the NDT Technician - ASNT

RRotary bolt hole eddy current (BHEC)is widely used throughout the aircraftindustry, both civilian and military, fordetection of fatigue cracks withinholes from which structural fastenershave been removed. Although theinspection is fairly straightforward toperform, its reliability is highlydependent on equipment setupparameters and signal interpretation.Parameters such as drive frequencyselection, filtering and phase angleadjustment are critical, but often arenot optimally employed. Improperselection of drive frequency, phaseadjustment and/or filters canintroduce excessive inspection noise asa result of being too sensitive tosignals from nonrelevant surfacefeatures or may result in suppressionof relevant responses, potentiallyallowing significant defects to remainundetected.

This article highlights the role offrequency selection, filtering andphase separation to: (1) maximizesignal-to-noise ratio, (2) achieveoptimum detection performance, and(3) reduce the propensity for falsepositives.

Equipment

The equipment used for BHECinspection typically includes a portableeddy current instrument, a hand heldhigh speed rotary scanner, eddycurrent probes that typically contain adifferential coil (or in some cases anabsolute eddy current coil) and theappropriate cables and referencestandards (Fig. 1). Modern eddycurrent instruments employ two basicmethods for displaying BHEC signals:(1) the impedance plane displayrepresenting the change in totalimpendence (the combination ofresistance and inductive reactance) interms of phase and amplitude, and(2) the sweep display where theamplitude of the signal represents the

* US Air Force Research Laboratory, Systems SupportDivision, Materials Integrity Branch, Wright Patterson AirForce Base, OH 45433; (937) 656-9151† Universal Technology Corporation, 1270 NorthFairfield Rd., Dayton, OH 45432; (937) 656-9149.

theNDT Technician

The American Society for Nondestructive Testingwww.asnt.org

FOCUS

Rotary Bolt Hole Eddy Current Inspection — Optimizing

Performance Through Filter Selection and Phase Analysisby John C. Brausch,* Kenneth J. LaCivita,* and Daniel C. Laufersweiler†

FOCUS continued on page 2.

TNT · January 2010 · 11Vol. 9, No. 1

Figure 1. Portable eddy currentinstruments: (a) typical setup,(b) probe with differential coil.

Eddy currentinstrument

Hand heldrotary scanner Bolt hole eddy current

probe

Referencestandard

Differential coil

((aa))

((bb))

vertical component of the signal displayed in theimpedance plane display, while the horizontal axisrepresents the rotational position of the rotating probe.

Selection of Inspection Parameters

The magnitude of the eddy current response from a crackis dependent on multiple factors including crack dimension(length and depth), crack location (edge or mid-borecracks), crack orientation relative to the probe scandirection, material properties (electrical conductivity andmagnetic permeability) and equipment variables such as coilconfiguration and probe drive frequency.

As a result of fastener removal, most fastener holes willcontain some amount of mechanical damage orcontamination (foreign material debris). These conditionscan induce inspection noise that may mask small indicationsand produce false indications known as false positives as well.When reaming of the hole is not permitted, a mild abrasivehoning of the hole is recommended to prepare the hole forinspection. However, even with careful hole preparation,the signal response of a small crack can be difficult todistinguish from random noise if optimum inspectionparameters are not established.

To many practitioners, the importance of criticalparameters such as test frequency, filters and phaseadjustment and their effect on signal response are notclearly understood. A sound understanding of theseconcepts is critical to establishing effective procedures andensuring their proper implementation. The followingdiscussion defines the effects of these parameters oninspection performance, provides a comparison of bothproper and improper application of these parameters andprovides recommendations for their implementation.

Probe Drive Frequency

Eddy current fields are confined to the surfaces ofconductive materials as a result of the skin effect. Selectionof the oscillating probe drive frequency establishes thedepth the eddy current field penetrates into the surface of agiven material. One standard depth of penetration is definedas the depth at which the eddy current field is reduced to37 percent of the field at the surface. The resulting depth ofpenetration is dependent on the material’s electricalconductivity and magnetic permeability (for ferromagneticmaterials) and is governed by the following equation:

Eq. 1

where:� = standard depth of penetration (in.),f = eddy current excitation frequency (Hz),

δµ σ

= 26

rf ⋅ ⋅

22 · Vol. 9, No. 1

FOCUS continued from page 1.

TTeecchh TToooonn

GFROM THE EDITOR

Greetings and best wishes for a happy and productive new year.The “Focus” article in this issue outlines the means to optimizeeddy current inspection of bolt holes in aircraft. AuthorsBrausch, LaCivita and Laufersweiler stress the importance ofproper equipment setup and signal interpretation.

“New Inspection Technology Heats Things Up” covers recentdevelopments in vibrothermography. Ongoing research beingconducted by Wayne State and Iowa State Universities holds

promise for improvements to thetechnique that will increase both speedand reliability in crack detection.

Lee Hulin describes the “ins and outs”of UT thickness gaging for marinevessels and the benefits of ASNTSection involvement for young NDTpractitioners in the “PractitionerProfile.”

Level I questions and answers for the“Crossword Challenge: Eddy Current Testing” have been adaptedfrom ASNT’s Electromagnetic Testing Method Questions & AnswersBook, second edition: Supplement to Recommended Practice No.SNT-TC-1A, Book E.

Hollis Humphries, TTNNTT EditorPO Box 28518, Columbus, Ohio 43228; (800) 222-2768 X206;

fax (614) 274-6899; e-mail <[email protected]>

µr = relative magnetic permeability (dimensionless),� = electrical conductivity (percent IACS).

Frequency selection is a critical parameter for maximizing thesignal-to-noise ratio. A frequency that is too low will result inreduced sensitivity to small surface cracks. A frequency that istoo high will elevate noise resulting from nonrelevant surfaceblemishes, scratches and burrs. As a general rule of thumb, thefrequency should be selected to establish one standard depth ofpenetration equal to approximately 0.25 mm (0.010 in.).

Filters

Eddy current instruments typically have the ability to adjustinternal filters to suppress noise and unwanted signal responses.The two types of filters that are commonly employed are thehigh pass filter (HPF) and the low pass filter (LPF). In somecases, band pass filters (BPF) that combine low and high passfilters are also available. By using these filters, the signal responsecan be restricted to a specific range of frequencies, resulting inless signal distortion and noise from nonrelevant sources (Fig. 2).The HPF allows high frequencies to pass, or suppresses lowfrequencies produced by slow or longer periodicity changescaused by hole ovality, nonrelevant surface blemishes and soforth. The LPF allows low frequencies to pass, or rejects highfrequencies typically caused by noise generated by the probe orinstrument.1

The filter frequencies referred to here are related to thefrequency response of the signal generated by the coil as itpasses over an indication at a given speed. It is essentially ameasure of the time (duration) that the coil is sensing a fatiguecrack and registering a response. Often referred to as the peakrotational frequency, this frequency is measured in hertz (Hz)and is independent of, and should not be confused with, theprobe excitation frequencies typically measured in kilohertz(kHz) or megahertz (MHz).

The peak rotational frequency can be determined if the coilsize and coil scan speed are known. For a BHEC inspection, thesurface speed can be approximated using the rotational speed ofthe scanner, measured in revolutions-per-second (rps).2

Eq. 2

Where surface speed is equal to scanner rpm × π × holediameter. For a shielded coil in high frequency applications, theeffective coil diameter can be taken as the coil outside diameter(OD). For unshielded coils, the effective coil diameter isapproximately the coil diameter plus four times the standarddepth of penetration.3

Effective coil diameterShielded coil: Deff = coil diameter (OD)

Unshielded coil: Deff = coil diameter (OD) + 4δ

Therefore, the peak rotational frequency for rotary bolt holeinspection can be estimated for any combination of the hole

diameter, rotational speed and effective coil diameter (Fig. 3).Although these examples identify the discontinuity frequency,

filters must be selected to allow a range of frequencies to passabove and below this frequency. Since filter design varies withinstrument model,4 and effective coil size can vary with probetypes, filter ranges must be verified experimentally by evaluatingthe amplitude and symmetry of the signal response for particularrotational speeds and hole diameters. An approach forsimplifying filter selection is to establish a fixed range of filtersettings for a range of hole diameters (Fig. 4) for a given scannerrotational speed and effective coil diameter.

An example of the effect of proper and improper filtering isillustrated in Fig. 5a and 5b. Improper filtering can result in adistortion of both the standard discontinuity response and crackresponse that is exhibited by an asymmetric signal about thebaseline. Also note that improper filter selection results inexcessive sensitivity to noise resulting from superficial holedamage.

Furthermore, the instrument gain level needed to achieve therequired sensitivity from the reference discontinuity can besignificantly greater for the un-optimized filter selection than forthe optimized filter selection, indicating a partial suppression ofthe signal of interest. This example clearly illustrates that properfilter adjustments can effectively reduce noise to the point whereit would likely not be confused with a crack signal, while thesymmetry of the response is dramatically improved.

Peak rotationalfrequency =

Surface speedEffective coiil diameter

,Hz

FOCUS continued on page 4.

TNT · January 2010 · 33

Figure 2. Filter selection to reduce signal distortion andnoise from nonrelevant sources by suppressing frequenciesabove and below acceptable range.

Rejectedby high

pass filter

Rejected bylow pass filter

Passed

0 200 400 600 800 1000

Ind

icat

ion

freq

uenc

y

Increasing signal

attenuation (dB)

Frequency (Hz)

Figure 3. Peak rotational frequency versus hole diameter forvarious rotational speeds in revolutions per minute and holediameters. Effective coil diameter is 1.588 mm (0.0625 in.).

2.54 5.08 7.62 10.16 12.7 15.2 17.8(0.1) (0.2) (0.3) (0.4) (0.5) (0.6) (0.7)

Frequency (Hz)

Legend500 rpm750 rpm1000 rpm1250 rpm1500 rpm

1000

800

600

400

200

0

Hole diameter, mm (in.)

Phase Angle

Apart from proper filter adjustment, further improvements insignal-to-noise ratio and signal evaluation can be achieved byrotating the phase so that the response of interest is maximizedwhile the contribution of liftoff resulting from hole ovality orsurface blemishes is minimized. To employ this approach,

sufficient separation must exist between the liftoff andresponse of interest. The amount of separation that can beachieved is governed by the material’s electrical and magneticproperties (conductivity and permeability) as well as selectionof the probe excitation frequency. For many commonaerospace metals, particularly aluminum and nickel based superalloys, significant phase angle separation can be achieved.

To achieve separation, a preferred method used in mostsurface scan impedance-plane instrument procedures, the“liftoff ” is generated by repeatedly lifting the probe from thereference standard and rotating the phase angle to achieve asubstantially horizontal liftoff response. The instrument gain isthen adjusted to achieve the required response/sensitivity inthe vertical direction. The same approach can be applied torotary bolt hole inspection by generating the liftoff responsewith the probe rotating against the flat surface of a referencestandard. This approach suppresses unwanted noise resultingfrom superficial scratches within the sweep display that couldnot be completely eliminated by filtering alone (Fig. 5b). Itshould also be noted that when using phase adjustment, theinstrument gain required to achieve the minimum requiredreference response may be significantly higher than withoutphase adjustment; however, the overall signal-to-noise issignificantly improved (Fig. 5c).

Signal Interpretation – Impedance Plane versus SweepDisplays

As previously demonstrated, nonrelevant noise can beminimized by appropriate filter selection and phase adjustment.When these parameters are properly established, relevant crack

indications will exhibit a pronounced phaseresponse clearly distinguishable from the liftoffline as shown in the impedance plane display(Fig. 5c). Indications that exhibit a horizontal oroff-phase response are often the result of holeovality or superficial hole damage.

When viewing the sweep display only,superficial hole damage can produce a responsethat can be misinterpreted as a crack-likeindication without benefit of the additionalinformation provided by the impedance planepresentation. By evaluating both the impedanceplane and sweep displays, relevant crackindications can be clearly distinguished fromnonrelevant surface conditions. These distinctionsare based on the periodicity and amplitude of theresponse exhibited in the sweep presentation andthe phase and amplitude information in theimpedance plane presentation.

Additional complications are presented whenforeign materials such as metal shavings fromfasteners and/or drill bits are embedded either on

FOCUS continued from page 3.

44 · Vol. 9, No. 1

Figure 4. Peak rotational frequency versus hole diameter for1.588 mm (0.0625 in.) diameter (OD) coil at 1500 rpm. Redand blue lines illustrate high and low pass frequency settingsadjusted for hole diameter.

2.54 5.08 7.62 10.16 12.7 15.2(0.1) (0.2) (0.3) (0.4) (0.5) (0.6)

Frequency (Hz)

Hole diameter, mm (in.)

LegendPeak rotational frequencyHigh pass filterLow pass filter

1600

1400

1200

1000

800

600

400

200

0

Figure 5. Effect of filter settings on signal response of reference standardnotch and coupon containing fatigue crack; (a) without proper filter or phaseadjustment, (b) with filter adjustment only, (c) with filter and phaseadjustment.

Instrument settingsPhase: 144 degreesGain: 65.9 dBLPF: 500 HzHPF: 200 HzRPM: 1500



Scratch Crack signal

Crack signal

Liftoff/noise

Fatigue crack couponReference standard

Sweep display Sweep display Impedance planedisplay

((aa))

((bb))

((cc))

Hole Diameter, mm (in.)

LPF (Hz)HPF (Hz)

3.97–5.56 >5.56–7.94 >7.94–11.11 >11.11–19.05(5/32–7/32) (>7/32–5/16) (>5/16–7/16) (>7/16–3/4)

500 500 700 1500150 200 300 500

the bore surface or at faying surfaces of multilayered structures.If the contamination is a metallic particle with significantlydifferent electrical conductivity and/or magnetic permeabilitycompared to the inspection material, the contamination willusually exhibit a distinct phase response clearly distinguishablefrom cracks. However, contaminations with similar electrical andmagnetic properties to the material under inspection can exhibitresponses (sweep and phase) that are indistinguishable fromthose of cracks. When these conditions are encountered, furthercleaning, honing or reaming of the hole may be required toeliminate this condition as causal.

Conclusions

The methods discussed in this article provide an approach foroptimizing the signal-to-noise ratiofor relevant indications such as fatiguecracks while suppressing nonrelevantsignals from sources such as holeovality and minor surface blemishes.As a result, use of eddy currentimpedance analysis in conjunctionwith appropriate frequency selection,filter application, as well as phaseadjustment and separation, provides avery effective tool for inspection offastener holes. This approach alsoimproves confidence in the inspectionresult while reducing the costassociated with unnecessary reworkresulting from false positiveindications. This body of work buildson the experience of the UnitedStates Air Force5 and the broadaerospace nondestructive inspectioncommunity and will continue toadvance as eddy current inspectiontechnologies evolve.

References

1. “The Use of Filters for EddyCurrent Testing.” Staveley SensorsBulletin. Issue SB991.1 Kennewick,WA: Staveley Sensors Inc. (April1999).

2. Nondestructive InspectionMethods, Basic Theory. TO33B-1-1(NAVAIR 01-1A-16) TM1-1500-335-23. Washington, DC:Department of Defense, UnitedStates Air Force (January 2005):Chapter 4.

3. ASNT Level III Study Guide: Eddy Current Testing Method.Columbus, OH: American Society for Nondestructive Testing(1983).

4. Hagemaier, D., M. Collingwood, and K. Nguyen. “EddyCurrent Standards – Cracks Versus Notches.” MaterialsEvaluation. Vol. 41, No. 7. Columbus, OH: American Societyfor Nondestructive Testing (June 1983): p 839-843.

5. LaCivita, K., J. Brausch, and D. Laufersweiler. “ImprovedBolt Hole Eddy Current Rotational Filtering Procedures andPhase-Analysis Based Interpretation to Reduce False Calls.”In-Service Inspection of Aircraft Topical Conference [Wichita,KS]. Columbus, OH: American Society for NondestructiveTesting (May 2006).


LLee Hulin uses ultrasonic testing to conduct gaging surveys onmarine vessels for NVI, LLC. The work is often arduous but Leeenjoys the challenge and looks forward to each job withenthusiasm.

Q: How did you get started in nondestructive testing?A: A friend was working in NDT. The work sounded

interesting so I applied with his company. The owner ofthe company I work for now actually worked there atthat time and when he started his company, he asked meto come work for him.

Q: How have you gotten NDT training?A: Training in my first job was in ultrasonic testing with

Krautkramer. I was certified Level I and Level II in UT.My training hours were on the job while working under aLevel II. My recent training has all been done throughour in-house Level III. He's very knowledgeable andusually does training every three months. He comes outwith us and audits us. We typically do a good 10 to20 hours of training — different field exercises,classroom training, and take all our tests too. I'mcurrently certified as a Level II in UT, MT and PT.

Q: Is ultrasonic testing the method you work with most?A: Yes. I also do magnetic particle testing but 90 percent of

the time, I do ultrasonic testing on marine vessels forABS (American Bureau of Shipping) and U.S. CoastGuard surveys.

Q: What kind of marine vessels do you inspect?

A: Really it's any vessel that's used in the waterways herethat has to be ABS class certified. We're basicallychecking erosion/corrosion. ABS is a classificationsociety with a guideline procedure. I work for thecustomer but I prepare my report for ABS. We are kindof like eyes and ears for the customer and ABS. Most ofthe vessels I inspect will come on the dry dock. That'srequired for a hold diminution marine survey — the kindof survey I provide. It determines how much of thevessel's metal has corroded. I've done everything fromvery, very small push boats — basically a small tug youmight see in the bayou pushing a small barge — up tobig barges or big vessels that go out in the Gulf ofMexico.

Q: Are the inspections made inside the vessel or outside?

A: Both. Outside structure, inside structure, the engineroom, inside the tanks — everything. Depending on thesurvey, we can be in and out of every area of the boat.For example, I did a ten-year-old vessel recently. It onlyrequired me to check a little bit of the main deck andthat was it. On the other hand, Monday I'm startinganother job where I'll have to check everything — allbulkheads, the top, the bottom, basically all the metal. I'llprobably take between 1,500 and 2,000 ultrasonicthickness readings on this vessel. We climb all over —the rudder room, the engine room, the ballast tanks, theliquid mud tanks — if there's a double bottom, like afalse bottom, you have to get in there. You drop downinto the tank and then there's an access hole inside thetank that allows you to crawl underneath the wholevessel. It can be very, very hot at times. This summer wasreally hot and we had to hook up blowers and takenecessary safety precautions.

Q: Do you encounter hazardous materials during inspections?

A: Our normal procedure is to examine a boat when it's drydocked but before we’re allowed to go in, a marine

Lee Hulin

PRACTITIONER PROFILE

66 · Vol. 9, No. 1

It’s not always black orwhite, this is bad or thisis good. A technicianreally has to decipherwhat’s going on.”

chemist has to certify each of the tanks. That's requiredby company policy. The marine chemist "sniffs" them tomake sure they're gas free and then puts a certificate withhis "John Hancock" down at the bottom where we cansee it. It certifies that everything has been checked, theH2S and oxygen levels, everything, and it's clear for us.

Q: UT is your primary test method. Do you use phased array?

A: Not at this time but we have purchased phased arrayequipment. I’ve been training to use it for the past fewmonths. Phased array is the coming thing. Somespeculate that it could even take over for what I do. But,my equipment is just a simple thickness gage to recordthe thickness of the metal. That's what the customerwants to see, the thickness of the metal, how much goodmetal is there. I put the probe on the metal, get thesound wave and the thickness reading at each spot andthen save it. When I get home, I upload everything to mycomputer. I sketch the side shell of the vessel or, if I canget my hands on blueprints, I’ll reduce them and usethose. Then I document the readings on the sketch.That's all sent in to our Lafayette [LA] office where it'sconverted to auto-cad and a data report listing all thepercentages of corrosion. Then it’s delivered to thecustomer digitally and in hardcopy.

Q: Do you work by yourself ?

A: I'm the technician and most of the time I work with anassistant.

Q: Is prep work needed before taking a thickness reading?

A: Prep work can actually be 90 percent of it. The metalhas to be smooth and flat to get good UT readings. I candraw it up really fast and be ready to shoot but, if themetal condition is not conducive to good readings, wehave to grind. My thickness gage is a through-coat gage.It can read through the thick primer that’s used to retardcorrosion. Sometimes I go in and it's a clean, smooth,freshly coated tank. My gage can shoot right through tothe metal. But, if the surface is corroded, there's a lot ofprep work to be done. Normally we use chippinghammers to chip down to bare metal and then we grindspots about the size of a quarter. We spray it with gel,put the probe on there and record the reading.

Q: Does your work require much travel?

A: I do travel a lot. I live in New Orleans, LA and most ofthe time I travel to shipyards in Larose, Lockport and

Amelia. An hour and a half normally gets me where Ineed to go. I’ve traveled to Florida, Alabama — we go allover. Wherever there are ports, vessels and waterways.

Q: What's the worst part of the work you do in NDT?

A: Probably the worst part for me is getting inside some ofthe tanks. The grinding wheel disturbs the environmentand kicks metal shavings everywhere. Even though amarine chemist has said it's okay, you never know. Youjust have to get blowers in there and sometimes you wearrespirators — not all the time but sometimes. Prep workcan be the most manual part of it. My assistant and Itake turns but we're going to end up grinding forthousands of shots on that barge. But it's not bad, wework together. I'm pretty much the main one who doesthis. My company has 100 or so people and 98 percentof them are X-ray techs. Two other technicians are ableto do marine vessels but they only break them off fromX-ray when I'm busy.

Q: How many vessels do you check in a month?

A: It's usually one or two vessels a week but it depends onthe severity of corrosion or how much they wantchecked. You never know how big they'll be or whenthey’ll be due. It could be a small job that takes twohours or it could take four days.

Q: What's the best part of your work?

A: I do enjoy my line of work. When I get called for a job,I'm kind of excited. I enjoy the people I work for. Theycare about my safety and that makes a difference to me.NDT is a great field and you can make a very decentliving doing it too.

Q: You're very active in the New Orleans Section. How has thatmembership benefitted you?

A: I think ASNT Section activities sharpen your skills a littlebit. I'm 28 years old and I've got about 4 or 5 yearscombined time in NDT. You can go to an ASNTmeeting and learn from guys that have been doing NDTfor 20, 25 years. There's a big gray area in NDT. It's notalways black or white, this is bad or this is good. Atechnician really has to decipher what's going on. Youhave to make sure that you're up to par and that youknow what you're doing.

Lee Hulin can be reached by phone at (504) 559-4992 or bye-mail at <[email protected]>.


88 · Vol. 9, No. 1

OOne reason many of us choose to enter thenondestructive inspection community is theopportunity to continually work with newtechnologies, whether that be improvements inexisting methods or the introduction of newinspection methods. Among many factorsdriving inspection developments are the needfor more sensitive inspections, improvements indetection reliability, reduced costs, and/orimprovements in speed. A technique known asvibrothermography has the potential to address allof these factors. While vibrothermography hasbeen around since the late 1970’s, advances ininfrared detection and computational speedhave renewed interest in the potential of thistechnique for inspection applications.

Basic Principles:

The technique, known alternatively asvibrothermography, sonic infrared inspection, andthermal acoustics, is based on infrared detectionof the frictional heat generated when two crackfaces rub together. The frictional heating resultsfrom application of external vibrational energy.Resurgence of the method in the U.S. has beenled by Wayne State University with theintroduction of ultrasonic welding guns as anexcitation source. Ultrasonic welding gunstypically operate at a prescribed frequency of20 kHz or 30 kHz. Primary elements in a typicalexperimental system (Fig. 1) include anexcitation source such as the welding gun, aninfrared camera to detect heat changes inducedby the excitation, and a laser vibrometer tomonitor vibration of the sample.

Typical Inspection Process

A typical inspection process involves shortpulses of energy at 20kHz or 30kHz for1 second. Infrared image acquisition is initiatedprior to excitation and continues for severalseconds to ensure that any heat generated at

* Center for NDE, Iowa State University; (515) 294-5227; [email protected].

** Center for NDE, Iowa State University; (512) 294-8659 [email protected]

crack surfaces will be captured. Other approaches to excitation havebeen developed which are broadband in nature. Figure 2 shows athermal image from a vibrothermography test performed at Iowa StateUniversity using the broadband excitation system developed there. Thisimage shows a crack near the trailing edge of a turbine vane.

Factors to Consider in Inspection Development

While the image in Figure 2 is quite impressive, before a new inspectionmethod can be implemented, the sensitivity, reliability, and repeatabilitymust be evaluated. Because this technique introduces energy into thepart, the nondestructive nature of the method must also be verified. Thatis to say, can the technique introduce damage into the part? Evaluation of

New Inspection Technology Heats Things UpLisa J.H. Brasche* and Stephen D. Holland**

Insight

Figure 1. Typical configuration for vibrothermography system.

Infrared camera

Figure 2. Thermal image of crack near trailing edge of turbine vane.

Sample under test

Laser vibrometer

Ultrasonic welding gun used for excitation


the technique indicates that using a couplantmaterial such as 90 weight cardstock betweenthe excitation source and the part under testprevents observable surface indentation. Limitson vibration-induced stresses are also needed toprevent crack initiation or growth. As with otherthermal based methods, the use of an emissivecoating such as black paint can help improvedetectability.

As is the case in other methods, crackmorphology and closure state can affectdetectability. Research efforts at Iowa StateUniversity are investigating the influence ofcrack closure stresses. Results are shown inFigure 3. As an externally applied stress is usedto open the crack, the closure state, and locus ofheating of the crack changes. The crack surfacesrub, generating heat, where the closure stressesare large enough to force the faces together butnot so large as to lock them in static friction.Cracks that are very open (surfaces not incontact) or very tightly closed (surface rubbingnot feasible) may not be readily detected.

It is also important to remember that, whileinfrared endoscopes are under development,inspection will be feasible for only thosesurfaces with visual access. Likewise, access isneeded for high-power insonification. Due tothe imaging nature of the process, rapidinspection of large areas is possible. Because ofthe diffusive nature of heat propagation,imaging resolution need only be proportional tothe size or depth of the defect.

Vibrothermographic response is alsodependent on geometry. Since the part undertest is vibrated to create the energy, a differentpart shape will cause different resonant patternsand hence lead to different sensitivity. Likewisecracks located in different positions withinthose resonance patterns will also have differentsensitivity. Efforts are underway to betterunderstand these effects, a step that will benecessary to implement the technique inproduction settings.

Current status:

The inspection method is being used for limited production inspectionfor both aviation and power generation applications. Additional researchand evaluation efforts are underway by the Federal AviationAdministration and the Air Force Research Laboratory to supportfuture implementation of the method.

Bibliography

1. Reifsnider, K., E. Henneke, and W. Stinchcomb. “The Mechanics ofVibrothermography.” Mechanics of Nondestructive Testing. NewYork, NY: Plenum Press (1980): p 249-276.

2. Maldague, X. Theory and Practice of Infrared Technology forNondestructive Testing. John Wiley and Sons: New York (2001).

3. Favro, L., X. Han, Z. Ouyang, G. Sun, H. Sui, and R. L. Thomas.“Infrared Imaging of Defects Heated by a Sonic Pulse.” Review ofScientific Instruments. Vol. 71, No. 6. New York, NY: AmericanInstitute of Physics (2000): p 2418-2421.

Figure 3. Closure state and locus of heating ofcrack changes as externally applied stress isused to open crack.

25 MPa

47 MPa

70 Pa

95 MPa

111M

Pa

144 MPa

163 MPa

Q: The article in The NDT Technician on "Understanding Near Vision Eye Tests"was welcome and informative. How about one in the near future on verifyingadequate gray scale differentiation for interpreters of radiographic film? Theredoesn't seem to be a lot of information available about what can be used for thistype of vision test. W.H., Attica, Indiana.

A: The ability of a person to differentiate between shades of gray is a requirementunique to the radiographic testing (RT) process. The question of how to do soand when it is required has been asked of the ASNT SNT-TC-1A InterpretationPanel in the past. The usual response is that how this test is performed is theresponsibility of the employer. There is clarification on when the shades of grayexamination is required in Interpreting SNT-TC-1A, Inquiries 08-2 and 08-3.*Inquiry 08-2 asks if a person seeking RT certification needs this test and theanswer was Yes. However, 08-3 asks if a person being certified in MT, where grayparticles will be used, needs the shades of gray test and the answer was No, andrefers the reader to the response in 08-2 in which the Panel states, "Whether totest for color differentiation or gray shade differentiation is determined by whichis appropriate for the method the individual is being certified in."

I have been looking for specific eye examination methods that address shades ofgray, but those I have found deal with larger objects in different shades of grayon a white background, not smaller objects and shade differentiations betweenvarying shades of gray which are needed for the RT process. Since film readersmust not only discern changes in shade but also a degree of sensitivity, it may benecessary to develop a shades of gray test specific to the RT process that willcheck contrast as well as sensitivity. I have some thoughts on this but will have todevelop them more fully before bringing them out for consideration.

Respectfully,James W. HoufSenior Manager, ASNT Technical Services Department

* Interpreting SNT-TC-1A can be found in PDF format in the members onlysection at <http://www.asnt.org>. A printed version can be purchased online atShopASNT, <http://www.asnt.org/shop/index.ihtml> or by contacting theASNT Book Department at (800) 222-2768. Reference item number 2040.

E-mail, fax or phone your questions for “Inbox” to the Editor: [email protected],fax (614) 274-6899, phone (800) 222-2768 X206.

INBOX

29. In a test coil with a double winding arrangement, one winding is referredto as the primary and the other is referred to as the _______.

30. An _______ coil arrangement can be used in probe, encircling and insidediameter coils.

31. A _______ coil uses one area of a test specimen as a reference standardand simultaneously compares it to another area on the same specimen.

32. The conductivity, permeability, and geometrical shape of a material willdetermine the _______ of the eddy currents induced in the material.

Down1. Self-comparison differential channels would be best for the detection

of _______ abrupt discontinuities.2. Term used to define a standardized condition in a device or system

that results in zero output.3. One principal advantage of

_______ probe eddy currenttesting is the ability to locate theexact position of discontinuities.

4. One or more turns or loops of aconductor wound so that anaxial magnetic field is producedwhen current passes through theconductor.

5. Small diameter probe coilswould typically be used fordetecting small _______discontinuities.

7. Ferromagnetic materials thathave been magnetically saturatedto suppress permeabilityvariations may retain a certainamount of the magnetizationcalled _______ magnetism.

9. In order to generate measurable eddy currents in a test specimen, thespecimen must be a _______.

Across3. Direct current saturation coils would most likely be used when testing

_______.6. When using an encircling coil with both primary and secondary windings,

the excitation alternating current is applied to the _______ winding.8. In the selection of an eddy current test frequency, there is a range of

suitable frequencies centered around the _______ frequency.10. When eddy current testing ferromagnetic materials, one method of

reducing _______ indications is to use a direct current saturation coil tomagnetically saturate the test specimen.

11. A coil’s magnetic field may be viewed as a distribution of lines of fluxaround the coil with the number of lines in a unit area defined as the flux_______.

15. Liftoff is utilized in measuring the thickness of _______ coatings.17. When inspecting products with

a uniform cross section, theeddy current signal produced asthe leading or trailing end ofthe product approaches the testcoil is called the _______ effect.

19. An alternating current flowingin a conductor will set up an_______ magnetic field aroundthe conductor.

20. When eddy current testing anonferrous specimen, adiscontinuity will decrease theeffective _______ of thespecimen.

22. Eddy currents always travel in_______ paths.

23. The impedance of a test coilwill _______ if the testfrequency increases.

24. The main purpose for spring-loaded eddy current probe coils is to_______ liftoff variations.

27. Test specimen used as a basis for calibrating test equipment or as acomparison when evaluating test results is referred to as a _______standard.

28. Large diameter surface scanning coils would most likely be used fordetecting changes in _______.

Crossword ChallengeEEddddyy CCuurrrreenntt TTeessttiinngg**EEddddyy CCuurrrreenntt TTeessttiinngg**

Crossword Challenge

* Content for “Crossword Challenge: Eddy Current Testing”adapted from Electromagnetic Testing Method Questions &Answers Book, second edition: Supplement to RecommendedPractice No. SNT-TC-1A, Book E.

1

4

8

12

15

22

24

27

28

29

32

30

31

25 26

23

13 14

19

20 21

16 17 18

9

10

5 6 7

11

2 3

Crossword Challenge continued on page 12.

1100 · Vol. 9, No. 1

the NDT Technician

Volume 9, Number 1 January 2010

Publisher : Wayne Holliday

Publications Manager : Tim Jones

Editor : Hollis Humphries

Technical Editor: Ricky L. Morgan

Review Board: W illiam W. Briody, Bruce G. Crouse,Anthony J. Gatti Sr., Edward E. Hall, James W. Houf, JocelynLanglois, Raymond G. Morasse, Ronald T. Nisbet, AngelaSwedlund

The NDT Technician: A Quarterly Publication for the NDT Practitioner(ISSN 1537-5919) is published quarterly by the American Society forNondestructive Testing, Inc. The TNT mission is to provide informationvaluable to NDT practitioners and a platform for discussion of issuesrelevant to their profession. ASNT exists to create a safer world by promoting the profession andtechnologies of nondestructive testing.Copyright© 2010 by the American Society for Nondestructive Testing, Inc. ASNT isnot responsible for the authenticity or accuracy of information herein. Publishedopinions and statements do not necessarily reflect the opinion of ASNT. Products orservices that are advertised or mentioned do not carry the endorsement orrecommendation of ASNT.

IRRSP, Materials Evaluation, NDT Handbook, Nondestructive Testing Handbook,The NDT Technician and www.asnt.org are trademarks of The American Society forNondestructive Testing, Inc. ACCP, ASNT, Level III Study Guide, Research inNondestructive Evaluation and RNDE are registered trademarks of the AmericanSociety for Nondestructive Testing, Inc.

the NDT TechnicianPO Box 28518 Columbus, Ohio 43228-0518

NONPROFITUS POSTAGE

PAIDST JOSEPH, MIPERMIT NO. 84The American Society for Nondestructive Testing

www.asnt.org

12. Rods, tubes and wire are commonly tested using _______ coils.13. A test system should include a probe coil when attempting to determine the exact

_______ location of a discontinuity in a tube.14. A test coil’s opposition to the flow of alternating current, causing the current to lag

behind the voltage, is called inductive _______.16. Changes in the ratio of inductive reactance to _______ cause phase shift in an eddy

current test coil.17. An entire cross-sectional area of a tube can be evaluated at one time when using an

_______ coil.18. The µ is commonly used to express _______.20. Process of comparing the reading or output of an instrument, device or dial with a

standard to determine the instrument’s accuracy, capacity or graduations.21. When testing tubing with a bobbin coil, most of the eddy currents flow around the

_______ diameter of the tubing.25. A secondary magnetic field produced by eddy currents induced in a nonmagnetic

conductor close to the inducing coils _______ the coil’s magnetic field.26. An increase in the impedance of a test coil will produce an increase in the _______

through the test coil.

Across

3.steel6.primary8.optimum

10.nonrelevant11.density15.nonconductive17.end19.alternating

20.conductivity22.circular23.increase24.minimize27.reference28.conductivity29.secondary30.absolute31.differential32.depth

Down

1.short2.null3.spinning4.coil5.surface7.residual9.conductor

12.encircling

13.circumferential14.reactance16.resistance17.encircling18.permeability20.calibration21.inside25.opposes26.voltage

Answers

Crossword Challenge continued from page 10.

Errata. Correct contact information for Michael White and William Mooz, contributors of “Crossword Challenge: LiquidPenetrant Testing” (October 2009), is as follows: Met-L-Chek Company, 1639 Euclid Street, Santa Monica, California 90404;phone (310) 450-1111; fax (310) 452-4046; <e-mail [email protected]>.

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