The NDT Technician: A Quarterly Publication for NDT

TTNNTTThe NDT Technician A Quarter ly Publ icat ion for the NDT Pract it ioner

W hen the requirements of ultrasonicinspection go beyondsingle-channel for the sake of speed

or coverage per pass, other instrumentconfigurations must be considered. The firstidea might be to stack up several similarinstruments, synchronize their clock or sweepcircuits, and have each unit drive its owntransducer. This gives total channelindependence and provides all the necessaryinformation, even if a different sound path isused for each channel. In some applications,cross talk between channels can occur if cleveraiming of transducers is not feasible. The costof such a system may be prohibitive even ifportable instruments are used. Weight and sizemay be problems as well.

Multiplexing, or high-speed automaticswitching of ultrasonic inspection fromchannel to channel while using one commonmainframe and cathode-ray tube (CRT), is notreally a new concept. But more companies areturning to it today as a way of cutting costs onelectronics and mechanical hardware, as wellas cutting time spent on inspection and raisingproductivity and quality.

Such systems are becoming very popular onthe production line. However, multiplexingmay not be the right choice in every case.Those who choose multiplexing should beaware of its limitations as well as its benefitsand carefully weigh the factors. Techniciansand new systems engineers may want somebackground information on multiplexers andtheir operation and limitations.

With only minor exceptions, mostoff-the-shelf multiplexers will work just finewith most conventional flaw instruments. It'sthe application that can cause problems.

Basic Calibration

Consider a basic four-channel multiplexer andany portable ultrasonic testing (UT) unit withan audible flaw gate, alarm, and analogrecording output. Figure 1 shows a typicalhookup and simple block specimen for testing.Gain on the UT unit is set at an optimum levelto start receiving signals from the artificialflaws for calibration. Then, individual preampgain controls on the multiplexer are adjusted soreference signals from each channel are thesame amplitude. Only one working channel at

a time should be observed to eliminateunwanted signals on the CRT from otherreflectors which may be present. This allows theUT unit's single gate to monitor all flawsequally. Sophisticated systems may use severalgates to monitor a variety of signal amplitudesand at different sound paths as well. Forsimplicity we will consider only one gate.

• • • • • • • • • • • • • • • • • • • • • • • • • • • • • •

Volume 4, Number 1 January 2005

Focus: Benefits and Limitations of Ultrasonic Testing Multiplexers . . . 1

From the Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

FYI: Practical Contact Ultrasonics — Angle Beam Scan Patterns

and Defect Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Feature: How Do You Define NDT? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Feature: Liquid Penetrant Process Is Not So Simple. . . . . . . . . . . . . . . . 10

Practitioner Profile: Leonard K. Cross . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

Tech Toon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

Inbox . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

A Publication of the American Society for Nondestructive Testing

FocusBenefits and Limitationsof Ultrasonic TestingMultiplexersby L. Byron Makarwich

CONTENTS

TTNNTTThe NDT Technician A Quarter ly Publ icat ion for the NDT Pract it ioner

Focus continued on page 2.

Scan direction

Transducer

T RT1 2 3 4

R

Test block

Figure 1. Typical hookup with simple blockspecimen for testing.

Four-channelmultiplexer

Ultrasonicinstrument

2 · 01/2005 · The NDT Technician

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FROM THE EDITOR

T he year 2004 was a banner year forThe NDT Technician. TNT’s new colorlayout met with much approval, ad sales

and subscriber lists increased dramatically and TNTbecame the first ASNT publication to be distributedelectronically. None of the above would have beenpossible without the ongoing support of TNT’soversight committee — the Technicians AdvisoryCommittee. TAC is a group of hardworking ASNTvolunteers that bring volunteer spirit and theirconsiderable expertise to this publication. Theysubmit articles, act as reviewers and advisors to TNTeditorial staff, suggest topics for interesting andrelevant materials and use their industryconnections to solicit the contributors for thosetopics. All of their efforts are determined by whatwill benefit you — the NDT technician — best. Onbehalf of TNT and ASNT, I would like to express mygratitude and thanks to the members of theTechnicians Advisory Committee.

Ricky L. Morgan – Committee chair, Smith EmeryCompany

William W. Briody, Froehling & Robertson, Inc.Bruce G. Crouse, Cowley Community CollegeEd E. Edgerton, K&E Enterprises, Inc.Anthony J. Gatti, Sr., Kingsbury, Inc.Jesse M. Granillo, R/D Tech, Inc.Edward E. Hall, Senior Aerospace SSPRichard A. Harrison, TEST NDT, Inc.James W. Houf, The American Society for

Nondestructive TestingEddy Messmer, Moraine Valley Community CollegeRaymond G. Morasse, Hyspan Precision ProductsRonald T. Nisbet, The Ronan Corporation

And many thanks to you — our readers and loyaladvertisers. Best wishes for a happy and prosperousnew year.

Hollis HumphriesTNT Editor

PO Box 28518, Columbus, Ohio 43228(800) 222-2768 X206; fax (614) 274-6899

<[email protected]>

After the initial amplitude setup (balancing)of all flaw channels, the UT unit's flaw gate mustbe positioned to encompass the desiredinterrogation area (of all flaw channels) alongthe CRT trace. The alarm threshold should thenbe set to the desired percentage of screen height.Let's assume, of course, that the flaw gate hasthe length range to accomplish this or that allsound paths are about equal. The gate shouldn'tbe monitoring frontwall, backwall,"wraparound," or any geometric targets. Whenall sound paths are kept the same length,multiplexer use remains simple. Such is the casein simple weld-line scanning, some immersiontesting, and other applications. A combination ofshear wave and longitudinal wave channels iswhere headaches can begin. Strict attention mustbe paid to the almost always present interfaceand backwall signals, unless of course, thosesignals are to be monitored.

Basic System and Applications

The pulse repetition rate of the UT unit forms thesystem clock and determines the actualmultiplexer switching speed. If, for instance, theUT unit's PRR is 1000 and a four-channelmultiplexer is used, the effective PRR per channelis 1000/4, or 250 pulses per second. Care must betaken not to inspect at too fast a rate for theapplication and surface speed. The greater thesurface speed inspected, the greater the distancebetween inspection points (points along theinspection line where a given transducer ispulsed). A too-fast inspection speed could resultin missed areas (dead spots between inspectionpoints). This formula will tell you the distancebetween those points: you must decide if it isgreater than you can live with:

where S is surface speed (in ft/min) and I is inchestraveled between pulses. Assume pinpoint beamwidth as a worst case and then use the known orapproximate beam width as a comparison (beconservative) to see if you still have overlappingcoverage.

Now, the UT unit's pulser simply acts as atriggering signal to cause a pulse output on themultiplexer from channel 1, with a correspondinglistening period to follow. This only lasts until theend of one sweep period or until the next UTunit's pulse occurs, which then triggers channel 2to pulse its transducer and listen for possibleflaws. Then channel 3 fires and listens, and so on,through 4, after which time the process starts allover again at channel 1 (Fig. 2). Raising the PRRof the UT unit can be done easily enoughthrough minor modifications to allow faster

SI

×=

1260

PRRchannel

Focus continued from page 1.

01/2005 · The NDT Technician · 3

inspection speeds and greater productivity.Remember the formula.

Alarm Gate Considerations

Another problem could result if you startincreasing inspection speed faster than thecapabilities of the present alarm circuitry. Suchan increase would affect recording output, too.All flaw channels are summed to a singleoutput, which is then routed to the UT unit viaexternal cables. When a channel sees a target,it will have to wait until its next turn to firebefore it can give another signal to the flawgate for processing. The flaw gate in mostportable and some lab instruments requiresmore than one sampling (or pulse repetition) toalarm properly. The problem is that inspectionspeeds may be so fast that the alarm circuitrymay never see more than one pulse comethrough it, and the target is past the soundbeam.

Now, you might say just raise the PRR tosolve that problem, right? Wrong! The peakdetector circuitry in most portable instrumentalarm modules contains too much capacitance,which makes it sluggish at faster-than-designspeeds. That is to say, single-pulse amplitudetracking is impossible. Add sample-and-holdcircuitry, and it's even more sluggish. Typically,when several pulses are needed to fully detectand alarm, alarm levels are reached, literally, bythe stairstep process visible in Fig. 3. Note thatthe level rises with each successive pulse. Afaster alarm capability is needed for fastersystems: a sluggish alarm would be seen as aweak link in such systems. A missed flaw is amissed flaw and can never be confirmed if itcan't be recorded or signaled by an alarm. If

inspection is being pushed to the speed limit,then the alarm's capabilities must be able toclosely follow signal amplitudes. An analogrecording output (or strip chart) will likewisenot track peak signal amplitude, for that too isa function of the peak detector circuitry andprogresses as a stairstep function.

Another scenario likely to cause problemswould be where each transducer is monitoringa backwall signal and a desired percentage lossof any one or more signals is needed to causean alarm. In this case if peak detectorcapacitance is not reduced (by a factor of about50) sufficient charge will remain on the circuitcapacitor so that a loss of any one or twochannels would never be seen (or alarmed).Although easily modifiable in most portables, itwould be better to go to a faster instrumentthat not only has single-rep alarm capabilitybut also resets itself at the end of the sweepperiod to be ready for the next pulse and anypossible new flaw conditions.

Further signal processing of the flaw gateoutput is sometimes done to separate it into itsindividual, corresponding analog flaw channels.This is referred to as demultiplexing. It involvestime sequencing or decoding of the gateanalog output to correspond exactly to eachrespective time domain of the multiplexedpulser/preamps. Off-the-shelf gates usuallyrequire some minor modifications to properlyinterface with a demultiplexer. With thistechnique, however, each channel could berecorded separately on a multichannel stripchart or stuffed into the memory of acomputer.

Environmental Noise

Airborne electrical noise can be a problemsooner or later — usually sooner. And this istrue of single-channel as well as multiplexinghardware. Even if ground loops wereconsidered in the design, solid-state relays wereused, and all necessary shielding againstelectromagnetic and radio-frequencyinterference was done, it seems noise"glitches" occasionally find their way into thealarm system. For this reason, prudent designengineers will design some noise immunity into

the alarm system to compensate for eitherenvironmental or self-inflicted noise or both.Generally, it's the higher gain systems thatexperience severe noise problems. Lower gainsystems don't seem to suffer as much. Again, itdepends on the application. A desirable featurewould be technician or engineer adjustablenoise immunity circuitry to compensate forindividual conditions. Obviously, a limit wouldneed to be imposed so alarms do work whenflaws are detected. Also, time sequence gatingof the preamps is a definite advantage forfurther noise reduction.

Sound Velocity

With most scanners, some type of medium isused to couple sound into the test specimenwithout introducing bubbles, which interruptthe sound beam. These media, whether water,oil, plastic, etc., typically slow down the soundby as much as a five-to-one ratio, compared tosteel. The longer the delay, the longer thelistening time necessary for each channel. Themedium helps determine how fast the systemcan run acoustically and electronically. Thisphenomenon will ultimately determine themaximum pulse repetition rate used for thetest. If large defects are to be found, largediameter transducers can be used but withsomewhat of a tradeoff in sensitivity loss versuscoverage gained with each pass of theinspection head. This may also help ensuremultiple detection of a flaw before atransducer has passed it. Nothing can really bedone to speed up the test except to shorten thedelay path. The problem would be the samewith single-channel but it is just tougher whenmultiplexing because of the long chain of delaypaths to be considered.

CRT Display

A CRT display of multiplexed signals lookssomewhat different than that of asingle-channel because multiplexed signalsshow up only once during several baselinetraces. For example, on a four-channel system,the baseline and gate traces continue to bedisplayed at the UT unit's repetition rate,whereas an echo from one multiplexer channeloccurs at only one-fourth of the repetition rate.Two distinct phenomena occur: the echo ismuch dimmer than the baseline trace and thereis not the usual break at the base of the echo asthere is on a single-channel instrument (Fig. 4).If balanced signals were to be received from allfour channels (with the same sound path) thephenomena would disappear. Note that theinitial pulse on the left in the multiplexeddisplay is not only distinctively brighter (bolder)but also has the characteristic break at its base.

Figure 2. Pulser firing diagram.

Pulser output

1st 2nd 3rd 4th 5thPulser sequenceof ultrasonictesting unit

MultiplexerChannel 1

Channel 2

Channel 3

Channel 4

Channel 5Repeat of

first channel

Figure 3. Successive pulses needed to triggeralarm per channel.

Ala

rm le

vel (

volt

s)1st

2

1

02nd 3rd 4th

Alarm level

5th

Focus continued on page 11.

A rticles one through five of this seriesdiscussed basic equipment setup andcalibration techniques commonly used

for contact angle beam testing. Remainingarticles in the series are focused on what isrequired once the UT operator is given aspecific part to inspect. In this article, fullpenetration plate butt welds and corner andT joint welds will be used as examples.Ultrasonic testing of nonwelded parts (castings,forgings and so on) is done in a similar manner,though the location reference points varydepending on the configuration of the partbeing inspected.

Determining Accessibility

The first step in setting up onsite for aninspection is to determine accessibility to thearea of interest for the weld to be inspected. Insome cases, the governing code or specificationmight also require ultrasonic inspection of thebase material for a short distance beyond thetoe of the weld. This requirement should betaken into consideration when selecting thewedge angle and determining the length ofthe scanning area.

Common Weld Configurations

Figure 1 shows three common weldconfigurations; butt welds in a plate, a cornerjoint made by two plate ends set at 90 degreeangles to each other and a T joint between twoplates (typically seen in beam-columnconnections). The surfaces adjacent to the weld(or in the case of corner and T welds, the facedirectly behind) are the surfaces from whichthe weld can be ultrasonically scanned. Asshown in all three figures, these surfaces arecommonly labeled Faces A, B and C asapplicable.

The two weld sketches in Fig. 1a showplates with two weld configurations, a single-Vbutt weld at the top and a double-V butt weldat the bottom. While the scanning surfaces forboth are labeled the same, the location of theroot of the weld is different and will affect thelocation of certain types of discontinuities.

Face A is on the plate side with the weld crownfor a single-V weld with Face B on the rootside. In the case of a double-V weld, Face A isusually the side of the weld most easilyaccessible.

Figure 1b shows a typical corner weld withthe Face A scanning surface on the weld crownside of the beveled plate, Face B on theopposite side of the same plate and Face Cdirectly opposite the beveled plate on thereverse side of the other plate. These facedesignations are the same for a typical T weldas shown in Fig. 1c. Scanning fromFace C is done with a straight beam transducerand is used primarily to detect laminar tears inthe vertical member behind the weld orsidewall lack of fusion at the vertical face ofthe weld groove as shown. In all inspections,the face from which the weld is beinginspected should be recorded on theinspection report form.

Determining the Scan Surface

Once the scanning face has been selected, it isnecessary to determine how much of thatsurface is required to perform a full coverageinspection of the area of interest. This isdetermined by material thickness and choice ofthe wedge angle if that choice is left up to theoperator and is not specified by code orspecification. In order to attain full volumetriccoverage, it is necessary to ensure that the weldarea is scanned by both the first and secondlegs of the sound beam. If an additionalportion of the base metal requires inspection,that distance must be added to the scanningarea also.Minimum Scanning Surface. Figure 2 shows theminimum scanning surface required on eachside of a plate butt weld to provide fullcoverage of the weld volume. In the figure,Face A is the scanning surface and the weld isbeing scanned from both sides. Notice that atposition A1, where the nose of the transducerhits the edge of the weld crown, the soundbeam only covers a small portion of the bottomof the weld. At position B1, from the otherside of the weld, only an additional smallvolume that was not covered by A1 is scanned.However, as the transducer is moved away fromthe weld, toward positions A2 and B2respectively, the sound beam will interrogatethe entire cross-section of the weld, resulting infull coverage of the weld volume. If thegoverning documents require that a portion ofthe base metal also be interrogated, then thatdistance must also be included when thescanning surface is prepared. Operators shouldkeep in mind that a 70-degree probe wouldrequire a much longer scan path than a45-degree probe. However, a 45-degree soundbeam may be too short to get the second legcompletely through the weld cross-section

FYIPractical ContactUltrasonics — AngleBeam Scan Patternsand Defect Locationby Jim Houf


Figure 1. Common weld configurations:(a) single-V and double-V butt welds in a plate,(b) corner joint formed by two plate ends at a90 degree angle, and (c) T joint between twoplates.

WeldcrownWeld

crown

Weldroot

Weldroot

Face A Face A

Face B Face B

Face A Face A

Face B Face B

Face AX = 0

Face C

Face B Beveledplate

Face A

X = 0

Face C

Face B

Beveledplate

(a)

(b)

(c)


before the nose of the probe hits the edge ofthe weld crown. Surface Preparation. The portion of the plateto be used as the scanning surface must be freeof any loose scale, rust or other foreignmaterial that might cause bad coupling andthereby prevent sound from entering the part.If weld spatter is present, it may be necessary touse a cold chisel to remove it. A 3 in. widechisel is wide enough to clean a large areaquickly and is a good tool for this purpose. Ifused properly, it will not gouge the platesurface. Once the scanning surfaces have beenprepared, couplant can be applied and theweld scanned.

Discontinuity Orientation

In order to detect both transverse and axiallyoriented discontinuities, the weld must bescanned in two directions from both sides ofthe weld.Axial Discontinuities. For axial discontinuities(those that run parallel to the weld length) thetransducer is aimed at the weld and movedback and forth as shown on side A of the platesurface in Fig. 3.

Transverse Discontinuities. For transverselyoriented discontinuities (those that run acrossthe width of the weld) the transducer shouldslide along the side of the weld in both

directions as shown on side B of the surface inFig. 3. Both types of scan are performed onboth sides A and B of the surface to ensure thatall orientations of discontinuities are found.

Manipulating the Probe

Overlap. As the probe is moved back and forth,the sound beam is required to overlap theprevious scan by some percentage. The scanpattern shown in Fig. 4 illustrates a scanpattern with a 50 percent overlap. If theamount of overlap is not detailed in governingdocuments, a 20 to 50 percent overlap is usuallyused. The easiest way to estimate thepercentage of overlap is to look at the tracksthe transducer makes in the couplant on thesurface of the plate.Oscillating the Probe. In addition to beingmoved back and forth, the probe should alsobe oscillated from side to side to detectdiscontinuities that may not be oriented atexactly 90 degrees to the sound beam. If nooscillation requirement is specified in thegoverning code or specification, a range ofapproximately 15 to 20 degrees can be used(see Fig. 4).Difficulties of Probe Manipulation. At firstglance, these manipulations may appearrelatively straightforward and not a complextask. One might assume that all that is requiredis to spread the couplant and then slide thetransducer back and forth far enough to ensurefull coverage by the sound beam whilewiggling the probe back and forth sideways.However, the combination of movements ismore difficult than first perceived and, inaddition, probe manipulation must be donewhile the operator is watching the CRT screenand not the probe. The process is furthercomplicated by the fact that operatorsroutinely use probes that vary from 0.25 in. to1.0 in. or greater in diameter. All of thesefactors make it difficult to maintain a goodscanning pattern with consistent overlap. It’shelpful to remember that proficiency improveswith practice.Scanning Rate. Most codes or standardsrequire that the scanning rate not exceed a

maximum travel speed, typically not more than6.0 in. per second. The actual scan speed willeither be detailed in the governingspecification or in one of the specificationreference documents. This restricted scan rateensures that any reflected sound has sufficienttime to return to the transducer before thetransducer moves on.

Reporting Discontinuity Locations

Using an XY Coordinate System. The final stepbefore beginning an inspection is to select a setof reference points so that any discontinuitiescan be accurately located. This is mostcommonly done with an XY coordinate system,where the X-axis represents the distance acrossthe weld and the Y-axis represents the distancealong the length of the weld. Figure 5 showstypical locations for these points on differenttypes of welds.Locating the Zero Point. As shown in Fig. 5a,the zero point for X on a plate weld is thecenterline of the weld and the zero point for Yis the left end of the weld. For corner andT welds, the zero point for X is at the back ofthe weld (Fig. 5b). For horizontal runs in pipe

Figure 2. Minimum scanning surface required on each side of a plate butt weld to provide fullcoverage of the weld volume.

Face A

A2 A1 B1 B2

Minimum scanning distance Minimum scanning distance

Figure 3. Scan patterns used to detect:(a) axially oriented discontinuities and(b) transverse discontinuities.

Side B

Side A

Transducer

Oneskip

distance

(a)

(b)

Figure 4. Scan pattern demonstrates50 percent overlap for each succeeding pass ofthe transducer. Transducer is simultaneouslyoscillated left and right to detect thosediscontinuities not perpendicular to soundbeam.

50 percent overlap

Scanningsurface

Scanningsurface

Weld

First pass

Second pass

Transduceroscillates

±15 to 20 degrees

FYI continued on page 6.

welds, it is common to place the Y referencepoint (Y = 0) at the top of the pipe (Fig. 5c) andin line with an elbow or fitting on vertical runs.The point at which X is equal to zero is againthe centerline of the weld. Note that somecodes and specifications do have specificconventions for these locations so they should

be checked prior to setting your own locatingmarks. If no specific location instructions aregiven, the operator should select one set ofconventions and record them on the reportform. It is also smart to mark the point atwhich Y equals zero on the weld. Then, ifrepairs are needed, the welder will know thepoint to measure from.

The distance of a discontinuity from Y = 0 isfairly obvious; the distance is measured fromthe left end of the weld to the nearest end of adiscontinuity. Therefore, a discontinuity thatstarts at 6.0 in. from the left end of the weldand ends 8.0 in. from the left end would belisted as a 2.0 in. indication at Y = 6.0 in.

Locating a defect on the X-axis requires alittle more thought. Centerline indications areeasy because they are at X = 0, as shown byindication 1 on the shaded weld in Fig. 6.Indication 2 is 0.5 in. from the centerline awayfrom the inspector and would be recorded asbeing at X = +0.5 in. Because indications 3 and4 are both on the operator side of X = zero,they would be recorded at X = –0.5 in. and–0.25 in., respectively. There is one note ofcaution when scanning the weld from theother side; the operator should make sure that

the ±X convention is not accidentally reversed.

Sizing the Discontinuity

Six Decibel Drop. When sizing a discontinuityfor length, the operator should refer to thegoverning documents for direction. However,if a specific method of determining the endpoint is not given, it is common practice to usethe six decibel drop method for making thatdetermination. If used, note of its use shouldbe recorded. The six decibel drop method refersto the fact that a 6 decibel (dB) decrease in gainresults in a 50 percent decrease in screenamplitude. The operator manipulates thetransducer to maximize the signal from anindication. When this is achieved, themaximized signal is then set to 80 percent fullscreen height (FSH) and the transducer ismoved slowly along parallel to the axis of thedefect until the screen signal drops to 40percent FSH, or 50 percent of the maximizedsignal (a 6 dB drop). A mark is then made at thecenterline of the transducer to denote that end

of the discontinuity. The transducer is thenmoved back toward the other end of thediscontinuity until the signal peaks at 80percent FSH and again drops to 40 percent atthe other end of the indication. This end ismarked as before using the centerline of thetransducer. The distance between the twomarks is considered the length of thediscontinuity.Irregularly Shaped Discontinuities. Note thatsince many discontinuities are irregular inshape, the operator should continue the sizingscan past the point at which a 50 percentamplitude drop is first seen. It is possible that adiscontinuity will have a varying orientationthat will drop off in amplitude at one pointthen increase in amplitude again farther awayfrom the center of the discontinuity. If thiscondition occurs and the scan is stopped whenthe signal first drops to 50 percent, therecorded length will not reflect the actualdiscontinuity length. If the acceptance-rejectioncriteria being used is based on a combination oflength and decibel rating, miscalculation of thelength could have serious results.

Discontinuity Depth

The depth of a discontinuity can be calculatedusing a trigonometric function based on thesound path or surface distance and theinspection angle. This can be read directly froma graphic UT calculator. On newer machines,depth calculation is an integral part of theprogramming and can be done by simplypushing a button. Regardless of how depth iscalculated, the operator should bear in mindthat depth calculation is based on the nominalwedge angle (45, 60 or 70 degrees). In addition,most codes and specifications allow thetransducer to vary within a range of±2 degrees. As a result, a three decimal placedepth number may not be as accurate as theoperator might think. It should also be notedthat the welder that must dig out the defectidentified by the UT operator will not be ableto remove the weld in 0.001 in. incrementswhen using a 0.375 in. diameter carbon air arcrod or 36 grit grinding wheel.

It has been the author's habit to report thedefect depth to the next lower full0.0625 (1/16) of an inch. Reporting a defect inthis manner is done for two reasons. Repeatedhandling of the transducer results in more wearat the nose of the wedge face, thus causing theangle to decrease over time. In addition,digging a little deeper the first time ensuresremoval of the defect and is quicker and lessexpensive than if a second attempt is required.TNT

Jim Houf is Senior Manager of ASNT’s TechnicalServices Department and administers all ASNTcertification programs. (800) 222-2768 X212,(614) 274-6899 fax, <[email protected]>.

Figure 5. Reference points used to locateindications in welds are based on anXY coordinate system with placement of zeropoints determined by weld configuration. Zeropoints for X and Y are shown in; (a) plateweld, (b) T weld and (c) pipe weld.

Scanning surfaceFace A


Y = 0

Y = 0

Y = 0

X = 0

Centerlineof weld

X = 0


Weld

Weld

+ XWeld centerline, X = 0

–0

(a)

(b)

(c)

Figure 6. Locations for indications on operatorside of weld centerline are consideredpositive; those on opposing side are negative.

Y = 0Operator

1

34

2

X = 0

– X

+ X0.5 in.

0.5 in.


FYI continued from page 5.


H ow do you explain NDT to someonethat has never heard the termbefore? It takes more than a little

explanation and a lot of examples. Even tothose of us involved in the field of NDT, thedefinition of what NDT is and isn’t can be alittle fuzzy. In addition, there are all thoseother initialisms. The following article has beenadapted from ASNT’s Nondestructive TestingHandbook series and is good solid text thatallows us to fine tune our understanding.

Nondestructive testing (NDT) has beendefined as comprising those methods used totest a part or material or system withoutimpairing future usefulness1 or affecting theability to fulfill the intended function.Generally applied to nonmedical investigationsof material integrity, nondestructive testmethods typically use an appropriate form ofenergy to determine material properties or toindicate the presence of materialdiscontinuities (surface, internal or concealed).A number of other technologies — for instance,radio astronomy, voltage and amperagemeasurement and rheometry (flowmeasurement) — are nondestructive but arenot used specifically to evaluate materialproperties. Radar and sonar are classified asnondestructive testing when used to inspectdams, for instance, but not when they are usedto chart a river bottom.

What NDT Isn’t

Nondestructive testing asks “Is there somethingwrong with this material?” In contrast,performance and proof tests ask “Does thiscomponent work?” It is not considerednondestructive testing when an inspectorchecks a circuit by running electric currentthrough it. Hydrostatic pressure testing is aform of proof testing that sometimes destroysthe test object.

Another gray area that invites variousinterpretations in defining nondestructivetesting is future usefulness. Some materialinvestigations involve taking a sample of thetest object for a test that is inherentlydestructive. A noncritical part of a pressurevessel may be scraped or shaved to get asample for electron microscopy, for example.Although future usefulness of the vessel is not

impaired by the loss of material, the procedureis inherently destructive and the shaving itself— in one sense the true test object — has beenremoved from service permanently.

The idea of future usefulness is relevant tothe quality control practice of sampling.Sampling (that is, less than 100 percent testingto draw inferences about the unsampled lots) isnondestructive testing if the tested sample isreturned to service. If steel bolts are tested toverify their alloy and then returned to service,the test is nondestructive. In contrast, even ifspectroscopy used in the chemical testing ofmany fluids is inherently nondestructive, thetesting is destructive if the samples are poureddown the drain after testing.

Nondestructive testing is not confined tocrack detection. Other discontinuities includeporosity, wall thinning from corrosion andmany sorts of disbonds.

Methods and Techniques

Nondestructive testing has also been definedby listing or classifying the varioustechniques.1-3 This approach to nondestructivetesting is practical in that it typically highlightsmethods in use by industry.

In the Nondestructive Testing Handbook,the word method is used for a group of testtechniques that share a form of probingenergy. The ultrasonic test method, forexample, uses acoustic waves faster than sound.Infrared and thermal testing and radiographictesting are two test methods that useelectromagnetic radiation, each in a definedwavelength range. A technique, in contrast, hasfeatures that adapt the method to theapplication. Through-transmission immersiontesting is a technique of the ultrasonic method,for example.

Purposes of NDT

Since the 1920s, the art of testing withoutdestroying the test object has developed from alaboratory curiosity to an indispensable tool ofproduction. No longer is visual examination theprincipal means of determining adequatequality. Nondestructive tests in great variety arein worldwide use to detect variations instructure, minute changes in surface finish, the

presence of cracks or other physicaldiscontinuities, to measure the thickness ofmaterials and coatings and to determine othercharacteristics of industrial products.

How is nondestructive testing useful? Whydoes industry buy the test equipment, pay thesubsequent costs of testing and even reshapemanufacturing to fit the needs and findings ofnondestructive testing? Modern nondestructivetesting (1) ensures product integrity and in turnreliability, (2) avoids failures, prevents accidentsand saves human life thereby protecting theinvestment and returns to the manufacturer,(4) ensures customer satisfaction and maintainsthe manufacturer’s reputation, (5) aids in betterproduct design, (6) controls manufacturingprocesses, (7) lowers manufacturing costs,(8) maintains uniform quality levels and(9) ensures operational readiness.

Classification of Methods

The National Materials Advisory Board (NMAB)Ad Hoc Committee on NondestructiveEvaluation adopted a system that classifiedtechniques into six major method categories:visual, penetrating radiation, magnetic-electrical, mechanical vibration, thermal andchemical/electrochemical.3 A modified version ispresented in the adjacent table.1

Each method can be completelycharacterized in terms of five principal factors:(1) energy source or medium used to probe theobject (such as X-rays, ultrasonic waves orthermal radiation); (2) nature of the signals,image or signature resulting from interactionwith the object (attenuation of X-rays orreflection of ultrasound, for example);(3) means of detecting or sensing resultantsignals (photoemulsion, piezoelectric crystal orinductance coil); (4) means of indicating orrecording signals (meter deflection, oscilloscopetrace or radiograph); and (5) basis forinterpreting the results (direct or indirectindication, qualitative or quantitative andpertinent dependencies).

The objective of each method is to provideinformation about one or more of thefollowing material parameters:(1) discontinuities and separations (cracks,voids, inclusions, delaminations and others);(2) structure or malstructure (crystallinestructure, grain size, segregation, misalignmentand others); (3) dimensions and metrology(thickness, diameter, gap size, discontinuity sizeand others); (4) physical and mechanicalproperties (reflectivity, conductivity, elasticmodulus, sonic velocity and others);(5) composition and chemical analysis (alloyidentification, impurities, elementaldistributions and others); (6) stress and dynamicresponse (residual stress, crack growth, wear,vibration and others); (7) signature analysis(image content, frequency spectrum, field

FeatureHow Do You Define NDT?


configuration and others); and (8) abnormalsources of heat.

The limitations of a method includeconditions to be met for method application(access, physical contact, preparation andothers) and requirements to adapt the probe orprobe medium to the object examined. Otherfactors limit the detection or characterizationof discontinuities, properties and otherattributes and limit interpretation of signals orimages generated.

Classification Relative to Test Object

Nondestructive testing techniques may beclassified according to how they detectindications relative to the surface of a testobject. Surface methods include liquidpenetrant testing, visual testing, grid testingand moiré testing. Surface/near-surfacemethods include tap, holographic,shearographic, magnetic particle andelectromagnetic testing. When surface ornear-surface methods are applied duringintermediate manufacturing processes, theyprovide preliminary assurance that volumetricmethods performed on the completed objector component will reveal few rejectablediscontinuities. Volumetric methods includeradiography, ultrasonic testing, acousticemission testing and less widely used methodssuch as acoustoultrasonic testing and magneticresonance imaging. Through-boundarytechniques include leak testing, some infraredthermographic techniques, airborne ultrasonictesting and certain techniques of acousticemission testing. Other less easily classified

methods are material identification, vibrationanalysis and strain gaging.

No one nondestructive testing method is allrevealing. In some cases, one method ortechnique may be adequate for testing aspecific object or component. However, in mostcases, it takes a series of test methods to do acomplete nondestructive test of an object orcomponent. For example, if surface cracks mustbe detected and eliminated and if the object orcomponent is made of ferromagnetic material,then magnetic particle testing would be theappropriate choice. If the material is aluminumor titanium, then the choice would be liquidpenetrant or electromagnetic testing. However,if internal discontinuities are to be detected,then ultrasonic testing or radiography wouldbe chosen. The exact technique in each casewould depend on the thickness and nature ofthe material and the types of discontinuitiesthat must be detected.

Nondestructive testing is sometimesreferred to by other similar and overlappingterms. The terminology may be specific to aparticular industry or perhaps implies anemphasis on one aspect of the testing processor on the community performing the test orexamination. The following definitions explainthe terms most often used in reference tonondestructive testing.· Nondestructive characterization (NDC) is a is

a branch of nondestructive testing concernedwith the description and prediction ofmaterial properties and behaviors ofcomponents and systems. NDC is used inmaterial identification and to assaymicrostructural characteristics — such as resin

curing, case hardening and stress — thathave a direct influence on the service life ofthe test object.

· Nondestructive evaluation (NDE) is anotherterm for nondestructive testing. In researchand academic communities, the wordevaluation is sometimes preferred because itimplies interpretation by knowledgeablepersonnel or systems.1

· Nondestructive examination (NDE) is used inthe utilities and nuclear industry. In theseindustries, examination is sometimespreferred because testing can implyperformance trials of pressure containmentor power generation systems.1

· Nondestructive inspection (NDI), anotherterm for nondestructive testing used in someindustries (utilities, aviation). The wordinspection often implies maintenance for acomponent that has been in service.1 TNT

References

1. Nondestructive Testing Handbook, secondedition: Vol. 10, Nondestructive TestingOverview. Columbus, OH: American societyfor Nondestructive Testing (1996).

2. Wenk, S.A. and R.C. McMaster. ChoosingNDT: Applications, Costs and Benefits ofNondestructive Testing in Your QualityAssurance Program. Columbus, OH: AmericanSociety for Nondestructive Testing (1987).

3. Nondestructive Testing Methods. TO33B-1-1(NAVAIR 01-1A-16) TM43-0103. Washington,DC: Department of Defense (June 1984).

Text for this article has been adapted from theNondestructive Testing Handbook, thirdedition: Volume 5, Electromagnetic Testing.

Nondestructive testing method categories.

Categories Objectives

Basic

Mechanical and optical color, cracks, dimensions, film thickness, gaging, reflectivity, strain distribution and magnitude, surface finish,surface flaws, through-cracks

Penetrating radiation cracks; density and chemistry variations; elemental distribution; foreign objects; inclusions; microporosity;misalignment; missing parts; segregation; service degradation; shrinkage; thickness; voids

Electromagnetic and electronic alloy content; anisotropy; cavities; cold work; local strain, hardness; composition; contamination; corrosion; cracks;crack depth; crystal structure; electrical conductivities; flakes; heat treatment; hot tears; inclusions; ionconcentrations; laps; lattice strain; layer thickness; moisture content; polarization; seams; segregation; shrinkage;state of cure; tensile strength; thickness; disbonds; voids

Sonic and ultrasonic crack initiation and propagation; cracks, voids; damping factor; degree of cure; degree of impregnation; degree ofsintering; delaminations; density; dimensions; elastic moduli; grain size; inclusions; mechanical degradation;misalignment; porosity; radiation degradation; structure of composites; surface stress; tensile, shear andcompressive strength; disbonds; wear

Infrared and thermal anisotropy, bonding; composition; emissivity; heat contours; plating thickness; porosity; reflectivity; stress; thermalconductivity; thickness; voids; cracks; delaminations; heat treatment; state of cure; moisture; corrosion

Chemical and analytical alloy identification; composition; cracks; elemental analysis and distribution; grain size; inclusions; macrostructure;porosity; segregation; surface anomalies

Auxiliary

Image generation dimensional variations; dynamic performance; anomaly characterization and definition; anomaly distribution;anomaly propagation; magnetic field configurations

Signal image analysis data selection, processing and display; anomaly mapping, correlation and identification; image enhancement;separation of multiple variables; signature analysis

N on-NDT personnel often refer toliquid penetrant testing (PT) as simpleto do. However, even trained NDT

technicians can and have performed PTincorrectly. Many variables can affect apenetrant test. Here’s a look at the process andareas where it could go wrong.

Temperature

You cannot perform PT in all temperatures;penetrant is a liquid and subject to the laws ofphysics. Some types of penetrant can be used onhigh temperature parts, but most are restrictedfrom use in lower temperatures (normally< 10 ºC [50 ºF]). Performing PT on a parttemperature below 10 ºC (50 ºF) is notappropriate. In cases where it is not possible toraise the temperature above 10 ºC (50 ºF), youmay qualify the inspection for the temperatureof intended use with customer acceptance.

Precleaning

This is one of the most critical steps in the PTprocess. The part’s surface must be free ofcontaminants that could interfere with flow ofpenetrant into any discontinuities open to thesurface. If coated with oil or grease, wiping thepart down with a rag is not going to do the job.For example, if the part’s surface was examinedby ultrasonics using a couplant before the test,consider steam cleaning the part beforeapplying the penetrant. Some codes specificallyrequire this.

Cleaning materials must also be taken intoaccount as they could damage the material beingcleaned. Think about whether they containprohibitive amounts of halogens, sulfur, ormercury, or if they were tested and certified forthe above. This is a requirement for PT. Cleaningmaterials may meet all of these requirementsand still cause damage if left on a part forfurther processing such as heat treatment. Thecustomer can reject an expensive completed partbecause a cleaner with sulfur content thatexceeded their requirements was used.

Penetrant Selection

Surface condition and number of parts, as wellas accessibility, size and type of discontinuity are

some of the factors that will guide your choice inthe type of penetrant. You would not want touse a solvent removable penetrant on a partwith a very rough surface or on a job thatrequires inspecting a lot of small parts. There areother types to use that would better suit theseneeds. In some cases your customer or code willtell you what type of penetrant to use. Is thepenetrant you are going to use (if it’s not solventremovable) being monitored by a processcontrol system, to ensure system effectiveness?Before applying the penetrant, there are manythings that could already have been done wrongif no attention were given to these criticalpreinspection processes.

Application

The entire inspection surface must be wettedwith the penetrant and kept wet during theentire dwell time. This means you may have toreapply penetrant during long dwell times. Thepenetrant can be applied by dipping, spraying,or brushing; however, remember that you haveto remove the penetrant. Furthermore, ifinspecting a small weld on a large part, it’s notnecessary to apply penetrant to the entire part.Health considerations may also be a factor ifpenetrant is applied in a confined space.Penetrant Dwell Time. This part of the processoften generates discussion on just how long thepenetrant has to remain on the part for aneffective inspection. ASME Section V, SE-165Table 2 lists different materials with dwell timesthat range from five to 10 minutes. I have seencompany procedures that range from 30 to60 minutes. The point is that there are differentdwell times depending on the code/procedurebeing following. Don’t assume that a fiveminute dwell time will always be acceptable;review the procedure or code governing theinspection being done at the time. I have alsoseen situations where dwell times in a companyprocedure were changed but PT personnel werestill using the old dwell times because they hadnot read their own procedures and wereunaware of changes.Penetrant Removal. Depending on the type ofpenetrant being used, the removal process canbe done in several ways — a water wash forwater washable penetrants, an emulsifier forpostemulsifiable penetrants, a solvent

dampened cloth for solvent removablepenetrants. The removal process can easily bedone incorrectly. When removing waterwashable penetrants, the temperature of thewater should be maintained in the range of10-38 ºC (50-100 ºF). The water spray pressureshould be no greater than 276 MPa (40 psi), soyou cannot just grab a water hose and removewater washable penetrant from a part. Forlipophilic and hydrophilic emulsifiers, severalfactors come into play. Emulsifier must beapplied correctly. For lipophilics, immerse thepart directly into the emulsifier with penetranton its surface. However, for hydrophilics the partshould be prerinsed with water prior toemulsification. Both have a certain dwell timethat the emulsifier can stay on the part, basedon the manufacturer or experimentally for eachspecific application. There are also temperatureand pressure restrictions for the water spray. Thesolvent removable penetrant is the one mostoften removed incorrectly. The surface penetrantshould be removed with a clean, lint free cloth,then a cloth should be dampened with solventto remove the rest of the surface penetrant.That sounds easy, but it is the most oftenimproperly executed removal process. In thepast, personnel have started the cleaning processwith greasy rags, soaked the cloth with solventand then applied it to the part, sprayed solventdirectly on the part, or used penetrantcontaminated cloth for final cleaning.Developer. When I went through my first PTtraining class over 30 years ago, I was told that Ishould see no trace of surface penetrant whenthe developer is applied — it should be on awhite background. In fact, during the practicalexam a couple of students failed because therewere shades of penetrant in the developer.Today, that process is incorrect; there should be aslight shading of penetrant in the developer,because that coloring indicates the part was notovercleaned. Most developers must be eithershaken, stirred, or agitated before use. This is animportant step; if you pick up a can of developerand spray it on without first shaking the can, theinspection becomes invalid.

The amount of developer applied is also verycritical — you do not want a thick, heavy coat ofdeveloper that could mask indications. A thinuniform coat is necessary. Dwell time isimportant; developer should stay on the part asrequired by the code/procedure being used.ASME, Section V, Article SE-165 states “Thelength of time the developer is to remain on thepart prior to examination should not be less than10 minutes. The maximum times typicallypermitted are two hours for aqueous and onehour for nonaqueous developers. Again, solventremoval is the process most often misperformed.NDT personnel have sprayed developer with thecan nozzle no more than 25 mm (1 in.) awayfrom the part causing developer to run down thepart, sprayed developer without shaking the can,


FeatureLiquid Penetrant ProcessIs Not So Simpleby Daniel E. Mace


applied insufficient developer, or applied thedeveloper and inspected the part too quickly.

Inspect the Part

Depending on the type of penetrant used,there will be light intensity requirements forboth visible and fluorescent examination. Forthe fluorescent part, there are ultraviolet warmup and visual adaptation times. Some codesrequire a minimum 550 lx (50 ftc) for visible dyePT, while others require a minimum of 1100 lux(100 ftc). An everyday flashlight will not workfor any of the above requirements. Forfluorescent lighting, the light wavelengthshould be in the range of 320 to 380 nm (1.2 ×10–5 to 1.5 × 10–5 in.). Light intensity should bechecked and recorded per the timeframe notedin the code/procedure. Important safetyprecautions must always be followed with anyuse of ultraviolet light.

I have seen precisely recorded records on theabove and intensity numbers were what theyshould have been, but, the filter on theblacklight was covered with a thick layer of dirtand grime. If going to the trouble ofdocumenting light intensities, do it right. It isalso important that there be a documentedvision test for the inspector within the past year.

Post Cleaning

Some codes state that post cleaning is onlynecessary in those cases where residualpenetrant or developer could interfere withsubsequent processing or service requirements.Other codes state that the penetrant materialsshall be removed as soon as possible and specifyhow to remove it. If inspecting an item that isnot covered by a code, your procedure shouldat least specify how to wipe off all PT materialsto avoid part corrosion. If leaving the developeron a part that is going to go through vapordegreasing, the developer could be baked ontothe part. Post cleaning materials come underthe same restrictions as precleaning materials.

Conclusion

The liquid penetrant process is not as simple asmost people think. It is only simple to doincorrectly. TNT

Daniel E. Mace has worked in the NDT communityfor more than 30 years utilizing MT, PT, UT, ET andRT NDT methods and is an ASNT Level III in PT, MTand UT. A member of ASNT for 15 years, he hasserved on the ASNT Certification ManagementCouncil. He’s also an AWS Certified Weld Inspector.(618) 269-4221, <[email protected]>.

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Closing

Advantages of multiplexed systems includegreater inspection coverage in each sweep ofthe test object, lower electronic and mechanicalsystem costs, faster setup and calibration time,reduced channel cross talk and, in manyapplications, no loss of inspection speed. Exceptfor practical considerations, the number ofchannels obtainable is unlimited. TNT

Byron Makarwich, owner of Scorpion Technology inAlbuquerque, New Mexico, has been designinginstrumentation and interfacing circuitry principallyfor ultrasonic testing equipment for more than18 years. (877) 252-7429, (505) 890-3931 fax,<[email protected]>.

Figure 4. Comparison of displays.

Typical display forsingle channel

Typical multiplexeddisplay, one channel

received

Focus continued from page 3.


PRACTITIONERPROFILELeonard K. Cross

Leonard Cross has been working in NDT for 22 years and is anInspection Supervisor for Smith Emery in San Francisco, California.His job is twofold: he performs NDT and, in many cases, heperforms third-party audits. The eastern span of the San Francisco-Oakland Bay Bridge in San Francisco, damaged in the 1989 LomaPrieta earthquake, is being replaced. The main structuralcomponents are being fabricated in the People’s Republic ofChina where the GB Standard is the mandatory national standardthat applies to everything from green tea to truck tires. Many butnot all GB standards are similar to, or identical to, standards usedin other countries. Since most of the specifications for the BayBridge job are according to ASNT’s SNT–TC–1A , an auditcomparing the two programs was needed.

Q: How did you first become involved in NDT?

A: I began by taking general engineering classes in college. Awelding instructor demonstrated NDT. I found that interestingand started taking NDT classes. My instruction included NDTinspection of the retrofit of some early WWII Liberty shipsoriginally built here in Oakland. Then, I went to Spartan School of

Aeronautics and leaned towardthe certification end in RT, UT,MT, PT, ET and leak testing.

Q: What have been yourmost interesting projects?

A: Currently, we’re workingon the San Francisco-OaklandBay Bridge which is basicallytwo bridges with Yerba BuenaIsland in the middle. Theportion that connects fromYerba Buena into Oakland isbeing replaced. I just returned

from China on that project. Another interesting project was thelarge trusses in the San Francisco International Terminal. Thatwas challenging because many of the pipe connections wereTKY connections. Portable digital UT equipment was used in theshop to test 100 percent of all the TKY welds. Automated UTwas performed on the jobsite only and was limited to the trusscord splices. Automated UT is set up with a track and scans byitself. It’s all computerized; the information is downloaded to alaptop for analysis. Then we go back for manual checks. Allother field welds were UT tested at 100 percent due to seismicrequirements. I’ve also worked on power plants in California,high rise buildings, tunnels, refineries. My work in China andKorea is to perform audits of quality systems and certification

for the International Accreditation Service (IAS) on thefabrication of structures. We also perform vendor surveillance,verifying quality systems of NDT, visual, etc. for various USmanufacturers with fabrication plants in China and Korea.

Q: What training or certification is required to perform audits?

A: To become a lead auditor, a combination of training and alsothe successful completion of an examination for ISO 9000: 2000and AS-9100B is required. To qualify for the course andexamination, I had to demonstrate fifteen years of experiencein the industry. I also feel that my practical experience with NDTand welding was very beneficial. To perform a successful audit,an auditor must be familiar with fabrication procedures.

Q: Describe an audit for us.

A: Basically, we perform audits of all the areas of the qualitymanagement system in accordance with ISO (InternationalOrganization for Standardization). We also verify NDTprocedures — all the way down to taking a completed memberor piece of equipment and tracing that component all the wayback to the mill test report. The mill test report is a certificateprovided by the steel mill to the customer identifying the typeand grade of steel. This reverse audit technique is referred to asa backward pass.

Q: What part of your work do you most enjoy?

A: New technology — we’ve just finished 40 hours of trainingon phased array UT. It’s the latest and greatest in terms of UTtechnology. It’s A, B, C and D scans showing a sectional view, atop view and a standard A scan. We’ve been working on it,developing procedures for approval in the structural industry.It’s commonly used now in the nuclear and pressure vesselindustries. I also enjoy the ongoing learning experience in mywork — working with different codes and different cultures. Asan example, I recently had the opportunity to meet one of theseventeen ASNT Level IIIs in China. He works for an aerospacecompany in China and he’s been retained by the contractorthat’s fabricating the Bay Bridge.

Q: How much of your work involves travel?

A: About 35 to 40 percent. I’ve worked down in Monterrey,Mexico on audits. Also, worked in Spain for a while. We’reseeing more and more work overseas at this point. It’sinteresting doing analysis on different NDT inspection programssuch as the Chinese inspection program, the GB Standard.


Q: How does that program compare with SNT-TC-1A?

A: Well, for example, on the Bay Bridge, we’ve just completedthe comparison of the GB Standard and ASNT’s RecommendedPractice No. SNT-TC-1A (2001). The specifications for the jobrequire certification per SNT-TC-1A. Because the bridge is beingfabricated in China and then sent back to San Francisco forerection, CALTRAN (California Department of Transportation)wanted a third party to perform a comparison between the twosystems to determine that the Chinese NDT code was close toequivalent in terms of training, qualifications, certifications, andsuch as required by SNT-TC-1A. We found that, in some areas,the Chinese program was equivalent; in some areas, it exceededour requirements; in some areas, it was not equivalent.

Q: What parts of the bridge are being manufactured in China?

A: All the primary girders, all the main structural components.

Q: Which NDT methods do you consider most significant fortrainees?

A: I think I would start with UT inspection and phased array.Phased array eliminates a lot of user error. The wonderful thingabout this new digital technology is that the guys taking NDTclasses in the next five years are going to see some really bigchanges in areas like thermal imaging, phased array and

automated UT. Everything is growing by leaps and bounds nowthat we are in the digital age.

Q: What advice would you offer an individual considering acareer in NDT?

A: I would get certified in as many different methods as I couldand try to work in some different fields in order to be a little bitmore diverse as an NDT inspector. For instance, I’ve performedNDT on pressure vessels which would apply to ASME (AmericanSociety of Mechanical Engineers) and I’ve worked in refinerieswhich would be API (American Petroleum Institute). It’simportant to be diverse for any of the various assignments thatpop up.

Q: How important is a college degree?

A: Well, I have an AS but that’s not required. It’s important tohave a lot of classes in NDT and to understand an industry. Takeadditional classes — continued education, inspection technologyclasses, computers. Become a certified welder for example. Thatworks very closely with evaluation of a weld.

Q: How has membership in ASNT benefited you?

A: Most importantly, my membership keeps me informed ofnew technology and techniques in NDT. TNT

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Tech Toon

“I don't know about you, Johnson, but I thinksequels are always worse than the first-run.”

Thanks ...for the positive comments!We’re glad to hear that you enjoyTech Toon and we appreciate thetopics suggested thus far. Nowwe’d like to hear from the rest ofyou. Here’s your chance to seeyour Tech Toon captionprofessionally rendered andpublished in TNT. Send yoursuggestion — if accepted, TNTwill pay seventy-five dollars andlist you as the source. We’ll alsomention your employer and ifyou’re an ASNT member, we’llmention your Section affiliationtoo. Contact:

TNT Editor

PO Box 28518, Columbus, Ohio 43228(800) 222-2768 X206fax (614) 274-6899<[email protected]>


Q. On radiographs of thintitanium components, I amfinding areas that appear assegregations within thematerial. My coworkersuggests we increase thekilovoltage and use leadscreens. The areasdisappeared when wechanged our process. Whatwas causing the indicationson the radiograph? B.C.,Arkansas City, Kansas

A. From your description, itappears you experiencedmottling, a form of scatteroriginating from largergrained materials. It’s nota common conditionencountered in radiography.Another way to verify thiscondition is to produce asecond radiograph with thecomponent tipped 5 degreesor so. If the componentcontains segregations, it willchange very little. However, ifdiffraction is causing theindication it will changemarkedly.

Q. Why are my hands red afterperforming wet magneticparticle inspection using astationary unit? E.H.,Burbank, California

A. The redness is a form ofdermatitis. Your skin isreacting to the suspensionfluid used in the machine. Amoisturizing lotion or handcream will offer some relief.

Q. My boss has told me that anAC yoke produces alongitudinal field and acircular field as well. Fromwhat I’ve learned duringtraining and in reading, theAC yoke only produces alongitudinal field. What doyou say? A.M., Pakistan

A. The magnetic field in a yokeis generated by an electriccoil wrapped around a steelbar in the yoke body which isattached to the two legs ofthe yoke. As a result, themagnetic field from the coil,which is longitudinal, travelsdown the legs and throughthe part. Therefore, themagnetic field that is in thepart is longitudinal.

However, there are someinstances when the appliedmagnetic field can be used incircular applications eventhough it is a longitudinalfield. An example of thiswould be if the legs of theyoke are applied to a pipetransverse to the axis of thepipe (like you were checkingfor transverse indications in around seam weld). In thatcase, the field flows aroundthe pipe circumference in thesame direction that circularmagnetization would flow ifthe pipe were inspected usinga head shot or a centralconductor. This use oflongitudinal magnetizationcan make it seem like thefield is circular, though in factit isn’t.

The yoke method ofmagnetization is known asindirect induction, meaningthat the magnetic field iscreated outside of the partand only the magnetic fieldenters the part. Directinduction, where the electriccurrent is passed through thepart (like a prod unit or ahead shot) results in circularmagnetization in the part.

E-mail, fax or phone questions forInbox to the Editor:<[email protected]>,phone (800) 222-2768 X206,fax (614) 274-6899.

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Volume 4, Number 1 January 2005

Publisher: Wayne HollidayPublications Manager: Paul McIntire

Editor: Hollis HumphriesTechnical Editor: Ricky L. Morgan

Review Board: William W. Briody, Bruce G. Crouse,Ed E. Edgerton, Anthony J. Gatti Sr., Jesse M. Granillo,Edward E. Hall, Richard A. Harrison, James W. Houf,Eddy Messmer, Raymond G. Morasse, Ronald T. Nisbet

The NDT Technician: A QuarterlyPublication for the NDT Practitioner(ISSN 1537-5919) is published quarterlyby the American Society forNondestructive Testing, Inc. The TNTmission is to provide information valuableto NDT practitioners and a platform fordiscussion of issues relevant to theirprofession. ASNT exists to create a safer world by promoting the professionand technologies of nondestructive testing.

Copyright © 2005 by the American Society for Nondestructive Testing, Inc.ASNT is not responsible for the authenticity or accuracy of informationherein. Published opinions and statements do not necessarily reflect theopinion of ASNT. Products or services that are advertised or mentioned donot carry the endorsement or recommendation of ASNT.

IRRSP, Level III Study Guide, Materials Evaluation, NDT Handbook,Nondestructive Testing Handbook, The NDT Technician and www.asnt.orgare trademarks of The American Society for Nondestructive Testing, Inc.ACCP, ASNT, Research in Nondestructive Evaluation and RNDE are registeredtrademarks of the American Society for Nondestructive Testing, Inc.

®

The NDT TechnicianPO Box 28518 Columbus, Ohio 43228-0518

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The NDT Technician: A Quarterly Publication for NDT