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o r d e r to r e d u c e test t i m e to b e a r a b l e l e v e l s , v e r y slow, p e r cycle fatigue c r a c k p r o p a g a t i o n r a t e s a r e u s u a l l y studied at high t e s t f r e q u e n c i e s on the o r d e r of about 1 kHz.l~ In what follows we r e p o r t and d i s cuss the use of a n even higher f r e q u e n c y (20 kHz) test technique 4"~ to d e t e r m i n e the slow da/dN c r a c k growth b e h a v i o r of a m o n o c r y s t a l l i n e n i c k e l - b a s e s u p e r a l l o y at v e r y low AK l e v e l s . The alloy chosen for this slow da/dN study is the (100) o r i e n t e d m o n o c r y s t a l l i n e n i c k e l - b a s e s u p e r a l l o y M a r - M - 2 0 0 . The m a i n r e a s o n for choosing this alloy is the a b s e n c e of low AK c r a c k growth data in a s y s t e m where c r a c k growth is c r y s t a l l o g r a p h i c a l l y cons t r a i n e d . T h i s high s t r e n g t h n i c k e l - b a s e s u p e r a l l o y , b e c a u s e of its high c o h e r e n t y ' p r e c i p i t a t e content (~56 v o l u m e pet), exhibits a n e x t r e m e l y p l a n a r mode of slip, and fatigue c r a c k i n g i n this alloy is e n t i r e l y along c r y s t a l l o g r a p h i c slip planes.~'~ Also, b e c a u s e its slip mode is p l a n a r e v e n at c o n v e n t i o n a l test f r e q u e n c i e s and s t r a i n r a t e s , its s t r a i n r a t e or fatigue f r e q u e n c y s e n s i t i v i t y of d e f o r m a t i o n is s e e n to be minimal.6-1~ It has b e e n f u r t h e r e s t a b l i s h e d that fatigue f r a c t u r e b e h a v i o r in a i r at u l t r a s o n i c f r e q u e n c i e s is d i f f e r e n t f r o m that at c o n v e n t i o n a l f r e q u e n c i e s only in that c r a c k growth d u r i n g high f r e q u e n c y loading is p e r h a p s slightly s l o w e r as a c o n s e q u e n c e of the supp r e s s i o n of a i r o r oxygen a d s o r p t i o n effects at the c r a c k t i p f Consequently, c o m p a r i s o n s a r e m o r e e a s i l y made b e t w e e n the high f r e q u e n c y r e s u l t s gene r a t e d in this study with those obtained or to be obt a i n e d at the m o r e c o n v e n t i o n a l f r e q u e n c i e s . F u r t h e r , p l a n a r slip m a t e r i a l s have the added advantage of poss e s s i n g low i n t e r n a l f r i c t i o n . 6 Low i n t e r n a l f r i c t i o n c h a r a c t e r i s t i c s a r e n e c e s s a r y to m i n i m i z e the h e a t i n g of the t e s t s p e c i m e n s d u r i n g u l t r a s o n i c r e s o n a n t loading, and p r e c l u d e s the use of cooling fluids which t h e m s e l v e s might affect the c r a c k growth p r o c e s s .11-14 EXPERIMENTAL DETAILS The d e t a i l s of the t e s t unit and its use to d e t e r m i n e S-N c u r v e s at high f r e q u e n c y have b e e n d e s c r i b e d SAMPATH PURUSHOTHAMANand JOHN K. TIEN are Research Associate and Professor, respectively,Henry Krumb School of Mines, Columbia University, New York, NY 10027. Man