The influence of deformation rate on the cyclic behavior of pure iron
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O N s t r a i n c y c l i n g m e t a l s , s t r e s s c h a n g e s m a y be obs e r v e d . T h e s e c h a n g e s , l a r g e at f i r s t , d i m i n i s h with c y c l e s until the flow s t r e s s b e c o m e s constant ( i . e . s a t u r a t i o n s t r e s s ) . R e c e n t work on low c y c l e , s t r a i n c o n t r o l l e d c y c l i c b e h a v i o r of i r o n has shown that the s a t u r a t i o n s t r e s s and the r e l a t e d d e f o r m a t i o n c e l l , o r s u b g r a i n , s i z e a r e r e l a t e d and independent of the s t r a i n h i s t o r y . 1 T h i s b e h a v i o r a p p e a r s t y p ic a l f o r a l l high stacking fault e n e r g y m e t a l s . 2'3 In this c a s e , the s a t u r a t i o n s t r e s s is e x p e c t e d to be a unique function of the e x t r i n s i c t e s t i n g p a r a m e t e r s such as s t r a i n a m p l i tude, d e f o r m a t i o n r a t e and t e m p e r a t u r e . 4 In i r o n , howe v e r , only the f i r s t of t h e s e v a r i a b l e s has been i n v e s t i gated to any depth. 1-3 The p r e s e n t i n v e s t i g a t i o n is c o n c e r n e d with the s ec o n d , n a m e l y the i n f l u e n c e of d e f o r m a t i o n r a t e on the c y c l i c b e h a v i o r of p o l y c r y s t a ll i ne iron. EXPERIMENTAL Annealed polycrystalline F e r r o v a c " E " iron s a m p l e s of about 100p g r a i n s i z e w e r e c y c l e d at r o o m t e m p e r a t u r e u n d er s t r a i n c o n t r o l in an M.T.S. c l o s e d loop u n i v e r s a l t e s t i n g m a c h i n e . The fatigue s p e c i m e n s 7 with ~- in. diam National F i n e t h r e a d e d ends f o r g r i p ping w e r e cut with a s m a l l gage length of 0.4 in. o v e r the m i n i m u m s e c t i o n of 0.375 in. d i a m . Throughout the s e r i e s of t e s t s , the t o t a l s t r a i n r a n g e (As T) was m a i n t a i n e d c o n s t a n t at 0.01 w h e r e a s d i f f e r e n t f r e q u e n c i e s ( f ) of 0.01, 0.1, 2, and 6 cps w e r e u s e d to v a r y the d e f o r m a t i o n r a t e . T h e s t r a i n was applied as a sine w a v e f o r m . F o r such a function, although the s t r a i n r a t e in e a c h individual c y c l e c h a n g e s c o n t i n u o u s l y , the t i m e taken to span the total s t r a i n r a n g e r e m a i n s constant f o r one c y c l e at each f r e q u e n c y . T hu s, for the p u r p o s e s of this study, the d e f o r m a t i o n r a t e (~) is defined as the total c y c l i c s t r a i n p e r second and
H. ABDEL-RAOUF and A. PLUMTREE are Research Assistant and Associate Professor, respectively, Materials Science Laboratory, Department of Mechanical Engineering, University of Waterloo, Waterloo, Ontario, Canada. Manuscript submitted October 19, 1970. METALLURGICAL TRANSACTIONS
m a y be e x p r e s s e d a s : 4) = 2 A E T f sec -~. The n u m b e r , 2, a p p e a r s in the equation s i n c e the total s t r a i n r a n g e is i m p o s e d t w i ce during each c y c l e . It should be noted that this r a t e equa
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