Frequency dependent low cycle fatigue crack propagation
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y e a r s high t e m p e r a t u r e low c y c l e fatigue h a s b e c o m e an i n c r e a s i n g l y i m p o r t a n t d e s i g n l i m i t a tion. An i m p o r t a n t a s p e c t of this p r o b l e m is the influe n c e of the c y c l i n g f r e q u e n c y . The u s e of high f r e quency l a b o r a t o r y t e s t s r e q u i r e s that f r e q u e n c y e f f e c t s be c o n s i d e r e d in o r d e r to m a k e the n e c e s s a r y e x t r a p o l a t i o n to the s e r v i c e t i m e s of i n t e r e s t . The influence of f r e q u e n c y is p a r t i c u l a r l y i m p o r t a n t at elevated temperatures because time dependent phen o m e n a such a s c r e e p and oxidation can have a p r o found influence on fatigue b e h a v i o r . One a p p r o a c h for p h e n o m e n o l o g i c a l l y d e s c r i b i n g the influence of f r e q u e n c y v a r i a t i o n s has b e e n to u s e a f r e q u e n c y m o d i f i e d low c y c l e fatigue law. Coffin h a s s h o w n 1-3 that the C o f f i n - M a n s o n low c y c l e fatigue e x p r e s s i o n can be m o d i f i e d to include the e f f e c t s of frequency, i.e., Aep(Nf Uk-1)/3 = c o n s t a n t [1] w h e r e A~p iS the p l a s t i c s t r a i n r a n g e of the t e s t , Nf i s the n u m b e r of c y c l e s to f a i l u r e , v the c y c l i c f r e q u e n c y , k is a f r e q u e n c y c o n s t a n t , and /3 i s the u s u a l C o f f i n - M a n s o n exponent. T h i s r e l a t i o n s h i p has been shown to be obeyed by quite a n u m b e r of m a t e r i a l s . One of the m o s t e x t e n s i v e l y s t u d i e d w a s A286, an i r o n - b a s e s u p e r a l l o y with wide r a n g i n g high t e m p e r a t u r e a p p l i c a t i o n s . Coffin has shown 3'4 that at l l 0 0 ~ in a i r , k = 0.65, which d e n o t e s a r a t h e r s t r o n g f r e quency e f f e c t . In a v a c u u m for v > 0.1 c p m , h o w e v e r , no f r e q u e n c y influence was noted (i.e., k = 1). T h i s i l l u s t r a t e s the i m p o r t a n c e of the e n v i r o n m e n t in d e t e r m i n i n g the f r e q u e n c y d e p e n d e n c e . Studying the f r e q u e n c y d e p e n d e n c e of the low c y c l e fatigue life d o e s not allow for a s e p a r a t i o n of n u c l e a tion and p r o p a g a t i o n p h e n o m e n a . The e x p e r i m e n t s d e s c r i b e d h e r e w e r e u n d e r t a k e n to p r o v i d e e x p l i c i t i n f o r m a t i o n about the influence of the c y c l i n g f r e q u e n c y on the c r a c k p r o p a g a t i o n r a t e . The u s e of c r a c k g r o w t h i n f o r m a t i o n for d e s i g n p u r p o s e s m a k e s such an a p p r o a c h quite n e c e s s a r y . One a d v a n t a g e of such an a p p r o a c h is that c o m p a r a t i v e l y few c y c l e s a r e r e q u i r e d to e s t a b l i s h the c r a c k g r o w t h r a t e , thus enabling the H. D. SOLOMON is Staff Member, Metallurgy and Ceramics Laboratory, Corporate Research and Development, General Ele
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