Forming limits of sandwich sheet materials
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INVESTIGATIONS into the formal~ility of f l a t - r o l l e d m o n o l i t h i c sheet m a t e r i a l s have c o n c e r n e d t h e m s e l v e s with one of two g e n e r a l a r e a s . On one hand, t h e r e have b e e n s t u d i e s of p r e s s p e r f o r m a n c e in o p e r a t i o n s such as deep d r a w i n g ~-~ and s t r e t c h i n g . 6-9 T e s t s such as the Swift cup test for deep d r a w a b i l i t y or the O l s e n cup t e s t for s t r e t c h a b i l i t y have been used as l a b o r a t o r y tools in this i n s t a n c e . The second m a j o r a r e a of i n v e s tigation has c o n c e r n e d itself with s t u d i e s of the modes of f a i l u r e of sheet m a t e r i a l s in these o p e r a t i o n s . F o r many e n g i n e e r i n g m a t e r i a l s , these f a i l u r e s u s u a l l y involve u n s t a b l e flow of m e t a l . In deep drawing, the i n s t a b i l i t y o c c u r s e i t h e r a s a r e s u l t of i n s u f f i c i e n t b l a n k holder force which gives r i s e to w r i n k l i n g of the flange 1~ or as a r e s u l t of loss of load c a r r y i n g a b i l i t y in the wall (tensile i n s t a b i l i t y ) which leads to localized n e c k i n g and f r a c t u r e . 1~,~2 In s t r e t c h i n g o p e r a t i o n s , f a i l u r e is often c o n t r o l l e d by t e n s i l e i n s t a b i l i t y as well. n Composite m a t e r i a l s which offer i m p r o v e m e n t s in c o r r o s i o n and fatigue p e r f o r m a n c e a r e b e i n g used inc r e a s i n g l y today. However, p e r f o r m a n c e of c o m p o s i t e s in sheet f o r m i n g o p e r a t i o n s has b e e n e x a m i n e d to a v e r y l i m i t e d extent. T h e s e s t u d i e s have dealt p r i m a r i l y with p r e s s f o r m a b i l i t y and have l a r g e l y involved sandwich sheet m a t e r i a l s . Hawkins and Wright a3 and H i r a i w a and Kondo ~ have studied the f o r m a b i l i t y of sandwich sheet m a t e r i a l s . By u s i n g the Swift and E r i c k s e n cup t e s t s , d r a w a b i l i t y and s t r e t c h a b i l i t y were found to depend heavily on the a r r a n g e m e n t of the l a y e r s of the l a m i nate. The effects of b e n d i n g and f r i c t i o n on the f o r m i n g S, L. SEMIAT1N,formerly graduate student, Department of Metallurgy and Materials Science, Carnegie-MellonUniversity, is now Research Scientist, MetalworkingSection, Battelle's Columbus Laboratories, Columbus, OH 43201. H. R. PIEHLER is Professor, Department of Metallurgy and Materials Science, Carnegie-MellonUniversity, Pittsburgh, PA 15213. Manuscript submitted December 18, 1978. METALLURGICAL TRANSACTIONS A
p r o c e s s complicate i n t e r p r e t a t i o n of t h e i r r e s u l t s , however, and definitive work to s e p a r a t e the influence of m a t e r i a l s - r e l a t e d f r o m p r o c e s s - r e l a t e d v a r i a b l e s on f o r m a b i l i t y is needed. In the p r e s e n t study, sandwich sheet m a t e r i a l f a i l u r e s r e s u l t i n g f r o
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