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An analysis is p r e s e n t e d which enables c r a c k propagation r a t e s under cyclic loading condiditions to be predicted f r o m static slow c r a c k growth p a r a m e t e r s . A c o m p a r i s o n of the predicted t i m e s to failure under c y c l i c conditions with available m e a s u r e d failure t i m e s , f o r s e v e r a l c e r a m i c m a t e r i a l s at ambient t e m p e r a t u r e s , suggests that there is no significant enhancement of the slow c r a c k growth r a t e due to cycling. This is v e r i f i e d in a s e r i e s of m e a s u r e m e n t s of slow c r a c k growth r a t e s under static and cyclic conditions. F O R static loading of metallic m a t e r i a l s , slow c r a c k growth can o c c u r , for example, due to s t r e s s c o r r o sion* and h y d r o g e n embrittlement, z For these p r o c e s ses, the c r a c k velocity ( d a / d t ) during c r a c k p r o p a g a tion depends p r i m a r i l y on the s t r e s s intensity factor, K I , and it is frequently o b s e r v e d that, s

where A and n a r e s y s t e m constants which depend only on the e n v i r o n m e n t and the t e m p e r a t u r e . When cyclic loading is imposed, an independent s o u r c e of slow c r a c k growth can function. This mode of c r a c k propagation depends p r i m a r i l y on the amplitude of the cycle, AKI, such that the c r a c k growth per cycle, d a / d N , is f r e quently given for a limited range of LxKI by, 4,s'e'7 _- B

I2]

where B and n* a r e s y s t e m constants. In c e r a m i c m a t e r i a l s , the existence of static (environmentally dependent) slow c r a c k growth is now well established, e,9 The r a t e of c r a c k propagation in the r e gion of p r a c t i c a l i n t e r e s t is d e t e r m i n e d exclusively by the s t r e s s intensity factor, K I , as it is for metallic m a t e r i a l s (Eq. [1]). Slow c r a c k growth under cyclic loading conditions is also observed, z~ But, it has f r e quently been suggested that t h e r e is no enhanced effect of cycling on the c r a c k propagation rate. It is intended in this paper to develop relations which p r e d i c t the c r a c k growth r a t e (or time to failure) under cyclic loading conditions f r o m the static slow c r a c k growth p a r a m e t e r s . This can then be c o m p a r e d with e x p e r i mental m e a s u r e m e n t s of c r a c k growth r a t e under c y clic conditions to establish whether t h e r e is an enhanced effect on the c r a c k growth r a t e due to cycling. The only available e x p e r i m e n t a l m e a s u r e m e n t s under c3~clic loading a r e t i m e - t o - f a i l u r e m e a s u r e m e n t s , Refs. 10 to 12. These a r e subject to considerable statistical variation and it is difficult to obtain effective c o m p a r i s o n s with static m e a s u r e m e n t s . A technique for obtaining m o r e p r e c i s e c o m p a r i s o n s is thus developed. It is b a s e d on c r a c k velocity m e a s u r e m e n t s for both static and cycli c loading situations.