Ductile fracture topography: Geometrical contributions and effects of hydrogen
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r e c e n t r e v i e w s~'~ h a v e c o n c l u d e d t h a t t h e only e s t a b l i s h e d type of n u c l e u s f o r m i c r o v o i d s i n d u c t i l e f r a c t u r e ( m i c r o v o i d c o a l e s c e n c e o r M V C ) of n o m i n a l l y single-phase m a t e r i a l s is t h e inclusion o r impurity/precipitate particle. A possible m e a n s o f t e s t i n g t h i s c o n c l u s i o n w o u l d be a n e x h a u s t i v e m e t a l l o g r a p h i c s u r v e y4 s e e k i n g a g r e e m e n t b e t w e e n t h e p a r t i c l e d i s t r i b u t i o n in t h e s t a r t i n g m a t e r i a l , a n d t h e obs e r v e d s i z e ( o r , e q u i v a l e n t l y , s p a c i n g ) of h a l f - v o i d s o r d i m p l e s on t h e d u c t i l e f r a c t u r e s u r f a c e . T h i s p a p e r a d d r e s s e s o n e o f t h e f a c t o r s w h i c h s h o u l d be t a k e n into a c c o u n t in a n y such c o m p a r i s o n , n a m e l y t h e g e o m e t r i c a l c h a n g e s in p a r t i c l e s p a c i n g w h i c h o c c u r w i t h i n c r e a s i n g s t r a i n . A r e l a t e d a p p l i c a t i o n of t h e s a m e g e o m e t r i c a l f a c t o r a r i s e s i n c o m p a r i s o n of d i m p l e s i z e s i n t e n s i l e t e s t s a n d f r a c t u r e t o u g h n e s s t e s t s on t h e same material; the s t r e s s s t a t e and l o c a l f r a c t u r e s t r a i n a r e d i f f e r e n t in t h e two k i n d s o f t e s t s , a n d g i v e r i s e t o d i f f e r e n c e s i n f r a c t u r e t o p o g r a p h y .5-7 A t h i r d and perhaps more complex application rel a t e s to t h e e f f e c t s o f h y d r o g e n o n ductile f r a c t u r e s . F o r m e t a l s w h i c h e x h i b i t MVC w i t h o u t h y d r o g e n , h y d r o g e n - a s s i s t e d f r a c t u r e is in m a n y c a s e s also t y p i f i e d by M V C . 8-~° M e a s u r e m e n t s of t h e d i a m e t e r o f d i m p l e s (microvoid " h a l v e s " ) on f r a c t u r e s u r f a c e s h a v e s h o w n t h a t d i m p l e s m a y be s m a l l e r ,8,~-13 u n c h a n g e d , ~4 o r l a r g e r 8,9,~5 i n h y d r o g e n - c o n t a i n i n g s p e c i m e n s . S i n c e i n a l l c a s e s r e p o r t e d t o d a t e the f r a c t u r e d u c t i l i t y ( r e d u c t i o n of a r e a , R A , o r t r u e f r a c t u r e s t r a i n , el) i s r e d u c e d by h y d r o g e n , the q u e s t i o n a r i s e s a s t o whether a specific relation e x i s t s between red u c e d ductility a n d d i m p l e size changes. Such a r e l a t i o n w o u l d be e x p e c t e d t o h a v e i m p l i c a t i o n s f o r t h e m e c h a n i s m ( s ) of h y d r o g e n e m b r i t t l e m e n t , %1°,~6 a t l e a s t f o r c a s e s of M V C . T h e physical b a s i s for the geometrical factor ref e r r e d t o a b o v e i s a s f o l l o w s . F o r r e a s o n s of c o n s t a n t p l a s t i c v o l u m e , a n y v o l u m e e l e m e n t w h i c h is s t r a i n e d ANTHONY W. THOMPSON is
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