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AFTERt h e r m o m e c h a n i c a l p r o c e s s i n g ,

certain m e t a s t a b l e a u s t e n i t i c s t e e l s have b e e n found to y i e l d u n u s u a l c o m b i n a t i o n s of s t r e n g t h , d u c t i l i t y , and toughn e s s 1-5 T h i s p a r t i c u l a r c l a s s of s t e e l s has b e e n g i v e n the d e s i g n a t i o n " T R I P " , which is an a c r o n y m f o r T R a n s f o r m a t i o n Induced _Plasticity. It has b e e n shown that the high t o u g h n e s s and d u c t i l i t y a s s o c i a t e d with t h e s e s t e e l s a r e due to the t r a n s f o r m a t i o n of m e t a s t a b l e a u s t e n i t e to m a r t e n s i t e d u r i n g the t e n s i l e t e s t i n g at t e m p e r a t u r e s below M D .1-6 A t a t e s t t e m p e r a t u r e below M D t h e r e is a c r i t i c a l s t r a i n or s t r e s s * beyond which the p a r e n t a u s t e n i t e *Whether the transformation is strata-induced or stress-induced from a fundamental point of viewis as yet unanswered. It is commonto refer to the transformation as strain-inducedwhenit occursafter yieldingof the austemte and stressinduced when it occurs beforeyieldingof the austemte. p a r t i a l l y t r a n s f o r m s to m a r t e n s i t e . T h e t r a n s f o r m a tion to m a r t e n s i t e c a u s e s s t r e n g t h e n i n g of the d e f o r m e d r e g i o n . B e c a u s e of t h i s , p l a s t i c i n s t a b i l i t y is p r e v e n t e d and the u n i f o r m elongation is i n c r e a s e d . T h e b e h a v i o r of t h e s e s t e e l s u n d e r monotonic loading has been studied e x t e n s i v e l y as a function of t e s t t e m p e r a t u r e ,2,s,7 and t h e r m o m e e h a n i c a l p r o e e s sing.l'2's In the p r e s e n t i n v e s t i g a t i o n , the fatigue c r a c k p r o pagation c h a r a c t e r i s t i c s of s e v e r a l T R I P s t e e l s w e r e studied as a function of the a u s t e n i t e s t a b i l i t y by c o r r e l a t i n g the r a t e of fatigue c r a c k p r o p a g a t i o n to the s t r e s s i n t e n s i t y p a r a m e t e r . F o r this p u r p o s e , the alloy c o m p o s i t i o n was c h o s e n as to have the Ms t e m p e r a t u r e w e l l b e lo w r o o m t e m p e r a t u r e and the M D t e m p e r a t u r e above r o o m t e m p e r a t u r e a f ter thermomechanical processing. 1 T h e f r a c t u r e m e c h a n i c s a p p r o a c h has b e e n w i d e l y u s e d to a n a l y z e fatigue c r a c k p r o p a g a t i o n 8-x4 and it is g e n e r a l l y found that the c r a c k p r o p a g a t i o n r a t e and s t r e s s i n t e n s i t y f a c t o r r a n g e a r e r e l a t e d by a p o w e r r e l a t i o n s h i p of the f o r m G. R. CHANANI and STEPHEN D. ANTOLOVICH are Post-Doctorial Fellow and Associate Professor of Materials Science, respectively, Department of Materials Science and Metallurgical Engineering, University of Cincinnati, Cincinnati, Ohio. W.W. GERBER1CH is Associate Professor, Department of Chemical Engineering and