The effect of combined low-cycle fatigue and creep on the life of austenitic stainless steels
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THEvalidity and the applicability of the so-called Coffin-Manson relationship, Eq. [1], for low-cycle fatigue at low temperatures are well established.
Nf
is the n u m b e r of c y c l e s to f r a c t u r e , Aep is the p l a s tic s t r a i n r a n g e , ot and C a r e constants, c~ is u s u a l l y about ~- and it has been widely shown that C is given by the condition that A6p is e q u a l to the t e n s i l e ductility, s when Nf e q u a l s ~. When the t e m p e r a t u r e is r a i s e d into the v i c i n i t y of half the m e l t i n g t e m p e r a t u r e (TM/2) , t i m e - d e p e n d e n t f a c t o r s will get i n c r e a s i n g l y i m p o r t a n t . This i m p l i e s that the fatigue life will be a f f e c t e d by the f r e q u e n c y u s e d in the t e s t and by hold p e r i o d s introduced into the test. The g e n e r a l o b s e r v a t i o n is that the life of the s a m p l e e x p r e s s e d as n u m b e r of c y c l e s to f r a c t u r e d e c r e a s e s as the f r e q u e n c y d e c r e a s e s or the hold p e r i o d s increase. 1-s As a result low-cycle fatigue at high temperatures cannot be described by the simple Coffin-Manson relationship, at least not without modification. To cope with this problem several methods have been tried, most of which in fact are modifications of the Coffin-Manson relationship. One method makes use of the above mentioned relationship between C and Cf, but makes C a variable. This is determined by the particular value of cf for a creep test carried out at the temperature and for a time corresponding to the fatigue test. 3'8 a is assumed to be constant and equal to ~. The implication of this method is that the so-called fatigue ductility, r defined by Eq. [ I] with a = ~- and the condition that
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