Dislocation structures adjacent to fatigue crack tips in stainless steel
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Dislocation Structures Adjacent to Fatigue Crack Tips in Stainless Steel JOBU AWATANI AND TETSURO SHIRAISHI
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Fig. 5--Effect of tensile load on radial crack pattern of ballistically damaged specimen. Note the appearance of several small radial cracks at the bottom of the impacted area; (a) before loading and, (b) during loading. s u c h a s o b s e r v e d on t h e f r a c t u r e s u r f a c e of n o t c h e d f a t i g u e s p e c i m e n s . ~ T h e c o m p l e x f r a c t u r e p a t h in r e l a t i o n to the o r i g i n a l b a l l i s t i c d a m a g e is shown in F i g . 3. Note how the d i r e c t i o n of c r a c k e x t e n s i o n has changed to a m o r e f a v o r a b l e one p e r p e n d i c u l a r to the loading direction. In c o n t r a s t to the c e n t e r - n o t c h p a n e l d i s c u s s e d a b o v e , the r e s i d u a l s t r e n g t h b e h a v i o r of b a l l i s t i c a l l y d a m a g e d p a n e l s has b e e n a n a l y z e d f r o m a f r a c t u r e m e c h a n i c s viewpoint a s s u m i n g that the c r a c k e x t e n d s METALLURGICAL TRANSACTIONS A
In p r e v i o u s work, ~ the a u t h o r s o b s e r v e d d i s l o c a tion s t r u c t u r e s in the i m m e d i a t e v i c i n i t y of the t i p s of the f a t i g u e c r a c k s f o r m e d in i r o n b u l k s p e c i m e n s . A f t e r that, we w e r e i n t e r e s t e d in o b s e r v a t i o n s of a l l o y s with low s t a c k i n g fault e n e r g y , b e c a u s e of unc o m m o n l y good fatigue p r o p e r t i e s of such a l l o y s c o m p a r e d with the s o l v e n t m e t a l . 2,3 In this p a p e r , o b s e r v a t i o n s a r e m a d e f o r 18-8 s t a i n l e s s s t e e l , the c h e m i c a l c o m p o s i t i o n of which i s F e 70.30, C 0.06, Si 0.66, Mn 1.52, P 0.032, S 0.018, Ni 8.89 and C r 18.52 in vet p e t . The s i z e of s p e c i m e n was s i m i l a r to that of the i r o n s p e c i m e n u s e d in the p r e v i o u s w o r k . ~ P r i o r to fatigue t e s t , the s p e c i m e n s w e r e s o l u t i o n - h e a t - t r e a t e d at 1373 K in a m b i e n t A r , w a t e r - q u e n c h e d and then lightly e l e c t r o p o l i s h e d , the final g r a i n s i z e b e i n g about 80 ~tm in d i a m . C r a c k s grown to about 2.0 m m f r o m the notch r o o t s w e r e e x a m i n e d , the g r o w t h r a t e s being about 0.01 ~tm p e r c y c l e in t h i s s t a g e . F o r t r a n s m i s sion e l e c t r o n m i c r o s c o p y , thin f o i l s w e r e p r e p a r e d f r o m such f a t i g u e d bulk s p e c i m e n s by thinning t h e m using j e t e l e c t r o p o l i s h i n g , r e g i o n s a s c l o s e a s 1 ~tm f r o m the c r a c k s b e i n g p r e s e r v e d in t h e s e f o i l s . An e x a m p l e of the c r a c k s a f t e r thinning is shown in F i g . 1. The d e t a i l s of the technique f o r thinning w e r e r e p o r t e d e l s e w h e r e . 4 F o r o b s e r v a t i o n s of i n t e r n a l s t r u c t u r e s , an e l e c t r o n m i c
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