The effect of high vacuum on the low cycle fatigue law
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[1]
It h a s been shown e x p e r i m e n t a l l y that the exponent ~3 is t e m p e r a t u r e s e n s i t i v e , and it is c o m m o n l y found that /3 i n c r e a s e s with i n c r e a s i n g t e m p e r a t u r e . A m o d e l for the b e h a v i o r h a s been d i s c u s s e d r e c e n t l y , 1 and i s b a s e d on the w e l l - d o c u m e n t e d concept that low c y c l e fatigue is p r i n c i p a l l y a c r a c k p r o p a g a t i o n p r o c e s s . F u r t h e r , it i s g e n e r a l l y found that the f r a c t u r e m o d e in c r a c k p r o p a g a t i o n c h a n g e s f r o m t r a n s g r a n u l a r to i n t e r g r a n u l a r with i n c r e a s i n g t e m p e r a t u r e . It w a s p r o p o s e d that in the r e g i m e w h e r e the f r a c t u r e m o d e is m i x e d , f o r a given f r e q u e n c y the s t r a i n r a t e a t t h e c r a c k t i p i n c r e a s e s with i n c r e a s i n g s t r a i n r a n g e and the p r o p o r tion of t r a n s g r a n u l a r f r a c t u r e should a l s o i n c r e a s e . At a s u f f i c i e n t l y high s t r a i n r a n g e , the f r a c t u r e p r o c e s s can then be e n t i r e l y t r a n s g r a n u l a r . It w a s then a r g u e d that t h i s b e h a v i o r would c a u s e a b r e a k in s l o p e in the p l a s t i c s t r a i n r a n g e - l i f e plot, such t h a t , a t high s t r a i n r a n g e s , /3 ~ 0.5 while at l o w e r s t r a i n r a n g e s / 3 > 0.5. E v d i e n c e w a s p r o v i d e d to s u p p o r t t h i s p o s i t i o n , a s shown in F i g . 1 f o r C1010 s t e e l at 600~ O t h e r w o r k 2 h a s shown that in the r e g i m e of a m i x e d f r a c t u r e m o d e , the effect of f r e q u e n c y m u s t b e a c counted f o r , and a c c o r d i n g l y t h e f a t i g u e law w a s m o d i fied such that in t h i s r e g i m e it b e c o m e s
(Nf uk-~)fl A~p =Ce
[2]
w h e r e u i s the f r e q u e n c y (cpm) and k i s an e x p e r i m e n t a l l y d e t e r m i n e d c o n s t a n t , independent of p l a s t i c s t r a i n but d e p e n d e n t on t e m p e r a t u r e . The t e r m Nfv k-~ has been d e f i n e d a s the f r e q u e n c y m o d i f i e d fatigue l i f e . P r o v i d e d s u f f i c i e n t t e s t r e s u l t s e x i s t f o r a given m a t e r i a l at d i f f e r e n t s t r a i n r a t e s o r f r e q u e n c i e s and s t r a i n r a n g e s at a given t e m p e r a t u r e , t h e s e r e s u l t s can be r e p r e s e n t e d by Eq. [2], a s s e e n t y p i c a l l y in F i g . 2. L. F. COFFIN, Jr., is with Research Staff, Corporate Research and Development, General Electric Co., Schnectady, N. Y. Manuscript submitted December 21, 1971. METALLURGICAL TRANSACTIONS
H e r e the t e s t r e s u l t s of B e r l i n g and Slot 3 a r e c o n v e r t e d to the f o r m of Eq. [2]. Note the effect of t e m p e r a t u r e on the exponent/3, and the c o n v e r g e n c e to a s i n g l e point. At the l o w e s t t e m p e r a t u r e , w h e r e the f r a c t u r e m
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