The shear mode of ductile fracture in materials with few inclusions
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DUCTILE
f r a c t u r e of c o m m e r c i a l m e t a l s and a l l o y s involves two b a s i c modes, the f i b r o u s mode and the s h e a r mode. F i b r o u s f r a c t u r e is c h a r a c t e r i z e d by the f o r m a t i o n and growth of voids f r o m n o n m e t a l l i c i n c l u s i o n s o r s e c o n d p h a s e s leading to a f r a c t u r e s u r face c o v e r e d with equiaxed d i m p l e s . The m e c h a n i s m of this type of f r a c t u r e is f a i r l y well u n d e r s t o o d . 1,2 The s h e a r mode of ductile f r a c t u r e , on the o t h e r hand, gives r i s e to f r a c t u r e s u r f a c e s c o v e r e d with s t r o n g l y o r i e n t e d p a r a b o l i c d i m p l e s . The m e c h a n i s m by which this type of f r a c t u r e o c c u r s is not so well u n d e r s t o o d , p a r t i c u l a r l y for s o m e c l a s s e s of m a t e r i a l s . The s h e a r mode of d u c t i l e f r a c t u r e o c c u r s in a n u m b e r of d i f f e r e n t s i t u a t i o n s i n c l u d i n g the o u t e r p o r t i o n of c u p - c o n e f r a c t u r e s , the t e n s i l e f r a c t u r e of s h e e t m a t e r i a l s and the f r a c t u r e of s o m e m a t e r i a l s u n d e r h y d r o s t a t i c p r e s s u r e . A g e n e r a l model for this type of f r a c t u r e has b e e n p r o p o s e d 2 which i n v o l v e s the d e v e l o p m e n t of z o n e s of heavy s h e a r d e f o r m a t i o n within which s h e e t s of voids f o r m , followed by f r a c t u r e due to the a x i a l t e n s i l e s t r e s s . No specific m e c h a n i s m for the f o r m a t i o n of the l a r g e n u m b e r of voids within the s h e a r zone was s u g g e s t e d by the model. The f o r m a t i o n of these voids can be s a t i s f a c t o r i l y explained in t e r m s of n u c l e a t i o n at i n c l u s i o n s for m a t e r i a l s which contain m a n y s m a l l i n c l u s i o n s , such as i n t e r n a l l y oxidized copper a l l o y s 3 and a l u m i n u m a l loys .4 A d d i t i o n a l f e a t u r e s of this type of f r a c t u r e which have b e c o m e a p p a r e n t f r o m s t u d i e s of t h e s e m a t e r i a l s a r e that, where t h e r e is a s t r o n g i n c l u s i o n m a t r i x i n t e r f a c e , such as in a l u m i n u m a l l o y s , voids develop only j u s t p r i o r to f r a c t u r e and that, i n any case, void c o a l e s c e n c e o c c u r s r a p i d l y and only c l o s e to the f r a c t u r e path. The m e c h a n i s m of f o r m a t i o n of s h e e t s of voids in i n c l u s i o n - f r e e m a t e r i a l has not b e e n s t u d i e d in d e t a i l . A n u m b e r of s t u d i e s have b e e n made of the p r o b l e m of n u c l e a t i o n of voids in the a b s e n c e of i n c l u s i o n s which s u g g e s t that c r a c k s could f o r m at g r a i n b o u n d a r i e s due to d i s l o c a t i o n p i l e - u p 5 or by c l u s t e r i n g of v a c a n cies p r o d u c e d by moving d i s l o c a t i o n s . 6 Also, c
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