Diffusion and deformation controlled creep crack growth
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AND
R. PILKINGTON
A m e c h a n i s m for c r e e p c r a c k growth is p r o p o s e d by which the c r a c k grows by f o r m a t i o n of g r a i n b o u n d a r y c a v i t i e s ahead of the c r a c k tip. Two c a s e s a r e c o n s i d e r e d ; f i r s t l y , when cavity growth is diffusion c o n t r o l l e d and secondly, where growth is d e f o r m a t i o n c o n t r o I l e d . The r e s u l t a n t c r a c k growth r a t e s p r e d i c t e d by these t h e o r e t i c a l models a r e c o m p a r e d with e x p e r i m e n t a l data.
1. INTRODUCTION
AN e x t e n s i v e a m o u n t of work has b e e n c a r r i e d
out on m e a s u r i n g e x p e r i m e n t a l c r e e p c r a c k growth r a t e s , in a n u m b e r of different e n g i n e e r i n g a l l o y s . l - l ~ The m a i n a i m of this work has b e e n to find the b e s t c o r r e l a t i o n b e t w e e n c r a c k growth r a t e s and v a r i o u s p a r a m e t e r s such as, s t r e s s i n t e n s i t y factor, net s e c t i o n s t r e s s , J i n t e g r a l , and C*, and a n u m b e r of r e v i e w s a r e now a v a i l a b l e on this topic, n'l~ One of the r e l a t i o n s h i p s f r e q u e n t l y found is V c = A K ~ where V c = c r a c k growth r a t e , K = s t r e s s i n t e n s i t y f a c t o r , and A and n a r e c o n s t a n t s . It is now r e a l i z e d 11 that this r e l a t i o n s h i p is only applicable to m a t e r i a l s which have a t e n d e n c y to exhibit low ductility d u r i n g c r e e p d e f o r m a t i o n , which may be t e r m e d the c r e e p b r i t t l e situation. U n f o r t u n a t e l y such c o r r e l a t i o n s t u d i e s r e v e a l v e r y little r e g a r d i n g the m i c r o s c o p i c a s p e c t s of the c r a c k growth p r o c e s s , and only a l i m i t e d n u m b e r of s t u d i e s have b e e n c o n c e r n e d with this a s p e c t . 5'1~ However r e c e n t l y , a t t e m p t s have b e e n made to t h e o r e t i c a l l y m o d e l the m e c h a n i s m of c r e e p c r a c k growth, ~4-2~ with the u l t i m a t e a i m of p r e d i c t i n g c r e e p c r a c k growth b e h a v i o r . Models p r o p o s e d so f a r tend to fall into two c a t e g o r i e s . F i r s t l y , those b a s e d on c o n s i d e r a t i o n of c r e e p d a m a g e that has f o r m e d ahead of the c r e e p c r a c k ; this may be i n t e r g r a n u l a r zs or i n t r a g r a n u l a r ~6 in c h a r a c t e r . Secondly, models which do not c o n s i d e r d a m a g e ahead of the c r a c k but i n s t e a d a s s u m e that growth o c c u r s by a v a c a n c y diffusion m e c h a n i s m r e s u l t i n g in a v a c a n c y flux to the c r a c k tip.iV-Z~ Most of the e x p e r i m e n t a l studies of the f r a c t u r e m e c h a n i s m s of c r e e p c r a c k growth mentioned above r e v e a l that the growth of a c r e e p c r a c k i s d i r e c t l y r e l a t e d to the b u i l d - u p of c a v i t a t i o n damage in the p l a s t i c zone ahead of the
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