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A N D J. W E E R T M A N

The a r e a a h e a d of the c r a c k t i p in s i n g l e c r y s t a l s of c o p p e r f a t i g u e d in p u l l - p u l l was s t u d i e d using o p t i c a l and e l e c t r o n m i c r o s c o p y t e c h n i q u e s . Through m i c r o h a r d n e s s m e a s u r e m e n t s , it was found that a zone of h i g h e r h a r d n e s s n e a r the fatigue c r a c k t i p c o r r e s p o n d s r o u g h l y to the s i z e of the f a t i g u e c r a c k p l a s t i c zone c a l c u l a t e d f r o m f r a c t u r e m e c h a n i c s criteria. Electron microscopy observations revealed a distinct dislocation structure ass o c i a t e d with t h i s zone of h i g h e r h a r d n e s s .

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p r e v i o u s work* we l o o k e d at the d i s l o c a t i o n s t r u c t u r e a s s o c i a t e d with m a t e r i a l a h e a d of the fatigue c r a c k t i p in c o p p e r s i n g l e c r y s t a l s p e c i m e n s . We found that, with d e c r e a s i n g d i s t a n c e f r o m the c r a c k tip, a t r a n s i tion o c c u r r e d in d i s l o c a t i o n m o r p h o l o g y that was c h a r a c t e r i z e d b y a change f r o m a r r a y s of l a r g e , i n c o m p l e t e c e l l s of r e l a t i v e l y low w a l l d e n s i t i e s to ones of s m a l l e r (by about a f a c t o r of two), w e l l - d e f i n e d c e l l s of r e l a t i v e l y high w a l l d e n s i t i e s . We a s s o c i a t e d t h i s t r a n s i t i o n with a s t r e s s buildup o c c u r r i n g n e a r the fatigue c r a c k tip. In the w o r k r e p o r t e d h e r e we have a g a i n c o n c e n t r a t e d on e x a m i n i n g the m a t e r i a l a h e a d of the fatigue c r a c k t i p in the hope of l e a r n i n g m o r e about how i t s s t r u c t u r e is r e l a t e d to fatigue c r a c k p r o p a g a t i o n . To t h i s end, we have c o r r e l a t e d the o b s e r v e d d i s l o c a t i o n c o n f i g u r a t i o n a l c h a n g e s with m e a s u r e d m i c r o h a r d n e s s c h a n g e s . ( M i c r o h a r d n e s s m e a s u r e m e n t s in the fatigue c r a c k tip a r e a have b e e n m a d e b y B a t h i a s and P e l l o u s ~ and b y Hahn et al s on s t e e l s . ) While it h a s b e e n g e n e r a l l y e s t a b l i s h e d t h a t softening is i n t i m a t e l y a s s o c i a t e d with fatigue c r a c k i n i t i a t i o n (Wilson and Mintz 4 p r e s e n t r e c e n t e v i d e n c e f o r t h i s ) , the r o l e of hardening in fatigue c r a c k propagation i s l e s s w e l l u n d e r s t o o d . Kang and Liu, s in fact, have found that w o r k - h a r d e n e d m i l d s t e e l s p e c i m e n s exhibit g r e a t e r r a t e s of fatigue c r a c k p r o p a g a t i o n than nonwork-hardened specimens. E X P E R I M E N T A L PROCEDURE B a r s i n g l e c r y s t a l s of p u r e c o p p e r (99.999 p c t OFHC) w e r e grown b y a m o d i f i e d B r i d g e m a n t e c h n i q u e and cut to s p e c i m e n s of d i m e n s i o n s 3 m m • 6 m m x 25 m m . A f t e r