Cyclic creep-rupture behavior of three high-temperature alloys
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O V E R the p a s t few y e a r s , we at the Lewis R e s e a r c h C e n t e r have b e e n developing methods for a n a l y z i n g and p r e d i c t i n g the r e s i s t a n c e of m a t e r i a l s to f a i l u r e by cyclic loading at e l e v a t e d t e m p e r a t u r e s in the highs t r a i n , l o w - c y c l e r a n g e . 1-7 The t e r m f a i l u r e by cyclic loading is u s e d i n s t e a d of fatigue s i n c e , as is often the c a s e , the cyclic f a i l u r e s at h i g h - t e m p e r a t u r e b e a r l i t tle or no r e s e m b l a n c e to c l a s s i c a l fatigue f a i l u r e s . I n s t e a d of being t r a n s g r a n u l a r , the c r a c k s f r e q u e n t ly follow g r a i n b o u n d a r i e s m a m e c h a n i s m of f a i l u r e n o r m a l l y a s s o c i a t e d with c r e e p . In fact, our p r o p o s e d p r o c e d u r e s for life a n a l y s i s and p r e d i c t i o n a r e b a s e d on the s u p p o s i t i o n that f a i l u r e s a r e g o v e r n e d by the a c c u m u l a t i o n of some f o r m of c r e e p d a m a g e at the one e x t r e m e or by the a c c u m u l a t i o n of fatigue damage at the other. In g e n e r a l , the f a i l u r e s r e s u l t f r o m some c o m b i n a t i o n of these d a m a g i n g m e c h a n i s m s , and a s i m p l e l i n e a r d a m a g e r u l e is u s e d as a v e h i c l e for d e t e r m i n i n g t h e i r c o m b i n e d effects. The need for cyclic c r e e p - r u p t u r e c u r v e s a r o s e out of a study of the r o l e of c r e e p in e l e v a t e d - t e m p e r a t u r e , l o w - c y c l e fatigue t e s t i n g conducted u n d e r s t r a i n c o n t r o l l e d c o n d i t i o n s . It was r e c o g n i z e d that some f o r m of c r e e p f a i l u r e c r i t e r i o n was r e q u i r e d to exp l a i n the f r e q u e n c y and t e m p e r a t u r e d e p e n d e n c e of s t r a i n - c y c l i n g f a i l u r e s e x p e r i e n c e d by s p e c i m e n s of L-605 and 316 s t a i n l e s s s t e e l . Looking f i r s t to the c o n v e n t i o n a l ( c o n s t a n t load), m o n o t o n i c , c r e e p - r u p t u r e c u r v e as a p o s s i b l e f a i l u r e c r i t e r i o n , it was found that cyclic l i v e s could be p r e d i c t e d c o r r e c t l y only f r o m a q u a l i t a t i v e s t a n d p o i n t . Computed lives in s o m e c a s e s w e r e s i g n i f i c a n t l y lower than o b s e r v e d , i n d i c a c a t i n g that such a f a i l u r e c r i t e r i o n is too s e v e r e and at b e s t can be u s e d as a lower bound on life. 4'5 T h i s GARY R. HALFORD is Research Engineer, Materials and Structures Division,NASA-LewisResearch Center, Cleveland,Ohio.44135. Manuscript submitted September 8, 1971. METALLURGICALTRANSACTIONS
r e s u l t can be r a t i o n a l i z e d as follows: d u r i n g a c o n s t a n t - l o a d r u p t u r e t e s t , the c r o s s - s e c t i o n a l a r e a d e c r e a s e s as the s p e c i m e n c r e e p s . The t r u e s t r e s s e x p e r i e
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