A new technique for examining microscopic fracture processes at crack tips
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M i c f rra c t uor e p rso c e scs e s oh a v e ps u c c eis s f u cl l y b e e n s t u d i e d i n the p a s t by e x a m i n i n g 1) p l a n e s e c t i o n s t h r o u g h h a l t e d c r a c k s a n d 2) s t r u c t u r e s o n f i n a l f r a c ture s u r f a c e s (fractography). T h e s e s t u d i e s have docu m e n t e d that a h i g h p r o p o r t i o n of c r a c k s p r o p o g a t e in a d i s c o n t i n u o u s m a n n e r o n the m i c r o s c o p i c s c a l e by o p e n i n g up a n d g r o w t h of c o u n t l e s s c r a c k s a n d v o i d s a h e a d of the m a i n c r a c k u n t i l they l i n k w i t h o n e a n o t h e r a n d with the m a i n c r a c k . T h e r e g i o n j u s t a h e a d of the p r o p o g a t i n g c r a c k tip h a s , t h e r e f o r e , a t t r a c t e d c o n s i d e r a b l e i n t e r e s t , a n d s t u d i e s o f the m i n u t e d e t a i l s of f r a c t u r e p r o c e s s e s h a v e b e e n m a d e by e l e c t r o n m i c r o s c o p y of c r o s s s e c t i o n s ( s t a n d a r d m e t a l l o g r a p h y a n d thin f o i l m i c r o s c o p y ) a n d f r a c t u r e s u r f a c e s . T h e s e c t i o n i n g t e c h n i q u e s , h o w e v e r , s u f f e r f r o m the d i s t r e s s i n g l y g o o d o d d s that a n y g i v e n s e c t i o n w i l l c o n t a i n a d i s a p p o i n t i n g l y s m a l l n u m b e r of the f e a t u r e s t h a t a r e sought, w h e r e a s fractography s u f f e r s from the disadv a n t a g e that the f r a c t u r i n g p r o c e s s e s have g o n e t o c o m p l e t i o n b e f o r e the e x a m i n a t i o n i s m a d e . In a n e f f o r t t o s u r m o u n t t h e s e d i f f i c u l t i e s , a n e w t e c h n i q u e h a s b e e n d e v i s e d in w h i c h the r e g i o n a h e a d of the c r a c k t i p i s s e c t i o n e d by f r a c t u r e a n d t h e n e x a m i n e d by f r a c t o g r a p h y . CHANGE-OF-MODE
s t e p s i n c r e a s e d w h e n the c l e a v a g e t e m p e r a t u r e w a s r a i s e d d u r i n g the f r a c t u r e o f z i n c . F i g . 1 i l l u s t r a t e s the n e w t e c h n i q u e w i t h a f a t i g u e p r e c r a c k e d s p e c i m e n ; Fig. l(a) s h o w s t h e c r o s s sect i o n of a c r a c k w h i c h h a s b e e n i n t e r r u p t e d a f t e r p r o p -
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