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1. INTRODUCTION C O M P O N E N T S of v a r i o u s m a c h i n e s and s t r u c t u r e s such as p r e s s u r e v e s s e l s for c o n v e n t i o n a l a s well as n u c l e a r power p l a n t s , a r e subjected to r e p e a t e d t h e r m a l s t r e s s e s r e s u l t i n g f r o m s t a r t u p , shutdown, or power v a r i a t i o n s d u r i n g the o p e r a t i o n s . The p r o b l e m s of high t e m p e r a t u r e l o w - c y c l e fatigue b e h a v i o r s a r e of g r e a t i m p o r t a n c e to the s t r e n g t h d e s i g n . U s u a l l y the fatigue life of the m a t e r i a l s is used as a c r i t e r i o n of the s t r e n g t h d e s i g n . 1 On the other hand, the data of the c r a c k p r o p a g a t i o n r a t e a r e also needed b e c a u s e the data of the fatigue life gives no i n f o r m a t i o n on the r e l a t i v e c y c l e s spent in c r a c k i n i t i a t i o n or p r o p a g a tion and on the r e m n a n t life of the component once c r a c k s have i n i t i a t e d . U n d e r h i g h - c y c l e fatigue conditions, c r a c k p r o p a g a tion b e h a v i o r has been a c t i v e l y i n v e s t i g a t e d for m a n y m a t e r i a l s s i n c e 1963, when P a r i s e t al 2 r e p r e s e n t e d the c r a c k p r o p a g a t i o n r a t e in t e r m s of the s t r e s s i n t e n s i t y f a c t o r . Under s t r a i n c o n t r o l l e d l o w - c y c l e fatigue conditions, however, the n o m i n a l s t r e s s is u s u a l l y far above the g e n e r a l yield s t r e s s and l i n e a r f r a c t u r e m e c h a n i c s is no l o n g e r applicable, b e c a u s e the applied cyclic s t r a i n r a n g e is l a r g e . To apply f r a c t u r e m e c h a n i c s to g e n e r a l yielding conditions, p a r a m e t e r s such as c r a c k opening d i s p l a c e m e n t or J - i n t e g r a l a r e p r o p o s e d . 3 It is v e r y difficuR to m e a s u r e t h e s e p a r a m e t e r s e x p e r i m e n t a l l y , and so t h e r e has not yet b e e n m a n y s t u d i e s in which these p a r a m e t e r s a r e applied to the s t r a i n c o n t r o l l e d l o w - c y c l e fatigue c r a c k propagation r a t e s . T o m k i n s 4 d e r i v e d a c r a c k p r o p a g a tion r a t e equation in s t r a i n c o n t r o l l e d l o w - c y c l e fatigue of ductile m e t a l s . B o e t t n e r e t a l ~ proposed the s t r a i n i n t e n s i t y f a c t o r in p l a c e of the s t r e s s i n t e n s i t y f a c t o r . In high t e m p e r a t u r e l o w - c y c l e fatigue, the s t r a i n - r a t e or hold t i m e effects of the fatigue life a r e a c t i v e l y i n v e s t i g a t e d , but t h e r e is not enough data on the c r a c k propagation rates. T h e r e a r e many methods a v a i l a b l e to m e a s u r e the fatigue c r a c k p r o p a g a t i o n r a t e s . In this study it is not p o s s i b l e to m e a s u r e the p r o p a g a t i o n r a t e of the s u r f a c e c r a c k of the s p e c i m e n , b e c a u s e the s p e c i m e n