Surface residual stresses, surface topography and the fatigue behavior of Ti-6AI-4V
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T H E s i g n i f i c a n t r o l e that s u r f a c e s p l a y i n d e t e r m i n i n g the f a t i g u e p r o p e r t i e s o f m e t a l s a n d a l l o y s h a s b e e n r e c o g n i z e d f o r s o m e t i m e . T h i s h a s l e d to t h e d e v e l o p m e n t o f s u r f a c e t r e a t m e n t s (e.g., s h o t p e e n i n g ) to i m p r o v e fatigue r e s i s t a n c e as well as i n n u m e r a b l e i n v e s t i g a t i o n s d i r e c t e d a t a n e v a l u a t i o n of the i n f l u e n c e of m a c h i n i n g p a r a m e t e r s on f a t i g u e p r o p e r t i e s . T h e s e s t u d i e s h a v e i d e n t i f i e d c e r t a i n f a c t o r s , 1-4 s u c h a s s u r f a c e t o p o g r a p h y a n d r e s i d u a l s t r e s s , that a r e i m p o r t a n t in c o n t r o l l i n g fatigue b e h a v i o r . T h e r e is l i t t l e o r no q u a n t i t a t i v e i n f o r m a t i o n , h o w e v e r , o n the r e l a t i v e i m p o r t a n c e of t h e s e f a c t o r s on f a t i g u e l i f e , n o r is t h e r e s u f f i c i e n t u n d e r s t a n d i n g o f t h e s t a b i l i t y of s u r f a c e r e s i d u a l s t r e s s e s u n d e r c y c l i c l o a d i n g a n d / or elevated temperature exposure. As component o p e r ating s t r e s s e s i n c r e a s e and critical flaw s i z e s correspondingly d e c r e a s e , improved nondestructive inspection methods are continuaUy b e i n g developed t o d e t e c t fatigue c r a c k s at a n e a r l y s t a g e in t h e i r e x i s t e n c e . T h e s e s h a l l o w c r a c k s a r e so m a r k e d l y i n f l u e n c e d by s u r f a c e p r o p e r t i e s t h a t p r e d i c t i o n of c r a c k a d v a n c e r e q u i r e s a c o m p l e t e u n d e r s t a n d i n g of t h e i n t e r a c t i o n o f s u r f a c e t o p o g r a p h y a n d r e s i d u a l s t r e s s w i t h the m i c r o c r a c k . For example, with a f r a c t u r e mechanics app r o a c h , the r e s i d u a l s t r e s s m u s t be t a k e n into a c c o u n t either explicitly through its influence o n effective s t r e s s intensity r a n g e or indirectly through its effect on stress r a t i o ? T h e w o r k r e p o r t e d in t h i s p a p e r is a n i n i t i a l a t t e m p t to i s o l a t e t h e r e l a t i v e i m p o r t a n c e o f s u r f a c e t o p o g r a p h y a n d s u r f a c e r e s i d u a l s t r e s s on t h e f a t i g u e p r o p e r t i e s of ~ / f l f o r g e d T i - 6 A 1 - 4 V . I t a l s o i n c l u d e s d i r e c t m e a s u r e m e n t of t h e s t a b i l i t y o f s u r f a c e r e s i d u a l s t r e s s e s u n d e r cyclic loading, elevated temperature exposure G. R. LEVERANT is Manager, Metallurgy, Southwest Research Institute, San Antonio, Texas, B. S. LANGER is with United Nuclear Corporation, Uncasville, Connecticut, A. YUEN is with Albany Medical College, Albany, New York, and S. W. HOPKINS is with Failure Analysis Associates, Palo Alto, California. T h e work was performed when all authors were affiliated wit
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