Mechanisms of fast fracture and arrest in steels
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Fig. 3--Interconnections between microcracks revealed by progressive sectioning. The depth of each section below the starting section is indicated. (a) and (b) are located approximately 0.5 and 0.1 in. from the arrested crack tip, respectively. f a c e and on the m i d s e c t i o n . T h i s d e f o r m a t i o n r e f l e c t s an i n c r e a s e in t o u g h n e s s and is r e l a t e d to the f o r m a tion of incipient s h e a r lips n e a r the s u r f a c e while the c r a c k tunneled on the m i d s e c t i o n . F i g s . 5 and 6 g i v e a b e t t e r p i c t u r e of the d i s t r i b u tion of d e f o r m a t i o n along the c r a c k path in the i n t e r i o r of the plate. The etching m a k e s it e a s y to identify l i g a m e n t s , p a r t i c u l a r l y b r o k e n l i g a m e n t s which a r e d i f ficult to d i s t i n g u i s h f r o m o t h e r i r r e g u l a r i t i e s . F i g . 5 i l l u s t r a t e s that the d e f o r m a t i o n is a l m o s t e n t i r e l y confined to l i g a m e n t s left behind by the a d v a n c i n g c r a c k front. As the c r a c k o p e n s , the l i g a m e n t s s t r e t c h and finally r u p t u r e at s o m e d i s t a n c e behind the c r a c k f r o n t . Most of the l i g a m e n t s evident in F i g . 6(b) a r e i n t a c t , while m a n y in F i g . 5 a r e r u p t u r e d [Fig. 5 is a 128-VOLUME 3, JANUARY 1972
s e c t i o n of the s a m e c r a c k shown in F i g . 6(b)]. R e g i o n s b e t w e e n l i g a m e n t s show v i r t u a l l y no signs of plasti," d e f o r m a t i o n - - s e e Section A of F i g . 5. T h e s e photog r a p h s s u g g e s t that the bulk of the p l a s t i c d e f o r m a t i o n and attending e n e r g y d i s s i p a t i o n a r e confined to i s o lated r e g i o n s which o c c u r behind r a t h e r than at the c r a c k f r o n t . It was a l s o a p p a r e n t that the l i g a m e n t s t h e m s e l v e s a r e not u n i f o r m . Both s m a l l l i g a m e n t s , and o c c a s i o n a l l a r g e d i s t u r b a n c e s which a b s o r b m u c h m o r e e n e r g y , such as the one b e t w e e n g and h in F i g . 5, a r e o b s e r v e d . Beyond the n o n u n i f o r m i t i e s a l r e a d y m e n t i o n e d , no s y s t e m a t i c v a r i a t i o n s in e n e r g y d i s s i p a tion along the f r a c t u r e path, f r o m the onset of p r o p a gation to the point of a r r e s t , could be i n f e r r e d f r o m the e t c h e d s e c t i o n s . S m a l l p l a s t i c z o n e s w e r e u s u a l l y METALLURGICAL TRANSACTIONS
(a)
Fig. 4--Development of surface deformation with increasing test temperature in Fe-3 Si. (a) -75~ (b) 0~ (c) 100~
(b)
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o b s e r v e d at the point of a r r e s t . The e x a m p l e r e p r o duced in F i g . 7 r e f l e c t s a K ~ 7 k s i ~ at the a r r e s t e d c r a c k tip,* and while this is c o m p a r a b l e to K a = 8 k s i *Previousstudies26 haveshown that p ~ 0.13 (K/Y)
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