The formation of cell structures in fatigued iron crystals
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transmission electron microscope investigations have studied the d i s l o c a t i o n a r r a n g e m e n t s gene r a t e d d u r i n g fatigue in m e t a l s . '-6 Depending upon m a t e r i a l p r o p e r t i e s , t e m p e r a t u r e , and s t r a i n a m p l i t u d e , well-defined cell structures, isolated tangles, patches of d e b r i s , or p l a n a r a r r a y s have b e e n o b s e r v e d . The r e l a t i o n s h i p of t h e s e d i s l o c a t i o n s t r u c t u r e s to o t h e r fatigue p h en o m en a such as s u r f a c e r o u g h e n i n g , c y c l i c s t r a i n h a r d e n i n g b e h a v i o r , and c r a c k i n i t i a t i o n is not fully u n d e r s t o o d and i s the s u b j e c t of many c o n t e m p o r a r y i n v e s t i g a t i o n s . 7- lo A r e c e n t t h e o r y p r o p o s e d by Nine and K u h lm an n W i l s d o r f 'b12 i n t e r r e l a t e s the fatigue f o r m e d c e l l s t r u c t u r e s and the s t r i a t i o n s a p p e a r i n g on the s u r f a c e s of fatigued fcc s p e c i m e n s . F e l t n e r and L a i r d '3'14 have studied the c y c l i c s t r e s s - s t r a i n r e s p o n s e of fcc m e t a l s and s u g g e s t e d a d i s l o c a t i o n m o d e l which a c c o u n t s f o r the s a t u r a t i o n p l a s t i c s t r a i n a m p l i t u d e by the m o v e m e n t of d i s l o c a t i o n s within the c o n f i n e s of a c e l l . Fatigue s t u d i e s in i r o n and s t e e l have b e e n n u m e r ous, but the m o r e r e c e n t ones of Gough, 15 M c C l i n t o c k , 1~ L i p s i t t and H o r n e , '7 H e m p e l , 18 Wood, R i e m a n n , and S a r g e n t , 19 K l e s n i l and L u k a s , 2~ M c G r a t h and B r a t ina, 22'e3 and Wei and B a k e r 24,es a r e of p a r t i c u l a r inte re st. H e m p e l showed that the S - N c u r v e was not s e n s i t i v e to c r y s t a l l o g r a p h i c o r i e n t a t i o n , that fatigue c r a c k s f o r m within fatigue slip b a n d s , and that the fatigue d e v e l o p e d s u r f a c e topography is i n f l u e n c e d by the c r y s t a l l o g r a p h y of the s p e c i m e n . Wood, R e i m a n n , and S a r g e n t have shown that m i c r o c r a c k s f o r m e d at the fatigue g e n e r a t e d s u b b o u n d a r i e s in p o l y c r y s t a l l i n e i r o n e s t a b l i s h i n g a link b e t w e e n the i n i t i a t i o n of fatigue c r a c k s and the fatigue f o r m e d substructures. M c G r a t h and B r a t i n a have shown that a c e l l s t r u c t u r e i s f o r m e d at high s t r e s s a m p l i t u d e s and p a t c h e s of d i s l o c a t i o n loops w e r e a l s o o b s e r v e d . S i m i l a r obs e r v a t i o n s w e r e r e p o r t e d by K l e s n i l and L u k a s , and Wei and B a k e r . The l a t t e r e x p e r i m e n t o r s have a l s o found d i s t i n c t l y d i f f e r e n t , d i s l o c a t i o n a r r a n g e m e n t s in a thin l a y e r , a p p r o x i m a t e l y 25 p thick, n e a r the s u r face of th
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