Fatigue of a glass bead blasted nickel-base superalloy
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T H I S i n v e s t i g a t i o n was u n d e r t a k e n to evaluate the r e l a t i v e and combined e f f e c t s of g l a s s bead b l a s t i n duced r e s i d u a l s t r e s s a n d s u r f a c e cold work on the high cycle (10s to l0 T cycle) fatigue s t r e n g t h of the wrought n i c k e l - b a s e s u p e r a l l o y U d i m e t 700. Room t e m p e r a t u r e fatigue c r a c k i n i t i a t i o n and p r o p a g a t i o n w e r e thus studied in e l e c t r o p o l i s h e d and in g l a s s b e a d blasted material. E X P E R I M E N T A L PROCEDURE All t e s t s p e c i m e n s , except the cold swaged s a m p l e s to be d e s c r i b e d below, w e r e m a c h i n e d from wrought U d i m e t 700* in e i t h e r the fully heat t r e a t e d t or solu*Nominalcomposition:Co, 18.5 pct;Cr, 15.0pct;Mo, 5.0 pct;A1,4.3 pct; Ti, 3.3 pct;C, 0.07 pct, B, 0.03 pct; N1,balance. tFull heat treat: 2140~ hr + 1975~ hr + 1550~ hr + 1400~ hr, forced air coolafter eachstep.
5 • 105 cycle i n t e r v a l s d u r i n g t e s t i n g ; if no fatigue damage was noted after s e v e r a l m i l l i o n c y c l e s , the s t r e s s l e v e l was i n c r e a s e d . When an active fatigue m i c r o c r a c k could be positively i d e n t i f i e d , u s u a l l y by noting a change in length when c o m p a r e d with p r e c e d ing or s u b s e q u e n t s u r f a c e r e p l i c a s , f a i l u r e was cons i d e r e d to have o c c u r r e d , and the s p e c i m e n was u s u a l l y r u n to complete f r a c t u r e . In s e v e r a l c a s e s the a v e r a g e c r a c k propagation r a t e s were noted. Fatigue s t r e n g t h of electropolished m a t e r i a l was taken a s the s t r e s s r e q u i r e d to i n i t i a t e a p r o p a g a t i n g c r a c k at a p p r o x i m a t e l y two m i l l i o n c y c l e s . In addition to d e t e r m i n i n g fatigue s t r e n g t h s , s o m e s a m p l e s were cycled at higher s t r e s s l e v e l s to d e t e r m i n e c r a c k p r o p a g a t i o n r a t e as a function of s t r e s s a m p l i t u d e
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tionized* condition. The g r a i n size a f t e r heat t r e a t *Solutlomze:2140~
hr, oil quench.
m e n t a v e r a g e d 0.009 in. d i a m . T e s t s p e c i m e n s were of t h r e e types: fiat c a n t i l e v e r r e v e r s e d b e n d i n g , axial t e n s i o n , and c o n s t a n t m o m e n t r o t a t i n g b e a m , Fig. 1. In addition, one s p e c i m e n with gage s e c t i o n d i m e n s i o n s of 0.375 in. diam by 1.00 in. long was t e s t e d in a x i a l c o m p r e s s i o n in o r d e r to d e t e r m i n e the effect of a c o m p r e s s i v e m e a n s t r e s s on fatigue c r a c k i n i t i a t i o n and propagation. All t e s t i n g was conducted at r o o m temperature. In o r d e r to obtain a cold worked s t r u c t u r e without the high r e s i d u a l s t r e s s which a c c o m p a n i e s g l a s s b e a d b l a s t i n g , a b a r of U d i m e t 700 was swaged at r o o m t e m p e
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