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S A F T - t h e e v o lu t io n o f a s ig n a lp r o c e s s i n g t e c h n o l o g y f o ru l t r a s o n i c t e s t i n g

S.R. D octor , T .E. Hal l and L.D. ReidT h i s p a p e r i s a r e v i e w o f t h e d e v e l o p m e n t o f t h e s y n t h e t ic a p e r t u r e f o c u s i n gt e c h n i q u e ( S A F T ) fo r use i n u l t r a so n i c t e s t i n g . T h e e q u a t i o n s f o r p r o c e s s i n g t h ed a t a u s i n g t h e S A F T a l g o r i t h m i n s e v e r a l m o d e s a r e d e s c r i b e d . T h i s p a p e r a lsoc o m p a r e s t h e r e s u l t s o b t a i n e d u s i n g f u l l 3 D process ing aga inst l i n e S A F Tprocess ing . I t t h e n b r i e f l y d i s c u s s e s w a y s t o r e d u c e t h e p r o c e s s i n g t i m e f o r t h eS A F T a l g o r i t h m .Keywords s y n t h e t i c a p e r t u r e f o c u s i n g t e c h n i q u e , u l tr a s o n i c te s t i n g

T h e b a s i c f u n c t i o n s o f a ll n o n - d e s t r u c t i v e t e s ti n g ( N D T )t e c h n i q u e s a r e t o d e t e c t f l a w s o r d e f e c t s w i t h i n s t r u c t u r a la n d f u n c t i o n al c o m p o n e n t s , a n d t o c l as s if y a n dc h a r a c t e r i z e t h e d e f e c t s b y t h e i r s i z e, s h a p e , o r i e n t a t i o n ,l o c a t i o n a n d c o m p o s i t i o n ( v o i d s , c r a c k s , la c k o f f u s io n ,e t c ) s o t h a t r a t i o n a l d e c i s i o n s a b o u t t h e u s e o f th ec o m p o n e n t c a n b e d e t e rm i n e d . U l t ra s o n i c t e s ti n g is o n e o ft h e N D T t e c h n i q u e s t h a t is u s e d f o r t h i s p u r p o s e .G e n e r a l l y , t h e d a t a a r e p r e s e n t e d t o t h e i n s p e c t o r i n t h ef o r m o f u n p r o c e s s e d r a d i o f r e q u e n c y ( R F ) s i g n a l s o r A -s c a n s . T h e s e A - s c a n s a r e v e r y d i ff i cu l t t o i n t e r p r e t w i t h o u te x t e n s i v e o p e r a t o r t r a i n i n g a n d m a y l e a d to u n r e l i a b l ei n s p e c t i o n s .I n t e r p r e ta t i o n m a y b e i m p r o v e d b y p r e s e n t in g a n i m a g ei n g r a p h i c f o r m c o n s i s t i n g o f a n i n t e g r a t i o n o f A - s c a n s . I fa n o n - f o c u s e d t r a n s d u c e r i s u s e d t o f o r m t h e im a g e , t h e nu n i f o r m r e s o l u t i o n w i l l b e p r e s e n t e d b u t i t w i ll b eu n f o c u s e d . A l t e r n a t i v e ly , a f o c u s e d t r a n s d u c e r m a y b eu s e d t o i m p r o v e t h e i m a g e r e s o l u t i o n , b u t t h e n s e v e r a lt r a n s d u c e rs m u s t b e e m p l o y e d t o i m a g e th e e n t ir e v o l u m ew i t h u n i f o r m l y h i g h r e s o lu t i o n . A n o t h e r a p p r o a c h i s t ou s e t h e s y n t h e t i c a p e r t u r e f o c u s i n g t e c h n i q u e ( S A F T ) t oa c c o m p l i s h t h i s o b j e c t i v e .T h e S A F T m e t h o d p e r f o r m s a f u ll v o l u m e t r i c c h a ra c t e r -i z a t i o n o f t h e p a rt . F u r t h e r m o r e , t h e v o l u m e i s i m a g e d a th i g h r e s o l u t i o n w i t h a h i g h s i g n a l - t o - n o i s e r a t i o w h i leo n l y u s i n g a s i n g le t r a n s d u c e r . T h e r e s o l u t i o n o f a lli m a g i n g s y s t e m s i s l i m i t e d b y t h e e f f ec t iv e a p e r t u r e a r e a ;t h a t i s, t h e a r e a o v e r w h i c h d a t a c a n b e c o l le c t ed . B e f o r es y n t h e t i c a p e r t u r e t e c h n i q u e s w e r e d e v e l o p e d , t h em a x i m u m a p e r t u r e w a s l i m i t e d b y t h e a b il i t y t o b u i l d a n dc o n t r o l t h e p h y s i c a l a p e r t u r e u s e d f o r d a t a c o l l e c ti o n ; f o re x a m p l e , i n o p t i c s a n d a s t r o n o m y , l a r g e - a p e r t u r et e l e s c o p e s w e r e l im i t e d b y l e n s / m i r r o r a b e r r a t i o n s a n da t m o s p h e r i c t u rb u l e n c e ; i n r a d a r a n d r a d i o a s t r o n o m y ,t h e p h y s i c a l s i z e o f t h e a n t e n n a w a s l i m i t e d b y t h e n e e df o r p o r t ab i l it y . T h e S A F T t e c h n i q u e is a n i m a g i n g m e t h o dw h i c h w a s d e v e l o p e d t o o v e r c o m e s o m e o f th e l i m i t a t io n si m p o s e d b y la r g e p h y s ic a l a p e r tu r e s a n d h a s b e e n

0 3 0 8 - 9 1 2 6 / 8 6 / 0 3 0 1 6 3 - 0 5 $ 3 . 0 0NDT INTERNATIONAL. VOL 19. NO 3. JUNE 1986

s u c c e s s f u ll y a p p l i e d i n a b r o a d r a n g e o f i m a g i n ga p p l i c a t i o n s , s u c h a s r a d a r , u l t r a s o n i c t e s ti n g , g e o p h y s i c a le x p l o r a t i o n , r a d i o a s t r o n o m y , etc.I n 1 9 74 th e U S N u c l e a r R e g u l a t o r y C o m m i s s i o n ( N R C )b e g a n d e v e l o p m e n t o f th e S A F T t e c h n o l o g y a t t h eU n i v e r s i t y o f M i c h i g a n l q . T h i s w o r k w a s d e d i c a t e d t od e v e l o p i n g t h e t e c h n o lo g y a n d d e m o n s t r a t i n g i n t h el a b o r a t o r y t h e c a p a b i l i t y o f th r e e - d i m e n s i o n a l ( 3 D ) S A F Tf o r u l tr a s o n i c t e st in g . I n a d d i t i o n , o t h e r r e s e a r c h e r sa r o u n d t h e w o r l d h a v e a l s o b e e n i n v o l v e d i n d e v e l o p i n gthe SAFT technologyl2-61.T h e P a c i f ic N o r t h w e s t L a b o r a t o r y ( P N L ) b e c a m ei n v o l v e d a s a c o n t r a c t o r t o t h e N R C t o a i d t e c h n o l o g yt r a n s f e r f r o m t h e l a b o r a t o r y t o c o m m e r c i a l i m p l e -m e n t a t i o n o f t h e t e c hn o l o gy . P N L ' s p r o g r a m m eo b j e c t iv e s a r e t o c o n s t r u c t a r e a l - t i m e S A F T f i el d s y s te m ,u s e t h is s y s t e m a t n u c l e a r p l a n t s t o c o n d u c t i n - s e rv i c ei n s p e c t i o n s , a n d g e n e r a t e a s u b s t a n t i a l e n g i n e e r i n gd a t a b a s e f o r g a i n i n g i n d u s t r i a l a c c e p t a n c e . A s p a r t o f t h ef ir s t y e a r o f t h e P N L p r o g r a m m e , a r e v i e w o f a l l a v a i l a b l eS A F T l i te r a t u r e w i t h a d i s c u s s i o n o f S A F T f u n d a m e n t a l sw a s c o n d u c t e d a n d r e p o r t e d b l. T h e r e m a i n d e r o f t h e f ir s ty e a r w a s d e v o t e d to c o n s t r u c t i n g a S A F T d a t a a c q u i s i t i o ns y s t e m f o r f ie l d u sa g e . T h i s d a t a a c q u i s i t i o n s y s t e m s t o r e dt h e u n p r o c e s s e d S A F T d a t a o n m a g n e t i c t a p e f o rs u b s e q u e n t p r o c e s s i n g a t P N L . D u r i n g t h e s e c o n d y e a r o ft h e p r o g r a m m e , t he d a t a a c q u i s i t i o n s y s t e m w a s t a k e n t ot h e f i e ld o n t w o o c c a s i o n s t o r e s o lv e i n c o n s i s t e n c i e s i n I S Ir e s u l t s l S I . A l s o d u r i n g t h e s e c o n d y e a r a s p e c i a l s c a n n i n gt e c h n i q u e u s i n g a s t a t i o n a r y t r a n s m i t t e r a n d a s c a n n i n gr e c ei v e r, c a l l e d t a n d e m S A F T ( T S A F T ) , w a sd ev el op ed l9 1. D u r i n g t h e t h i r d y e a r o f t h e p r o g r a m m ee m p h a s i s w a s p l a c e d o n t h e d e v e l o p m e n t o f fi el d s y st e ms o f t w a r e f o r t h e d e f e c t d e t e c t i o n m o d e a n d t h e d e f e c ta n a l y s i s o r c h a r a c t e r i z a t i o n m o d e . I n a d d i t i o n , a s p e c ia l -p u r p o s e p r o c e s s o r f o r S A F T p r o c e s s i n g t h e u l t r a s o n i cd a t a a t r a te s t o a c h i e v e r e a l - t i m e o p e r a t i o n w a s d e v e l o p e da n d e v a l u a t e d . P r o c e s s i n g r a t e s o f t e n A - s c a n s p e r s e c o n do r g r e a t e r a r e c o n s i d e r e d r e a l t i m e . T h i s s p e c i a l - p u r p o s e

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processor was integrated into the full SAFT-UT fieldsystem and does provide real-time operation for thinmaterial (up to 50 mm thick). Work over the next severalyears will focus on generating system performance data,defining system operating limits and expanding thespecial-purpose processor for real-time operation on thickmaterials.This paper concentrates on the signal processing aspectsof the SAFT system and a compari son of the processingmodes full 3D SAFT, line SAFT, in conjuction withthreshold selective processing.S A F T f i e l d s y s t e mThe SAFT field system has been tailored for datacollection and processing of UT information acquiredfrom inspecting light water reactor components such aspiping, pressure vessels and nozzles. The system, shownin Figure 1, has been designed to use commerciallyavailable components and is packaged in three racks1524 mm tall. In Figure 1 the sys tem termin al is shown atthe far left, while the left-most rack of equip ment contai nsthe UT and scanner control electronics. The middle rackcontains the Ramtek display, the VAX 11/730 computerand the disc drive. The rack on the far right contains thereal-time SAFT processor and magnetic tape for datastorage, and finally, the remaining small box to the farright is the Ramtek display electronics. Figure 2 is aschematic diagram of the system showing the connectionsbetween the system components.S A F T t h e o r ySynthetic aperture focusing refers to a process in whichthe focal properties of a large-aperture focused transd ucerare mimicked or synthesized by a series of measurementsmade using a small-aperture transducer that has beenscanned over a large area. In other words, SAFTprocessing is just simulating a focused transducer: andsince this is performed in computer software, the SAFTsystem contains a large number of focused transducerequivalents available on command.SAFT has several potential advantages over physicalfocusing techniques. SAFT has a great deal of flexibility indata collection a nd image formation. If a test object ofirregular size and s hape must be inspected, the data can beprocessed in a way that minimizes image error caused by

Fig. 1 SAFT f ie ld sys tem hardware

Pre-amplifiera n d m e c h a n i c a ls c a n n e r

61 m c a b le

J ac~:;ffimn

O p e r a t o rc o n s o l e

minicomputerHigh speeddata link

MA X host computer I

IR e a l t im e ] SAFTp r o c e s s o r

Fig. 2 Conceptual d iagram of SAFT f ie ld sys tem

refraction. To make a similar correction with a physical-focus imaging system would require a transducer withdifferent focal properties to be designed for every differentpart and zone of the part to be inspected.For the typical data collection scheme used with SAFT,either a focused transducer is positioned with the focalspot located at the surface of the part to be scanned or asmall contact transducer is employed. These two con-figura tions simul ate a point-like source /receiver in tha t abroad, divergent ultrasonic beam is launched into theobject being scanned. As the transducer is scanned overthe surface of the object, a series of A-scans are rec ordedfor each position of the transducer. Every reflectorproduces a collection of echoes in the A-scan records.Each elementary reflector produces a characteristichyperbolic curve. The apex and the curvature of thehyperbola are completely determined by the depth of thereflector within the test object. This deterministicrelationship between the echo location in the recorded A-scans and the location of reflectors within the test objectmakes it possible to reconstruct a processed image fromthe raw data by the coherent summation process.If the sca nnin g and the surface geometries are known, it ispossible to predict accurately the shape of the locus ofechoes for each point within the test object. The process o fcoherent su mmatio n involves shifting the raw A-scans bythe predicted time delay and sum min g of the raw RF echo

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d a t a . T h i s p r o c e s s i s e q u i v a l e n t t o a s p a t i a l - t e m p o r a lm a t c h e d f i lt e r f o r e a c h p o i n t w i t h i n t h e v o l u m e t o b ei m a g e d . T h e t i m e - c o m p e n s a t e d A - s c a n s a r e t h e na v e r a g e d o v e r a n u m b e r o f po s i ti o n s , w h i c h a r ed e t e r m i n e d b y t h e e f f ec t iv e a p e r t u r e o f t h e t r a n s d u c e r . I ft h e p o i n t b e i n g i m a g e d c o r r e l a t e s w i t h a lo c u s o f A - s c a ne c h o e s , t h e n a l l o f t h e A - s c a n s w i l l a d d i n p h a s e d u r i n g t h ep r o c e s s o f s p a t i a l a v e r a g i n g t o p r o d u c e a w e l l - d e f in e ds i g n al . I f t h e p o i n t b e i n g i m a g e d d o e s n o t c o r r e l a t e w i t h al o c u s o f A - s c a n e c h o e s , t h e n d e s t r u c t i v e i n t e r f e r e n c e o ft h e A - s c a n s w i l l t a k e p l a c e , a n d t h e s p a t i a l a v e r a g e w i l l b en e a r l y z e ro . T h e r e f o r e , th e r e i s b o t h a n i m p r o v e m e n t i ns i g n a l le v e l a n d a r e d u c t i o n i n n o i s e l ev e l p r o d u c i n g a ni m a g e w i t h a r e s u l t a n t h i g h s i g n a l - t o - n o i s e r at i o.S AF T p u l s e - e c h o o p e ra t in g m o d eUn d e r s ta n d in g o f t h e p r i n c i p le s o f t h e SA F T p u lse -e ch oimag ing mode is imp or tan t in o rde r to in te rp re t accura te lyt h e i m a g e s p r e s e n t e d b y t h e S A F T p r o c e s s . T h e p u l s e -e c h o m o d e i s d e f i n e d a s t h e u s e o f a si n g le t r a n s d u c e r f o rb o t h s o u r c e a n d r e c e i v e r s c a n n i n g ; ie , t he s a m e t r a n s d u c e rt h a t i l l u m i n a t e s t h e o b j e c t a l s o r e c e iv e s t h e r e t u r n e c h o .F o r f i n i t e p a r t s , g e o m e t r i c a l c o n s t r a i n t s a n d f i n i t e d e p t ho f m a t e r i a l f o r a n g l e d b e a m i n s p e c t i o n s , m u l t i p l e s o u n dp a t h s c a n c o n t r i b u t e t o t h e p r o c e s s e d i m a g e s .I n t h e p u l s e - e c h o m o d e t h e r e a r e t h r e e p a t h s t h a t c a nc o n t r i b u t e r e f l e c t i o n e c h o e s i n t h e d a t a . T h e s e p a t h s a r es h o w n i n F i g u r e 3 . O n e n e e d s t o b e a w a r e o f t h e p h y s i c s o ft h e s i t u a t i o n t o a v o i d m i s i n t e r p r e t i n g i m a g e s o r t o u s e t hem u l t i p l e p a t h i n f o r m a t i o n t o b u i l d g r e a t e r a c c u r a c y i n t ot h e f i n a l i m a g e . I n F i g u r e 3 th e m a t e r i a l i s o f t h i c k n e s s Ta n d t h e d e f e c t o f i n t e r e s t i s l o c a t e d a t X = 0 a n d a t a d e p t h Di n t h e m a t e r i a l .T h e f ir s t p a t h i s t h e d i r e c t b a c k - s c a t t e r e d p a t h ( P ~ - P , )f r o m t h e s o u n d f i el d e n t r a n c e i n t o t h e p a r t , t o t h e d e f e c ta n d b a c k t o t h e r e c e i v e r . A n a l y s i s s h o w s t h a t t h e d a t aa l o n g t h i s p a t h w i l l b e d e t e r m i n i s t i c ; t h a t i s , o n e u n i q u eo b j e c t d e p t h c o r r e s p o n d s t o o n e u n i q u e h y p e r b o l i c cu r v e .T h i s c u r v e i s d e f i n e d b y t h e e q u a t i o n :

F ( X , D ) = ( O z + X2) /z (1)T h i s i s f o r t h e t w o - d i m e n s i o n a l c a s e ( l in e S A F T ) . F o r t h et h r e e - d i m e n s i o n a l c a s e ( 3 D S A F T ) , t h e e q u a t i o n i s:

F ( X , Y , D ) = ( D z + X 2 + y2),/z (2)T h e s e c o n d s o u n d p a t h i s (P 2 - t- P 3 + P 1 ) f r o m t h e s o u n df ie l d e n t r a n c e p o i n t o f t h e p a r t , to t h e b a c k s u r f a c e , t o th ed e f e c t a n d f i n a l l y t o t h e r e c e i v e r . T h e s o u n d p a t h i s t h es u m o f t h e l e n g t h o f t h e r a y p a t h s P I , P 2 a n d P 3. T h ee q u a t i o n f o r t h e t w o - d i m e n s i o n a l c a s e ( li n e S A P ' T ) is :

Ix\l X = 0

x , e / o a

Fig. 3 Angled-beam sound field paths for pulse-echo mode with a d e f e c tat X = 0 and at a depth D in a material of thickne ss T

F ( X , D , T ) = ( D 2 + ) ( 2 ) ' /z + [ ( 2 T - D ) 2 + X 2] / 2 (3 )T h e e q u a t i o n f o r t h e t h r e e - d i m e n s i o n a l c a s e i s:

F(X , } I, D, 7 ) = (D 2 + X z + y2)z/2+ [(2T -- D) z + X z + Y2ll/Z (4)

Th i s p a th i s i de n t i c a l t o t he r e ve r se se que n c e o f (P~ + P3 + / '2 ) .T h i s p a t h l e a d s t o a m b i g u i t y s i n c e t he s u m o f a l l t h e p a t h si s b a s i c a l l y a c o n s t a n t i n d e p e n d e n t o f t h e t r a n s d u c e rl o c a ti o n l l l . T h i s s e c o n d p a t h c o n t r i b u t e s a v e r y l a r g er e t u r n e c h o s i n c e i t r e l i e s o n t h e s t r o n g f o r w a r d - s c a t t e r e ds i g n a l r a t h e r t h a n t h e w e a k e r b a c k - s c a t t e r e d s i g n a l .H o w e v e r , t h e a m b i g u i t y t h a t e x i s t s w i t h t h e d a t a ( a n di m a g e ) r e s u lt s in l o s s o f a n y u n i q u e i n f o r m a t i o n a b o u t t h ed e p t h o f p l a n a r d e f e c t s b u t d o e s p r o v i d e r e d u n d a n c y f o rd e t e c t io n o f s u r f a c e - b r e a k i n g o r n e a r - s u r f a c e d e f e c ts .T h e t h i r d s o u n d p a t h t o b e c o n s i d e r e d i s f r o m t h e s o u n df i e ld e n t r a n c e p o i n t o n t h e p a r t , t o t h e b a c k s u r f a c e , t o th ed e f ec t , b a c k - s c a t t e r f r o m t h e d e f e c t t o t h e b a c k s u r f a c e a n dr e t u r n t o t h e r e c e i v e r o r ( Pz + P 3 + P 3 + P 2 ) - T h i s p a t hf o r m s t h e m i r r o r i m a g e o f t h e d e f e c t a n d r e li e s o n t h eu s u a l l y w e a k b a c k - s c a t t e r e d s i g na l . T h e e q u a t i o n f o r t h et w o - d i m e n s i o n a l c a s e (l i n e S A F T ) i s :

F ( X , D , T) = [ (2T - - D )2 + X2] /z (5)T h e e q u a t i o n f o r t h e th r e e - d i m e n s i o n a l c a s e is :

F(X, II, D, 7) = [ (2T -- D) z + X z + y2] ,/2 (6)S i n c e a m i r r o r i m a g e i s f o r m e d b y t h is p a t h w h e n i t i sc o n s i d e r e d a d i r e c t p a t h , t h e p r o c e s s e d i m a g e w i ll c o n t a i nt w o d e f ec t s . T o e l i m i n a t e t h i s e f f ec t th e r e c o n s t r u c t i o na l g o r i t h m m u s t h a v e t h e a b il i ty to r e m o v e t h is r e d u n d a n ti m a g e . W i t h t h e P N L S A F T s y s t e m , t h e b a c k s u r f a c el o c a t i o n i s i n c o r p o r a t e d i n t o t h e i m a g e r e c o n s t r u c t i o np r o c e d u r e t o e l i m i n a t e t h i s m i r r o r e f f e c t .D e p e n d i n g o n t h e d e f e ct t y pe , o r i e n t a t i o n a n d o t h e rc h a r a c t er i s t ic s , o n e o f t h e t h r e e p a t h s w i ll d o m i n a t e t h er e c o r d e d e x p e r i m e n t a l d a t a . T h e g e n e r a l - p u r p o s e S A F Ta l g o r i t h m m u s t b e c a p a b l e o f r e c o n s t r u c t i n g a l l t h r e e o ft h e p r o p a g a t i o n p a t h s f o r a n g l e - b e a m d a t a .Process ing schemesT h e S A F T p r o c e s s i s e x t r e m e l y c o m p u t e r - i n t e n s i v e i n t h a tit re q u i re s a l a r ge a m o u n t o f c o m p u t e r m e m o r y a n dr e p e ti t iv e c a l c u l a t io n s . F u r t h e r m o r e , c o m p u t e r a r c h i t e c -t u r e d o e s n o t l e n d i t s e l f t o s t r e a m l i n i n g t h e S A F Tc a l c u l a t i o n s . I n o r d e r t o p l a c e t h i s i n t o p e r s p e c t i v e , a ne x a m p l e w i l l b e u s e d t o i l l u s t r a t e t h e p o i n t .T h e r e s o l u t i o n b l o c k s h o w n i n F i g u r e 4 w a s s c a n n e d u s i n gt h e S A F T s y s t e m . T h e t r a n s d u c e r e m p l o y e d i s a 2.2 5 M H z ,K - B A e r o t e c h , 6 m m ( 0.2 5 i n ) d i a m e t e r p r o b e . T h et r a n s d u c e r s c a n s t h e s u r f a c e o f t h e b l o c k c o n t i n u o u s l ya c q u i r i n g d a t a o n a g r i d t h a t h a s a b o u t o n e - w a v e l e n g t hs p a c i n g s o f 1 . 2 5 m m ( 0. 05 i n ) . A 7 5 m m ( 3 i n ) b y 75 m m( 3 i n ) a r e a i s s c a n n e d p r o d u c i n g a to t a l o f 3 60 0 A - s c a n s .T h e A - s c a n i s s a m p l e d a t t h e r a t e o f 2 0 M H z , e i g h t t im e st h e t r a n s d u c e r c e n t r e f r e q u e n c y . T h e A - s c a n r e c o r d s a r e8 06 p o i n t s l o n g . T h i s l e a d s t o a f il e c o n t a i n i n g 2 9 01 6 0 0s a m p l e d d a t a p o i n t s ( 8 - b i t i n t e g e rs ) .T h i s d a t a f i l e c a n b e p r o c e s s e d i n a n u m b e r o f w a y s o n t h eS A F T s y s t em . T h e f ir s t i s t o u s e t h e f u ll t h r e e - d i m e n s i o n a la l g o r i th m t h a t w a s d e s c r ib e d a b o v e . T h e n u m b e r o f

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3 . 1(0,

Aluminiurn

1 .98 mm 10.078 n}

Nnd view ~ ,y pa t tern o f f la t- topdr i l l holes0.99(0 .039 in ) ~ ;71 .1mm.H o le d iameter ~ (2 .8 in)4.78 mm10.188 in) Side viewF i g. 4 R e s o l u t i o n b l o c k f o r S A F T e v a l u a t i o n s h o w i n g t h e e n d v i e w o f t h eplane c o n ta i n i n g t h e t o p s o f t h e f l a t - b o t t o m holes and the side v i e w d r a w nto i l lus t ra te for s impl ic i ty depth i n f o r ma t i o n b u t n o t t h e c o mp l e x Y 'p a t t e r nexecutions of the inner loop (defined as adding an off-centre A-scan point to a point on the A-scan beingprocessed) required is the very large number 770 093 696and whi ch equates to a CPU time of 9 hours 33 minutesand 50 seconds. The results from this processing areshown in Figure 5 and in colou r in Plate 2 on p 162. Thedisplayed perspective is at the angle of the sound fieldillumination, looking perpendicular to a plane conta iningthe tops of the fiat-bottomed holes.This data file can also be processed in the line SAFT modeusing the equations defined above. The numb er o f inner-loop executions is now greatly reduced to 58 523 688 andrequires only 42 minutes and 54 seconds of CPU time. Theresults from the line SAFT processing are shown in Figure 6and in colour in Plate 2 on p 162. It is very apparent that theimages (Figures 5 and 6) have two different resolutionfigures. Both of the images clearly demonstrate thatdefects would be detected by each processing mode, butthe three-dimensional SAFT allows the separate holes tobe seen distinctly. The line SAFT has the greatestresolution loss as expected in the tranverse direction.There is obviously a significant trade-off in processingrate v r s us resolution performance when using a singlecomputer (CPU) for performing the processing.One of the major factors hindering the transference andacceptance of the SAFT technology for industrial use isthe computer-intensive nature of the process. There areseveral ways that the SAFT processing can be performedto overcome this problem. The line SAFF mode v r s u s thefull 3D mode is one way. A second is called selective

F i g. 5 T h r e e - d i me n s i o n a l S A F T p e rfo r ma n c e o n r e s o l u t io n b l o c k

F ig . 6 L ine SAFT resu l ts on reso lu t ion b lock

processing. In selective processing the operator of thesystem can select a threshold such that at each sampledpoint o f the A-scan a test is performed. If the A-scan valueexceeds this level, then the data are processed normally. If,however, the A-scan value is below the threshold, then thevalue is not processed but simply scaled and placed in theprocessed file. This way no data are lost, but there is asubstantial improvement in the processing time.In the above example if a threshold of 20 dB below themaximum signal from the flat-bottomed holes is selectedas the threshold, t hen there will be a significant reductionin the processing times as shown in Table 1. The three-dimensional SAFT case is improved by a reduction in thenum ber of inner-loo p executions to 20 160 632 which is afactor of 38.2, while the processing time is reduced to 21minutes and 57 seconds or a factor of 26.14. In the lineSAFT case the number of inner-loop calculations isreduced to 3 895 145 or a factor of 15.02, while theprocessing time is reduced to 9 minutes a nd 28 seconds ora factor of only 4.3. The improvement is substantial, butthe factors of improvement are not linearly related to oneanother because there are two variables impacting theprocessing time. One is the overhead required for thecalculations and the second is the expanding apertureused for the processing. This 20 dB level has been shown

T a b l e 1 . S A F T p r o c e s si n g t i m e s 3 6 0 0 A - s c a n s,8 0 2 p o i n t s / A - s c a n , f i le s iz e = 2 9 0 1 6 0 0 p o i n t s)N u m b e r o f C P U t im ei n n e r - lo o p r e q u i r e d f o re x e c u t i o n s p r o c e s s i n g

F u ll S A F T , a ll d a t a 7 7 0 0 9 3 6 9 6p r o c e s s e dF u ll S A F T , 2 0 1 6 0 6 3 2- 2 0 d B se le c t iv ep r o c e s s i n gL ine SAFT, a l ld a t a p r o c e s s e dL i n e S A F T , - 2 0 d Bselectivep r o c e s s i n g

9 h 3 3 m in 5 0 s2 1 m i n 5 7 s

5 8 5 2 3 6 8 8 4 2 m in 5 4 s

3 8 9 5 1 4 5 9 m in 5 8 s

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e x p e r i m e n t a l l y t o h a v e l i tt l e e f fe c t o n t h e r e s u l t a n t i m a g ec o n t e n t , f o r t h e t y p i c a l 3 0 l e ve l s o f c o l o u r o r b l a c k a n dw h i t e u s e d f o r t h e d i s p l a y . T h i s m e a n s t h a t t h e f i r st t h r e el o w e s t l e v e ls m a y h a v e s o m e c h a n g e s . O n e p r o c e d u r e u s e db y P N L i s t o u s e t h e 20 d B t h r e s h o l d ; a n d i f a d e f e c t isd e t e c t e d , t h e n r e p r o c e s s t h e s t o r e d u n p r o c e s s e d d a t aw i t h o u t a t h r e s h o l d o n l y i n t h e z o n e c l o s e t o t h ei n d i c a t i o n f o r t h e f i n a l a n a l y s i s .A l t e r n a t i v e a c c e l e r a ti o n t e c h n i q u e s h a v e a l s o b e e np u r s u e d t h r o u g h t h e u s e o f s p e c i a l - p u r p o s e p r oc e ss o rs l , ~ 2 1.P N L i s a l s o d e v e l o p in g a S A F T p r o c e s s o r b a s e d o n u s i n gc o m m e r c i a l l y a v a i l a b l e b o a r d s c o n t a i n i n g t h e 68 00 0f a m i l y o f m i c r o p r o c e s s o r s . T h i s p r o c e s s o r u s e s a p a r a l l e lp r o c e s s i n g s c h e m e i n w h i c h t h e S A F T o p e r a t i o n i s p e r f or -m e d b y a n u m b e r o f C P U s . E s t i m a t e s b a s e d o n m o d e l l i n ga n d b e n c h m a r k i n g a tw o - C P U p a r a l l e l a r c h i te c t u r e h a ves h o w n t h a t t he p r o c e s s o r c a n a c h i e v e a d d i t i o n a l r e a l i s t ics p e e d i m p r o v e m e n t s r a n g i n g f r o m 6 t o 1 0 0 . T h e r a n g e o fi m p r o v e m e n t i s a r e s u l t o f o n l y a c c e l e r a t i n g t h e i n n e r -l o o p c o m p u t a t i o n t i m e b e c a u s e t h e p e r c e n t a g e o f t i m es p e n t o n i n n e r - l o o p o p e r a t i o n s i s g r e a te r f or t h i c k t h a nt h i n m a t e r i a l s . H o w e v e r , t h e s e f i g u r e s w i l l r e q u i r e v e r i f i -c a t i o n t h r o u g h l a b o r a t o r y t e s t i n g o f t h e f u ll h a r d w a r ee m p l o y i n g t e n 6 80 20 C P U b o a r d s .

S u m m a r yT h e S A F T t e c h n o l o g y h a s e v o l v e d o v e r t h e y e a r s t o t h ep o i n t w h e r e i t i s n o w a t o ol t h a t c a n p r o v i d e a d d i t i o n a li m p r o v e m e n t s i n u l t r a s o n i c t e s t i ng t h r o u g h h i g h -r e s o l u t i o n i m a g i n g . T h i s h a s b e e n a c c o m p l i s h e d e n t i r e l yw i th c o m m e r c i a l l y a v a i l a b l e e q u i p m e n t . T h e S A F Ta l g o r i t h m h a s a l s o u n d e r g o n e e v o l u t i o n a n d i s f l e x i b lyd e s i g n e d s o th a t i t c a n o p e r a t e i n t h e a n g l e - b e a m m o d ew i t h v a r i o u s t y p e s o f i m a g e p r o c e s s i n g a v a i l a b l e ( 3 D , l i ne ,p u l s e - e c h o , t a n d e m ) .T h e m a j o r i m p r o v e m e n t h a s b e e n i n th e p r o c e s s i ng s p e e dw i t h f u r t h e r i m p r o v e m e n t f a c t o rs o f 6 t o 1 00 a n t i c i p a t e du s i n g s p e c i a l - p u r p o s e p r o c e s s o r s .

A c k n o w l e d g e m e n tT h i s w o r k w a s s u p p o r t e d b y t h e U S N u c l e a r R e g u l a t o r yC o m m i s s i o n u n d e r C o n t r a c t D E - A C 0 6 - 7 6 R L O 1 83 0; D r J .M u s c a r a i s t h e N R C P r o g r a m M a n a g e r , N R C F I N B 23 67 .

R e f e r e n c e sl Freder ick J .R. Se yde l J.A. and Fairchild R.C. Improvedultrasonic non-destructive testing of pressure vessels U SN u c l e a r R e g u l at o r y C o m m i s s i o n R e p o r t N U R E G - O 0 0 7 - 1 (January1976)2 Highmore, P.J. , Willets, A.J. a n d C l o u g h P. ~Circe - a versa tileu l t rason ic d ig i ta l data record ing , analysis and d isp lay system'I n s t i t u t e o f M e c h a n i c a l E n gi n ee r in g , C1 5 4 / 8 2 (1982) pp 209-2153 Jackson , J .L . 'P rogram fo r f ie ld val idat ion o f the syn thet ic

aper tu re focusing techn ique fo r u l t rason ic tes t ing (SA Fr- UT ) 'U S N u c l e a r R e gu l a t o ry C o m m i s s i o n R e p o r t N U R E G / C R - 1 8 8 5 1 , 2,3 (1981)4 Kino G.S . Cor l D . Benne t t S . and P e t e r son K. Real t imesyn thet ic aper tu re imag ing system' Ultrasonics Syrup Proc , Cat8 0 C H - 1 6 0 2 - 2 S 7 . I E E E (1980)5 Schmitz, V., Mu ller, W. a n d S c h a f e r G. Classification andreconst ruct ion o f defects by comb ined aco ust ical ho lographyan d l i n e -S A F F P ro e o f G e r m a n y - U n i t e d S t a t e s W o r k s h o p o nR e se arc h a n d D e ve l o p me n t o f Ne w P ro c e d ure s i n ND T SpringerVerlag, Berlin (1983)6 Seydel, J.A. ~Ultrasonic synthetic apertu re focusing techniq uesin NDT Research Tech niques in Nond estruc t ive Test ing 6 ed R.S.Sharpe, Academic Press, New York (1983)7 Busse, L.J., Collins, H.D . and Doc t or S.R. 'Review andd iscussion o f the developm ent o f syn thet ic aper tu re focusingtechn ique fo r u l t rason ic tes t ing (SAFT-UT) U S N u c l e a rR e g u l a to r y C o m m i s s i on R e p o r t N U R E G / C R - 3 6 2 5 . P N L - 4 9 5 7(March 1984)8 D o c t o r S . R . H a l l T . E . C r a w f or d S . L . a n d V a n H o u t e n L . P .'Advanced u l t rason ic imag ing o f p ip ing in Dresden andVermont Yankee re a c t o rs ' 8 t h I n t Co n f o n S t ru c t u ra l M e c h a n i c s i nR e a c t o r T e c h n o l o g y D 1/7 (Augu st 1985) pp 225-2309 Doctor, S.R. , Crawford, S.L. and Hall, T.E. SAF T-U T fieldexperience 1985 Pressure Vesse ls an d Piping Co nf N ew Orleans.LA 98-1 (Ju ne 1985) pp 203-21010 Busse, L.J., Collins, H.D . and Doctor, S.R. 'Th e eme rgin gtechno logy o f syn thet ic a per tu re focusing fo r u l t rason ic test ing1 9 84 P ressure V e sse l s a n d P i p i n g Co n f S a n A n t o n i o . T X84-PUD-122 (June 1984)1 1 G a n a p a t h y S . S c h m u l t B . W u W . S . D e n n e h y T . G .M o a y e s i N . a n d Kelly, P. ~Design and deve lopm ent of a spec ialpurpose SA FT system fo r nondest ruct ive evaluat ion o f nuclearreacto r vessels and pipin g components U S N u c l e a r R e g u l a t o ryC o m m i s s i o n R e p o r t N U R E G / C R - 4 3 6 5 (August 1985)12 Polky, J.N . and Mi l l e r D.D. 'An u l t ra-h igh speed res idueprocesso r for SAFT inspect ion system ima ge enhancem ent ' U SN u c l e a r R e g u l a to r y C o m m i s s i o n R e p o r t N U R E G / C R - 4 1 7 0(March 1985)

A u t h o r sT h e a u t h o r s a r e a t P a c i f i c N o r t h w e s t L a b o r a t o r y ,O p e r a t e d b y B a tt e l l e M e m o r i a l I n s t i t u te , R i c h l a n d , W A9 9 3 5 2 , U S A .

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