Flaw Size Evaluation in Immersed Ultrasonic Testing

8/14/2019 Flaw Size Evaluation in Immersed Ultrasonic Testing

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F l a w s iz e e v a l u a t i o n in i m m e r s e d u l t r a s o n i c t e s t i n gJ . D . H i s l o p *

T h e f a c t o r s w h i c h a f fe c t t h e e v a l u a t i o n o f f l a w s i z e b y e c h oa m p l i t u d e m e a s u r e m e n t s a r e d i s c u s s e d a n d f u n d a m e n t a lw e a k n e s s e s i n th e f i a t - b o t t o m e d h o l e t e s t b l o ck s y s t e m a r eo u t l in e d . T h e n e e d f o r k n o w l e d ge o f o p e r a t i n g f re q u e n c y i ss t r e s s e d a n d t h e co n s e q u e n t c o r r e c t i o n s r e q u i r e d a r e e x -p l a i n e d . F i n a l l y s c a n n i n g m e t h o d s r a t h e r t h a n a m p l i t u d em e a s u r e m e n t s a r e p r o p o s e d a s o f f e r in g a w a y o f e l i m i n a t i n gm o s t o f t h e u n c e r t a i n t i e s i n fl a w s i z e e v a l u a t i o n

F L A T - B O T T O M E D H O L E S T A N DA R D SI n t h e m a n u f a c t u r e o f f o r g i n g s a n d w r o u g h t p r o d u c t s , i n c l u -s i o n s a n d s h r i n k a g e p i p e i n t h e o r i g i n a l c a s t i n g o t a r e f l a t -t e n e d b y t h e u p s e t t in g a c t i o n o f t h e f o r g in g p r o c e s s . T h er e s u l t i n g f l a w s g e n e r a l l y l i e a l o n g t h e d i r e c t i o n o f g r a i n f l owa n d a r e o f te n s u b s t a n t i a l l y f l a t a n d p a r a l l e l t o t h e m a j o rf o r g i n g s u r f a c e s . N d t w o r k e r s s o o n s a w t h a t a r t i f i c i a l f l a w sm i g h t b e u s e d i n n o n - d e s t r u c t i v e a s s e s s m e n t o f f l a w s i z e ,p r o v i d e d t h e i r g e o m e t r y s i m u l a t e d t h a t o f a c t u a l f l a w s . F l a t -b o t t o m e d h o l e s d r i l l e d i n m e t a l b l o c k s h a v e l o n g b e e n e s t a b -l i s h e d , e s p e c i a l l y i n th e a l u m i n i u m a n d a i r c r a f t i n d u s t r i e s ,f o r e s t i m a t i n g f l aw s i z e a n d e s t a b l i s h i n g q u a l i t y a c c e p t a n c el e v e l s .T h e f i r s t a l u m i n i u m b l o c k s w e r e m a d e a n d te s t e d i n 1 95 2 b yW . C . H i t t, 1 a n d a f t e r e x t e n s i v e p r a c t i c a l t e s t s z w e r e s t a n -d a r d i z e d b y t h e A m e r i c a n S o c i e ty f o r T e s t i n g a n d M a t e r i a l s . 3T h e s y s t e m i s n o w w i d e l y u s e d , a l t h o u g h t h e m u l t i p l i c i t y o fb l o c k s w h i c h a r e n e e d e d i n p r a c t i c e , a n d t h e d i f f ic u l ty o f p r o -d u c i n g s t a n d a r d i z e d s e t s p a r t i c u l a r l y i n t h e h e a v i e r a ll o y s )h a v e b e e n r e s p o n s i b l e f o r a t t e m p t s t o r e p l a c e t h e f l a t-b o t t o m e d h o l e s t a n d a r d b y o t h e r t y p e s o f r e f l e c t o r . H e m i s -p h e r i c a l - e n d e d h o l e s i n m e t a l b l o c k s 4 a n d s m a l l p l a t i n u md i s c s i n f u s e d g l a s s b l o c k s h a v e b e e n s u g g e s t e d , a n d b o t hh a v e t h e i r o w n m e r i t s a n d d e m e r i t s . I t i s u n l ik e l y t h a t t e s tb l o c k s w i l l b e c o m p l e t e l y e l i m i n a t e d f o r u l t r a s o n i c c o n t a c tt e s t i n g , a l th o u g h t h e i r p e r f o r m a n c e c a n c e r t a i n ly b e i m p r o v e db y c o n s i d e r i n g s o m e o f t h e o p e r a t i o n a l c o r r e c t i o n s w h i c h w il lb e d e s c r i b e d i n t h i s p a p e r .A t t e m p t s h a v e b e e n m a d e , n o t a bl y i n G e r m a n y , t o q ua n t i fy t h ef l a t r e f l e c t o r s y s t e m w i t h o ut a c t u a l l y u s i n g t e s t b l o c k s . T h eK r a u t k r ~ m e r A V G d i a g r a m a l l o w s fl aw s i z e e s t i m a t i o n sb a s e d o n o b s e r v e d e c h o h e i g h t s a n d a s e r i e s o f s i m p l e c a l c u -l a t i o n s .

T H E D E C I B E L U N I TI t h a s b e e n ommon e s p e c i a l l y i n t h e U S A ) t o r e f e r t o f l a we c h o e s i n t e r m s o f t h e i r h e i g h t o n t h e c a t h o d e r a y t u b es c r e en a s m e a s u r e d i n m i l l i m e t r e s o r i nc he s o r e x p r e s s e da s a p e r c e n t a g e o f t h e m a x i m u m s c r e e n h e i gh t o r o f t h eheight of a fixed dat um. Th e reproducibility of all thes em e t h o d s d e p e n d s o n a l w a y s u s i n g t he s a m e t y p e o f f l a w d e -t e ct o r o r r e l y i n g o n t h e U n e a r i t y o f t h e a m pl i f i e r r e s p o n s e .

* D i v i s i o n a l Q u a l i ty E n g i n e e r N D T ), R o l l s R o y c e L t d, A e r oE n g i n e D i v i s i o n , D e r b y , E n g l a n d

E l a b o r a t e c h e c k s h a v e b e e n d e v i s e d t o t e s t l i n e a r i t y , 3 b u tt h e r e i s a m u c h e a s i e r w a y .T h e i n t r o d u c t i o n i n E u r o p e o f t h e c a l i b r a t e d a t t e n u a t o r , w h i c hc a n r e d u c e t h e s i z e o f t h e d i s p l a y e d s i g n a l i n a w a y w h i c h i si n d e p e n d e n t o f a m p l i f i e r l i n e a r i t y , h a s o v e r c o m e t h e s e p r o b -l e m s o f c o m p a r i s o n . W h e n a f l a w s i g n a l i s d e t e c t e d , t h ea t t e n u a t o r i s u s e d t o a d j u s t i t s h e i g h t t o a f i x e d d a t u m l e v e ls o t h a t t h e fl a w e c h o a m p l i t u d e c a n b e e x p r e s s e d u n a m b i g u -o u s l y, a nd r e p e a t e d l y , a s d e c i b e l s a b o v e o r b e l o w t h i s l e v e l .I f t h e e q u i p m e n t h a s b e e n s e t u p s o t h a t a s t a n d a r d t a r g e ta l s o g i v e s a n e c h o t o t h i s l e v e l t h e n f la w e c h o e s c a n a l l b er e f e r r e d b a c k t o t h e s t a n d a r d a s s o m a n y d B a b o v e o r b e l owi t . T h e d B s c a l e h a s a f l o a t i n g z e r o , s i n c e i t i s a u n i t o fc o m p a r i s o n , a n d t h i s z e r o c a n b e c h o s e n t o c o r r e s p o n d t o t h ee c h o f r o m a c o n v e n i e n t s t a n d a r d t a r g e t .I h a v e u s e d t h i s s y s t e m t h r o u g h o u t t h i s p a p e r , e x c e p t w h e r em a k i n g s p e c i f i c c o m p a r i s o n w i t h f l a t - b o t t o m e d h o l e s . A l ld i s c u s s i o n s c a n b e c a r r i e d o u t i n t h e s a m e l a n g u a g e , i n d e p e n -d e n t l y of t h e c h a r a c t e r i s t i c s o f t r a n s d u c e r s a n d f l a w d e t e c -t o r s . F i g 1 s h o w s a c u r v e r e l a t i n g t h e d e c i b e l s c a l e t ot y p i c a l f l a t - b o t t o m e d h o l e s . T h e r e i s a g a p o f 6 d B b e t w e e nt a r g e t s w i t h a r e a s i n t he r a t i o 2 : 1 a n d o f 1 2 dB b e t w e e n t a r -g e t s w i t h l i n e a r d i m e n s i o n s i n th e r a t i o 2 : 1 . T h e f i g u r e i sm a r k e d t o s h o w t he d B le v e l s c o r r e s p o n d i n g t o t h e s t a n d a r dt a r g e t s o f 2, 3 , 4 , 5 a n d 6 / s4 in d i a m e t e r f l a t - b o t t o m e d h o l e s ;

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F i g 1 T h i s c u r v e r e l a t e s t h e d e c i b e l s c a l e t o t y p i c a l f l a t -b o t t o m e d h o l e s . T h e r e i s a 6 d B g a p b e t w e e n t a r g e t sw i t h a r e a s i n t h e r a t i o : 1 , a n d 1 2 d B b e t w e e n t a r g e t sw i t h l i n e a r d i m e n s i o n s i n th e r a t i o 2 : 1

n o n - d e s t r u c t i v e t e s t i n g A u g u s t 1 96 9 1 83


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thes e diffe renc es apply to all ma ter ial s. Fig 2 shows how thecommonly used percentage scal e is related to dBs. The gapbet ween the 100 and 50 ech oes is equal to that betw een50 and 25/o, and so on. T he s am e 6dB diffe ren ce is foundfor all echoes with a r elat ive amplitude of 2 : 1.B E A M P A T T E R N S A N D V A R I A T I O N O F S E N S I T I V I T Y W I T HD I S T A N C EA c o m p l e t e s e t o f f l a t - b o t t o m e d h o l e t e s t b l o c k s p r o v i d e s :1 Stan dard artificial targets at different depths in a set of

b l o c k s o f t h e s a m e a l lo y a s t h a t u n d e r t e s t s o t h a t c o m -p a r i s o n s c a n b e m a d e b e t w e e n r e a l f l a w s a n d ar ti fi c ia ltargets at equivalent positions in the ultrasonic b ea m

2 A set of different target sizes so that reflection equalityc a n b e e s t a b l i s h e d b e t w e e n a d e t e c t e d f l a w a n d a s e l e c t e dstandard target.

Th e sou nd intensity in the ultrasonic b e am varies with dis-tance so that the reflected signal fr om a given size of targetv a r i e s w it h it s s e p a ra t i o n f r o m t he tr a n s d uc e r . W h e n b e a mp a t t e r n s w e r e e x p l o r e d i n d e ta i l t h e s h a p e o f t h e e n e r g yf i el d r a d i a t e d f r o m a n u l t ra s on i c p r o b e w a s d i v i d e d i n to t h en o w w e l l - k n o w n n e a r a n d f a r z o ne s . E x p e r i m e n t s s h o w e ds i gn i fi c a nt d i f f e r e n ce s b e t w e e n t h e t h eo r et i ca l b e a m s h a p e sa n d r e al i ty . I n g e n e r a l t h e s e w e r e a ll d u e t o n o n - u n i f o r mvibration of the transdu cer and in particular the mech ani cald a m p i n g o f t he t r a n s d u c e r n e c e s s a r y t o p r o d u c e p u l s e s o fe n e r g y s h o r t e n o u g h f o r a d e q u a t e f l a w r es o lu t io n .V a r i o u s m e t h o d s h a v e b e e n u s e d t o a s s e s s b e a m c h a r ac t e r is -t ic s a l m o s t a l w a y s b y p l o tt i ng t h e b e a m i n w a t e r u s i n g s m a l ltargets to delineate the intensity variations along the be ama x i s a n d i n p l a n e s a t ri g h t a n g l e s t o it. M e t h o d s o f b e a mv i su a li z at i on h a v e b e e n u s e d 6 7 w i t h e m p h a s i s u s ua l ly p l a c e don the predictability and unifo rmity of the emitted b ea m.B o t h m e t h o d s d e s c r i b e d i n t h e r e f e r e n c e s a l l o w o n e t o d e t e r -m i n e a n o m a l i e s o f b e a m p a t te r n a n d h e n c e s e le ct t r a n s d u c e r sfor practical applications but they ar e not calibrationmet hod s. A quantitative as se ss me nt is essential for evaluat-ing pro bes for a particular job.S i n c e m o s t o f t h e b e a m e v al u at i on p r o b l e m s a r e g e o m e t r i c a lit helps to use a sta ndard target wh ich is relatively insensi-tive to incident angle--hence the wid esp rea d u se of sphericaltargets since they are easily reproducib le a nd are insensi-t i ve t o e x a ct b e a m a l i g n m e n t s .3 v F i g 3 s h o w s a p l o t o f t h ev a ri a ti o n o f s o u n d i n te n si t y a l o n g t h e b e a m a x i s i n w a t e r p r o -d u c e d w i t h s u c h a t a r g et . T h i s c a n c o n v e n i e n t l y b e c a l l ed ad i s t a n c e - a m p l i t u d e c u r v e s i n c e it r e p r e s e n t s t h e w a y t h ee c h o a m p l i t u d e f r o m a s m a l l t a rg e t v a r i e s w i t h d i s t a n ce i ns o m e c h o se n m e d i u m .

THE DISTANCE-AMPLITUDE CURVESome features of the d is tance-ampli tude curve are impor tantand need elaborating. The point at which maximum sensi-tivity is achieved, and which can be desc ribe d loosely as theend of the nea r zone, is c ommonly designa ted in Europe bythe te rm N-point and in the USA by ~o. The positi on of thispoint in the beam is determined by both the diameter of thetra nsd uce r (D) and its effective operating frequenc y (f) . I tsactual distance from the probe (N) is governed by the velo-city of sound (V) in the test medium. The commonly acceptedformula N = D2f /4V generally es t imate s N as l arger than thevalue determined exper imentally ; the reasons for th is willbe d iscussed la te r .The effective sensitivity falls off on both sides of the N-pointbut rather more s teeply nearer the transducer (near zone)than beyond the N-point (far zone). The part of the beam inwhich the axial sensitivity never fails more than 6 dB belowthe maxi mum can convenientl y be called the effect ive workingrange of the probe, since inside it the echo from a given flawwill never be less than 50 of the maxim um. Thus a pre -fer re d water gap can be determined as the d is tance to thenearer '6dB down' point and a limit set to the metal thick-ness which can be ef fectively tes ted by ref erenc e to thefurth er point. The geome try of the curve is governed by thesame considerat ions that deter mine the posit ion of theN-point . Thus if N is gre ate r for a par t ic u lar probe thecurve is more spread out; i f N is shor ter there is a much

Linear oxnplif er0 ~ I00%-2~dB ~ 75%

- 6 d B 50

-12dB ~ 2 5 %-18dB~ 12Yz%

LineardB scale

t ii i , i i i i i i i ii i i i i ii i ii 0 1 1- 6dB 25/,

-12dB 12Yz%-18dB

Fig 2 Relation of dB scales and the commonly used percen-tage scale . The same 6dB difference is found for allechoes with a rela tive amplitude of 2 : 1

2r n1 o 0

-2o E - t .o~ - 6

~ ph e-W ater p o t h - ~ W o r k i n g range ,J "

N =I I I I I I I 1 I2 4 6 8 10 12 14 16 18Depth of reflector n wQter in ] 20 22

Fig 3 This d is tan ce-ampli tude curve is a p lot of the var iat ionof sound intensity along the beam ax is in water, pro-duced with a spherical target. The N-point is the pointof maximum sensi t iv i ty (Probe data: nominal d iameter20ram, nominal frequency 6MHz, measured frequency5.2MHz, ef fective d iameter 17. lmm)

steeper reduction in sensitivity on both sides of the N-point.Fig 4 illustrates this; note that changes in the value of N canbe caused by changes in ei ther f requency or d i ameter . Twoprobe s with diffe rent values of D and f, but the sa me value ofthe product Daf, would have, in the same medium, exactlysimilar d is tance-ampli tude curves , ignor ing for the momentchanges of mater ia l at tenuation with f requency.Fig 4 lists the values of N, pre fe rre d water gap, and workingrange in water and in s teel for the three probes i l lus trated .The working range for o ther engineer ing metals is s ubstan-t ial ly the same as for s teel s ince their sound velocit ies al lapproximate to that for s teel .DISTANCE-AMPLITUDE CURVES IN DIFFERENTMATERIALSThe shape of an ultrasonic beam in one material is exactlylike that in another except that the horizontal scale of thedis tance-ampl i tude curve is expanded or c ontracted in pro-por t ion to the relat ive sound velocity . Therefo re one curvecan be used to represent beam behaviour in al l mater ials ,converting from one to another by multiplying the distancescale by a simple factor. Fig 4 shows this for water,alumlnium alloys, s teel etc . Fig 5 i l lus trat es the point d if -ferently by showing tra nsve rse sections of the beam from agiven transd ucer operating respect ively in water and in amate rial , such as stee l, with a sound velocity four times

184 non-d est ruct ive testi ng August 1969


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grea ter than in water . This p ictor ial v iew of beam s pread isrelated to the shape of the d is t ance-ampl i tude curve; themore rapid spread c orre sponds to a smalle r value of N anda s t eepe r fal l-of f of sensit iv i ty in the far zone.I t is a lso possib le to deduce the beam shape and perform ancecharacter is t ics for composite sound paths of two or moredif ferent media. The s implest way is to conver t al l mater ialth ickne sses to an equivalent water th i ckness , so that al l d is-tances can be added s imply . A typical example is the calcu-lation of an effect ive N-point position in a meta l block to beimm ersi on tes ted . The water path (W) must be subtractedfrom the exper imenta l value of N in water , and the d if fer encedivided by 4.0 (ratio of velocities) to convert a water thick-ness to an equivalent one in steel. Fig 6 shows how this isdone. The diagram of a longitudinal beam section in Fig 7shows how to ar r i ve at the same result by consider ing re-f r ac t ion at the wate r /met a l in te r f ace.THE KRAUTKRAMER AVG DIAGRAMFig 8 shows the Krautkr~ mer AVG diagram, f irs t de scr i bedin 1959.8 Thi s app roach to flaw evaluation, designe d to elimi -nate the need for actual te st blocks, has been slow of acc ep-tance, except possibly in Germany and by some more adven-turous indiv iduals in Br i tain . The d iagram is s traightforward

2 3

o , ,Depth in atu rninium a t ioys [ in]0 1 2 3 4I I I I IDepth in s tee l . , n icker and t i tan i u m arrays [ in ]D e p t h i n w a t e r [ i n ]

I

2 0 2 ?

Fig 4 Distance-a mpli tude curves for three probes, showinghow the shape of the curve varies with the N-pointCurve 1--N = 4in, pr ef er re d wa ter gap = 23/4inWorking range = 33/4in water (lin steelapprox)Curve 2--N = 6in , pref er re d wate r gap = 41/4inWorking range = 53/4in water (lZ/2in steelapprox)Curve 3--N = 10in, pr ef er re d wat er gap = 7in,Working range = 9~/2in (21/2 steel approx)

once the physical character is t ics of beam shapes are appre-ciated. The basic concept s are re duced ran ge (A} (distan ce oftran sduce r to f law expres sed as a mult ip le of N) , reducedsize (G) ( f law diame ter expr esse d as a f raction of the probediam eter ), and comp aris on of the flaw echo with that from therear surface of a f lat p late (Y) for s tandardization pur poses .This greatly s implif ies f law size evaluation in terms of anideal f lat-bottomed target . There are a number of var iant sof the d iagram to su it par t icu la r types of inspection . ..........................:.:.:.:::::...:,: ~ / / . . ~ ~ / < / / / . / . . . - . / / / ~ / / ~ / ~ .iE E E }}~~ N , 5 i n o f w a t e r + ~ , i n o f s t e e [ ~: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :

i i i i i i i i i i i i i i i i i l i l i l i i i i i i i i i i i l i i i i i i i i i i~. ,~ E EE ~E E ~Ei i i i i i l } i i i i i i i i i i ii i i l i l i l i li l i i i i i i

: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :0 1 2 3 4 5 0 -5 1 1 -5 2 2 '5 3[ i n c h e s ]

Fig 6 The s impl est way to deduce beam shape and perfor -mance for composite sound paths is to conver t al lmater i al t h icknes ses to an equivalent water th icknessso that all distances can be added simply

. . . .. . . . . : . . . .

t . . ================================:::::::::::::: i: :i:i:i: :i:Pro b e :E~E EI E E E ~ E E EE~E E

0 1 2 3 / , 5 6 7 B 9 10Dis tance [ inches]

Fig 7 The same re sult can also be deduced by consid eringref r ac t ion at the wate r /meta l in te r f ace

L o . ., ~ . ~ ~ " ~ " : 2 2 . : LLL:2:L :L :~ I ; . L : . : : : 2 -= : -= ,=~: / , " : I: : : : = = : = = = = : : ' = = : : : = = : = : : = = = : = : = : ~..............bMHz ~l I" . . . . : : : : - : . . . . . . . . . . . . . . . . . , , _ _ _ . Ip~..:2: :::::::::::::i::i:i:i:::::::::%::::::::::::i:::::: :~:::::%:::::::::::::~ :~::::::::::::~::::::::::: :~:~:..:.~E:.~

[i n] 0 2 /. 6 8 10 12 1/, 16 18 2010turn

[ i n ] 0 : ~ . 6 8 o 2 ~ 6 18

Fig 5 Beam spre ad is related to the shape of the d is tance -amplitude curve . This figure shows, in a diffe rent wayfrom Fig 4, how one curve can be used to representbeam behaviour in al l mat er ials , conver t ing f rom oneto another by multiplying the distance scale by asimple factor

o o.e . . . . . . . . . . . . " . . " " " ~

' 0 " t ~20 b ,3 . 30 0"2

U~ z.0 ]-1< 50

6Oi I i I i i i i i I0"1 0"2 0'4 06 1"0 2 I4

",," xO~

6 810 20 40 60 80Distance in near zones

Fig 8 The Krautkr~/mer AVG diagram is designed to elimin-ate the need for actual test blocks

non-destructive testing August 1969 185


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There is not space here to descr ibe the use of the d iagramin detai l ; those in tere sted should refer to the English tra ns-lations of the original and subsequent publications byDr Krautkr~'mer.9, zo Test res ult s have been publ ished z zpurporting to show that the method is not accurate, and anumber of unpublished investigations have also shown evi-dence of unreliabili ty. As Bradfi eld has pointed out, Z2 someof these results are due to concentrat ing investigation workon the near zone; Dr Krautkr h'mer has himse lf warned thater ro rs are l ikely to be large in th is region . I t is also in th iszone that test blocks are inhere ntly le ast useful, owing to therapid fluctuation of probe sensitivity for a small change ofmater ial th ickness . Tests in my own depar tme nt have shownthat, provided the probe is chosen so that ta rgets are s i tuatedin its far zone, the res ult s given by the AVG diag ram a greewell with actual target s izes . I t is always easy in imm ersi ontesting to adjust the water path until a detected flaw lies inthe far zone; the same result can be achieved in contact tes t-ing, if nece ssa ry by changing the probe for one of sma ll erdia met er. Flaw distanc es between 1.5 and 3N are found to bethe most sat isfactory .Two probe c hara cter is t ics must be known before the AVGdia gram can be used, the diam ete r D and the value of N.Other wor kers have apparently been content to accept thenominal values for these, as calculated f rom the reputed f re -quency and diamet er of the tr ans duc er. We have found itessential to determine both by practical measurement. Thisinvolves constructing a d is ta nce-ampli tude curve in water ,preferabl y with a spher ical target and by a meas ureme nt ofef fective f requency as outl ined later . From these two quanti-ties, and a knowledge of the velocity of sound in water , it iseasy to calculate the ef fective probe d iamet er f rom the re-lationship N = D2f/4V. We have found that D is a lmos talways less than the nominal d iameter , sometimes by asmuch as 15/o, and Fig 3 il lu st ra te s a '20m m' p robe which isef fectively only 17 . lmm in d iame ter .Table 1 shows typical results of tests to check the AVG dia-gram. The left hand column shows the true t arget size,column 1 the estimate from the AVG diagram using thenominal value of D and the cal culate d value of N, and column2 the revise d est imate using a me asure d N and a calculatedD based on a d irect f requency measurement. These resultsare sele cte d from many made in my own depart ment, all ofwhich agree remarkably well with f lat-bottomed hole s izes .Thus we believe that anyone who is still convinced of thevalue of f lat-bottomed targets for s ta ndardization shouldseriously consider using one piece of paper instead of 72 ormore metal tes t b locks.

Table I Typical results of tes ts to check the AVGdia gram(see text)2Trues ize Es t imated Es t imated[in] siz e [in] Er ro r [ 1 siz e [in] Er ro r [ 1

0.047 0.036 20 0.045 40.078 0.065 16 0.08 0 30.125 0. I00 20 0.12 2 30.188 0.146 22 0.18 0 4Probe data: nominal f requency 4 .0MHz, nominal d iamet er20mm, calculated N 10.6in , measu red N 8 .0 in , measuredfrequency 3 .5MHz, ef fective d iamete r 18 .5mm

ATTENUATION EFFECTS ON PROBE PERFORMANCECURVESAttenuation is the property inherent in all material of absorb-ing and scattering energy from a beam of ultrasound. Fig 9shows three curves of the performance of a given probe. Thesolid line is the distance-amplitude curve in water deter-mined by any small target the dotted curve is the probe per-formance in such materials as forged steel and the dashedcurve is the corresponding performance in a heavily attenuat-ing material e.g. one of the nickel-base alloys. For clarity

~ 8-1216

-2C _Wa~r_ Opa~n -,8

I 3 4 5 61 Dept2h n steer ond Inco 901 in] I12 16 20 24 28 32 36Depth in water [in]

~ ' o o=" \ " " ' " ' " . . S teel." ' , .~ \ \ ~ ""-.ater "'....

\ Inco 901I I I i I

Fig 9 Distance-ampli tude curves for the same probe in s teel(attenuation 0dB/in), water (0.22dB/in) and Inco 901(2.0dB/in ). The diffe renc es in the far zone are due tothe d if ferent at tenuation in the three tes t media

all three hor izontal d is tance scales have been rat ionalized toapproximately coincident N-points and the f lat -bottomed holetarge t in steel ha s been chosen to equate, at the test fre -quency, with the s tandard spher ical target used to determi nethe curve in wat er. We have found for exa mple that at5.0MHz, the r eflec tivi ty of a 0. 050in flat -bott omed hole insteel is equivalent to that of a 0.100in diameter sphere inwater .Although the curves agree closely in the near zone and nearthe N-point , they d if fer increasingly in the extrem e far zone.This is due to the different attenuation in the three testmedia. A good quality forged ste el has, for all practi cal pur -poses , zero at tenuation; in n ickel -base al l oys at tenuation canbe substantial , especial ly at h igher f requencies . Waterusually l ies som ewhere between the two. The d if ferences aref requency- sensit ive, s ince at tenuation depends on f requency,and the d ivergences shown would increase at h igher f requen-cies and decrease at lower f requencies .It is desi rabl e th ere fore that, as in the case of the AVG dia-gram, a d is tance-ampl i tude curve determ ined in water shouldbe correc ted for water at tenuation to produce what is ess en-tially a curve for a me dium in which the attenuation is ze ro.Since at tenuation in water is tempe ratur e sensit ive as wellas varying with f requency the dete rmination of water at tenu-ation involves measurement of the water temperature in thetank used for obtain ing the d is ta nce-ampli t ude curve.Mater ials such as forged aluminium alloys and fer r i t ic s teelshave inherently low attenuation, and few substantial err orsare produced by providing standard test blocks in one alloyto repr esent al l s imil ar mater ia ls . Usually any componentmade from a similar alloy will have attenuation almost iden-t ical to that of the tes t-b lock mater ial ; th is is the taci tassumption in any tes t-b lock sys tem of f law evaluation . Thisassumption has been explored practical ly , and tes ts on o thercommonly used al loys--par t icu lar ly n ic kel-base and t i tanium--show considerable variation of attenuation, not only betweenforgings of nominally s i mila r type, but also between d if ferentpoin ts in a s ingle forg ing . Table 2 g ives represent at ivefigur es for typical alloys in the forg ed condition, but widevariation can occur depending both on the forging techniquesand heat treatment procedures .The only general so lu tion to th is problem is to mea sure theattenuation of mater ial prese nted to the probe, at al l poin ts,so that appropr iate c orrec tions can be made to the workingsensit iv i ty for tes t scanning and to the s ize of echoesascr i bed to par t icu lar f laws when they are detected . This isdealt with more fu lly later .INTERFACE EFFECTSInterface effects are much less predictable than attenuation.Essentially they are associated with the electronic design ofthe flaw detector. All amplifiers and to some extent the

186 non-destructive testing August 1969


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T a b l e 2 Typical attenuation values in dB/i nch for alloys inthe forged conditionFrequency [MHz]2.25 4.0 5.0 5.5

Steel* 0 0 0 0 0 0 0 0Aluminium 0.0 0.0 0.0 0.0Inco 901 0.5 1.5 3.0 3.7Titanium 0.0 0.8 I . 5 I . 8* 'Zero-attenuating ' s tandard (see tex t)trans ducer i tself , are 'paral ysed ' af ter receiv ing a largeultrasonic s ignal . Thus even af ter the mechanical v ibrat ionof the trans ducer has been damped out the amplif i er cannotfully amplify a signal immedi ate ly following. The exactphysica l caus es ar e complex, but the phenomenon can usefullybe treated as an amplif ier b locking and recovery ef fect . This ,rather than the simple time duration of the blocking echo,.isthe main r eason for the sensit i v i ty dead zone in practica lflaw detection.In contact testing the initial pulse is very l arge, and even inimmersion tes t ing the surface echo is much larger than anysmal l f law echoes that might be closely following. Echoamplitude is measurably reduced even af ter a degree of sepa-rat ion f rom the surface greater than that normally consideredas the extent of the practical dead-zone. Fig 10 shows theamplitude of echoes f rom a s tandard target at var ious d is -tances below an entry surfa ce. The var iat ion shown isentir ely due to the surfa ce blocking effect, since the wa tergaps have been adjusted so that the target is always at theN-point in the geometr ical beam. The received s ignal is s t i l laffected even down to depths of 11/2in or more in steel.This ef fect thus fur ther al ters the beam shape determined inmeta ls, and Fig 9 shows the extent of the variat ion in a typicalexample. The axial sensit iv i ty curve is depr esse d close to ametal in ter face, so that the dotted curve for s teel l ies s l ightlybelow the so lid curve det ermine d in water , where there is nointe rfac e blocking. Thi s effect, coupled with water attenuation,causes the curve determined in water to d if fer f rom theequivalent ones in metal. Fig 9 shows the sum of the twoeffects ; however sensit iv i ty d i f ferences in the working rangefor practical tes t i ng are smal l , and normally res tr i cte d to 2or 3dB.The in ter face ef fect causes s l ight under-est i mation of f lawsize close to the in ter face when using a curve dete rmined inwater while in the far zone attenuation differences can lead toerr ors in ei ther d irection . Flaws tend to be over -est ima tedin s teel s ince i t is generally le ss at tenuating than water , butin a n ic kel-base al loy, for example, f laws could be unde rest i -mated . I t is however easy enough to correc t for these ef fectsonce they have been established quantitatively.EVALUATION O F MATERIAL A T T E N U A T I O NVariat ions of attenuation ar e to be expec ted in thick nickel-bas e alloy forgings since ea ch pa rt of the forging is subjectt o a va r i a bl e a m o u n t o f h ot w o r k . T h e w o r k i n g t e m p e r a t u r ediffers acco rdin g to the different cooling effects of die sur -f a c e s a n d m a t e r i a l s e ct i on s . H e n c e it i s i m p o s s i b l e t o e v a l u -ate flaw si ze effectivel y without a full knowledge of theattenuation ex perie nced by the ultrasound beam, and theseeffects must be known in the region immediately adjacent toany detected flaw which is to be evaluated.These ef fects can be corrected for in measurements made inte rm s of both flaf-bottom ed hole targ ets and with the AVGdiagram. At least one mater ial is requir ed which can bemade up in a range of d if ferent sectional th ic knesses , andwhich is effectivel y non-attenuat ing at the frequency con-cerned. Within the range of practic al tes t f requencies forour imm ersi on work (1 .5-6MHz) good quali ty forged fer r i t i cs tain l ess s teel c omes in to th is category and al l our at tenua-t ion meas urem ents are made in ter ms of th is mater ial .( 'Ze ro-a tte nuat ion' blocks are made from a 12/o chro miumcre ep-r esi s t i ng s tain l ess s teel (Rex 448). Similar mater ia lspe ci fic ati ons ar e S 62, AISI 410 and SAE 51410.)

0-1-2

~ 3-4

-5 I I I I I I I I I1 / 4 1 / 2 3 / 4 1 1 1 / 4 I / 2 2S e p a r a t i o n o f 0 - 0 5 0 in f i a t - b o t t o m e d h o . e f r o m s u r f a c e

Fig 10 Varia tion in the amplitude of echoe s from a stan dardtarget at var ious d is t ances below an entry surface,caused by the in ter face ref lect ion and amplif ierblocking

10

3

25

0

Fig 11

Z-: ... o S t e e l' " ' " ' " ' " " " . . . . . . . . . - - - - . . . . . .. T o p~ . , . . b " . o . . . . . o . . . . . . . . . " o - . e c h o

" - . . - . ) u n d e r t e s t ]Inco 901 1 =I I I ' m e a s u r e d

1 2 3 4 S 6 7M a t e r i a l t h i c k n e s s [ i n ]M e a s u r e m e n t o f m a t e r i a l a t t e n u a t io n (s e e t e x t ) . Z ist h e z e r o t h i c k n e s s p o i n t f o r s t e e l a n d I n c o 9 0 1 .( P r o be d a t a: f r e q u e n c y 5 . 0 M H z , d i a m e t e r 2 0 m m )

Each probe is evaluated in i t ial ly with this s et of s teel b locksranging in thickn ess from 1 to 6in in lin steps . A dist ance -ampli tude curve is produced in water and the corre spondingwater gap for the best working range is determined. Usingth is water gap , a ser i es of mea sure ment s is made of thedif ference between f ront and back surface echoes for each ofthe six blocks. Thes e Points are plotted as in Fig 11, and itis generally found that a straight line can be drawn fairlyeasily through the six points. This line is projected to theleft to cut the vertical (dB) axis; the point of intersection Zis cal led the zero- th i cknes s poin t and the corres ponding dBfigure g ives the relat ive re f lect iv i ty correcti on at the water /meta l in te r f ace.The s lope of the l ine repre sent s the geomet r ical chara cte r i-s t ics of the beam with respe ct to the echo f rom an extendedflat plate and, provide d that the steel u sed is e ffectivel y non-attenuating, no component of the slope will be due to themetal as such. However , a measu reme nt made with the sameprobe on another mater ial , e .g . a n ickel -base ahoy, g ives apoint such as A in Fig 11, corr espon ding to the differencebetween front and back echoes for the 4in thickness ofmater ial under tes t . We can assume that the acoustic im-pedance for n i ckel-ba se al loys is equal to that of s teel(approximate impedances for some commonly used mater ialsare given in Table 3); so if we join A to Z we get a line whic his character is t ic of the average at tenuation of th is mater ial .T a b l e 3 Approximate acoustic impedances for somecommonly used mater ialsSteel, nickel alloysAluminium alloysTitanium alloysWater

470-490 x 104 g/ se c/ cm 2170 x l04 g/s ec /c m 2280 l04 g/ se c/ cm 215 x 107 g/ se c/ cm 2

non- des tru cti ve testing August 1969 187


6/10

We can determine the attenuation at any point such as B bymeasur ing the separation of the s teel and n ickel-base al loylines. In the example given the attenuation of the nickel-base alloy is 3dB/in. (Note that engi neer s commonly ref erto at tenuation in ter ms of the ref lect ion inch . For pul se-echo work the total path distance is twice the indicated path,so that real at tenuation f igures are s tr ict l y only half thosequoted in this paper. For flaw corre cti on calculations theyhave to be doubled again, and on the whole it is eas ier foroperators to think in terms of the one-way distances.)

MEASUREMENTS IN MATERIAI~ OF DI FFERENTACOUSTIC IMPEDANCEFor alloys with a different acoustic impedance the zerothickness point is different. Fig 12 shows the lines foraluminium and titanium alloys and in the absence of signifi-cant mater ial at tenuation these are paral lel to the l ine drawnfor s teel . This is to be expected s ince under these condit ionsthe slope of the curve is solely a function of the geometry ofthe beam, which is constant for a given probe. The di fferentin tercepts on the ver t ical ax is represent the relat ive correc-t ions that must be made for surface ref lect iv i t ies caused bythe d if ferent acoustic impedances . Between s teel andti tan ium alloys the correct ion factor is approximately 3dB;a fur ther 3dB are necessary to correct for aluminium alloys.If we now take a 4in sample of a titanium alloy with somemater ial at tenuation and make f ront and rear surface echomeas ureme nts as before we obtain in general a point suchas C in Fig 12. If we connect C to the corres pondi ng zero-th ickness poin t we can deter mine the mate r ial at tenuationjust as before. In the example shown it is 1.5d B/inch. Notethat we need not use a series of titanium or aluminium alloyblocks to determine the appropr iate zero- t h ickne ss poin t .In ter face correction f igures relat ive to s teel for thecommonly used alloys are:

Nickel alloys 0.0dBTitanium alloys 3.0dBAluminium alloys 6.0dB

Once a set of steel blocks has been used to prepare the basicline for ea ch probe, the rel ated l ines for the other alloyscan be drawn as in Fig 12.REAL FLAW SIZE AND THE EQUIVALENT FLAT-BOTTOMED HOLEA number of workers have investigated the relat ionship be-tween apparent f law size me asur ed in terms of f lat-bott omedholes , and the real s ize de termine d when the f law is subse-quently f ract ured .Claydon ~ found that correcti on factors f r om xl to x5 wererequired for flaws in alumintum alloy forgings when com-paring the area of real f laws with the area of the equivalentflat -bott omed holes. Shar pe s re sult s 14 are given in Fig 13;the mean corre ction factor is x 4 .4 for f law area (x 2 .1 ford iameter) in forged s teel relat ive to the equivalent f lat-bottomed hole. We have carried out investigations of thissort over many years, and have always found that defectsaf ter f racture are considerably larger than expected f romthe flat -bott omed hole evaluation. Unfortunately we obtainedmany of the ear ly re sults before we appreciat ed the s ignif i-cance of frequency and we can no longer go back over theresults and correct for f requency ef fects .DISCREPANCIES IN FLAW ESTIMATIONS WITHDIFFERENT PROBESDuring r ecent investigations in to better techniques for es t i -mating flaw size we noted cer tain d iscre pancie s in the s izeof the flat- botto med hole with a reflec tivi ty equivalent to oneselect ed f law, when tes ts were carr ied out with d if ferentprobe/ f law- dete ctor combinations. We investigated th is morefully , and the re sults were much more in terest ing than anyobtained during f ractu re exer cise s; they led us to the conclu-s ion that there were great d if f icu lt ies in using tes t b locks(or any substi tu te for them) without physical measu reme nt ofoperati ng frequency, and that the resul ts could in fact bequite i naccurate.

Z erot h i c k , , e s s + . , y . . . . . J L Z _ .points ~ :~ "~ : " -o- --. .... . ~ """ """. .. .. , ~ . . , ~ . . . . .. . .. T , ta n , u r n a l t o ; " - " ~15 --.-.. o o . . . . .. o . . . . . . o . . t

oa,Z-_ 20"(3m-~ 25

30 , I0 1 2

A t t e n u a t i o nr 0 . 0 d B / i n

~ S t e e l/ ' / t Q~ . . ~ , A t t enu a t i on" l(arh 'Q-~/o l~ ~ l ' S dB / i n

I 13 4 5 6 7[ in ]Fig 12 The zero thickness point is different for alloys withdif ferent acoustic impedances. The l ines foralumintum and titanium alloys are parallel to thatfor s teel in the absence of s ignif icant mater i al at tenu-ation

NI l l"c~u"1o

0"150

0.120

0-09 0

0 " 0 6 0

0 ' 0 3 0

IIi W o r s t l i n e

iiiiIi

IIii .

I t o/ / o . . j~ ' / i ; ss

s t r a i g h tf i n e

SSSs ~ O p t i m u r ns l i n eS

SSSS

I IF s I I I0 0-030 0.060 0-090 0.120D i a r n e t e r o f f l a t - b o t t o m e d h o l e [ i n c h e s ]Fig 13 Compa ris on of actual defe cts in stee l forgings andf lat-bottomed holes g iv ing identical indications. Cor-rection factors : worst l ine x 6 .0 on d iameter , beststra ight line 2.1 on diam ete r, optimum line x 1.0on d iameter

These anomalous results f ir s t came to l ight dur ing theevaluation of identical defects by d if ferent labora tor ies usingnominally s imila r f law detection equipment. Es timat ed f lawsizes var ied so much that f laws considere d fu lly acceptableby one inspection depar tment were being rejected by anotheralthough each was working to the same specifications andquali ty s tandards . Investigation of the probe/f law-detectorcombinations used revealed that nominally s imilar probes,which should have been operat ing at 5MHz, were in fact

188 non-d estr ucti ve testi ng August 1969


7/10

operati ng at 5.5MHz and 3.8MHz. Up to that time there hadbeen no suggestion that it was not perfectly valid to evaluateflaw sizes with any probe and obtain an identical answerindependently of the specia l c harac ter i s t i cs of each probeused. We found that when one selec ted flaw was subject toquanti tat ive assessment i t appeared equivalent to d if ferents tandard t argets when tes ted at d if ferent f requencies .Figs 14, 15 and 16 show how the received echo amplitude inte rms of a flat -bott omed hole varie s with changing frequency ,for a number of differ ent size s and types of flaw. Thechanges e xpr ess ed in dBs are independent of absolute flawsize over the range studied. In steel (Fig 14) for example,the f laws appear larger at the h igher f requencies and smal-ler at the lower ones. For one typical flaw the equivalentflat-bottomed hole at 51/4MHz is 0.080in and at 2~/4MHz only0. 030in--a difference of 18dB. Flaws in certain titaniumforgings Fig 15) show a precisely opposite variation. Oneselected flaw, for example, appears equivalent to the 0. 047inflat-bottomed hole at 51/4MHz and to 0. 125in at 21/4MHz a14dB difference but in the opposite sense to the results insteel. The results for nickel-base alloys Fig 16) areequally surprising and similar to those in steel. We havemade similar tests to examine this effect for typical flawsin aluminium alloys, and have found the results to be verylike those for steel.The curves in Figs 15 and 16 were made by comparing realflaws with flat-bottomed hole test blocks making fullcorrection for attenuation differencesbetween the forgingand the test block. Hence he variability in the size estima-tions is entirely due to the change of reflectivity with fre-quency. Over the range 2-5.5MHz differences in the sizeestimation of the same flaw are as much as 19dB equiva-lent to a diameter ratio of almost 3 : 1) in the worst case.These and similar results for all flaws investigated in fouralloy types showhow much the test-block system is builton shifting sand--the shifting sand of varying relative reflec-tivity at different frequencies.

POSSIBLE ALTERNATIVE REFLECTION STANDARDSSince these re sults undermine completely any remainingconfidence in the use of flat-bottomed hole targets fors tandardization , we s tar ted an exercis e to determine whetherany other practical artif icial target would be any better. Wecompared the reflectivity of typical flaws in steel withstandard ref lectors of var ious types: a spher ical target , aflat -ended rod, a stre tch ed wire and a flat surfac e. All thetargets were imm erse d in water , and a range of f requencieswas used for each one. The res ults indicated some degreeof f requency sensit iv i ty for each ref lect or , most marked forthe spherical target and comparable to that for the flat-bottomed hole for all the others.The behaviour of the spheri cal targe t is not unexpected; it iswell known that spheres appreciably larger than the wave-length used reflect energy largely independently of the fre-quency. Other types of reflec tor, espe cia lly those with af lat characte r is t ic shape, are to some extent f requency-sensi tive in their refle ctiv itie s, and this explains why thediscrepancy with f requency changes is generally greater forthe sphe ric al target. However, for the titanium forgingsinvesti gated, and because of the anomalous behaviour of theflaws in this alloy, variati on with res pec t to the spheri caltarget is less than for the o thers . All these result s under-line how essential it is to know the operational frequency ofa probe before making any attempt to arrive at an intelligentand rel iab le es t imate of equivalent ref lect iv i ty , whatever typeof ar t if ical s tandard is used .

MEASUREMENT OF FREQUENCYOur investigations showed that we needed some way ofmeasur i ng the ef fective operating f requency of probes. Butwhat is meant by the operating frequency of a probe workingin a puls e-e cho sys te m? A full evaluation of the subjectwould entai l e laborate spe ctrum analy tical work . Howeverfor our investigations it was adequate in practice to displaythe echo from a st andar d r efl ect or 15 on a high quality oscil -loscope with accurate time cal ibrati on, and study the cha rac -ter is t ics of the wave patterns . The s tandard ref lector can

usefully be a sphe re or a flat plate in water . Differe ncesbetween the echoes f rom th is and o ther s tandard targets arenot significant to the degree of accura cy re quired. An aver -age frequency can be determined by counting the centregroup of individual cycl es; typical pulses are shown in Fig 17.AN ALTERNATIVE APPROACH TO FLAW SIZEMEASUREMENTTests have been carried out to examine the possibility of areturn to the early technique of moving the probe to scan the

0r

Eooqe

i11

~ 4

+lS+10

>~ -5

-10-15

V a r i a t i on o f ap p a r en t s i ze w i t h f r equ enc yf i a t -b o t t o m e d h o l e s t a n d a r d }

Q, 'oss

I I I I2 3 4 5[ M H z ]Fig 14 Flaws in forged steel

co lo

4Oe~

c{D

DC rt13

Fig 15

+15

EO

._>aGa

+I0

+5

V ar i a t i on o f ap p a r en t s i z e w i t h f r equ enc yf l ab-bot tome d ho[e s tandard)

- 5

-10

-15

,%\E)

\ \ \

I I I I I2 3 4 5 6[ M H z ]Flaws in forged titanium alloy

+154

7/~ - +I06/64

. a~ - >

17uJ -10-15

V ar i a t ion o f ap p a r en t s i z e w i t h f requ enc yf l a t - bo t t om ed ho t e s t anda r d )

s

I I I I i2 3 5 6[ M H z ]

Fi g 16 Fla ws in for ged Inca 901

non-destructive testing August 1969 189


8/10

b

Fig 17 T y p i ca l p u l ses f ro m a s t an d ard f l a t p l a te r e f l ec t o rin water . These can be used to deter mine ave ragefrequency by count ing the cen t re g roup of ind iv idualcycles (cal ib rat ion : 0 .5 sec/cm)(a) 2.0MHz pulse (b) 5.3MHz pulse

edge of a f law to determine i t s ou t l ines and pr incipal d imen-sions . Th is techn ique, o r ig inal ly manual ly app l ied , wasnecessar i ly crude and inaccurate . A good deal o f ref inementhas to be app l ied to equ ipment designed for f law t ra vers ework to ob tain wor thwhi le and reproducib le resu l t s .No rmal i mm ers i o n t e s t i n g eq u i p men t can t r av e r se a p ro b ewith the mini mum of backlas h to an accu rac y of 0. 001in. Itis conve nient to use a techn ique l ike that use d in contacttes t ing ; the edge of the defect i s e s t im ated as the posi t ionwhere the received flaw echo falls to 6dB below the maxi-mum. The f law echo i s ma ximiz ed by t i l t ing the p robe i fnece ssa ry , and the p robe i s t rave rse d in both hor izon taldire ctio ns unti l the echo fails to this fixed level. Such anapproach has one b ig advantage over any method that de-p en d s on measu r i n g r e f l ec t i on amp l i t u d es . I t i s r easo n ab l eto expect that flaw scanning mea sure men ts are independentof the absolute refle ctivi ty of a discontinuit y, provided th ati t s re f lect iv i ty i s co nsis te n t acros s the fu l l wid th . I t wouldalso be possib le to use s t andard f la t -bo t t omed targets inan y mat e r i a l t o ca l i b ra t e a f law t r av e r se sy s t em, a l t ho u g hthe abso lu te ref lect iv i ty o f such ar t i f ic ial ref le ctors i sappreciab ly g reater than that o f most real f laws.Under these condi t ions the en t ry surface in ter face and i t se f f ect o n amp l i f i e r p a ra l y s i s a r e a l so u n i mp or t an t . S i mi -lar ly changes in mater ial a t tenuat ion af fect ing the relat ivesensit ivity at the posit ion of the flaw will not have anybear ing on the echo var iat ion across the defect . Any reduc-t ion in sensi t iv i ty due to ei ther in ter face ef fects o r m ate r ialattenuation will apply equally to all parts of the flaw echoand hence wi l l no t a / feet the f inal t raverse measurementresu l t s . F ig 18 i l lus t rates th is po in t .We car rie d out init ial te sts to find wher e the flaw should bei n t he g eo met r i ca l b eam fo r b es t d i sc r i mi n a t i o n . T h e N-

point se em ed the mo st likely ince it repres ents the natrowest par t o f the beam emit te d by a f la t t ra nsduc er , andtes t s c onf i rme d th is . In general , i t i s possib le to ad just thewater gap between probe and specimen so that the defectunder invest i gat ion i s s i tuated at an equ ivalen t dep th equalto N. F ig 19 i l l ust rat es how this achieved for a probe (withN = l l in in water) which is used to evaluate two flaws, l inand 2in below the surface. Water gaps of 7 and 3in respec-tively place the two defec ts at N, the point of ma xim umsensi t iv i ty . S ince bo th f laws are at equ ivalen t posi t ions inthe beam, the evaluat ion can be co mpare d d i rect ly wi th aca l i b r a t i o n cu rv e b ased o n measu re men t s o f f l a t -b o t t omedholes ly ing in the same posi t ion .We have const ruc ted cal ib rat ion c urves fo r a range ofprobes to determine the possib i l i t ies o f app ly ing the t ra-ver se technique. A typ ical resu l t i s shown in F ig 20. Thet rue d iameter o f the f la t target i s p lo t ted against the cor-responding t rave rse d imens ion between the 6dB downpoin ts . T he curve gets s t eepe r as the f law gets smal ler , andeventual ly approaches a l imi t on the hor izon tal ax is whichdepends on the ind iv idual p robe cha rac ter i s t ic s . Th is curveis typ ical of a wide range of p ract ic al e xampl es . The bestpor t ion fo r f law measurement i s where the g rad ien t i s near45 , when a smal l change in p robe t r ave rse mea sure men tcorr espon ds to a smal l change in es t i mate d flaw s ize. Wherethe curve i s s teep the er ror in the es t imate o f real d iameterwould be too great.Fig 20 shows that a probe with a flat transducer of 20mmdiameter , operat ing at a f requency of 5MHz, and wi th a cor-responding N-poin t a t l l i n o f water , can be used to ass esstar get s down to 0. 100in (6/64in) in dia met er . However fra c-tu re t es t s have shown that real defects may be 2, 3 o r mor et imes the l inear d imensions o f the equ ivalen t f la t -bo t tomedhole g iv ing equ ivalen t ref lect iv i ty , and thus a qual i ty accep-tance s tandard based on the ampl i tude of ref lect ion f rom a:~/6.iin fla t-bo ttom ed hole could be m aint aine d when tra ns-fer r ed to the f law t rav erse technique, a t the equ ivalen t of afla t-bo ttom ed hole at least 6/64in in dia met er. This is justwi th in the range fo r which good ass ess ment s can be made inp rac t i ce .F O C U S S E D P R O B E SS i n c e t he b e a m f r o m a f la t t r a n s d u c e r h a s a p oi n t o f m a x i -m u m s e ns i ti v it y it c a n b e c o n s i d e r e d t o h a v e s o m e d e g r e e o ff o cu s si n g. T h i s i s t h e p o i n t a t w h i c h t r a v e r s e m e a s u r e m e n t sare mo st satisfactorily ma de whi ch sug gests that a deliber-a t el y f o c u s s e d t r a n s d u c e r c o u l d b e u s e d f o r e v e n b e tt e rdiscrimination.F i g 2 1 s h o w s h o w f l a w s h a v e b e e n p l a c e d a t t h e f o ca l p o in tsituated at a depth of 3/4in steel wh en using a 2in wat er gap.

T nce

Ma x e ch o Ech o a tf r o m f l a w e d g e

Codl3d i f f e r e n c e

M a x e c h o E c ho a tf r o m f l a w e d g eb

Fig 18 In ter face and at tenuat ion ef fects do not af fect p robet raverse measurements , s ince they app ly equal ly tothe echoes f rom al l par t s o f the f law(a) No correct ions(b) 3.6d B total fro m att enuation, blocking and fre-quency ef fects

190 non-de st ruc t ive tes t ing August 1969


9/10

/ L ~ ~ J | i ll 1~ 'l 0[ ' : :: :: :: : . .I . . . . . . . . . . l m l , , : , . . . .l l ~ l l : - - n~' - - : - - : - - ~ :~1 " ' : : : : : : : ~ ~ . : : : : : : : : : :~L ::i:]~:i:i:~:iiiii:i:i: :;:i:i$~:i:: :::l:i:i:i i :~ i t : : : : : : : i : :ii i i i: : l i: : : : i : : l ~ i ~ i ~ i ~ i i ~ : : i : :: : : : :: i l i: ~ : ~ : i

i i i i i i i i i i i ; i : : i i i i i i i ; : : i i i : : ; : : i : : i : :

F i g 1 9 I t i s g e n e r a l l y p o s s i b l e t o a d j u s t t h e p r o b e / s p e c i m e nw a t e r g a p s o t h a t t h e fl a w i s s i t u a t e d a t a d e p t he q u a l t o N ( p r o b e d a ta : f r e q u e n c y 5 . 5 M H z , d i a m e t e r2 2 r a m , N - p o i n t i n w a t e r l l i n )

F i g 2 2 i s a c a l i b r a t i o n c u r v e f o r s u c h a p r o b e w i t h a f o c a ll e n g t h o f Si n i n w a t e r . T h e m i n i m u m f l a w s i z e w h i c h c a n bee s t i m a t e d a c c u r a t e l y i s a p p r e c i a b l y r e d u ce d , an d e s t i m a t e so f f l a w s a r e n o w p r a c t i c a b l e d o w n t o 0 . 0 5 0 i n d i a m e t e r . T h et r a v e r s e f i g u r e s f o r t h i s p r o b e a r e a l m o s t e x a c t l y e q u a l t ot h e fl a w s i z e a b o v e 0 . 0 9 0 i n , s o t h a t n o c a l i b r a t i o n c u r v ew o u l d b e r e q u i r e d f o r t h i s r a n g e . H o w e v e r , t h e fl a w m u s tn o t b e t o o f a r b e l o w t h e s u r f a c e t o b e p l a c e d a t t h e e f f e c t i v ef o c a l p o i n t . S i n c e f o c a l l e n g t h s u p t o 1 0 in o f w a t e r a r e p r a c -t i c a l, a n d a f o c u s s e d t r a n s d u c e r c a n b e u s e d v e r y c l o s e t ot h e s u r f a c e o f t h e c o m p o n e n t u n d e r t e s t , f l a w s u p t o a b o u t2 1/ 4i n d e e p c a n b e p l a c e d a t t h e f o c a l s p o t .I n t h e e x t r e m e c a s e s t h e p r o b e i s p l a c e d s o c l o s e t o t h e s u r -f a c e t h a t m u l t i p l e f ro n t e c h o e s a r i s e . P r o v i d e d t h e f l a we c h o i s r e c o g n i s a b l e a n d c l e a r l y s e p a r a t e d f r o m t h e s e , t r a -v e r s e m e a s u r e m e n t s a r e s t i l l p r a c t i c a b l e . W a t e r g a p s d ow nt o z/ 4in h a v e b e e n u s e d f o r m e a s u r e m e n t s w i t h fo c u s s e dp r o b e s , b y a d j u s t i n g t h e p r o b e p o s i t i o n s l i g h t l y s o t h a t t h ef l a w e c h o i s c l e a r l y r e c o g n i s a b l e a m o n g t h e r e p e a t f r o n ts u r f a c e e c h o e s .P O S S I B L E E R R O R S I N P R O B E T R A V E R S EM E A S U R E M E N T SA t r a v e r s e s c a n e l i m i n a t e s t h e m a j o r d i f fi c u l t i e s a s s o c i a t e dw i t h f l a w estimation by a m p l i t u de m e a s u r e m e n t ( m a t e r i a lattenuation frequency s e n s i t i v e r e f l e c t i v i t i e s , a n d i n t e r f a c ee f f e c ts ) . S i n c e f l aw t r a v e r s e i s e s s e n t i a l l y a g e o m e t r i c a lp r o c e s s t h e on ly p r o b l e m s w h i c h m i g h t a r i s e m u s t b e g e o -m e t r i c a l i n o r i g i n . T h e s h a p e s o f u l t r a s o n i c b e a m s c a n bem e a s u r e d , a n d t h e s h a p e s o f s u r f a c e s a r e k n o w n , s o t h e o nl yr e a l u n k n o w n s a r e a s s o c i a t e d w i t h th e g e o m e t r y o f th e fl a wi t s e l f .I f t h e f l a w is p l a n a r t h e n th e m e a s u r e m e n t s h o u l d be c a p a b l eo f h i g h a c c u r a c y . F o r f l a w s w h i c h a r e l a r g e l y s p h e r i c a l t h et r a v e r s e f i g u r e s m a y l e a d t o 20/ e r r o r s i n l i n e a r d i m e n s i o n ,b u t s u c h f l a w s a r e u n l i k e ly i n m o s t f o r g e d m a t e r i a l . I f t h ef l aw i s n o t t ru l y s p h e r i c a l b u t l e n t i c u l a r , w h i c h i s m u c h m o r el i ke l y , t h e e r r o r s i n t r a v e r s e m e a s u r e m e n t w i l l b e m i n i m a l ;t e s t s w i t h s l i g h t ly r a d i u s e d a r t i f i c i a l d e f e c t s h a v e s h o w ne r r o r s o f o n l y 5 / . F o r f l a w s w h i c h h a v e b e e n d i s t o r t e d b yt h e f o r g i n g o p e r a t i o n s o t h a t t h e y a r e c o n v e x o n on e s i d e a n dc o n c a v e o n t h e o th e r , d i f f e r e n c e s i n t h e t r a v e r s e e s t i m a t e sf r o m o p p o s i t e s i d e s o f t h e f o r g i n g w i l l g i v e a c l u e t o t h i sc o n d i ti o n , a n d a n a v e r a g e f i g u r e w i l l b e l i t t l e d i f f e r e n t f r o mt h e t r u e s i z e .A l l t h e s e q u a l i f i c a t i o n s a r e b a s e d o n ly o n t h e c h a r a c t e r i s t i cs h a p e o f t h e fl a w , a n d n o t o n i ts a c o u s t i c p r o p e r t i e s , n o r t h ea c o u s t i c p r o p e r t i e s o f t h e m a t e r i a l i n w h i c h i t i s s i t u a t e d .S i n c e v a r i a t i o n s i n t h e s e a c o u s t i c p r o p e r t i e s a r e u s u a l l yu n k n ow n a n d l a r g e l y u n p r e d i c t a b l e , t h e p o t e n t i a l s o u r c e s o fe r r o r a r e g r e a t ly r e d u c e d b y t r a v e r s e m e a s u r e m e n t s . O n lyf l a w s h a p e w i l l i n f l u en c e t h e s i z e e s t i m a t e s m a d e b y p r o b et r a v e r s e , a n d t h i s c a n u s u a l l y b e q u a n t if i e d i f t h e e r r o r si n t r o d u c e d by a s s u m i n g t r u e p l a n a r i t y a r e g r e a t e r t h a n c a n

0"2000"175

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I I I I ( ~ I I I i i0-150 0"200 0.250 03 00 0"350T rav e rse scan [ i n ]C a l i b r a t i o n c u r v e f o r a f l a t p r o b e ; t h e t r u e d i a m e t e r so f f l a t t a r g e t s i n a n y m a t e r i a l a r e p l o t te d a g a i n s t t h ec o r r e s p o n d i n g d i m e n s i o n b e t w e e n t h e 6 d B d o w np o i n t s . T h e b e s t p a r t o f t h e c u r v e f o r fl a w m e a s u r e -m e n t i s w h e r e t h e g r a d i e n t i s n e a r 4 5 (p r o b e d a t a :f r e q u e n c y 5 . 2 5 M H z , d i a m e t e r 2 0 r am , N = 1 1 . 5 in ; a l lm e a s u r e m e n t s m a d e a t N )

1

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F i g 2 1 M e t h o d f o r p l a c i n g f l a w a t f o c a l p o i n t o f a f o c u s s e dt r a n s d u c e r ( s e e t e x t ) ( P r o b e d a t a : f o c a l le n g t h 5 i ni n w a t e r , = 2 i n w a t e r + 3 /4 in s t e e l )

0.200

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T r a v e r s e s c a n [ i n ]F i g 2 2 C a l i b r a t i o n c u r v e f o r a f o c u s s e d p r o b e . T h e m i n i -m u m f l aw s i z e f o r a c c u r a t e e s t i m a t i o n is a p p r e c i a b l ys m a l l e r t h a n th a t in F i g 2 0 . ( P r o b e d a t a : f r e q u e n c y

4 . 5 M H z , f o c a l l e n g t h 5 i n o f w a t e r , d i a m e t e r 1 5 r a m )n o n - d e s t r u c t i v e t e s t i n g A u g u s t 1 9 69 1 91


10/10

b e to l e r a t e d . H o w e v e r , u l t r a s o n i c p r a c t i t i o n e r s h a v e f o rm a n y y e a r s u s u a l l y t o l e r a t e d m u c h w i d e r u n c e r t a i n t i e s i na m p l i t u d e - b a s e d m e a s u r e m e n t s a n d h a v e a c h i e v ed a p r a c -t i c a l s c h e m e o f q u a l it y a c c e p t a n c e s t a n d a r d s .T h i s m u s t b e b e c a u s e t h e m a r g i n s o f s a f e t y h a v e b e e n g r e a ta n d h a v e t h e r e f o r e a l l ow e d g r o s s e r r o r s i n s i z e e s t i m a t e st o b e a b s o r b e d s a f e ly . I f w e a r e t o a p p r o a c h m u c h c l o s e r t ot h e s a f e ty l i m i t s , a n d in th e a e r o s p a c e b u s i n e s s t h i s i s av i t a l n e c e s s i t y , i t i s i n e v i t a b l e t h a t g r e a t e r a c c u r a c y i n f l aws i z e e s t i m a t i o n m u s t b e a c h ie v e d . O n l y b y k n o w i n g m o r ea b o u t th e f l aw s w e d i s c o v e r c a n w e b e g in t o p r e d i c t s o m e -t h i n g a b o u t t h e i r s i g n i f i c a n c e t o t h e e n g i n e e r . T h e i r p o s i t i o nc a n b e e s t i m a t e d w i t h g r e a t a c c u r a c y ; t h e i r n a t u r e c a nu s u a l l y b e i n f e r r e d f r o m m e t a l l u r g i c a l e v i d e n c e ; it is o n l yt h e i r t r u e p h y s i c a l s i z e w h i c h h a s e l u d e d n d t p r a c t i t i o n e r sf o r s o l o n g .

A C K N O W L E D G E M E N T SI w a n t t o t h a n k t h e D i r e c t o r s o f R o l l s R o y c e L t d f o r p e r -m i s s i o n t o p u b l i s h t h i s a r t i c l e ; t h e v i e w s e x p r e s s e d a n d th eo p i n i o n s c o n t a i n e d i n it a r e m y o w n .I a ls o w a n t t o a c kn o w l e d g e t h e a s s i s t a n c e p r o v i d e d b yR . L . H o r t o n a n d v a r i o u s c o l l e a g u e s i n t h e R o l ls R o y c e D i v i -s i o n a l N D T L a b o r a t o r y i n c a r r y i n g o u t m o s t o f t h e e x p e r i -m e n t a l w o r k a n d h e l p i ng i n t he p r e p a r a t i o n o f i l l u s t r a t i o n s .M y s i n c e r e t h a n k s a r e a l s o d u e t o W . C . H i t t , D . G . W . C l a y d o n ,R . S. S h a r p e , D r J . K r a u t k r i i m e r , D . O . S p r o u le , J . E . B o b b i na n d G . B r a d f i e l d f o r i n f o r m a t i o n a n d c o m m e n t s c o n t a i n e d inv a r i o u s p r i v a t e c o m m u n i c a t i o n s .

R E F E R E N C E SI H i t t , . C . P r o g r e s s i n t h e f i e l d o f n o n - d e s t r u c t i v e

t e st i n g t h r o u g h t h e u s e o f u l t ra s o ni c s , P r o c e e d i n g s o ft h e A S T M S y m p o s i u m ( J u n e 1 9 5 2) p p 5 3 - 7 52 H i t t, W . C . U l t r a s o n i c fl a w e v a l u a t i o n , W e s t e r nM a c h i n e r y a n d S t e e l W o r l d ( A p r i l 1 9 5 7 ) p p 9 5 - 9 93 A S T M S p e c i f i c a t i o n E 1 2 7 - 6 4

4 B r a d f i e l d , G . U l t r a s o n i c f l aw d e t e c t i o n i n m e t a l s LC h a p t e r 1 2 i n P h y s i c a l e x a m i n a t i o n o f m e t a l s , e di t e db y C h a l m e r s a n d Q u a r r e l l , A r n o l d , L o n d o n ( 2 nd e d it i o n,1 9 6 0 )5 C h r i s t i e , D . G . S t r e s s w a v e p r o p a g a t i o n a s a p p l i ed t ot h e d e t e c t io n o f f l a w s b y u l t r a s o n i c i n s p e c t i o n , P r o -g r e s s i n N o n - D e s t r u c t i v e T e s t i n g , V o l I , H e y w o od ,L o n d o n ( 1 95 8 ) p p 3 5 - 5 66 H o d g k i n s on , W . L . I s o s o n o g r a p h y , U l t r a s o n i c s , V o l 4 ,N o 3 ( J u l y 1 9 6 6 ) p p 1 3 8 - 1 4 27 M c E l r o y , J . T . I d e n t i f ic a t i o n a n d m e a s u r e m e n t o fu l t r a s o n i c s e a r c h u n i t c h a r a c t e r i s t i c s , M a t e r i a l sE v a l u a t i o n ( J u n e 1 96 7 ) p p 1 2 9 - 1 3 78 K r a u t k r ~ i m e r , J . D e t e r m i n a t i o n o f t h e s i z e o f d e f e c t sb y th e u l t r a s o n i c i m p u l s e e c h o m e t h o d , A r c h l y f i i r d a sE i s e n h u t t e n w e s e n , V o l 3 0 , N o 1 1 ( 1 95 9 ) p 6 9 3 . E n g l i s hv e r s i o n : B r i t i s h J o u r n a l o f A p p l i ed P h y s i c s , V o l 1 0,N o 6 ( 1 9 5 9 ) p 2 4 09 W e l l s , C . D . H a v e w e th e a n s w e r to t h e n e e d f o r r e c o r -d i n g b o th u l t r a s o n i c w e l d t e s t i n g a n d s e n s i t i v i t y , B r i t i s hJ o u r n a l o f N o n - D e s t r u c t i v e T e s t i n g , V o l 1 0 , N o 4 (1 9 68 )p p 7 8 - 8 6

1 0 W e l l s , C . D . A c l o s e r l o o k a t u l t r a s o n i c f l aw d e t e c ti o nc a l i b r a t i o n , B r i t i s h J o u r n a l o f N o n - D e s t r u c t i v e T e s t i n g ,V o l 8 , N o 4 ( 1 9 6 6) p p 8 4 - 9 11 1 B a s t i e n , P . D i f f i c u l t i e s i n t h e u l t r a s o n i c e v a l u a t i o n o fd e f e c t s i z e , N o n - D e s t r u c t i v e T e s t i n g , V o l 1 , N o 3 (1 9 6 8)p p 1 4 7 - 1 5 11 2 B r a d f i e l d , G . C o r r e l a t i n g e c h o a n d f l a w m a g n i t u d e s ,L e t t e r t o th e E d i t o r o f N o n - D e s t r u c t i v e T e s t i n g , V o l 1 ,N o 5 ( 1 9 6 8 ) p p 3 1 7 - 3 1 81 3 C l a y d o n , D . G . W . D e f e c t a s s e s s m e n t u s i n g u l t r a s o n i cw a v e s , P r o g r e s s i n N o n - D e s t r u c t i v e T e s t i n g , V o l 2H e y w o o d , L o n d o n ( 1 9 5 9 ) p p 1 6 5 - 1 8 71 4 S h a r p e , R . S . P r o b l e m s o f d e f e c t l o c a t i o n a n d s i z ea s s e s s m e n t i n u l t r a s o n i c i n s p e c t i o n , B r i s t o l S i d d e l e yE n g i n e s L t d I n t e r n a l R e p o r t ( S e p t e m b e r 1 95 8)

1 92 n o n - d e s t r u c t i v e t e s t i n g A u g u s t 1 96 9

Flaw Size Evaluation in Immersed Ultrasonic Testing

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