inspectionequipment. Finally, at the turn of the century in the late 1990sand early 2000s, giant magnetoresistive‡ sensors were utilized toallow multi-frequency techniques at very low frequencies toprobe for flaws deep in multi-layer metallic aircraft structures.

PHYSICAL PRINCIPLESEddy current NDT is based on the principals of electromagneticinduction for inducing eddy currents§ in a material or partplaced in or adjacent to one or more alternating flux fieldinduction coils.[2] The system is operated at very low powerlevels to minimize heating and temperature changes. The loopcurrents induced in the material produce an additional magneticfield, and a sensor is used to measure the total magnetic fieldnear the specimen. The value of the total magnetic field dependson several factors including the following:

• Geometry of the induction coil• Geometry of the specimen• Current and frequency in the coil• Electrical conductivity of the specimen• Magnetic permeability of the specimen

Selecting a Nondestructive Testing Method, Part III: Eddy Current Testing

7

H. Thomas YolkenTRI/AustinAustin, TX

Figure 1. Eddy CurrentInspection of an F-16Fighting Falcon Aircraft.NDT is Responsible forEarly Engine WearDetection to AccomplishMissions in a More CostEffective Way.

This edition of TechSolutions is the third installment in a series dedicated to the subject of nondestructive testing. TechSolutions 1, published in Vol. 1, No. 2 of the AMMTIAC Quarterly, introduced the concept of nondestructive testing and provided brief descriptions of the various techniques currently available. TechSolutions 2, published in Vol. 1,No. 3 of the AMMTIAC Quarterly focused in on the most common nondestructive testing technique: Visual Inspection.This current article continues the series and covers another very common technique: Eddy Current Testing. Once the series on nondestructive testing methods is complete, we will combine all of the articles into a valuable desk reference onnondestructive testing and place it on our website. - Editor

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The AMMTIAC Quarterly, Volume 1, Number 48

How Eddy Current Testing WorksA crack in the surface, or near the surface of the specimeninterrupts the current flowing in the specimen (i.e., it locallychanges the electrical conductivity) and causes a change in the adjacent magnetic field. The induction coil is scanned overthe specimen, and the magnetic field is measured by a sensorand recorded. In another approach, there is no second orsensing coil, and the reluctance** is measured directly in theexciting or induction coil to locate a crack.

Figure 2 shows the principal elements of four types oftypical eddy current systems. Figure 2 (a) shows a simplearrangement, in which voltage across the coil is monitored.Figure 2 (b) shows a typical impedance bridge. Figure 2 (c)shows an impedance bridge with dual coils, and Figure 2 (d)shows an impedance bridge with dual coils and a referencesample in the second cell.

The location of the eddy currents in the specimen in the z,or depth direction, is a function of the frequency. As thefrequency is increased, the eddy currents are increasinglyconcentrated near the surface of the specimen, and as thefrequency is decreased the eddy currents increase their

penetration into the specimen. Employing a variety offrequencies to probe different depths in the specimen can bevery useful for analyzing a greater volume of the specimen.

Types of DiscontinuitiesThere are a number of different discontinuities that can bedetected with eddy current NDT. In metallic structures, welds,fatigue cracks, voids, hidden corrosion and stress corrosioncracks can be detected (Figure 3) and the size of such defectscan also be determined. The geometry of the part and thedefect location dictate the size of the flaw that can be detected.For example, automated and manual eddy current inspectionof gas turbine engine disks can reliably detect cracks as small as0.023 inches in length in boltholes of seventh stage compressordisks.[3] Defects such as delaminations, voids and brokenfibers from impact damage can be detected in graphite epoxycomposites. While in carbon/carbon composites for hightemperature use, eddy current NDT can be used to determinethe thickness of the silicon carbide (SiC) coating used oncarbon/carbon composite for oxidation protection. In addition,voids caused by oxidation between the SiC coating and thecarbon/carbon base can be detected and carbon loss due tooxidation can be determined using eddy current NDT. Eddycurrent NDT can be used on conducting materials includingmetals, alloys, carbon/epoxy composites, carbon/carboncomposites, and metallic matrix composites.

INSPECTION REQUIREMENTSThere are no special facility requirements for eddy currentNDT, and portable instrumentation is available for fieldapplications such as aircraft inspection, as shown in Figure 4.Rugged eddy current equipment is also available for use inmanufacturing environments to inspect metallic products as they are being processed. There is no special materialpreparation for testing, but a smooth surface producesoptimum results. Eddy current equipment is calibrated usingphysical calibration standards made of the same material withthe same geometry as the part to be tested. Electrodischargemachining (EDM) notches, drilled holes, etc., can serve as

Figure 2. Four Types of Eddy Current Instruments. (a) A Simple Arrangement, in which Voltage across the Coil is Monitored. (b) Typical Impedance Bridge. (c) Impedance Bridge with Dual Coils. (d) Impedance Bridge with Dual Coils and a Reference Sample in the Second Coil.[2] (Reprinted with permission of ASM International®. All rights reserved. www.asminternational.org)

Figure 3. A SeniorAirman Uses EddyCurrent to LocateSurface Cracks on aC-5 Flange. (Phototaken by Staff Sgt.Matt McGovern andprovided courtesyof the US Air Force)

Resistor

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ac VR

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ac V

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flaws, and several sizes should be used to encompass the actualflaw sizes expected. Figure 5 shows several fabricateddiscontinuities used as standards in eddy current inspection.

Real flaws such as fatigue cracks, stress corrosion cracks, etc.,are required for improved accuracy in sizing of defects. Thedistance of the inspection coil from the surface of the sample,called “liftoff ” must also be carefully controlled. The inter-pretation of results using the modern, computer-based eddycurrent equipment is straightforward with both a display screenshowing the results and the computer recording the data.

PRACTICAL CONSIDERATIONSCommercial, off-the-shelf eddy current equipment is availablethat is very portable and user-friendly. Training for the eddycurrent testing technique is somewhat straightforward (Figure6). However, certified NDT inspectors are either recommendedor required and the training for a certified inspector involvesmore in-depth training than just how to use the instrument and

interpret the results. In the US, eddy current inspectors can becertified by NAS 410 from the Aerospace Industries Associationof America, or by the American Society for NondestructiveTesting (ASNT). Eddy current instruments range in cost fromabout $10,000 to about $30,000.

In additional to conventional eddy current NDT techniques,remote field eddy current inspection capability was developed toinspect tubular metallic products from the inside of the tube.This technique, illustrated in Figure 7, provides a means ofinspecting the outside of the tube wall with only an interioreddy current probe. The technique is applicable to any metallicmaterial, but has been primarily applied to ferromagneticmaterials since the wall of the tube must be magneticallysaturated. The outside of the tube or pipe can be inspected forcorrosion/erosion wall thinning, pitting, and cracks. Thetechnique is equally sensitive to axial and circumferential flaws.The major disadvantage is that when applied to nonmagneticmaterials the sensitivity is generally decreased.

Advantages and Disadvantages of Eddy Current TestingThere are several advantages to using the method of eddycurrent testing. These typically include:

• Reasonable cost• Availability of a wide variety of commercial,

off-the-shelf instruments• Automation potential• Good sensitivity to small flaws at or near the surface

of the sample• Capability for quantitative flaw sizing• Portable equipment

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Figure 4. A Royal AirForce Airman Checksfor Potential Flaws on an Aircraft UsingEddy Current. (Phototaken by Staff Sgt.Rhiannon Willard andprovided courtesy ofthe US Air Force)

Figure 5. Several Fabricated Discontinuities Used as ReferenceStandards in Eddy Current Inspection.[2] (Reprinted with permission of ASM International®. All rights reserved.www.asminternational.org)

Figure 6. A US AirForce Airman Non-destructive InspectionApprentice Checks forDefects on a TrainingAircraft Part by Performing an EddyCurrent Inspection(Photo taken by StaffSgt. Michael B. Kellerand providedcourtesy of theUS Air Force)

Figure 7. Schematic Showing Location of Remote-Field Zone inRelation to Exciter Coil and Direct Coupling Zone.[2] (Reprintedwith permission of ASM International®. All rights reserved.www.asminternational.org)

Figure 8. Standard Depths of Penetration as a Function of Frequencies Used in Eddy Current Inspection for Several Metals of Various Electrical Conductivities. [2] (Reprinted with permission of ASM International®. All rights reserved.www.asminternational.org)

Filed Transverse Notch

Milled orElectrical Discharge

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The AMMTIAC Quarterly, Volume 1, Number 410

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The disadvantages of eddy current NDT include the lack of capability to detect flaws that are deep in thick sectionmetallic structures and the restriction for application to onlyconducting materials. Figure 8 gives the standard depths of penetration of eddy currents as a function of frequency for several metals of various electrical conductivities.

SELECTED EXAMPLES OF EDDY CURRENT APPLICATIONSEddy current NDT is widely used to inspect for corrosion

and cracking in airplane wing skins at rivet holes and inaircraft frames.[4] Modern commercial eddy currentinstrumentation capable of operating down to 60 Hz withsmall eddy current probes is now available to detect smallfatigue cracks below the surface in aircraft airframes withmore sensitivity than X-ray radiography. Fatigue cracks canalso be detected in layered structures such as an aircraftwindow belt splice, which is illustrated in Figure 9 (a). The technique produces easily interpreted crack responses

EddyCurrent

Summary

Figure 9. Detection of Crack in Second Layer by Scanning OverFasteners with a 15 mm (0.6 in.) Probe at 1 kHz: (a) ScanningProcedure; (b) No Crack Response; (c) Crack Response.[4] (Copyright 2004 © The American Society for Nondestructive Testing, Inc. Reprinted with permission from NDT Handbook,third edition: Vol. 5, Electromagnetic Testing.)

Figure 10. Typical Applications for Low Frequency Eddy CurrentTesting: (a) Interstitial Corrosion; (b) Thinning.[4] (Copyright 2004© The American Society for Nondestructive Testing, Inc. Reprintedwith permission from NDT Handbook, third edition: Vol. 5, Electromagnetic Testing.)

Discontinuity types • Cracks(e.g., what types the method can detect) • Holes

• Corrosion• Damage in C/C composites

Size of discontinuities • Depends on geometry, etc., but down to 0.020” for jet turbine engine fatigue cracks at or near the surface

Limitations • Material must be electrically conducting• Depth of inspection limited by frequency of eddy currents used • Not effective for cracks deep in thick sectioned metallic structures

Advantages • Can detect discontinuities that do not break the surface • Quantitative flaw sizing• Can be automated for repetitive inspections

Inspector training (level • Usually required, with commercial or military training schools and/or availability) available

Inspector certification • Depends on application required • Certification available through the American Society for Non-

destructive Inspection (ASNT)

Equipment • Except for very simple equipment, most have computer control and data logging, automated scanning is also available, single frequency or multi-frequency equipment is available

Relative cost • Can be labor intensive if used manually on large areas such as of inspection airplane skins and rivets, equipment cost vary from a modest amount

to $100,000 or more

Aluminum Fasteners

ProbeUp(a) (a)

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on a screen display, as shown in Figure 9 (b & c).These cracks were detected in the first row of rivets above

the longitudinal belt splice of aircraft windows. The cracksinitiated at the fastener holes in the internal (second layer) skinand grew in a longitudinal direction. Corrosion of multiplayeraircraft skins can also be readily detected with eddy currenttechniques, as shown in Figure 10.

Eddy current techniques are also in widespread use todetect fatigue cracks in critical aircraft jet engines components,such as blades and turbine disk during overhaul. Fordiscontinuities more than 0.07 in. (1.8 mm) long fluorescentpenetrant inspection will usually suffice. However, for cracksbelow 0.07 in. (1.8 mm) in length, eddy current NDT isusually required.

CONCLUSIONSEddy current NDT is a mature technology with widespreadavailability of user-friendly, affordable, commercial, off-the-shelf equipment. It can be used on conducting materials andcan detect many types of discontinuities. Eddy current testinghas enjoyed considerable success in a number of applicationsincluding, for example, inspection of nuclear reactor heatexchanger tubes, aircraft engine and metal skin components,and in the manufacturing plant inspection of a variety ofmetallic components. In addition, eddy current NDT is widelyused to inspect welds along with X-ray radiography andultrasonic testing.

NOTES & REFERENCES* Electromagnetic induction is the generation of an electrical voltageacross a conducting material through stimulation by an applied alternating magnetic field.

† Tests used to quickly sort metal alloys with differing chemical oralloying compositions.

‡ Giant magnetoresistance is a phenomenon where the application of a magnetic field reduces the electrical resistance of certain materials by a significant margin.

§ An eddy current is an electrical current that flows in a circular path or loop and is induced by an applied magnetic field.

** Reluctance in a magnetic system is akin to resistance in an electrical system.

[1] C.J. Hellier, “Eddy Current Testing,” Handbook of NondestructiveEvaluation, McGraw-Hill, NY, 2001, pp. 8.1-8.7.[2] “Eddy Current Inspection,” ASM Metals Handbook, Ninth Edition, Vol. 17, Nondestructive Inspection and Quality Control,1989, ASM International, Metals Park, OH, pp. 164-194.

[3] Nondestructive Evaluation Capabilities Data Book, 3rd Edition,Appendix A, Eddy Current Inspection, NTIAC, DB-97-02, November 1997.[4] D.J. Hagemaier, “Low Frequency Eddy Current Testing of Aircraft Structures,” Nondestructive Testing Handbook, Third Edition, Volume 5; S.S. Udpa and P.O. Moore, editors; AmericanSociety for Nondestructive Testing, 2004, pp. 481-485.

GENERAL REFERENCESN. Eua-Anant, X. Cai, L. Udpa, J. Chao and I. Elshafiey, “CrackDetection in Eddy Current Images of Jet Engine Disks,” Review ofProgress in Quantitative Nondestructive Evaluation, Vol. 19A. AmericanInstitute of Physics, Melville, NY, May 2000, pp. 773-780.

E.M. Franklin “Eddy Current Inspection,” Materials Evaluation,Vol. 40, No. 10, American Society for Nondestructive Testing,Columbus, OH, September 1982, pp. 1008-1010.

N. Goldfine, V. Zilbertstein, K. Walrath, E. Hill and C. Paraizaman,“Inspection of Gas Turbine Components Using Conformable MWMEddy-Current Sensors,” ASNT Fall Conference and Quality TestingShow–2000: Paper Summaries Book, American Society for Non-destructive Testing, Columbus, OH, November 2000, pp. 29-31.

D.J.Hagemaier, “Nondestructive Testing of Aging Aircraft,” FAAAging Aircraft Workshop, American Society for Nondestructive Testing,Columbus, OH, 1990, pp. 4-12.

D.J. Hagemaier, and G. Klark, “Eddy Current Detection of ShortCracks under Installed Fasteners,” Materials Evaluation, Vol. 55, No. 1, American Society for Nondestructive Testing, Columbus, OH,January 1997, pp. 25-30.

A.W. Irvine, “Quick Eddy Current Inspection of Aircraft Wheels,”Materials Evaluation, Vol. 55, No. 5, American Society for Non-destructive Testing, Columbus, OH, May 1997, p. 573.

B. Lepine, D. Forsyth, S. Guiguere and S. Dubois, “Comparison ofPulsed Eddy Current NDT to Conventional Eddy Current Testing,”ASNT Spring Conference and 8th Annual Research Symposium PaperSummaries, American Society for Nondestructive Testing, Columbus,OH, March 1999, p. 124.

“MIL-STD-1537C, Electrical Conductivity Test for Verification of Heat Treatment of Aluminum Alloys, Eddy Current Method,” United States Department of Defense, Arlington, VA, 2002.

D. Moore, J. Mihelic and J.D. Barnes, “Crack Detection on HC-130H Aircraft Using Low Frequency Eddy Current,” The 9th Asia-Pacific Conference on Nondestructive Testing in Conjunctionwith ASNT’s 1998 Spring Conference and 7th Annual Research Symposium, American Society for Nondestructive Testing, Columbus,OH, March 1998, pp. 202-205.

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