The creep fracture of wrought nickel-base alloys by a fracture mechanics approach
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T H E c r e e p b e h a v i o r of n i c k e l - b a s e s u p e r a l l o y s has long b e e n a s u b j e c t of i n t e r e s t . In p a r t i c u l a r the p r o p e r t i e s of notched s a m p l e s often has b e e n of c o n c e r n . However, the e a r l i e r s t u d i e s of the notch r u p t u r e p r o p e r t i e s of n i c k e l - b a s e alloys have always employed m a chine notched s a m p l e s . While t e s t s of this kind a r e adequate for r e v e a l i n g d i f f e r e n c e s between c o m p o s i tions, heat t r e a t m e n t s , etc., they do not lend t h e m s e l v e s to f r a c t u r e m e c h a n i c s a n a l y s i s . D u r i n g the past decade f r a c t u r e m e c h a n i c s p r o c e d u r e s have b e c o m e e s t a b l i s h e d for d e a l i n g with m a n y f r a c t u r e p r o b l e m s . Within the past y e a r or so, i n c r e a s i n g a t t e n t i o n has been focused on the use of f r a c t u r e m e c h a n i c s t e s t t e c h n i q u e s for the study of c r e e p f r a c t u r e . 1-~ The o b j e c t i v e of the p r e s e n t work was to study the c r e e p f a i l u r e c h a r a c t e r i s t i c s of n i c k e l - b a s e alloys by a f r a c t u r e m e c h a n i c s approach. Six d i f f e r e n t wrought alloys w e r e t e s t e d in the t e m p e r a t u r e r a n g e 500 to 750~ The i n t e n t was not to e x a m i n e a s i n g l e alloy in detail, but to s u r v e y the p r o p e r t i e s of a n u m b e r of r e p r e s e n t a t i v e alloys in o r d e r to develop an o v e r a l l p i c t u r e of the f r a c t u r e b e h a v i o r of t h e s e types of m a t e r i a l s . Included in this study a r e the g e n e r a l c r a c k growth c h a r a c t e r i s t i c s , the effects of p r o c e s s i n g and heat t r e a t m e n t v a r i a t i o n s , and the r e l a t i o n s h i p between the p r o p e r t i e s of notched and smooth s p e c i m e n s .
p e r a t u r e had s t a b i l i z e d the load was applied through the l e v e r a r m s y s t e m of the c r e e p stand. Although the t e s t s a m p l e was r e l a t i v e l y s m a l l , in all c a s e s the loads w e r e such that the t h i c k n e s s t of the s a m p l e obeyed the r e l a t i o n s h i p :
where K is the i n i t i a l s t r e s s i n t e n s i t y and Y the yield s t r e n g t h ; i.e., the ASTM c r i t e r i o n for plane s t r a i n loading was obeyed. In addition, d u r i n g slow c r a c k growth the f r a c t u r e was v e r y flat with no s h e a r lip f o r m a t i o n . D i m e n s i o n a l m e a s u r e m e n t s after t e s t i n g showed no p l a s t i c flow in the t h i c k n e s s d i r e c t i o n . T h e r e f o r e it is b e l i e v e d that plane s t r a i n loading c o n ditions w e r e achieved d u r i n g slow c r a c k growth. The r e s u l t s w e r e e x a m i n e d by both s t r e s s i n t e n s i t y and net s e c t i o n s t r e s s c a l c u l a t i o n s . In g e n e r a l much m o r e c o n s i s t e n t r e s u l t s w e r e obtained u
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