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E. W. HART

S t r e s s r e l a x a t i o n t e s t s have been made in the t e m p e r a t u r e r a n g e 200 ~ to 400~ on two m a t e r i a l s , the Mg-A1 e u t e c t i c alloy and c o m m e r c i a l p u r i t y z i r c o n i u m . The m a t e r i a l s r e p r e s e n t r e s p e c t i v e l y high and low h o m o l o g o u s t e m p e r a t u r e r e g i m e s . The n o v e l f e a t u r e s of the t e s t s a r e the use of high speed, high s e n s i t i v i t y digital m e a s u r e m e n t t e c h n i q u e s and the d i r e c t r e d u c t i o n of the data to s t r e s s - s t r a i n rate c u r v e s . It was p o s s i b l e in this way to o b t a i n p h e n o m e n o l o g i c a l i n f o r m a t i o n on the m a t e r i a l b e h a v i o r o v e r a v e r y l a r g e r a n g e of s t r a i n r a t e with v e r y few s p e c i m e n s . C o m p a r i s o n was made with r e s u l t s obtained by m o r e c o n v e n t i o n a l d i f f e r e n t i a l s t r a i n r a t e t e s t s . The t e s t l e n d s itself well to e s t a b l i s h i n g the p h e n o m e n o l o g y of m e c h a n i c a l b e h a v i o r of m e t a l s .

T H E load r e l a x a t i o n t e s t for m e t a l s in its m o s t u s u a l f o r m c o n s i s t s in loading a test s p e c i m e n in t e n s i o n or o r c o m p r e s s i o n in a tensile t e s t i n g m a c h i n e to some p r e d e t e r m i n e d load l e v e l , then stopping the c r o s s h e a d m o t i o n , and s u b s e q u e n t l y r e c o r d i n g the load as a f u n c tion of t i m e at fixed c r o s s h e a d p o s i t i o n . The r e s u l t a n t l o a d - t i m e r e c o r d is dependent both on the p l a s t i c p r o p e r t i e s of the s p e c i m e n and on the e l a s t i c p r o p e r t i e s of the t e s t i n g m a c h i n e and s p e c i m e n . Although it i s a s t r a i g h t f o r w a r d p r o c e d u r e to e x t r a c t the m a t e r i a l dependent p r o p e r t i e s f r o m the l o a d - t i m e r e c o r d , this i s r a r e l y done in c u r r e n t u s e s of the t e s t . I n s t e a d , a p r e d i c t e d l o a d - t i m e b e h a v i o r i s d e r i v e d f r o m some theory for the m a t e r i a l b e h a v i o r , and that b e h a v i o r is then c o m p a r e d with the m e a s u r e d load time r e c o r d . This p r o c e d u r e g e n e r a l l y o b s c u r e s the fact that the p r o p e r t y that is m e a s u r e d in the t e s t is in f a c t a m a t e r i a l p r o p e r t y that i s i n d e p e n d e n t of any s p e c i a l theory chosen to e x p l a i n it. We s h a l l d e m o n s t r a t e in this p a p e r how the e x p e r i m e n t a l data of the load r e laxation t e s t can be a n a l y z e d p r a c t i c a l l y to yield explicit s t r e s s - s t r a i n rate data. As it i s generally done, the relaxation test yields results that cover barely two decades of strain rate in any one run. This can be seen in the exceptional published cases 1'2 where load relaxation data were converted into s t r e