Cavity formation from inclusions in ductile fracture
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a p p e a r s that J o s e p h Henry, l of e l e c t r o - m a g n e t i s m f a m e , h a s r e c o g n i z e d a s e a r l y a s 1855 t h a t m e t a l s f r a c t u r e p r e m a t u r e l y b y a p r o c e s s of i n t e r n a l n e c k i n g when e x t e n d e d b y s t r e t c h i n g . He a d v i s e d that w i r e d r a w i n g and r o l l i n g w e r e p r e f e r a b l e o p e r a t i o n s (for t h i s and o t h e r i n t e r e s t i n g h i s t o r i c a l p e r s p e c t i v e s on d e f o r m a t i o n p r o c e s s i n g s e e BackofenZ). In m o r e r e c e n t t i m e s P u t t i c k s t r a c e d the c a u s e of d u c t i l e f r a c t u r e to the d e v e l o p m e n t of h o l e s f r o m i n c l u s i o n s (for a s u m m a r y of e a r l i e r v i e w s on d u c t i l e f r a c t u r e and t h e i r i n a d e q u a c y s e e O r o wan4). The p r o c e s s h a s s i n c e b e e n i n v e s t i g a t e d e x t e n s i v e l y both e x p e r i m e n t a l l y and t h e o r e t i c a l l y . The c u r r e n t l e v e l of u n d e r s t a n d i n g of the r o l e of i n c l u s i o n s in d u c t i l e f r a c t u r e h a s b e e n r e v i e w e d b y R o s e n f i e l d . s It h a s now b e e n g e n e r a l l y e s t a b l i s h e d that once i n t e r n a l cavities are nucleated from inclusions or second phase p a r t i c l e s , t h e y can be p l a s t i c a l l y e x p a n d e d u n d e r v a r i ous c o m b i n a t i o n s of s h e a r s t r e s s and n e g a t i v e p r e s s u r e . A n a l y s e s of h o m o g e n e o u s p l a s t i c c a v i t a t i o n b y M c C l i n tock, 6 R i c e and T r a c y ~ and o t h e r s have e l u c i d a t e d the i m p o r t a n c e of n e g a t i v e p r e s s u r e s in the flow f i e l d in h a s t e n i n g the p l a s t i c hole e x p a n s i o n p r o c e s s . C o m p a r i son of such a n a l y s e s with e x p e r i m e n t s have shown that local ductile fracture requires considerably smaller a v e r a g e p l a s t i c s t r a i n a s a r e s u l t of l o c a l i z a t i o n of d i l a t i o n a l d e f o r m a t i o n into z o n e s , 8 followed b y f o r m a t i o n A. S. ARGON is Professor of Mechanical Engineering and J. IM is Graduate Student at the Massachusetts Institute of Technology, Cambridge, Mass., 02139. R. SAFOGLU is with the Department of Mining and Metallurgy, Technical University of Instanbul, Istanbul, Turkey. This work has been presented in part orally at the Third International Conference on Fracture in Munich, Germany, April 1973. Manuscript submitted November 5, 1973. METALLURGICAL TRANSACTIONS A
and p r o p a g a t i o n of c r a c k s w h e r e the hole e x p a n s i o n p r o c e s s i s s h a r p l y confined into p o r t i o n s of the h i g h l y s t r a i n e d zone in f r o n t of the c r a c k . 9'1~ Although t h e s e p r o c e s s e s of t e r m i n a l d u c t i l e s e p a r a t i o n a r e not y e t fully u n d e r s t o o d , they a r e in a f a r b e t t e r l e v e l of d e v e l opment than the i n
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