The Dynamic Yield Behavior of Annealed and Cold-Worked Fe-0.17 Pct Ti Alloy
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S H O C K c o m p r e s s i o n p r o v i d e s a unique m e a n s f o r studying the d y n a m i c y i e l d i n g of m a t e r i a l s at s t r a i n r a t e s a p p r o a c h i n g l 0 s s e c -1. In such e x p e r i m e n t s a p l a n e shock wave i s i n t r o d u c e d into a s p e c i m e n by e i t h e r e x p l o s i v e p l a n e wave l o a d i n g o r p l a t e i m p a c t . The shock w a v e i n i t i a l l y c r e a t e s a s t a t e of u n i a x i a l s t r a i n * n e a r the c e n t e r of the s p e c i m e n ; c o n s e q u e n t l y *Uniaxlalstrata refersto loadingconditionsin wtuchonlyone of the principal strain components~snonzero. it e x p e r i e n c e s both s p h e r i c a l and d e v i a t o r i c s t r e s s e s . If the induced d e v i a t o r i c s t r e s s e s a r e s u f f i c i e n t l y l a r g e , the m a t e r i a l w i l l y i e l d and p l a s t i c a l l y d e f o r m . A s a r e s u l t of t h i s p l a s t i c d e f o r m a t i o n , the s i n g l e shock wave i n i t i a l l y i 0 t r o d u c e d into a s p e c i m e n w i l l d e c o m p o s e into two s e p a r a t e w a v e s . T h e p l a s t i c w a v e o r w a v e of p e r m a n e n t d e f o r m a t i o n is p r e c e d e d by an e l a s t i c p r e c u r s o r wave which has an a s s o c i a t e d a m p l i tude c a l l e d the Hugoniot e l a s t i c l i m i t (HEL). T h e HEL can be r e l a t e d to the d y n a m i c y i e l d s t r e n g t h of t h e material. Since l o a d s a r e a p p l i e d v e r y r a p i d l y , m a n y c h a r a c t e r i s t i c s of the e l a s t i c p r e c u r s o r exhibit r a t e - d e p e n d ent e f f e c t s . F o r e x a m p l e , r a t e e f f e c t s r e s u l t in the H E L of m a n y m a t e r i a l s being 2 to 7 t i m e s g r e a t e r than y i e l d s t r e n g t h s i n f e r r e d f r o m q u a s i s t a t i c , u n i a x i a l s t r e s s e x p e r i m e n t s . I-7 A n o t h e r i n d i c a t i o n of r a t e e f f e c t s i s a d e c r e a s e in the a m p l i t u d e of t h e e l a s t i c p r e c u r s o r with i n c r e a s i n g p r o p a g a t i o n d i s t a n c e , the s o - c a l l e d e l a s t i c p r e c u r s o r d e c a y . 1 7 7 -~~ Many m a t e r i a l s a l s o exhibit a sudden d e c r e a s e in s t r e s s behind the e l a s t i c p r e c u r s o r , a y i e l d d r o p J -s Significant p r o g r e s s h a s been m a d e r e c e n t l y t o w a r d e x p l a i n i n g d y n a m i c y i e l d p h e n o m e n a by m o d e l s b a s e d on s i m p l e d i s l o c a t i o n d y n a m i c s c o n c e p t s . Q u a l i t a t i v e l y , at l e a s t , the r a t e - d e p e n d e n c e of the y i e l d s t r e s s , the e l a s t i c p r e c u r s o r d e c a y , and the y i e l d d r o p have been modeled considering dislocation multiplication and glide u n d e r s h o c k - l o a d i n g c o n d i t i o n s J - ~ The m o d e l s a r e g e n e r a l l y fit to e x p e r i m e n t a l d a t a by a d j u s t i n g p a r a m e t e r s r e l a t i n g d i s l
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