Kinetics of environmental fatigue crack growth in a nickel-copper alloy: Part II. In hydrogen
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S. PURUSHOTHAMAN, AND O. BUCK
J. K. TIEN,
A s y s t e m a t i c m a t r i x of f a ti g u e c r a c k g r o w t h r a t e data in a h y d r o g e n e n v i r o n m e n t has b e e n g e n e r a t e d in a n i c k e l - c o p p e r a l l o y and c o m p a r e d with the b a s e line data in the m i l d e r oxygen and v a c u u m e n v i r o n m e n t s d e s c r i b e d in the p r e c e d i n g p a p e r . It is found that c r a c k growth in the h y d r o g e n e n v i r o n m e n t is c h a r a c t e r i z e d by high v a l u e s of the P a r i s e q u a t i o n exponent and f a s t e r c r a c k p r o p a g a t i o n r a t e s as c o m p a r e d to t h o s e in the m i l d e r e n v i r o n m e n t s . F r a c t o g r a p h i c e x a m i n a t i o n shows that b r i t t l e i n t e r g r a n u l a r s e p a r a t i o n o c c u r s s u p e r i m p o s e d on an o t h e r w i s e d u ct i l e c r a c k mode in the h y d r o g e n t e s t s . Q u a n t i t a t i v e f r a c t o g r a p h i c a n a l y s i s of the h y d r o g e n a f f e c t e d f r a c t u r e s u r f a c e s i n d i c a t e s that the p e r c e n t a g e of i n t e r g r a n u l a r f a i l u r e ( a r e a f r a c t i o n ) is a uniquely r e l a t e d function of the m e a n s t r e s s i n t e n s i t y r a t h e r than the m a x i m u m s t r e s s i n t e n s i t y l e v e l of f at i g u e loading. T h i s d e p e n d e n c e is d i s c u s s e d in t e r m s of d i s l o c a t i o n s w e e p - i n t r a n s p o r t of hyd r o g e n deep into the p l a s t i c zone d u r i n g fatigue c y c l i n g .
IN P a r t
11 of this t w o - p a r t p a p e r we d e t e r m i n e d , in v a c u u m and oxygen, the f u n c t i o n a l d e p e n d e n c e of fatigue c r a c k growth r a t e s on v a r i o u s loading p a r a m e t e r s f o r M o n el K- 5 0 0 * . F o r e x a m p l e , it was found that in
s p e c i m e n s e x h i b i t e d a r e d u c t i o n in a r e a d e c r e a s e f r o m 45 to 2 pct. Th e e x p e r i m e n t a l p r o c e d u r e followed in this w o r k was as d e s c r i b e d in P a r t I of this p a p e r .1
*RegisteredTrademark, The International NickelCompany,Inc. both e n v i r o n m e n t s the growth r a t e s a r e s e n s i b l y independent of the s t r e s s i n t e n s i t y r a t i o R = K m i n / K max. H o w e v e r , the fatigue c r a c k growth r a t e s f o r a g i v en applied s t r e s s i n t e n s i t y r a n g e w e r e found to be h i g h e r in oxygen than in v a c u u m , although no b r i t t l e m o d e s , s u p e r i m p o s e d on the d u c t i l e f r a c t u r e s u r f a c e , o c c u r r e d in e i t h e r e n v i r o n m e n t . F u r t h e r , the P a r i s eq u ation exponent, m, was d e t e r m i n e d to be about 2.2 in oxygen and 3.4 in v a c u u m when data g e n e r a t e d at a l l R v a l u e s w e r e c o n s i d e r e d . B a s e d on f r a c t o g r a p h i c obs e r v a t i o n s , it was s u g g e s t e d that the o b s e r v e d b e h a v i o r in oxygen and v a c u u m cou
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