Fatigue crack propagation at elevated temperatures in solid solution strengthened superalloys
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HE a p p l i c a t i o n of f a i l s a f e d e s i g n p r i n c i p l e s to s t r u c t u r e s o p e r a t i n g at e l e v a t e d t e m p e r a t u r e s h a s been, thus f a r , r e s t r i c t e d by the l a c k of fatigue c r a c k growth d a t a and by the q u e s t i o n a b l e v a l i d i t y of a p p l y ing the p r i n c i p l e s of l i n e a r e l a s t i c f r a c t u r e m e c h a n i c s to m a t e r i a l s having a t i m e d e p e n d e n t m e c h a n i c a l b e h a v i o r . In o r d e r to e l i m i n a t e t h e s e two r e s t r i c t i o n s it w i l l be i m p o r t a n t to u n d e r t a k e s o m e e x t e n s i v e r e s e a r c h w o r k to u n d e r s t a n d the m e c h a n i s m s of fatigue c r a c k growth at e l e v a t e d t e m p e r a t u r e s in both the e l a s t i c p l a s t i c and in the fully p l a s t i c condition. C u r r e n t und e r s t a n d i n g of fatigue c r a c k growth at e l e v a t e d t e m p e r a t u r e s has been r e v i e w e d r e c e n t l y by Shahinian, ~ J a m e s , 2'3'4 Speidel, 5 Coffin, 6 Solomon, 7 Haig, 8 and P e l l o u x and O h m u r a . 9 M o s t of t h e s e r e v i e w s which a r e l i m i t e d to a few a l l o y s , ( a u s t e n i t i c s t e e l s u s u a l l y ) , d i s c u s s the effect of f r e q u e n c y and t e s t t e m p e r a t u r e on the d a / d N - ~ d a t a without a c c o u n t i n g f o r the d e t a i l e d m e c h a n i s m s of c r a c k p r o p a g a t i o n . In the w o r k r e p o r t e d h e r e we have a t t e m p t e d to identify the d i f f e r ent m e c h a n i s m s of fatigue c r a c k growth of t h r e e s o l i d s o l u t i o n s t r e n g t h e n e d s u p e r a l l o y s o v e r a r a n g e of t e m p e r a t u r e s and f r e q u e n c i e s c o r r e l a t i n g the growth r a t e s to the f r a c t u r e s u r f a c e f e a t u r e s . The s o l i d s o l u t i o n s u p e r a l l o y s w e r e s e l e c t e d for t h e i r s t r u c t u r a l s t a b i l i t y at e l e v a t e d t e m p e r a t u r e s and f o r t h e i r e x c e l l e n t o x i d a tion r e s i s t a n c e , which l i m i t s the o x i d a t i o n d a m a g e of the f r a c t u r e f e a t u r e s . The fatigue c r a c k growth r a t e s w e r e m e a s u r e d f r o m 10 "~ m / c y c l e to 10 "~ m / c y c l e a s a function of t e s t t e m p e r a t u r e , f r e q u e n c y , s t r e s s w a v e f o r m and s t r e s s r a t i o R (R = Kmi n / K m a x ) . C o r r e l a t i o n s b e t w e e n the m i c r o s c o p i c f r a c t o g r a p h i c f e a t u r e s and the m e a s u r e d m a c r o s c o p i c c r a c k growth r a t e s p r o v i d e a good b a s i s f o r
D. A JABLONSKI and J. V. CARISELLA are Research Assistants, and R. M. PELLOUX is Professor, Massachusetts Institute of Technology, Cambridge, MA 02139. Manuscript submitted March 14, 1977. METALLURGICAL TRANSACTIONS A
the u n d e r s t a n d i n g of the m e c h a n i s m s of fatigue c r a c k growth at e l e v a t e d t e m p e r a t u r e s
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