The effects of crack tip plastic flow directions upon microscopic dimple shapes
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a s i n g l e m i c r o v o i d , and 2) the a n a l y s e s a r e m o s t c l e a r l y and s i m p I y shown in the f o r m of i d e a l i z e d c i r c l e s and o p e n - e n d e d p a r a b o l a - l i k e c u r v e s w h e r e a s the d i m p l e s t h e m s e l v e s s e l d o m have t h e s e p a r t i c u l a r s h a p e s . F i g . 1 i l l u s t r a t e s the eight o b s e r v e d m a t i n g d i m p l e s h a p e s and s i x o t h e r d i m p l e s h a p e s which have not yet b e e n r e p o r t e d but which p r o b a b l y e x i s t . The t h r e e f r a c t u r e m o d e s - - I , II and I I I - - a r e t h o s e used in l i n e a r e l a s t i c f r a c t u r e m e c h a n i c s . 2,a C o m b i n e d m o d e s in r e l a t i o n to m i e r o v o i d growth have b e e n i n t r o d u c e d e a r l i e r 1'4 but the c o n c e p t n e e d s f u r t h e r e m p h a s i s . It should be i m m e d i a t e l y s t a t e d t h a t though the R o m a n n u m e r a l f r a c t u r e m o d e s a r e c o m m o n l y u s e d to i d e n t i f y the m a c r o s c o p i c l o a d i n g a r r a n g e m e n t s , and that although the s k e t c h e s of l o a d i n g a r r a n g e m e n t s in F i g . 1 a p p e a r as m a c r o s c o p i c s p e c i m e n s , we a r e only i n t e r e s t e d h e r e in the e x t r e m e l y l o c a l p l a s t i c flow d i r e c t i o n s a s s o c i a t e d with the f o r m a tion of i n d i v i d u a l v o i d s r i g h t at the c r a c k tip. The d i m e n s i o n s of the s k e t c h e d s p e c i m e n s , then, a r e no m o r e than a few m i c r o n s each. In a l l of the s k e t c h e s and f r a c t o g r a p h s in this p a p e r , the c r a c k is g r o w i n g f r o m left to r i g h t . The t h i r d c o l umn in F i g . 1 is of p r i m a r y i n t e r e s t . T h i s shows the in situ m a t i n g s h a p e s of the d i m p l e s b e f o r e the s u r f a c e s a r e o p e n e d up. The d i r e c t i o n of v i e w i n g for t h e s e d i m p l e s is f r o m the top of the s p e c i m e n s shown in the s e c o n d column. The b o t t o m f r a c t u r e s u r f a c e thus is v i e w e d f r o m o u t s i d e the f r a c t u r e s u r f a c e t h r o u g h the top half of the s p e c i m e n , while the top f r a c t u r e s u r f a c e is v i e w e d f r o m i n s i d e the f r a c t u r e s u r f a c e , a l s o t h r o u g h the top half of the s p e c i m e n . In Column 3, the d i m p l e s on the b o t t o m s u r f a c e a r e shown a s s o l i d l i n e s , and the d i m p l e s on the top s u r f a c e a s d a s h e d l i n e s . The f r a c t u r e s u r f a c e s t h e m s e l v e s a r e shown in Column 4. The m a n n e r of opening the c r a c k (going f r o m Column 3 to Column 4) is l i k e opening a book when the binding is a w a y f r o m the v i e w e r . F i g . 2 is o f f e r e d a s a f u r t h e r a i d in v i s u a l i z i n g the opening s e n s e . A hinge is i m a g ined to e x i s t above the d i m p l e s in Column 3, c o r r e sponding to
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