Elastic-plastic fracture by homogeneous microvoid coalescence tearing along alternating shear planes
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In the p r e s e n t work the p r i m a r y goal was to d e t e r m i n e the m e c h a n i s m by which m i c r o v o i d c o a l e s c e n c e in advance of the tips of c r a c k s r e s u l t s in the o b s e r v e d r i d g e s and v a l l e y s . Most of the o b s e r v a t i o n s were made at high m a g n i f i c a t i o n s u s i n g t r a n s m i s s i o n e l e c t r o n m i c r o s c o p y of r e p l i c a s and s c a n n i n g e l e c t r o n m i c r o scopy of the actual f r a c t u r e s u r f a c e s . E l e c t r o n fractography precision matching analysis procedures were used to study the m a t i n g s u r f a c e f e a t u r e s on the two sides of the f r a c t u r e path. Since s e v e r a l of the key o b s e r v a t i o n s involved dimple s h a p e s , a b r i e f d e s c r i p t i o n of the o r i g i n of these shapes is in o r d e r . T h e r e a r e t h r e e b a s i c types of d i m p l e s as shown in Fig. l:S 1) equiaxed d i m p l e s , which form when the s u r f a c e s pull d i r e c t l y away f r o m one a n o t h e r in a d i r e c t i o n p e r p e n d i c u l a r to the plane of the local c r a c k , c r e a t i n g rounded d i m p l e s of the s a m e shape on both f r a c t u r e s u r f a c e s , Fig. l(a); 2) s h e a r d i m p l e s , which f o r m when the two s u r f a c e s a r e being displaced in opposing d i r e c t i o n s p a r a l l e l to the plane of s h e a r as well as b e i n g s e p a r a t e d in d i r e c tions p e r p e n d i c u l a r to the p l a n e , Fig. l(b); and 3) t e a r d i m p l e s , which f o r m when a s t r a i n g r a d i e n t e x i s t s at the c r a c k tip which c a u s e s the voids to be opened up f i r s t on the s i d e s n e a r e s t the c r a c k tip, Fig. l(c). Both s h e a r d i m p l e s and t e a r d i m p l e s a r e open ended, r e s e m b l i n g p a r a b o l a s ; but a r e r e a d i l y d i s t i n g u i s h a b l e b e c a u s e the p a r a b o l i c d i m p l e s on m a t c h i n g s u r f a c e s point in opposite d i r e c t i o n s for s h e a r d i m p l e s , but in the s a m e d i r e c t i o n - - toward the c r a c k o r i g i n -- for tear dimples. The a n a l y s e s of c r a c k tip p r o c e s s e s in the p r e s e n t work r e q u i r e d a m o r e detailed c h a r a c t e r i z a t i o n of dimple shapes than the above b e c a u s e p r e c i s i o n m a t c h i n g t e c h n i q u e s r e v e a l e d that the m i c r o f r a c t u r e events ocVOLUME 4, APRIL 1973-1145
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Fig. 1--Three basic types of dimples. For each mode, the sketches show from left to right: material stressed almost to the point of local rupture; local rupture; and the directional sense of the dimples on the fracture surfaces. The arrows indicate the direction and character of plastic strins. Equiaxed dimples are formed in (a), shear dimples in (b), and tear dimples
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