Effect of microstructure on notch fatigue properties of Ti-6Al-4V
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is g e n e r a l l y held that the s m o o t h fatigue b e h a v i o r of a l l o y s is m o r e s e n s i t i v e to c h a n g e s in m i c r o s t r u c t u r e than notched fatigue. ~ The r e a s o n given for this o b s e r v a t i o n is the fact t h a t d u r i n g the i n i t i a t i o n s t a g e of the fatigue c r a c k , the p l a s t i c z o n e a t the front of the c r a c k in a s m o o t h s p e c i m e n is s m a l l e r than that in a notched s p e c i m e n and t h e r e f o r e m o r e s e n s i t i v e to m i c r o s t r u c t u r a l f e a t u r e s . Since, in t i t a n i u m a l l o y s , it i s p o s s i b l e to o b t a i n m a n y d i f f e r e n t m i c r o s t r u c t u r e s in a s i n g l e a l l o y through d i f f e r i n g c o m b i n a t i o n s of p r o c e s s i n g and h e a t t r e a t i n g (Hall e t a l ) , ~ the s m o o t h f a t i g u e b e h a v i o r of t i t a n i u m should be v e r y much d e p e n dent on the p r i o r h i s t o r y of the m a t e r i a l . The fact that p r o c e s s i n g and h e a t t r e a t i n g can c o n t r o l the s m o o t h fatigue b e h a v i o r of t i t a n i u m through c h a n g e s in m i c r o s t r u c t u r e , h a s i n d e e d b e e n shown b y Bowen and Stubbington,~'4's by L u c a s , 6'~ and by M a r g o l i n , e t a l , 8 In addition, e x t e n s i v e e x a m i n a t i o n of the fatigue b e h a v i o r of c o m p o n e n t s and j o i n t s in a i r f r a m e s by C r i e h low ~176showed that, unlike s t e e l s o r a l u m i n u m a l l o y s , t i t a n i u m a l l o y s showed s i g n i f i c a n t s c a t t e r in t h e i r f a tigue b e h a v i o r . In fact the s c a t t e r f o r the t i t a n i u m a l l o y s i s a l m o s t two t i m e s g r e a t e r than that of a l u m i num a l l o y s and t h r e e t i m e s g r e a t e r than that of s t e e l s . This p r o p o r t i o n w a s found to b e a p p r o x i m a t e l y the s a m e for both c o n s t a n t a m p l i t u d e and s p e c t r u m l o a d ing t e s t c o n d i t i o n s . T h e s e o b s e r v a t i o n s on c o m p o n e n t s w e r e s u p p o r t e d by t e s t d a t a on notched coupons and the s a m e c o n c l u s i o n w a s r e a c h e d in w o r k done b y W h i t t a k e r . ~ ' ~a The p u r p o s e of t h i s i n v e s t i g a t i o n w a s to d e t e r m i n e the d e g r e e to which m i c r o s t r u c t u r a l m o d i f i c a t i o n s a f fect the notch fatigue p r o p e r t i e s of the m o s t c o m m o n l y used t i t a n i u m a l l o y in a i r f r a m e c o n s t r u c t i o n , T i - 6 A 1 4 u M i c r o s t r u c t u r a l v a r i a t i o n s in a c t u a l p r a c t i c e can be due not only to d i f f e r i n g p r o c e s s i n g / h e a t t r e a t m e n t p r o c e d u r e s , but a l s o due to s e c t i o n s i z e e f f e c t s in a s i n g l e component. Bowen and Stubbington, ~ for e x a m ple, showed that s p e c i m e n s f r o m d i f f e r e n t l o c a t i o n s in a l a r g e Ti-6A1-4V s e c t i o n gave
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