A critical strain model for the creep fracture of nickel-base superalloys
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AND
R. H. KANE
F r a c t u r e t o u g h n e s s s a m p l e s of NIMONIC 115 w e r e c r e e p t e s t e d at 704~ in Mode I (tension) and Mode III (torsion) loading. In Mode III loading the r u p t u r e lives were two o r d e r s of magnitude s h o r t e r than in Mode I. T h e effects of loading mode a r e shown to a g r e e with p r e d i c t i o n s b a s e d on a c r i t i c a l s t r a i n f r a c t u r e model. E a r l i e r t e s t r e s u l t s with a n u m b e r of different s u p e r a l l o y s also a r e c o n s i s t e n t with a s t r a i n c o n t r o l l e d f r a c t u r e model. I m proved r e s i s t a n c e to c r a c k growth d u r i n g c r e e p at i n t e r m e d i a t e t e m p e r a t u r e s can be achieved by i n c r e a s i n g Young's m o d u l u s , yield strength, g r a i n s i z e and the c r i t i c a l s t r a i n value. C R A C K growth and f r a c t u r e d u r i n g c r e e p loading have long been topics of c o n s i d e r a b l e i n t e r e s t . In the past s e v e r a l y e a r s , f r a c t u r e m e c h a n i c s t e c h n i q u e s have begun to be used to study t h e s e p r o b l e m s .1-9 R e cently, the f r a c t u r e 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 loys in the t e m p e r a t u r e r a n g e 500 to 800~ was exa m i n e d u s i n g fatigue p r e c r a c k e d compact t e n s i o n s a m p l e s , t~ I n t e r g r a n u l a r c r e e p c r a c k growth at low s t r e s s i n t e n s i t i e s was found to p r o c e e d by a p r o c e s s i n v o l v i n g m i c r o c r a c k f o r m a t i o n on g r a i n b o u n d a r i e s just ahead of the m a i n c r a c k . The g e n e r a l r e s u l t s s u g g e s t e d that f r a c t u r e m e c h a n i c s could be u s e d to a n a l y z e these f r a c t u r e s . T h i s a s s u m p t i o n was s u p p o r t e d by a l a t e r study showing that the c r e e p f r a c t u r e c h a r a c t e r i s t i c s of one of these alloys was unaffected by s a m p l e t h i c k n e s s in sheet s a m p l e s as thin as 0.5 m m . n While the e a r l i e r work identified some of the m e t a l l u r g i c a l p a r a m e t e r s that were i m p o r t a n t , no m o d e l was p r e s e n t e d to e x p l a i n the wide v a r i a t i o n s i n c r a c k i n g s u s c e p t i b i l i t y . In this study s a m p l e s of a n i c k e l - b a s e s u p e r a l l o y having the s a m e t e s t s e c t i o n g e o m e t r y w e r e creep t e s t e d by e i t h e r Mode I (tension) or Mode llI (torsion) loading. The r e s u l t s a r e shown to be c o n s i s t e n t with the c r i t i c a l s t r a i n f r a c t u r e c r i t e r i a developed by M c C l i n t o c k and I r w i n ~z and G e r b e r i c h ? 3 Some of the e a r l i e r t e s t r e s u l t s obtained with a n u m b e r of s u p e r a l l o y s a r e then e x a m i n e d i n light of this model. EXPERIMENTAL PROCEDURE AND RESULTS T e s t s a m p l e s w e r e t a k e n f r o m an extruded s e c t i o n of NIMONIC| 115 that had been heat t r e a t e d
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