Crack initiation in laminar composite armor steel
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Crack Initiation in Laminar Composite Armor Steel R. CHAIT AND R. PAPIRNO M E T A L L U R G I C A L L Y bonded 2 - l a y e r p l a t e c o m p o s i t e s with a v e r y h a r d f r o n t a l f a c e (Rcd0) and a t o u g h e r , l o w e r h a r d n e s s backup p o r t i o n (RcS0) c a n i m p r o v e b a l l i s t i c p e r f o r m a n c e ) It i s p o s s i b l e that t h i s m a t e r i a l , a l s o c a l l e d dual h a r d n e s s o r dual p r o p e r t y s t e e l , could b e u s e d in a l o a d c a r r y i n g c a p a c i t y while at the s a m e time providing ballistic protection. Therefore, recent i n v e s t i g a t i o n s have f o c u s e d on the t e n s i l e p r o p e r t i e s , fatigue c r a c k p r o p a g a t i o n and S - N b e h a v i o r , and s t r u c t u r a l d e g r a d a t i o n due to b a l l i s t i c d a m a g e , a-4 It is the p u r p o s e of the p r e s e n t w o r k to d e t a i l a n o t h e r a s p e c t of m e c h a n i c a l b e h a v i o r of t h i s m a t e r i a l , n a m e l y c r a c k i n i t i a t i o n under c y c l i c loading. The c o m p o s i t i o n of the m a t e r i a l u s e d f o r t h i s s t u d y is shown in T a b l e I. Note that the c o m p o s i t i o n d i f f e r ence b e t w e e n the s i d e s l i e s in the p e t C. A f l a t 0.465 in. (11.8 m m ) wide, 0.195 in. (4.95 ram) thick, double notched (Kt = 2.0) s p e c i m e n was u s e d . The h a r d and soft p o r t i o n of the c o m p o s i t e s p e c i m e n w e r e a p p r o x i m a t e l y of equal t h i c k n e s s . The s p e c i m e n was c y c l e d b e t w e e n a v e r a g e s t r e s s l e v e l s of 0 and 105 k s i ('/20 N / m s) in a 60,000 lb. (267,000 N) c a p a c i t y s c r e w o p e r a t e d , c y c l i c loading, W i e d e m a r m - B a l d w i n t e s t i n g m a c h i n e . The a v e r a g e s t r e s s i s d e f i n e d h e r e a s the l o a d d i v i d e d b y the net c r o s s - s e c t i o n a l a r e a of the notch. The c y c l i c t i m e f o r c r a c k i n i t i a t i o n was d e t e r m i n e d u s i n g an a u t o m a l l e s y s t e m d e v e l o p e d b y P a p i r n o and P a r k e r 5 t h a t p e r i o d i c a l l y took f l a s h p h o t o m i c r o g r a p h s , (X12), of the r o o t of the notch a s s c h e m a t i c a l l y shown in F i g . 1. In t h i s m a n n e r p i c t u r e s w e r e t a k e n and f u r t h e r e n l a r g e d , (X120), to d e t e r m i n e the f o r m a tion of a w e l l - d e f i n e d c r a c k without i n t e r r u p t i n g the
R. CHAIT is Acting Chief, Materials Properties Branch, Materials Manufacturing and Testing Technology Division and R. PAPIRNO is Research Mechanical Engineer, Engineering Mechanics Division, Mechanics Research Laboratory, Army Materials and Mechanics Research Center, Watertown, Mass. 02172. Manuscript submitted November 14, 1972. METALLURGICAL TRANSACTIONS
Table I. Composition Side
C
hard 0.53 soft 0.28
Si
P
S
AI
Mn
Cr
V
Mo
Ni
0.27 0.27
0.006 0.006
0.005 0.007
0.05 0.03
0.87 0.87
0.76 0.7
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