An evaluation of the ring test for strain-rate-sensitive materials
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J. C. CHESNUTT,
The d e v e l o p m e n t of i s o t h e r m a l f o r g i n g and s u p e r p l a s t i c f o r m i n g of t i t a n i u m alloys has c r e a t e d a need for a n u n d e r s t a n d i n g of w o r k p i e c e / t o o l i n g i n t e r a c t i o n s for s t r a i n - r a t e s e n s i t i v e workpiece m a t e r i a l s . One key i n t e r a c t i o n is e x p r e s s e d as the coefficient of f r i c t i o n . L o w e r coefficients c o n t r i b u t e to lower d e f o r m a t i o n load r e q u i r e m e n t s and to m o r e u n i f o r m die filling. Knowledge of the coefficient of f r i c t i o n is i m p o r t a n t for these p r a c t i c a l r e a s o n s , and it is also e s s e n t i a l for f o r m u l a t i n g a c c u r a t e a n a l y t i c a l models of the d e f o r m a t i o n p r o c e s s . The f r i c t i o n f a c t o r between a g r o s s l y d e f o r m i n g workpiece and its tooling is r e l a t e d to l u b r i c a t i o n , d e f o r m a t i o n s t a t e , and d e f o r m a t i o n conditions (temp e r a t u r e , r a t e , and so forth). The r i n g t e s t , developed by Male and Cockcroft ~ in 1964, p r o v i d e s a m e a n s for m e a s u r i n g the f r i c t i o n factor u n d e r a p p r o x i m a t e l y the s a m e l u b r i c a t i o n , d e f o r m a t i o n , and d e f o r m a t i o n s t a t e , as e n c o u n t e r e d i n m a n y i s o t h e r m a l f o r g i n g or o t h e r p l a s t i c d e f o r m a t i o n p r o c e s s e s . A thin r i n g of the workpiece m a t e r i a l is c o m p r e s s e d , with a p p r o p r i ate l u b r i c a t i o n , to a p r e d e t e r m i n e d r e d u c t i o n in thickn e s s between the t o o l i n g m a t e r i a l s , and the i n n e r dia m e t e r of the r i n g is m e a s u r e d a f t e r the test is c o m pleted. T h e n the i n n e r d i a m e t e r for a s e r i e s of thickn e s s r e d u c t i o n s is plotted a g a i n s t r e d u c t i o n . The f r i c t i o n f a c t o r is then d e t e r m i n e d through the use of a t h e o r e t i c a l l y d e r i v e d c a l i b r a t i o n chart.* *Generally,with the ringtest, the frictionis expressedby the frictionfactor, m = interfaceshearstrength/shearyield stress,rather thanwitha Coloumbcoefficient of friction,/z A m a j o r r e q u i r e m e n t for s u c c e s s f u l a p p l i c a t i o n of the method is an a n a l y t i c a l s o l u t i o n to the r i n g - f o r g i n g p r o b l e m to p r o v i d e the n e c e s s a r y c a l i b r a t i o n chart. A good d e a l of effort has b e e n d i r e c t e d to such s o l u t i o n s for v a r i o u s m e t a l c o n s t i t u t i v e p r o p e r t i e s , but to our knowledge no such s o l u t i o n is a v a i l a b l e for s t r a i n r a t e s e n s i t i v e m a t e r i a l s . It is the p u r p o s e of this c o m m u n i c a t i o n to r e p o r t the r e s u l t s of a s o l u t i o n for a c o n s t i t u t i v e equation a p p l i c a b l e to a s t r a i n - r a t e s e n s i t i v e m a t e r i
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