A new approach to the prediction of low-cycle fatigue data
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AND
J. B. CONWAY
L o w - c y c l e fatigue data for a n n e a l e d AISI Type 304, 316, and 348 s t a i n l e s s s t e e l to 816~ have been a n a l y z e d and a new r e l a t i o n s h i p has b e e n identified a s s o c i a t i n g fatigue data with s h o r t t e r m t e n s i l e b e h a v i o r . A l o g a r i t h m i c plot of p l a s t i c s t r a i n range v s t i m e to f r a c t u r e was shown to be l i n e a r with a slope of m i n u s unity. T h i s line was a l s o found to o r i g i n a t e at the point c o r r e s p o n d i n g to the t e n s i l e d u c t i l i t y . A s i m i l a r a n a l y s i s for e l a s t i c s t r a i n range data r e v e a l e d a r e l a t i o n s h i p with the s t r a i n - h a r d e n i n g exponent m e a s u r e d in a s h o r t - t e r m t e n s i l e t e s t at the s a m e t e m p e r a t u r e and s t r a i n rate. T h e s e c o r r e l a t i o n s have b e e n c o m b i n e d to p r o vide an equation for p r e d i c t i n g the r e l a t i o n s h i p between total s t r a i n r a n g e and c y c t e s to f r a c t u r e . T h i s approach has b e e n t e r m e d the Method of C h a r a c t e r i s t i c Slopes. A r e c e n t study 1 suggested a new c o r r e l a t i o n for p l a s t i c s t r a i n range data obtained in l o w - c y c l e fatigue t e s t s of s e v e r a l s t a i n l e s s s t e e l s to 650~ It was shown that a l o g a r i t h m i c plot of p l a s t i c s t r a i n range vs f r a c t u r e time was l i n e a r and exhibited a slope of m i n u s one. F u r t h e r m o r e , it was pointed out that this line p a s s e d through the point c o r r e s p o n d i n g to the t e n sile ductility (el = In[100/(100 - RA)] where RA is the p e r c e n t reduction in a r e a m e a s u r e d in a s h o r t - t e r m t e n s i l e test). Data for s e v e r a l s t a i n l e s s s t e e l s at t e m p e r a t u r e s to 650~ and for v a r i o u s s t r a i n r a t e s were shown to be c o n s i s t e n t with this approach. Data for i r r a d i a t e d AISI 316 s t a i n l e s s s t e e l were also found to be in e x c e l l e n t a g r e e m e n t with this new c o r r e l a tion p r o c e d u r e . B a s e d on the above c o r r e l a t i o n f e a t u r e s it was shown 1 that for all the fatigue data %3 for AISI 304, 316 and 348 s t a i n l e s s s t e e l to 650~ the p l a s t i c s t r a i n r a n g e , Aep, could be e x p r e s s e d as:
:
kT-/
[1]
where 4t is the s t r a i n r a t e , f i s the c y c l i n g f r e q u e n c y , and N f is the c y c l e s to f r a c t u r e . Since ~t = 2fAe t for a t r i a n g u l a r s t r a i n wave f o r m , where Ae t i s the total s t r a i n r a n g e , it follows from Eq. [1] that:
AEp A E t N f - s2
[2]
This r e l a t i o n s h i p s p e c i f i e s that the p r o d u c t of fatigue life, p l a s t i c s t r a i n r a n g e , and total s t r a i n range is c o n s t a n t for a given m a t e r i a l at a given t e s t condition. Once the t e n s i l e ductility is d e t e r m i n e d , the value
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