Initiation of fatigue cracks in 4340 steel
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F. N. K U S E N B E R G E R
E l e c t r o n m i c r o s c o p y of s u r f a c e r e p l i c a s h a s b e e n e m p l o y e d to d e t e r m i n e the m e t a l l u r g i c a l m i c r o m e c h a n i c s of fatigue c r a c k i n i t i a t i o n at s u r f a c e and s h a l l o w s u b s u r f a c e i n c l u s i o n s in 4340 s t e e l . T h e e x p e r i m e n t a l r e s u l t s a r e c o n s i d e r e d in t e r m s of c u r r e n t t h e o r e t i c a l a n a l y s e s of e l a s t i c s t r e s s d i s t r i b u t i o n s a r o u n d h a r d i n c l u s i o n s i m b e d d e d in soft m a t r i c e s . A m o d e l f o r c r a c k n u c l e a t i o n at s u r f a c e i n c l u s i o n s i s s u g g e s t e d , b a s e d upon debonding of the i n c l u s i o n / m a t r i x i n t e r f a c e at the t e n s i l e p o l e of the i n c l u s i o n , followed by g r o w t h of the debond s e a m t o w a r d the i n c l u s i o n " e q u a t o r " . Subsequent g e n e r a t i o n of a m a t r i x c r a c k i s a s s o c i a t e d with the i n i t i a t i o n of " p o i n t " s u r f a c e d e f e c t s s o m e d i s t a n c e f r o m the i n c l u s i o n / m a t r i x b o u n d a r y , but in the p l a n e of the i n c l u s i o n " e q u a t o r " . S u r f a c e s l i p p l a y s no d i s c e r n i b l e r o l e in n u c l e a t i o n of s u r f a c e i n c l u s i o n s , but s l i p b a n d s a r e f o r m e d at c r a c k s i t e s a b o v e s u b s u r f a c e i n c l u s i o n s . H o w e v e r , the m i c r o c r a c k s continue to f o r m t h r o u g h the l i n k - u p of point s u r f a c e d e f e c t s .
THEi m p o r t a n t
r o l e p l a y e d by n o n m e t a l l i c i n c l u s i o n s a s i n i t i a t i o n s i t e s f o r fatigue c r a c k s in high s t r e n g t h a l l o y s h a s b e e n d o c u m e n t e d e x t e n s i v e l y , z-9 One of the m o s t w i d e l y s t u d i e d a l l o y s in this r e g a r d h a s b e e n 4340 s t e e l . H o w e v e r , a s y e t no c o n c l u s i v e e v i d e n c e of the m i c r o m e c h a n i s m r e s p o n s i b l e f o r c r a c k i n i t i a t i o n at the inclusion-matrix interface has been presented, A few e x a m p l e s suffice to i n d i c a t e the i m p o r t a n c e of i n c l u s i o n s a s fatigue s t r e s s r a i s e r s in high s t r e n g t h a l l o y s . F o r s e v e r a l s t e e l s , including 4340, E p r e m i a n and Mehl* c o n c l u d e d that i n c l u s i o n s w e r e p r o b a b l y the m o s t i m p o r t a n t s i n g l e f a c t o r in e s t a b l i s h i n g the e x t e n t of s t a t i s t i c a l v a r i a t i o n s in fatigue life. S i m i l a r l y , C u m m i n g s e t a l . , 2 r e p o r t e d that of 540 s p e c i m e n s of 4340 and 4350 s t e e l having s i n g l e n u c l e u s f r a c t u r e s , 99 p e t of the fatigue f a i l u r e s w e r e a t t r i b u t a b l e to i n c l u s i o n r e l a t e d c r a c k n u c l e a t i o n . E r h a r d t e t a l . s and G r o s s k r e u t z and S h a
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