Influence of metallurgical factors on the fast fracture energy absorption rates
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In the p r e s e n t i n v e s t i g a t i o n , two c o m m e r c i a l s t e e l s w e r e c h o s e n so as to p r o v i d e s i g n i f i c a n t l y d i f f e r e n t c a r b o n c o n t e n t s while v a r y i n g the o t h e r a l l o y i n g e l e m e n t s a s l i t t l e a s f e a s i b l e : SAE4340 and 0-1 tool s t e e l ( c o m p o s i t i o n s a r e given in T a b l e I). In the p r e s ent p a p e r , two a d d i t i o n a l t e m p e r i n g t e m p e r a t u r e s of 4340 a r e added to those p r e v i o u s l y studied. ~-~ The 0-1 s t e e l is v e r y c l o s e to the " P i t h o " s t e e l s t u d i e d by Congleton, et al. 9-11 PROCEDURE The e x p e r i m e n t s w e r e c a r r i e d out using the w e d g e loaded DCB s p e c i m e n with p r o c e d u r e s and s p e c i m e n d i m e n s i o n s i d e n t i c a l to those e m p l o y e d in the p r e v i o u s p a p e r . 4 F o r both s t e e l s the d y n a m i c b e a m - o n - e l a s t i c foundation a n a l y s i s ~-s was u s e d to c a l c u l a t e d y n a m i c t o u g h n e s s and KQ, the t r e s s i n t e n s i t y at c r a c k e x t e n sion. The SAE4340 s a m p l e s w e r e too thin to s a t i s f y plane s t r a i n r e q u i r e m e n t s (ASTM E399) and g e n e r a l l y exhibited s i g n i f i c a n t a m o u n t s of s h e a r lip f o r m a t i o n . A c c o r d i n g l y , the t e r m , KD, i s used to d e s c r i b e the d y n a m i c m i x e d - m o d e f r a c t u r e t o u g h n e s s of 4340. F o r ali but one of the 4340 s a m p l e s and in a f e w o f t h e 0 - 1 s a m p l e s the c r a c k s a r r e s t e d within the s p e c i m e n . This p r o v i d e s a s e c o n d e s t i m a t e of KD u s i n g the r e l a tion 2 KD = ~KQK a
[1]
K a is the s t a t i c a l l y c a l c u l a t e d s t r e s s i n t e n s i t y at c r a c k a r r e s t b a s e d on the i n i t i a l d i s p l a c e m e n t and the c r a c k length at a r r e s t . RESULTS Dynamic Toughness 4340 Steel. Two t e m p e r i n g t e m p e r a t u r e s w e r e u s e d , 370~ and 425~ H o w e v e r , only the h i g h e r of t h e s e was s t u d i e d in d e t a i l . As b e f o r e , a the c r a c k s p r o p a gated at a c o n s t a n t v e l o c i t y b e f o r e d e c e l e r a t i n g and a r r e s t i n g . The d y n a m i c f r a c t u r e t o u g h n e s s e s a f t e r t e m p e r i n g at 425~ CTable II) a r e , on a v e r a g e , s l i g h t l y s m a l l e r than those f o r the 205~ t e m p e r , while the z e r o - v e l o c i t y value is s o m e w h a t h i g h e r as shown in Fig. 2. W h i l e t h e r e i s c o n s i d e r a b l e s c a t t e r around the s m o o t h c u r v e d r a w n so a s to e s t i m a t e the t r e n d of the 425~ p o i n t s in Fig. 2, T a b l e II shows that d u p l i c a t e s a m p l e s a r e quite r e p r o d u c i b l e . The only e x c e p t i o n is the twofold d i s c r e p a n c y in v e l o c i t y b e t w e e n the two VOLUME 7A, JANUARY 1976-49
samples with 0.36 mm diam starting notches a
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