Mechanism of anisotropic lamellar fractures
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T E X T U R E b a n d i n g is c o m m o n in sheet s t e e l s , e s p e c i a l l y in highly alloyed s t e e l s . ''~ A n i s o t r o p i c p l a s tic flow d u r i n g t e n s i l e s t r a i n i n g is known to cause c o l d - r o l l e d and a n n e a l e d sheet of f e r r i t i c s t a i n l e s s s t e e l to exhibit " r i d g i n g " or " r o p i n g " (a c o r r u g a t e d surface) in the plane of the s h e e t , ' even in the a b s e n c e of banding f r o m g r a i n size, c a r b i d e s , or i n c l u s i o n s . It has b e e n o b s e r v e d that a n i s o t r o p i c banding m a y a p p e a r in l a y e r e d or r i b b o n - l i k e f o r m ',~ when the c u b e - o n face (CF) t e x t u r e component is imbedded as l a y e r s in a c u b e - o n - c o r n e r (CC) m a t r i x , Fig. l(a). 2,3 In a r e cent d i s c u s s i o n on the r i d g i n g phenomenon, 4 it was s u g g e s t e d that this type of a n i s o t r o p i c b a n d i n g may cause i n t e r r a c i a l s p l i t t i n g or d e l a m i n a t i o n in the plane of the sheet when s t r a i n e d at low t e m p e r a t u r e or at high speed. In the p r e s e n t i n v e s t i g a t i o n , a n a n i s o t r o p i c t e x t u r e a n a l y s i s and m e t a l l o g r a p h i c study was p e r f o r m e d on light- and h e a v y - g a g e AISI Type 430 s t a i n l e s s - s t e e l sheet to d e m o n s t r a t e that the a n i s o t o p i c p l a s t i c flow which c a u s e s thin sheet to r i d g e on the s u r f a c e may also cause c r a c k s to i n i t i a t e and to propagate p a r a l l e l to the sheet s u r f a c e , r e s u l t i n g i n l a m e l l a r f r a c t u r e of plate p r o d u c t . It is b e l i e v e d that this m e c h a n i s m cont r i b u t e s to the " s p l i t t i n g " which is s o m e t i m e s obs e r v e d in a l l o y - s t e e l p l a t e s .
mm) sheet, and the other f r o m c o l d - r o l l e d and a n n e a l e d 0 . 0 2 0 - i n . - t h i c k (0.5 mm) sheet. The c o m p o s i tion of each s a m p l e is shown in T a b l e I. R o o m - t e m p e r a t u r e t e n s i o n t e s t s were used to s t r a i n both sheets, and t e n s i o n t e s t s at - 320~ (- 196~ were used to s t r a i n the t h i c k e r sheet. Both t e s t s were conducted at a s t r a i n r a t e of 0.167 s - ' . Also, a high s t r a i n r a t e of 200 s - ' was used to b r e a k some s a m p l e s of thin sheet a t - 100~ (-73~ The l o w - t e m p e r a t u r e t e s t s were used to lower the d u c t i l i t y of the s t e e l and thus to e n s u r e that l a m e l l a r f r a c t u r e would o c c u r if the
MATERIALS AND E X P E R I M E N T A L WORK The two Type 430 s t a i n l e s s - s t e e l sheet s a m p l e s used in this i n v e s t i g a t i o n were p r o d u c e d f r o m two diff e r e n t c o m m e r c i a l heats. One s a m p l e was obtained f r o m h o t - r o l l e d and a n n e a l e d 0.120 i n . - t h i c k (3.05 HUNG-CHICHAO is Research Consultant, United States Steel Corporation, Monroeville,PA 151
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