Effects of strain path changes on the formability of sheet metals
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ORMTNG l i m i t d i a g r a m s (FLD), 1'2 which define m a x i m u m allowable s t r a i n levels d u r i n g sheet m e t a l f o r m i n g , have found i n c r e a s e d usage in sheet s t a m p ing a n a l y s i s , T h e s e d i a g r a m s show the magnitude of m a j o r s t r a i n before the onset of a localized neck as a function of m i n o r s t r a i n in the sheet s u r f a c e (Fig. 1). Usually, s t r a i n paths d u r i n g one step f o r m ing o p e r a t i o n s a r e n e a r l y p r o p o r t i o n a l , i.e. they m a i n t a i n a n e a r l y constant ratio of m a j o r to m i n o r s t r a i n as indicated in Fig. 1. L a b o r a t o r y punch s t r e t c h i n g t e s t s also produce such n e a r l y l i n e a r s t r a i n paths, 3 and c o n s e q u e n t l y l a b o r a t o r y F L D s match the p r a c t i c a l ones quite well. In p r a c t i c e , the s t a m p i n g of sheet m e t a l components often involves multiple o p e r a t i o n s . Such o p e r a t i o n s may v a r y f r o m a d r a w - t y p e ( e x t e n s i o n - c o n t r a c t i o n ) to a s t r e t c h - t y p e ( e x t e n s i o n - e x t e n s i o n ) d e f o r m a t i o n in the sheet plane. D u r i n g s t a m p i n g , c e r t a i n m a t e r i a l locations in a p a r t may e x p e r i e n c e a change f r o m one type of d e f o r m a t i o n to the other. In a single o p e r a t i o n such changes a r e g e n e r a l l y gradual; however, in a multiple o p e r a t i o n a b r u p t changes can take place. T h e r e is evidence in the l i t e r a t u r e %5 that changes in the mode of d e f o r m a t i o n can a l t e r the f o r m a b i l i t y of sheet s t e e l . B i a x i a l p r e s t r a i n i n g has b e e n found to r e d u c e the u n i a x i a l ductility of low c a r b o n s t e e l when c o m p a r e d with its ductility in continuous u n i a x i a l tens i l e d e f o r m a t i o n . 4 It was shown that such effects might arise from increased hardening under biaxial tension, r e s u l t i n g in a p r e m a t u r e loss of s t a b i l i t y when the d e f o r m a t i o n mode is changed f r o m b i a x i a l to u n i a x i a l t e n s i o n . The f o r m i n g l i m i t d i a g r a m s p r e s e n t e d by Kobayashi, Ishigaki and Abe 5 provide f u r t h e r s u p p o r t to this effect. In addition, they show that p r i o r s t r a i n ing along a u n i a x i a l t e n s i l e path e n h a n c e s the s u b s e quent b i a x i a l ductility as m e a s u r e d f r o m f o r m i n g l i m i t diagrams. T h e s e effects a p p e a r to be i m p o r t a n t in sheet m e t a l JOSEPH V. LAUKONISis Senior Research Physicist, Physics Department, General Motors Research Laboratories, Warren, MI 48090, and AMIT K. GHOSH, formerly with General Motors Research Laboratories, is now a member of Technical Staff, Rockwell International Science Center, Thousand Oaks, CA 91360. Manuscript submitted February 2, 1978. METALLURGICALTRANSACTIONSA
f o r m i n g p r a c t i c e , p a r t i c u l a r l y in the d e s i g n
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