The dependence of the fracture toughness of mi steel on temperature and crack velocity
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BILEK
The dependence of the d y n a m i c p l a n e - s t r a i n f r a c t u r e t o u g h n e s s , Kid, on t e m p e r a t u r e and c r a c k velocity was m e a s u r e d for p r o p a g a t i n g c r a c k s in 1020 s t e e l . The d y n a m i c s of c r a c k p r o p a g a t i o n in d o u b l e - c a n t i l e v e r e d s p e c i m e n s was r e c o r d e d u s i n g e l e c t r o r e s i s t i v ity t e c h n i q u e s . The f r a c t u r e s u r f a c e e n e r g y was found by c o m p a r i n g the c r a c k p r o p a g a tion to s o l u t i o n s of c r a c k motion in wedged-open c a n t i l e v e r e d s p e c i m e n s . The Kid b e h a vior was i n v e s t i g a t e d over a r a n g e of t e m p e r a t u r e s from - 1 9 6 ~ to - 5 0 ~ and c r a c k v e l o c i t i e s of 3 • 10 -3 to 5 • 10 -2 of 4--ff-~. The r a t e and t e m p e r a t u r e dependence of Kid over the r a n g e of T and vc i n v e s t i g a t e d is well d e s c r i b e d by:
l i K e d = - - a ( l n Vo//vc)T + B w h e r e A, B, and v0 a r e e x p e r i m e n t a l c o n s t a n t s . A d y n a m i c value of KI d was 70 pct of KIc at the s a m e t e m p e r a t u r e , although in the t e m p e r a t u r e and c r a c k velocity r a n g e i n v e s t i gated the specific f r a c t u r e s u r f a c e e n e r g y v a r i e s by a factor of 6. The t e m p e r a t u r e T T =_ B/A In (Vo/Vc) for which 1/K~ d = 0 is s i m i l a r to Charpy i m p a c t t r a n s i t i o n t e m p e r a t u r e v a l u e s when vc = 3 • 10-3 E ~ . If the p l a n e - s t r a i n s t r e s s condition could be m a i n t a i n e d , then T T would define a b r i t t l e - d u c t i l e t r a n s i t i o n t e m p e r a t u r e for d y n a m i c p l a n e - s t r a i n f r a c t u r e t o u g h n e s s . The c o n s t a n t s A and B a r e i n t e r p r e t e d by u n d e r s t a n d i n g the p l a s t i c e n e r g y d i s s i p a t e d by a moving c r a c k .
THE static
p l a n e - s t r a i n f r a c t u r e t o u g h n e s s , KIc, q u a n t i t a t i v e l y r e l a t e s the m e c h a n i c s of c r a c k p r o p a g a tion in a p l a n e - s t r a i n s t r e s s field to the g e o m e t r y and loading a r r a n g e m e n t of the s p e c i m e n . It is not an i n t r i n s i c m a t e r i a l p r o p e r t y but is i n f l u e n c e d by and i n deed d e r i v a b l e (ideally) f r o m m e c h a n i c a l and m i c r o s t r u c t u r a l p r o p e r t i e s , x Its value is c o n t r o l l e d p r i m a r i l y by the m a t e r i a l ' s a b i l i t y to a b s o r b e n e r g y by p l a s t i c flow n e a r the c r a c k tip and by the i n t r i n s i c b l u n t i n g of the c r a c k which p a r t l y r e l i e v e s t e n s i l e s t r e s s e s . A b s o lute t h e o r e t i c a l v a l u e s of Krc a r e not t r a c t a b l e b e c a u s e p l a s t i c i t y t h e o r y cannot a c c u r a t e l y p r e d i c t the extent of t r i a x i a l i t y , h a r d e n i n g , and the effects of b l u n t i n g in the flow field v e r y n e a r
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