The strain-dependence of plastic strain ratio (r m value) of deep drawing sheet steels determined by simple tension test
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v a l u e of r m obtained. S o m e of t h e i r r e s u l t s c l e a r l y i n d i cat ed a s t r a i n d e p e n d e n c e of the r value, but no a t t e m p t w as made to f u r t h e r e x p l o r e this p h e n o m e n o n by any of t h e s e a u t h o r s . C o n s i d e r i n g the f a c t that t e x t u r e d s h e e t s ex hi bi t ani s o t r o p i c p r o p e r t i e s , the c h a r a c t e r i s t i c s of p l a s t i c d e f o r m a t i o n , such as y i e l d and w o r k h a r d e n i n g , in a d e e p d r a w i n g s t e e l s h e e t would be e x p e c t e d to be d i f f e r e n t f o r its t h i c k n e s s d i r e c t i o n f r o m t h o se f o r its width d i r e c t i o n . Thus, for n e a r l y r a n d o m or w e a k l y t e x t u r e d s p e c i m e n s , no s i g n i f i c a n t v a r i a t i o n s in the r v a l u e s would h a v e been o b s e r v e d u p o n s t r a i n i n g to v a r i o u s e x t e n s i o n s within the r a n g e of u n i f o r m d e f o r m a t i o n , s i n c e t h e i r p l a s t i c p r o p e r t i e s a r e a l m o s t i s o t r o p i c . H o w e v e r , for s p e c i m e n s having f a i r l y s t r o n g t e x t u r e s , such as deep d r a w i n g s h e e t s t e e l s d e v e l o p e d in r e c e n t y e a r s , the above c r i t e r i o n may no l o n g e r be valid, b e c a u s e the c h a r a c t e r i s t i c s of p l a s t i c d e f o r m a t i o n m ay be c o n s i d e r a b l y d i f f e r e n t f o r the t h i c k n e s s d i r e c t i o n f r o m that f o r the width d i r e c t i o n . Th e p r e s e n t i n v e s t i g a t i o n w a s u n d e r t a k e n to t e s t this idea. Three laboratory produced low-carbon sheet steels, fully r e c r y s t a l l i z e d with a m e a n g r a i n d i a m of 0.02 to 0.03 m m , w e r e u s e d for the p r e s e n t i n v e s t i g a t i o n . T h e i r r m v a l u e s * as d e t e r m i n e d by s t r a i n i n g to 15 pct e l o n *rm = (r0 + 2r4s + r~o)/4.
gation, c o r r e s p o n d i n g a p p r o x i m a t e l y to the midpoint of u n i f o r m d e f o r m a t i o n , w e r e b et w een 2.30 and 2.50. T he s p e c i m e n s w e r e t e s t e d to v a r i o u s s t r a i n l e v e l s , f r o m s l i g h t l y beyond the L i i d e r s s t r a i n to m a x i m u m u n i f o r m e l o n g a t i o n . At each s t a g e of the s t r a i n i n g , the t e s t i n g w as i n t e r r u p t e d for m e a s u r e m e n t s of the s t r a i n s in the length and width d i r e c t i o n s of the s p e c i m e n , f r o m w h i ch the s t r a i n in the t h i c k n e s s d i r e c t i o n can be c a l c u lated. Immediately after these measurements, testing w as r e s u m e d to the next h i g h e r s t r a i n l e v e l , and finally to the m a x i m u m u n i f o r m e l o n g a t i o n . A l o a d - e l o n g a t i o n c u r v e , typical of such a t e s t i n g s e q u e n c e is shown in HSUN HU F i g . 1. No d e t e c t a b l e i n c r e a s e in flow s t r e s s due to s t r a i n - a g i n g upon r e t e s t i n g
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