ASME TOFD Interpretation Manual

8/10/2019 ASME TOFD Interpretation Manual

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N-410 2OO4 ECTION V N-421

A06 APPENDIX N - TIME OF FLIGHTDTFFRACTTON TOFD)

INTERPRETATION

N.410 SCOPE

This Appendix s to be used as an aid for the nterpreta-tion of Time of Flight Diffraction (TOFD) ultrasonicimages. Diffraction s a common ultrasonic phenomenonand occurs under much broader conditions han ustlongitudinal-longitudinal diffraction as used in typicalTOFD examinations. his interpretation uide s primar-ily aimed at longitudinal-longitudinal iffraction TOFDsetups using separated ransducers n either side of theweld on a plate, pipe, or curved vessel. Other possibilitiesinclude:

(aJ shear-shear iffraction(b) longitudinal-shear iffraction(c) single ransducer iffraction (called back diffrac-

tion or the tip-echo method(c) twin transducer TOFD with both transducers n

the same side of the flaw/weld(d) complex nspections, .g., nozzles

N-420N-421

GENERAL

TOFD Images - Data Visualization

(a) TOFD data s routinely displayed as a grayscaleimage of the digitized A-scan. Figure N-421 a) shows he

grayscale erivation of an A-scan (or waveform) signal.(b) TOFD images are generated by the stacking of

these grayscale ransformed A-scans as shown in Fig.N-421(b). The ateral wave and backwall signals are visi-ble as continuous multicycle lines. The midwall flawshown consists of a visible upper and lower tip signal.

These show as intermediate multicycle signals betweenthe lateral wave and the backwall.

(c) TOFD grayscale mages display phase changes,some signals are whi te-black-whi te ; o thers are

black-white-black. his permits dentification f the wavesource flaw top or bottom, etc.), as well as being usedfor flaw sizing. Depending on the phase of the incidentpulse usually a negative oltage), he ateral wave wouldbe positive, then the first diffracted (upper tip) signalnegative. he second iffracted lower ip) signal positive,and he backwall signal negative. his s shown schemati-cally in Fig. N-421(c). This phase nformation is veryuseful or signal nterpretation; onsequently, F signalsand unrectified signals are used for TOFD. The phaseintbrmation s used or corectly identifying signals usu-ally the top and bottom of flaws, f they can be differenti-

ated), and for determining he correct ocation or depthmeasurements.

(d) An actual OFD mage s shown n Fig. N-421(d),with flaws. The time-base s horizontal and the axis ofmotion is vertical [the same as the schematic n Fig.N-42l (c)1.The ateral wave s the airly strong multicyclepulse at eft, and he backwall he strong multicycle pulseat right. The flaws show as multicycle gray and whitereflections etween he lateral and backwall signals. Thescan shows several separate laws (incomplete usion,porosity, and slag). The ultrasonic noise usually comesfrom grain reflections, which limits the practical re -quency that can be used. TOFD scans may only showthe lateral wave (OD) and backwall (ID), with noise.There is also ultrasonic nfbrmation available past thebackwall (typically shear wave diffractions), but this issenerallv not used.

Ampl i tude

wt n

I m e

F I G .N - 4 2 1 ( a ) S C H E M AT I C H O W I N G AV E F 0 R M R A N S F 0 R M AT I 0 N N TOG R AY S C A L E

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ARTICLE 4 - NONMANDATORY APPENDICES

-rt,'A--y--*,*i:_

Upper surface Back wall

F I G . N - 4 2 1 ( b ) S C H E M AT I C H O W I N GG E N E R AT I O N F GR AY S C A L E - S C A N R O M M U LT I P L E A - S C A N S

F I G .N - 4 2 1 ( c ) S C H E M AT I C H O W I N G TANDARD O F D E T U P N D D I S P L AY I T H WAV E F O R MA N DS I G N A L H A S E S

LW

I:]:]

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N-450

Incomple te us ionat root

Porosity

lncomple te idewa l lfus ion

F I G .N - 4 2 1 ( d ) TO F D I S P L AY I T H F L AW SO F

ANDD I S P L AY E D - S C A N . I M E S H O R I Z O N TA L NDTHE A X I SM O T I O NS V E RT I C A L

2OO4 ECTION V N-454

N-450 PROCEDURE

N-451 Measurement ToolsTOFD variables re probe spacing, material hickness,

sound velocity, ransducer elay, and ateral wave transitand backwall eflection arrival ime. Not all the variablesneed o be known or flaw sizing. For example, alibrationusing ust the ateral wave front wall or OD) and backwall(lD) signals an be performed without knowing he trans-ducers delay, separation, r velocity. The arrival time,Fig. N-45 , of the lateral wave (tr) and the backwallsignal (t2) are entered nto the compu ter software andcursors are then displayed or automated izing.

N-452 Flaw Position Errors

Flaws will not always be symmetrically laced betweenthe transmitter nd receiver ransducers. ormally, a sin-gle pair oftransducers s used, entered n the weld axis.However, multiple TOFD sets an be used, articularly onheavy wall vessels, nd offsets are used o give improveddetection. Also, flaws do not normally occur on the weldcenterline. Either way, the flaws will not be positionedsymmetrically, Fig. N-452(a ) and this will be a sourceor error n location and sizins.

There will be positional and sizing errors associatedwith a noncentered law, as shown in Fig. N-452(b).However, these errors will be small. and generally aretolerable ince he maximum error due o off-axis positionis less han 07o and he error s actually smaller yet sinceboth the top and bottom of the flaw are offset by similaramounts. The biggest sizing problems occur with smallflaws near he backwall. Exact error values will deoendon the inspection parameters.

N-453 Measuring Flaw Length

Flaw lengths parallel to the surface can be measuredfrom the TOFD image by fitting hyperbolic cursors tothe ends of the flaws (see Fig. N-453).

N-454 Measuring Flaw Depth

Flaw height perpendicular o the surface can be mea-sured rom the TOFD image by fitting cursors on the topand bottom tip signals. The following are two examplesof depth measurements f weld flaws n a I in. (25 mm)thick plate. Figure N-454(a) is midwall lack of fusionand Fig. N-454(b) s a centerline rack. Note that TOFDsignals are not linear, so midwall flaws show n the upper

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ARTICLE .1 NONMANDATORY APPENDICES

A-scan

A^l l l l n

|4

t i i , t ,v v l t utt tz

CursorsBuild- in

t 1 , 2 = d 1 , d 2 a n d h a r e a u t o m a t i c a l l yca cu a t ed .

F I G .N - 4 5 1 M E A S U R E M E N T O O L S O R L AW H E I G H T S

Tra smitter

t X

F I G .N - 4 5 2 ( a ) S C H E M AT I C H O W I N G HE D E T E C T I O NF 0 F F - A X I S L AW S

Transmitter

Flaw Position Uncertainty

G E N E R A L N O T E : I n p r a c t i c e , h e m a x i m u m r r o r o n a b s o l u t e e p t h p o s i t i o n i e s b e l o w 1 0 % . T h e e r r o r o n h e i g h t s t i m a t i o n f i n t e r n a l( smal l ) l aws s neg l ig ib le . e ca re fu l o f sma l l r aws s i tua ted a t the backwal l .

F I G .N - 4 5 2 ( b ) M E A S U R E M E N T RRORS R O M L AW P O S I T I O N N C E RTA I N T Y

D-scan

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N-454

third region of the image. t is possible o linearize heTOFD scans by computer software.

N-480 EVALUATION

This section shows a vari ety of TOFD images andthe interpretation/explanation. nfortunately, here aresignificant variations amongst laws and TOFD setups

N-481

and displays, so the following images should be used asa guide only. Evaluator experience nd analysis kills arevery important as well.

N-481 Single Flaw Images

(a) Point flaws [Fig. N-481(a)], like porosity, showup as single multicycle points between he lateral andbackwall signals. Point flaws typically display a single

200,1SECTION V

F I G .N - 4 5 3 TO F D M A G E S H O W I N G Y P E R B O L I C TA I L S F R O M H E ENDSO F A F L AW M A G EU S E D OM E A S U R E L AW E N G T H

0 .43 n.{ 1 1 m m }

0.59 n .( 1 5m m )

Late awave

Bottomecno

Backwal lecno

Topecno

FIG.N-454(a ) OFD MAGE HOWINGOP ND BOTTOMIFFRACTEDIGNALSROM IDWALLLAW NDA-SCANNTERPRETATIOI \

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ARTICI-E 4 - NONMANDATORY APPENDICES

0 .62 n .

( 1 5 . 7m m )

0 .88 n .122 .4mml

Frontwa l l

Tops i g n a l

Bottoms igna

F I G .N - 4 5 4 ( b ) TO F D M A G ES H O W I N G O P ANDB O T TO M I F F R A C T E D I G N A L S R O M E N T E R L I N E R A C I (ANDA - S C A N N T E R P R E TAT I O N

Lateralwave

Backwall

F I G .N - 4 8 I ( a ) S C H E M AT I C S F M A G EG E N E R AT I O N ,C A N AT T E R N , AV E F O R M , N DTO F DD I S P L AYS H O W I N G H E M A G E FT H E P O I N T L AW

l nd ica t ion

tIII

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N-481 2OO4 ECTION V N-481

Back wall echo

F I G .N - 4 8 1 ( b ) S C H E M AT I C S F M A G EG E N E R AT I O N ,L AWL O C AT I O N ,NDTO F D I S P L AY H O W I N G HEI M A G EO F T H E N S I D E I D )S U R FA C E - B R E A I ( I N GL AW

Lateral

i - - - f - - - - :\ /t t\ i\ . /\ l\ - \

l - . '

TOFD signal since law heights are smaller han he ring-down of the pulse (usually a few millimeters, dependingon the transducer requency and damping). Point flawsusually show parabolic tails where he signal drops of ftowards he backwall.

(b) Ins ide (ID) far-surface-breaking laws [Fig .N-48 (b)l shows no interruption of the lateral wave, asignal near the backwall, and a related nterruption orbreak of the backwall (depending on flaw size).

(c) Near-surface-breaking laws [Fig . N-481(c)]shows perturbations n the lateral wave. The flaw breaksthe latera l wave, so TOFD can be used o determine f

the flaw is surface-breaking r not. The lower signal canthen be used o measure he depth of the flaw. If the flawis not surface-breaking, .e., ust subsurface, he lateralwave will not be broken. If the flaw is near-subsurfaceand shallow (that is, less than the ringing time of thelateral wave or a few millimeters deep), hen he law willprobably be nvisible o TOFD. The mage also displays anumber of signals rom point flaws.

(d) Midwall flaws [Fig. N-481(d)l show complete at-eral and backwall signals, plus diffraction signals iomthe top and bottom of the flaw. The flaw tip echoes

provide a very good profile of the actual law. Flaw sizescan be readi ly black-whi te , whi le the lower echo sblack-white-black. lso note he hyperbolic curve hat seasily visible at he eft end of the top echo; his s similarto the effect rom a point flaw [see N-481(a)] and permitsaccurate ength measurement f flaws [see N-450(a)].

If a midwall flaw is shallow, .e., less han the trans-ducer pulse ring-down (a few millimeters), he top andbottom tip signals cannot be separated. nder these cir-cumstances. t is not possible o differentiate he op fromthe bottom of the flaw, so he evaluato{ an only say hatthe law s less han he ingdown distance which depends

on transducer requency and damping, etc.).(e) Lack of root penetration see Fig. N-481(e)] is

similar to an inside (ID) far-surface-breaking law [seeN-481(b)1. This flaw gives a strong diffracted signal (ormore correctly, a reflected ignal) with a phase nversionfrom the backwall signal. Note that whether signals arediffracted or reflected s not important for TOFD charac-terization; he analysis and sizing s the same. Also noteeven hough here s a perturbation f the backwall signal,the backwall s still visible across he whole flaw. Thismaterial also shows small point flaws and some srain

Receiver

N o b a c k w a l l e c h o

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Surface-breaking la w

F I G .N - 4 8 1 ( c ) S C H E M AT I C S F I M A G EG E N E R AT I 0 N .I M A G EO FT H E O U T S I D E O D )

InF I G .N - 4 8 1 ( d ) S C H E M AT I C S F F L AWL O C AT I O N ,I G N A L S , NDTO F DD I S P L AY H O W I N G H E M A G EO FTHE M I D WA L L L AW

F L AW L O C AT I O N ,NDTO F D I S P L AY H O W I N G H ES UR FA C E . B R E A I ( I N GL AW

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N-481

FIG. N-481(e) FLAW L0CATI0N

2OO4 ECTION V

A N DT O F D I S P L AY H O W I N G H E M A G EP E N E T R AT I O N

OFTHELACI( FROOT

N-481

F I G .N - 4 8 1 ( f ) F L AW L O C AT I O NNDTO F DD I S P L AY

noise, which is quite common. TOFD typically overem-phasizes mallpoint laws, which are normally undetected

by conventional shear wave pulse-echo echniques.(/) Concave oot flaws fsee Fig. N-481(f)] are similar

to lack of root penetration. he top of the flaw is visiblein the TOFD image, as well as the general shape. Thebackwall signal shows some perturbation as expected.

(g) Sidewall ack of fusion see ig. N-a8l (g)] s simi-lar to a midwall flaw Isee N-48 (d)] with two dilferences.First, the flaw is angled along the fusion line. so TOFDis effectively ndependent f orientation, which is not aproblem or TOFD. Second, he upper law signal s partlyburied n the lateral wave for this oarticular law. In this

S H O W I N G H E M A G EO F THE CONCAVE O O T L AW

instance, he upper ip signal s detectable ince he ateralwave signal amplitude s noticeably ncreased. owever,

if this were not the case, hen the evaluator would beunable o accurately measure he flaw depth.

(ft) Porosity see Fig. N-481(h)] appears s a series ofhyperbolic curves of varying amplitudes, imilar to thepoint flaw [see N-481(a)1. he TOFD hyperbolic curvesare superimposed ince he ndividual porosity pores areclosely spaced. his does not permit accurate nalysis, utthe unique nature of the mage permits characterization fthe signals as multiple small point flaws, .e., porosity.

(i) Transverse racks [see Fig. N-481(i)] are similarto a point flaw lsee N-481(a)1. he TOFD scan displays

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FIG. N-481(g) FLAW L0CATI0N, OFD DISPLAYFLAW,

ARTICLE 4 NONMANDATORY APPENDICES

lhS H O W I N G H E M A G EO F THE M I D WA L L A C I (O F F U S I O NANDTHE A - S C A N

F I G .N - 4 8 I ( h ) F L AW L O C AT I O NNDTO F D I S P L AY H O W I N G H E M A G EO F T H E P O R O S I T Y

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2OO4 ECTION V

F I G .N - 4 8 1 ( i ) F L AW L O C AT I O NNDTO F DD I S P L AY H O W I N G H E M A G EO F T H E T R A N S V E R S E R A C K

Transmitter

4Receiver

Lateral C---

1&

\ 1 / /

o'oy Back wall

Wry- -:-ffi

F I G .N - 4 8 1 ( J ) S C H E M AT I C S F M A G EG E N E R AT I O N ,L AWL O C AT I O N NDTO F D I S P L AY H O W I N G HEI M A G E FT H E N T E R PA S S A C I ( F F U S I O N

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N-481

1 Incomple tes idewa l l us ion

a typical hyperbola. Normally, t woul d not be possible odifferentiate ransverse racks rom near- urf'ace orosi yusing TOFD; further nspection would be needed.

(7) Interpass ack of fusion see Fig. N-481(j)] shows

as a single, high amplitude signal n the midwall region.Ifthe signal s long, t is easily differentiated rom poros-ity or point sources. t is not possible o distinguish hetop and bottom, as these do not exist as such. Note theexpected phase change rom the lateral wave. Interpasslack of fusion signals are generally benign.

N-482 Multiple Flaw Images

TOFD images of flawed welds contain bur flaws each.N-482.1 Plate 1 [Fig. N-482(a)]


3 - S l a g

f r o m o n e e n d .( 1 0 0 - 1 2 5 m m ) .

( 2 3 ' 7 - 2 6 7 m m ) .

4 - Incomple tefusion at rool

advantages f TOFD (midwall flaw detection, law siz-ing), the limitations due to dead zones, and that:

(a) the sidewall ncomplete usion shows up clearly,as does he slag

(b) the incomplete usion at the root was not easilydetected, hough t did disturb he backwall. This in notsurprising n the backwall dead zone due o a shear-sheardiffracted wave. This example llustrates he potentialvalue of using nfbrmation ater n the time base, ut thisis outside he scope of this interpretation manual.

(c) the root crack s not visible at all due to the back-wall dead zone

N-482.2 Ptate 2 [Fig. N-482(b)]

G E N E R A L O T E S :l . l n c o m p l e t e u s i o n t r o o t ( l e f t ) : 0 . 6 - . 8 n . ( 1 5 - 4 5 m m ) f r o m

o n e e n d .

2 . To e c r a c k t o p e f t ) : 3 - 4 i n . ( 8 0 - 1 0 0 m m ) .3 . P o r o s i t y : 5 . 5 - 6 . 2 5 n . ( 1 4 0 1 6 0 m m ) .4 . I n c o m p l e t e i d e w a l l u s i o n u p p e r i g h t ) : - 8 - 9 . 2 5 i n . ( 2 0 0

z 5 \ m m ) .

N-482.2

F I G .N - 4 8 2 ( a ) S C H E M AT I C F F L AWL 0 C AT I O N S NDT 0 F D M A G E S H O W I N G H E L AT E R A L AV E ,B A C I ( WA L L ,NDTHREEO F T H E F O U R L AW S

G E N R A L N O T E S :1 . R o o t c r a c k r i g h t ) : I . 6 - 2 . 5 i n . ( 4 0 - 6 4 m m )2 . I n c o m p l e t e i d e w a l l u s i o n m i d - l e f t ) : 4 - 5 i n .

3 . S l a g : 6 . 4 - 1 2 i n . ( 1 6 3 - 1 8 3 m m ) .4 . l n c o m p l e t e u s i o n t r o o t ( l e f t ) : 9 . 3 - 1 0 . 5 n .

Figure N-482(a) c lear ly l lus t ra tes he s igni f icant

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N-482.2 2OO4 ECTIONV

1 - Porosity

N-483

1 - Incomple tefusion at root

Figure N-482(b) shows hat:(a) all four flaws are detectable(b) the ncomplete usion at the root shows up clearly

in this scan because t is deeper. oth the backwall pertur-bation and the flaw tip signals are clear.

(c) the crown toe crack is clearly visible, both bycomplete disruption of the ateral wave and by the bottomtip signal. Both the incomplete usion at the root andcrown toe crack are identifiable as surf-ace reaking by

the disruption of the lateral wave and backwall signal,respectively.

(d) the porosity s visible as a series of signals. Thiscluster ofporosity would be difficult to characterize rop-erly using he TOFD scan alone, since t could be identi-fied as slag or a planar law.

(e) the incomplete sidewall fusion is clearly visibleand could be easily sized using cursors.

N-483 Typical Problems With TOFDInterpretation

TOFD images can be corupted by incorrect setups orother problems such as electrical noise. The followingimages were all made on the sarne plate to show someof the typical problems hat can occur. Starting irst withan acceptable can, and then subsequent cans made toshow various comrptions of this image.

(a) Acceptable Scan IFig. N-483(a)]. The gain andgate setting are reasonable, nd the electrical noise isminimal.

(b) Incorrect Lovv Gain Setting IFig. N-183(b)]. Thelateral wave and some of the diffracted signals re starting

4 - Incomple tes idewa l l u s ion

to disappear. At yet lower gain levels, some of the dif-fiacted signals would become undetectable.

(c) Incorrect High Gain Setting IFig. N-483(c)]. Thenoise level increases o obscure he diffracted signals;this can ead o reduced robability of detection, nd poorsizing. High noise evels can also arise rom large grains.In this case. he solution s to reduce he ultrasonic re-quency.

(d) Conect gate settings are critical, because TOFDA-scans are not that easy to interpret since there aremultiple visible signals. As a minimum, the gates shouldencompass he lateral wave and ongitudinal wave back-wall signal; he gate can extend o the shear wave back-wall, if required. Typically, the best signal to use as aguide is the first (longitudinal wave) backwall, since tis strong and always present (assuming he transducerseparation s reasonably orrect). The following figuresshow examples of incorrect gate positioning, which willinherently ead to poor flaw detection.

The first example, Fig. N-483(d)(1), shows the gateset oo early, he ateral wave s visible, and he backwalli s not . Any ins ide (ID) near-backwal l laws wi l l bemissed.

The second xample, Fig. N-483(d)(2), shows he gateset too late. The lateral wave is not visible. The firstsignal s the backwall, and the second ignal s the shearwave backwall. With this setup, all the outside (OD)near-surf'ace laws will be missed.

The third example, Fig. N-483(dX3), s with the gateset oo long. Though his is not technically ncorrect, heimage will show he diffiacted backwall shear-shear ave

F I G .N - 4 8 2 ( b ) S C H E M AT I C F F L AWL O C AT I O N SN DTO F D I S P L AY H O W I N G HE L AT E R A L AV E ,

B A C I ( WA L L ,ND F O U R L AW S

2 - Toe crack

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OD surface-breaking la w

Lateral wave

Buried law

Region ofporosityoften difficultto detect

Backwall

FIG.N .483 (a ) CCEPTABLEOISE EVELS, LAWS, ATERAL AVE, ND ONGITUDINALAVE ACKWALL

Signa l s

becominginv i s ib l ei n h i sarea.

F I G .N - 4 8 3 ( b ) TO F D M A G EW I T HG A I NTO O O W

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FIG.N-483(d ) (2 ) OFDMAGE ITH HEGATE ET OO ATE

F I G .N - 4 8 3 ( d ) ( 3 ) O F D M A G E W I T HT H E G AT ES E T TO O O N G

L-WaVe

backwall

S-wavebackwalls igna l

Lateral wave

L-wavebackwalls ig a

S-wavebackwall

s igna l

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F I G .N - 4 8 3 ( g ) TO F D M A G EW I T HTRANSDUCERS O TC E N T E R E D NTHE W E L DA X I S

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N-483

signal. These S-S waves may show additional and con-firmatory information. The diffracted shear waves showthe porosity more clearly than the diffracted longitudinalwaves and there s a strong mode-converted ignal thatoccursjust before he shear wave gate, which could causeinterpretation problems. Normally, the gate is set fairlyshort o enclose nly the ateral wave and he ongitudinalwave backwall to clarify interpretation.

(e) Incorrect too far apart) ransducer eparation Fig.N-483(e)l results n the backwall signal becoming dis-torted, the lateral wave becomes weaker, and some ofthe diffracted signal amplitudes drop.

(f) Incorrect too close ogether) ransducer eparation[Fig. N-483(f)] results n the lateral waves becoming

N-483

stronger, and the backwal l weaker. Again, the TOFDirnage of the flaws is poor.

(g) If the ransducers re not centered n the weld [Fig.N-483(g)1, he diffracted signal amplitudes will decline othe point where flaw detection s seriously mpaired.

(ft) Noise evels [Fig. N-483(h)] can seriously mpairTOFD interyretation. Noise can come rom a number ofsources such as electrical, ultrasonic, grains, and cou-pling. Typically, ultrasonic and grain noise appears ni -versally across he TOFD image. Electrical noise appearsas an nterference attem, depending n the noise source.Once the occurrence of the electrical noise ncreasesbeyond a certain point, nterpretation ecomes ssentiallyimpossible.

2OO4 ECTION V

F I G .N - 4 8 3 ( h ) TO F D M A G ES H O W I N G L E C T R I C A LO I S E N T E R F E R E N C E

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ASME TOFD Interpretation Manual