Low-cycle fatigue and cyclic stress-strain behavior of incoloy 800
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CONWAY, J. T . B E R L I N G , AND R2 H. STENTZ
S t r a i n - c o n t r o l l e d l o w - c y c l e f a t i g u e t e s t s of s o l u t i o n - a n n e a l e d Incoloy 800 w e r e p e r f o r m e d at t e m p e r a t u r e s of 538 ~ 649 ~ 704 ~ and 760~ u s i n g a x i a l s t r a i n r a t e s of 4 • 10 -3 and 4 • 10 -4 s e c -~. A few h o l d - t i m e t e s t s w e r e a l s o p e r f o r m e d to i n d i c a t e a n o t i c e a b l e r e d u c tion in fatigue life at hold t i m e s of 10 and 60 m i n . A c o m p a r i s o n of t h e s e f a t i g u e d a t a with s i m i l a r r e s u l t s for AISI 304 s t a i n l e s s s t e e l i n d i c a t e s e s s e n t i a l l y i d e n t i c a l b e h a v i o r . An e x t e n s i v e study i s m a d e of the c y c l i c s t r e s s - s t r a i n b e h a v i o r of Incoloy 800 and t h e r e l a t i o n s h i p b e t w e e n the c y c l i c s t r a i n - h a r d e n i n g exponent and fatigue b e h a v i o r i s c o n f i r m e d . E x p o n e n t s on Nf in the e l a s t i c and p l a s t i c s t r a i n r a n g e t e r m s of the t o t a l s t r a i n r a n g e equation a r e i d e n t i f i e d and c o m p a r e d with t h o s e u s e d in the U n i v e r s a l S l o p e s equation.
I N C O L O Y 800 i s a h i g h - n i c k e l a u s t e n i t i c s t e e l with the n o m i n a l c o m p o s i t i o n : F e - 2 0 C r - 3 2 N i - 0 . 7 5 M n 0.35 S i - 0 . 3 Cu-0.3 A1-0.3 Ti. It i s a c o m m e r c i a l l y i m p o r t a n t a l l o y b e c a u s e of i t s high t e m p e r a t u r e s t r e n g t h and its oxidation resistance. Recent applications for t h i s m a t e r i a l in t h e f a b r i c a t i o n of heat e x c h a n g e r s f o r u s e in g a s - c o o l e d n u c l e a r r e a c t o r s have n e c e s s i t a t e d a m o r e d e t a i l e d e v a l u a t i o n of i t s l o w - c y c l e fatigue and c y c l i c s t r e s s - s t r a i n c h a r a c t e r i s t i c s . Since no such d a t a h a v e b e e n r e p o r t e d f o r t h i s a l l o y the p r e s e n t program was initiated. M a t e r i a l w a s o b t a i n e d a s s o l u t i o n - a n n e a l e d (1 h r at 1145~ b a r s t o c k and s p e c i m e n s w e r e m a c h i n e d to c o n f o r m to the h o u r g l a s s s h a p e e m p l o y e d ~-3 in p r e v i o u s s t u d i e s of t h i s t y p e . T e s t s w e r e p e r f o r m e d 13 in a i r at 538 ~ 649 ~ 704 ~ and 760~ u s i n g s t r a i n r a t e s of 4 • 10 -3 and 4 • 10 -4 s e c -1 to define the fatigue l i f e in t h e r a n g e of 100 to 100,000 c y c l e s . In a d d i t i o n , a few h o l d t i m e t e s t s w e r e p e r f o r m e d to p r o v i d e s o m e i n d i c a t i o n of the effect of t h i s v a r i a b l e on c y c l i c fatigue life.
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Specially developed3 servo-controlled, closed-loop, h y d r a u l i c a l l y - a c t u a t e d , p u s h - p u l l fatigue t e s t i n g m a c h i n e s w e r e e m p l o y e d in a x i a l s t r a i n c o n t r o l . A d i a m e t r a l e x t e n s o m e t e r w a s
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