strengthening and fracture in fatigue (approaches for achieving high fatigue strength)
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T H E c o r r e l a t i o n b e t w e e n fatigue s t r e n g t h and u l t i m a t e t e n s i l e s t r e n g t h , F i g . 1, p r o b a b l y r e p r e s e n t s the f i r s t r e l a t i o n s h i p to b e found b e t w e e n fatigue r e s i s t a n c e and a controllable, material parameter. This correlation, h o w e v e r , d o e s not p r o v i d e an a b s o l u t e g u i d e l i n e to t h e d e v e l o p m e n t of f a t i g u e - r e s i s t a n t m a t e r i a l s , b e c a u s e t h e r e a r e s o m e n o t a b l e e x c e p t i o n s (e.g., high s t r e n g t h a l u m i n u m a l l o y s ) and b e c a u s e such f a c t o r s such a s notch s e n s i t i v i t y , e n v i r o n m e n t a l a t t a c k , and b e h a v i o r u n d e r high c y c l i c s t r e s s a m p l i t u d e s a r e not included in the c o r r e l a t i o n . N o r i s t h e p o s s i b i l i t y t a k e n into a c c o u n t t h a t a m a t e r i a l m a y b e r e q u i r e d to r e s i s t c y c l i c strain r a t h e r than c y c l i c s t r e s s - - t h a t i s , w h e t h e r the m a t e r i a l is to be u s e d in a c o m p l i a n t r a t h e r than a stiff s t r u c t u r e . H e n c e , it h a s b e e n n e c e s s a r y f o r t h e fatigue s p e c i a l i s t to s e a r c h f o r b e t t e r i n d i c e s of fatigue r e s i s t a n c e in o r d e r f o r r e a l p r o g r e s s to be m a d e in materials development and selection. T h e s e a r c h f o r m e a n i n g f u l g u i d e l i n e s to fatigue r e s i s t a n c e h a s b e e n g r e a t l y a i d e d by s e v e r a l d e v e l o p ments during the past decade. First, extensive res e a r c h into the m e c h a n i s m s of fatigue h a s p r o v i d e d the b a s i c u n d e r s t a n d i n g n e c e s s a r y f o r d e s c r i b i n g f a t i g u e in t e r m s of s p e c i f i c s , such a s h a r d e n i n g / s o f t e n ing, c r a c k i n i t i a t i o n , and c r a c k p r o p a g a t i o n . T h i s k n o w l e d g e h a s a l s o p r o v i d e d m o d e l s on which t h e o r i e s of f a t i g u e c a n b e b u i l t - - t h e o r i e s which r e l a t e f a t i g u e to macroscopic material parameters. Mechanisms res e a r c h h a s i d e n t i f i e d m i c r o s t r u c t u r a l f e a t u r e s which a r e i m p o r t a n t to fatigue f a i l u r e ; f o r e x a m p l e , s l i p mode, inclusions, second phases. A second important d e v e l o p m e n t h a s b e e n t h e e m e r g e n c e of new o b s e r v a t i o n a l t e c h n i q u e s such a s m i c r o p r o b e a n a l y s i s and t r a n s m i s s i o n and s c a n n i n g e l e c t r o n m i c r o s c o p y . T h i r d , new c l o s e d - l o o p fatigue t e s t i n g m a c h i n e s have f r e e d e x p e r i m e n t e r s f r o m the c o n s t r a i n t s of c o n s t a n t load, c o n s t a n t d e f l e c t i o n , and r e s o n a n t m a c h i n e s . With t h i s new f r e e d o m h a s c o m e t h e a b i l i t y to c h a r a c t e r i z e the J. C. GROSSKREUTZ, formerly with Mid
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