About the Cover: Coating applicators apply a layer of primer to the underbelly of an F-117 Nighthawkbefore it is painted gray. (Photo taken by Airman 1st Class Vanessa Laboy and providedcourtesy of the US Air Force)

Our feature article in this issue reminded methat it was about a year and a half ago that I firstsaw a demonstration of the virtual spray systemat the STAR4D® exhibit, which is the IowaWaste Reduction Center’s (IWRC) innovative

training tool for honing the skillsof spray gun operators. It was oneof the few “hands-on” exhibits atthe conference I was attending. Anumber of attendees stood in linewaiting to have their turn at the

simulator. Many of the participants, including anumber of my colleagues, congregated aftertheir turn, proudly comparing their scores todetermine who had “sprayed” their virtual panelbest. In this case, scores were based on a numberof factors, including how uniform the coatingwas applied or by the amount of waste generat-ed. Having operated manual, air-assisted spraysystems myself earlier in my career, I alreadyknew firsthand that manual spraying of coatingsis still mostly an art, and that no amount ofautomation or level of technology will ever total-ly replace it. There will always be situationswhere manual application is necessary.

To most at the conference, the main attractionof the STAR4D® (Spray Technique Analysis andResearch for Defense) exhibit was in all likeli-hood its hands-on demonstration, which wouldnaturally appeal to engineers and other technicalpersonnel. However, the experience crystallizedin my head a very different thought as I walkedaway from the exhibit. Most technology we have

developed throughout the history of civilizationhas either expanded our capabilities to affect theworld around us (more so than that of an unaid-ed human) or has assumed some function thatwould otherwise be performed by people. Whilethe technologies featured in the article are notunique in this respect, they are part of the com-paratively small group of technologies thatspecifically acknowledge the importance ofhumans and foster their interaction rather thandiminish or supplant them. In such systems, theprimary intent is to foster and enhance the indi-vidual’s ability to master a technology, and notto replace them with it.

As technologists, I think we sometimes losesight of this basic precept. In our haste and zealto create enabling technologies and systems toaid and enhance the nation’s defense capabilities,it is easy to overlook the human element that isthe very essence of the warfighter. Therefore, wemust first take the human factor into account ineverything we do. Technology is meant to servepeople, and not the other way around. Let us allremember those men and women who comprisethe front line of the nation’s defenses, and whohave far more invested in the endeavor. Whilewe all strive in our own way to define, develop,design, manufacture, field, or maintain the sys-tems that serve the warfighter, we must remem-ber that we ultimately serve those people whoare the warfighter.

Chris GrethleinDeputy Director

Always Remember

the Human Factor

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INTRODUCTIONThe effectiveness of US military forces is largely dependent on thereadiness of the equipment and vehicles they use. One process inparticular that plays an important role in the overall readiness ofmilitary equipment is the spray application of coatings. This is significant because it is such a pervasive process within the ArmedForces. Spray-applied coatings are used for many types of equip-ment, components and vehicles including aircraft, ground vehi-cles, water-borne vessels and ordnance. For example, if a coatinghas to be re-applied because it was poorly applied in the first-place, re-deployment of the vehicle will be delayed, therebystraining military readiness. Coating re-application also results inincreased usage of labor and materials, thus increasing overallmaintenance costs. Improved coating application quality there-fore results in reduced cost and increased readiness.

Spray Application by the DoDThe DoD has implemented some of the most advanced coatingsystems ever created. However, the performance of these coatingsis dependent upon the effectiveness of their application. Sincecoatings are designed to perform specific tasks, they are typicallyvery expensive. By using high performance coatings efficiently,the DoD can realize significant monetary savings through thereduction in coating purchases, material rework and waste dispos-al. Training has been the conventional route to improving thequality of coating application, but advances in both spray appli-cation technologies and training programs have brought the qual-ity of coating application to the next level. This article describessome of the nuances of the coating application process that affectthe quality of the coating and also presents a special training pro-gram and new technologies that have been developed to helpcoating applicators improve the quality of their coatings.

THE COATING APPLICATION PROCESSApplying coatings using a spray gun (Figure 1) is a skill that canonly be mastered through effective hands-on training and years ofexperience. However, the fundamental skills and knowledgerequired for acceptable DoD coating application can be passed on through effective education and training. Coating applicationeffectiveness can generally be evaluated based upon three factors of application outcomes: efficiency, accuracy and consistency. Inaddition to these three factors, spray applicators must also possessthe knowledge required to effectively prepare surfaces and coatingmaterials and safely dispose of the waste products which could contain hazardous materials.

Spray Gun SettingsIn order to efficiently apply coatings, spray applicators mustunderstand and follow several techniques. One of the most

important preparatory steps is properly setting up a spray gun sothat it operates at its maximum efficiency. A common mistake insetting up the gun is using excessive air pressure, which reducesthe amount of paint that adheres to the part. Properly adjustingthe fan pattern, air pressure and fluid flow can drastically affectthe efficiency with which coatings are applied to substrates.

Coating Application ParametersOther techniques that can improve efficiency include the reduc-tion of lead and lag space* and maintaining the proper spray gun-to-substrate distance. Also of primary importance is maintainingan appropriate speed with which the spray gun is moved whiletaking into consideration theparticular properties of thecoating being applied. Sprayapplicators must learn aboutthe properties of certain coat-ings and how fast they shouldbe applied. This knowledge canthen be converted into skillthrough training and experi-ence, because ultimately, thisskill and knowledge can resultin more accurate mil-build(coating thickness applied tosubstrate). As a result of inap-propriate mil build, coatingdefects can develop in the formsof corrosion or poor abrasionresistance. Thus, an accurateand consistent mil-build is themost important factor affectingcoating performance and dura-bility of the complete coatingsystem. Techniques that can betaught to improve consistencyinclude identifying and producing the proper fifty percent over-lap with each spray pass and maintaining an orientation of hold-ing the spray gun perpendicular to the substrate.

All of these factors play a pivotal role with regard to the endproduct and the coating quality. Beginning at the preparation stageand continuing through the entire application process, the imple-mentation of these techniques assure an efficient, accurate and con-sistent finish on materials. Perhaps the best way to quickly achievethis level of proficiency is through proper and effective training.However, to supplement and enhance training, several technologieshave been developed to help coating applicators cultivate and main-tain proper techniques for the coating application process.

Lea Ann SchellhornIowa Waste Reduction Center

Cedar Falls, IA

http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 1, Number 4 3

Figure 1. An Anti-Terrorism BombBlast Protective Coating is sprayedonto the Interior Walls of an AirForce Base facility. (Photo taken by Airman 1st Class Nicholas Pilchand provided courtesy of the US Air Force)

The AMMTIAC Quarterly, Volume 1, Number 44

LASER-GUIDED SPRAY GUN ATTACHMENTEfficiently using raw coating materials can lead to significant sav-ings when large quantities of coatings are required. This leads tothe question of how can coatings be used more efficiently? Oneanswer to this question is maintaining a proper spray gun dis-tance. When spray guns are too far away from the substrate thereis an increased likelihood of coating over-atomization†. One resultof over-atomization is that the coating material can bounce offthe substrate, which results in wasted material. Improving theconsistency with which substrates are coated can reduce theoccurrence of several problematic coating application results. Twoof the most easily recognized results of an inconsistent finish aresags and light spots. Sags result in areas with a wet paint filmbuild that is overly heavy causing the paint to drip down the sur-face, and light spots can cause certain coatings, such as chemicalagent resistant coatings (CARC), to lose their effectiveness. Oftenthese problems can be attributed to a change in the distance thespray gun was held from the substrate.

The Iowa Waste Reduction Center (IWRC) has combined avail-able technologies to help applicators mitigate these problems. Toensure applicators maintain a consistent distance between the noz-zle of the spray gun and the substrate, IWRC created a special laserdevice which attaches to any spray gun. The laser-guided spray gunattachment utilizes a class 3A device laser beam‡ that passesthrough a beam splitter producing two separate beams, which areprojected onto the substrate being painted, as shown in Figure 2.As the applicator moves the gun toward or away from the substratethe two laser dots move in relation to each other. When the twolaser dots converge and result in one dot on the substrate, this indi-cates to the applicator that the paint from the spray gun is travel-ing the optimal distance from the spray gun tip to the substrate.When the distance between the spray gun and substrate changes,the laser dot will separate from one dot to two dots. The spray gunsettings can be adjusted by an experienced applicator to determinethe optimal distance at which the lasers converge. This visual feedback serves as an aid for maintaining a consistent finish anduniform thickness on the substrate and in the long-term helps

prevent coating failure.An additional advantage of this sys-

tem is that applicators have been able togauge exactly where the paint is goingto land using the converged dots of thelaser to target the gun. Increased con-trol over where paint is sprayed candecrease the amount of paint sprayedoff of the edge of the part and improvetransfer efficiency§. Increased transferefficiency leads to a reduction involatile organic compound (VOC)emissions, hazardous wastes and ozonecausing air pollution. IWRC data haveshown that transfer efficiencies can beincreased by almost forty percent when

using this laser-guided spray gun attachment.In addition, the laser attachment has demonstrated another

advantage in that it has helped to more precisely control over-lapping spray passes at the optimal fifty percent overlap. When afifty percent overlap is achieved, coating application defects suchas tiger striping** are eliminated.

The laser-guided spray gun attachment has been shown toimprove the technique of spray applicators, while decreasingcosts, materials, waste and time. With this new technology, sprayapplicators have demonstrated that they can consistently apply ahigh quality finish while reducing the environmental impact.

TRAININGEffective training is rarely provided to commercial and industrialspray applicators. In most cases, an applicator is handed a spraygun with minimal knowledge and is forced to either learn on theirown through trial-and-error or while watching their peers. Thislack of standardized fundamental training can lead to a significantgap in the knowledge and skill level of applicators.

Extensive hands-on training is necessary to help applicatorsunderstand, practice and become proficient with proper coatingapplication techniques. Traditionally, however, there have beenmany drawbacks to providing effective hands-on training to coat-ing applicators. These typically include the amount of time, labor,space, material, hazardous wastes, and safety precautions that arerequired, as well as the cost that is incurred when conductingtraining. Such obstacles and expenses lead to less than optimalrates of training for incoming spray technicians. Consequently,these coating applicators are often trained on the job, possiblylearning bad habits from more experienced applicators, who alsomay not have been trained properly. To help overcome some ofthese obstacles and costs a special training program was created toaddress the issues and provide the DoD with a resource for train-ing incoming and experienced applicators.

A New Approach to TrainingThe Spray Technique Analysis and Research for Defense(STAR4D®)†† program was developed and implemented to pro-vide individual, hands-on training to military spray applicators,focusing on developing proper techniques as well as improvingefficiency and reducing waste. This program, which was estab-lished by IWRC, has provided military personnel with the education and resources needed to develop quality painting tech-niques. The program has utilized state-of-the-art technologiesand hands-on painter training to not only help improve coatingdurability and performance, but also to save money andresources by implementing waste reduction initiatives.

This training program for military personnel has taught appli-cators how to effectively spray specific components, equipment, vehicles and surfaces to maximize coating efficiency and minimizeenvironmental pollution. Sample training results, which areshown in Table 1, compare the transfer efficiency of coating appli-cators before and after training. Other data are also shown in thetable, including the amount of environmentally harmful VOCsreleased. The test results show that the average transfer efficiencyfor 254 students improved from 56 percent to 66 percent aftertraining. Additionally, the amount of coatings is shown to havebeen reduced by 13 percent among the students trained. [1]

Military personnel have traveled to the facility in Cedar Falls,Iowa, to learn advanced spray techniques, master new technolo-gies and examine alternative technologies that allow spray appli-cators to improve quality. As a result, military spray techniciansfrom 65 sites in 40 states and territories have received instructionin the form of training, equipment and waste reduction advice.

The program educates trainees about the fundamentals of coat-

Figure 2. Two Laser Beams Help the Coating ApplicatorMaintain Proper Distance from the Substrate and Keep the Coating on Target.

http://ammtiac.alionscience.com The AMMTIAC Quarterly, Volume 1, Number 4 5

ings and spray techniques. Furthermore, in order to give applica-tors training that was relevant to their job, the program makesevery attempt to use the same coatings and equipment that theytypically use on-site (Figure 3). The training staff also demon-strates some of the new or existing technologies which may provemore efficient for their process.

During the training course, applicators go through a mix ofclassroom, hands-on and simulation training. A pre-test and post-test are administered which allows the trainees to see their growthand improvements from the first day to the last. By focusing onstrategies and techniques that enable military spray applicators touse less material and improve spray technique, applicators are ableto return to their paint facilities and produce a quality finish whilemost likely lowering the costs and environmental implicationsassociated with painting.

Coating Application ResearchIn addition to training, IWRC has been conducting numerous studies and research relating to painting and coating operations.In response to anticipated local and federal restrictions on volatileorganic compound (VOC) and hazardous air pollutant (HAP)emissions, the DoD is in the process of replacing its current sol-vent-borne chemical agent resistant coatings with water dis-persible CARC (WD-CARC II), which are in compliance withthese restrictions. The STAR4D® training program is currentlyproviding assistance in examining the compatibility of the newtypes of WD-CARC II and is investigating the possible use ofinfrared curing in order to reduce potentially problematic curetimes. Supplemental to these applied studies on the performanceof newly developed CARC, a cost analysis regarding the place-ment of solvent-borne CARC to WD-CARC II was recently com-

pleted for the DoD. Studies of WD-CARC II dry times have alsobeen conducted for the National Guard Bureau in the past year.IWRC has also developed a data compilation containing CARCinformation for the Army. This CD has been distributed to vari-ous military bases across the United States and is continuallybeing updated with new information.

IWRC has spent the past two years developing a standardizedpainter training course for Air Force spray applications. In itsentirety, the course consists of instructor manuals, student man-uals and presentations for 13 units that encompass the processof coating application. Six military bases were visited with thepurpose of determining the factors and problems that needed to be addressed and incorporated into an effective training pro-gram. Based upon the research at the bases, the IWRC designedthe program to meet those needs and developed the instructormanuals accordingly.

Other Coatings Projects and TrainingIWRC is also involved in projects and studies that involve theuse of black lights for the inspection of coating mil-build and the development of a painter certification course. By discoveringnew ways to examine acceptable mil-build, such as using a blacklight, a reduction in corrosion could result by reducing theoccurrence of light spots. Additionally, the goals of the paintercertification course will be to educate and help select spray applicators for the DoD.

The program offers an interactive website to complement thetraining program. The site, www.star4d.org, serves as a centralizedlocation where painters of the US Armed Services can find usefulinformation.

Virtual Spray TrainingTo eliminate many of the drawbacks of conventional spray appli-cation training, an innovative approach was developed that uti-lizes virtual reality technologies such as simulated paint andsurfaces. Spray applicators are now able to view and interact withreal spray application equipment while simulating the actualapplication process onto a virtual surface, as shown in Figure 4.The conventional training requirements of a paint booth, safetyequipment, solvents or a regulatory permit are no longer neces-sary. The time spent to prepare the surface, mix the coatings andcleanup are also factors that no longer must be considered in thetraining process.

Simulated spraying is very similar to actual spray applicationbut with additional benefits. The virtual training system uses areal spray gun which is uniquely instrumented to allow applica-tors to control flow rate and fan size in the same manner as theydo on other spray guns. Those factors can also be displayed on thesimulator screen to enable applicators to completely understandhow the spray gun operates.

How the Virtual Training System WorksWhen the spray gun trigger is pulled, the position and orientationof the spray gun is automatically tracked in relation to the virtu-al surface. A software program translates the data input and com-municates it to a projector which then displays the spray patternonto the screen.

Along with the benefits of learning the spray gun operation,applicators are also provided with instantaneous feedback

Table 1. STAR4D® Results from Training Classes to Date [1].No. of Students: Pre-Training Post-Training Difference Percent

254 Change

Transfer Efficiency 55.56% 66.12% 10.56% 19.01%

Amount of Coatings 0.15 0.13 0.02 13.33%Used (gal)

Amount of VOCs 0.45 0.39 0.06 12.57%Released (lbs)

Amount of Coating 0.06 0.05 0.01 16.67%Material Used per Thickness Applied (gal/mil)

Figure 3. A Military Technician Practices Proper Spray Technique onthe Hood of a Vehicle.

regarding their spray pattern and overall application. Mil-buildaverage, ounces of paint sprayed and elapsed time spent spray-ing are also displayed on the screen. Regarding the actual application, transfer efficiency and overspray are also displayed.All of these factors combine to give applicators a quantitativeassessment of their performance and ultimately the overall effectiveness of the training.

While all of the factors explained thus far are important, pos-sibly the most useful display factor is the use of two differentpaint accumulation modes: the single-color and the multi-coloraccumulation mode (Figure 5). Single-color paint mode is anapproximation of the actual appearance of paint. In this mode,shades of a single color are used to indicate paint thickness. Inmulti-color accumulation mode, multiple shades of three colorsare used to indicate the thickness of the paint. In this mode, thesurface color represents an area with no paint. Shades of blue represent values below the minimum thickness setting. Shades ofgreen represent values within the minimum and maximum target thickness range. Shades of red represent thickness valuesthat exceed the maximum target.

AdvantagesThis system has allowed for realistic hands-on painter training tobe conducted in a more suitable setting. For example, the virtualtraining can be conducted in a classroom with more studentscompared to conventional training. The instructors can providedemonstrations encompassing spray gun setup and proper appli-cation and then supervise the students as they practice in the sameroom. This allows the instructor to identify strengths and weak-nesses of each trainee.

The virtual spray application training is beneficial becauseapplicators are presented with an unlimited amount of practiceparts and materials while completely eliminating the byproductsof hazardous waste or air emissions. Applicators are able to

explore coating application and accumulation in an entirely newway, while improving the motor skills necessary for applying highperformance coatings appropriately.

SUMMARYThe military spray application training program, STAR4D®,at the IWRC continues to meet the needs of military spray applicators by continually updating their knowledge-base andincreasing their technological capabilities through tools such asthe laser-guided spray gun attachment and the virtual trainingsystem. With the help of this program and these technologies, the DoD continues to properly train and educate spray applica-tors. As a result, coatings applied to real-life military systems andequipment will be of higher quality to ensure these systems are ready in the time of need.

NOTES AND REFERENCE* Lead is the distance between the point where the material leaves the partand the point where the spray gun trigger is released; lag is the distancebetween the point where the spray gun is triggered off of the part and thepoint where the material leaving the spray gun lands on the part. † Over-atomization results when the paint droplet size becomes too smalland does not adhere or bounces off the substrate.‡ A class 3A laser is one that is produced for continuous operations but does not require operator training. § Transfer efficiency is the amount of coating material deposited on the substrate divided by the amount of coating material sprayed.** Tiger striping is a defect that occurs as a result of an inconsistent filmbuild.†† STAR4D® is a registered trademark of the Iowa Waste Reduction Center.

[1] STAR4D® Training Class Database, Iowa Waste Reduction Center, University of Northern Iowa, http://www.star4d.org/database.cfm.

The AMMTIAC Quarterly, Volume 1, Number 46

Lea Ann Schellhorn is a member of the staff at the Iowa Waste Reduction Center (IWRC). As a part of Iowa’s 1987 Groundwater Protec-tion Act, the IWRC was established as a service of the University of Northern Iowa (UNI) in Cedar Falls, Iowa. The IWRC receives stateand federal funding to provide small Iowa businesses with the guidance and resources necessary to keep up-to-date with the constantlychanging environmental regulations. A portion of the Center has focused on the painting and coating industry for over 10 years. Not only istraining conducted on-site, but the staff continuously keep up-to-date with knowledge and advancements in the industry. The IWRC is contin-uously striving to expand their resources and services to provide equipment and tools to assist spray technicians in their day-to-day routines.

Figure 4.Demonstrationof the VirtualSpray Appli-cation TrainingSystem.

Figure 5. Two Viewing Modes of the Virtual Training System: SingleColor Mode (Left) and Accumulation Mode (Right). Various ThicknessLevels Are Visible in the Accumulation Mode.

techsolutions 3

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

techsolutions 3

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

Voltmeter

TestSample

InspectionCoil

ac VR

Ground Ground Ground

Voltmeter

TestSample

InspectionCoil

BalancingImpedance

ac V

R R

R

Ground Ground

Voltmeter

TestSample

InspectionCoil

Coil(BalancingImpedance)

Coil(BalancingImpedance)

ReferenceSample

ac V

R R

RR

Ground Ground

Voltmeter

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InspectionCoil

ac V

R R

RR

Ground Ground

(a) (b) (c) (d)

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

Machined Longitudinal Notch

Milled orElectrical Discharge

Machined Transverse NotchDrilled Hole

Tube OD DirectCoupling

Zone

Remote Field ZoneTube ID

Exciter Coil Detector Array

10 102 103 104 105 106 107 108

Frequency, Hz

10

1

0.1

0.01

0.001

0.0001

Dep

thof

Pene

tratio

n,in

.

Graphite

Titanium

Stainless Steel

Aluminum

Ingot Iron

High-Alloy Steel

Copper

The AMMTIAC Quarterly, Volume 1, Number 410

techsolutions 3

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)

(b)(b) (c)

Forward

ScanDirection

Nonmetallic Guide

Liftoff Aluminum

Fasteners

Liftoff Aluminum

Crack Response

Resistance R(Relative Scale)

Resistance R(Relative Scale)

Reac

tanc

eX

(Rel

ativ

eSc

ale)

Reac

tanc

eX

(Rel

ativ

eSc

ale)

Crack

1.2 mm(0.05 in.)

0.6 mm (0.025 in.)0.4 mm (0.085 in.)

Probe

Corrosion

Corrosion

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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AMMTIAC’s National Materials Information System(NAMIS) now houses the many volumes of valuable datagenerated under the $152M Air Force Composite Afford-ability Initiative (CAI). By selecting NAMIS as its datadepository, the Air Force benefited from thedocument storage, database access and secu-rity mechanisms for managing and accessingexport controlled information that NAMISprovides. AMMTIAC developed a data mod-ule and an interface to enable authorized users toaccess the information. In addition, CAI was featured inthe lead article of the AMMTIAC Quarterly, Vol. 1, No. 3.

In the mid-1990s, the Air Force Research Laboratory’sMaterials and Manufacturing Directorate (AFRL/ML)initiated an 11-year, $152M effort to breach the afford-ability barriers that were preventing widespread use ofcomposite structures in air vehicles. This effort involvedthe entire industrial base of aerospace primes, as well as theAir Force and the Navy. All participants agreed to a single

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participating aerospace primes related to matur-ing improved manufacturing processes, advanced materi-als and structural design. The database captures thematerials and manufacturing pedigree of composite testdata by preserving metadata such as pretest preparations,test conditions, manufacturing processes used, and thecomposite architecture. The database includes high valuedata from full-scale development projects that have result-ed in transition to real systems, such as the F-35, X-45,and the C-17. AMMTIAC provides a means to maintainthe data for future use by Army, Navy, and Air Force man-ufacturing technology programs, as well as weapon systemprogram offices.

The CAI Database will continue to provide companydesigners, engineers, and weapon system program officeswith the data they require to introduce advanced compos-ite technologies into current and future weapon systems.The database will also minimize the financial burden oftesting required to implement these new technologies, aswell as reduce the risk and accelerate the transition of thetechnologies. Qualified users can access this data throughNAMIS at http://namis.alionscience.com.

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2007 Electronic Materials Conference06/20/07 - 06/22/07Notre Dame, INContact: TMS184 Thorn Hill RdWarrendale, PA 15086 Phone: 724.776.9000 ext 243Email: mtgserv@tms.orgWeb Link: www.tms.org

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