Fracture mechanics and surface chemistry studies of subcritical crack growth in AISI 4340 steel
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assisted subcritical crack growth in high-strength steels and other high-strength alloys (particularly in hydrogen and in hydrogenous environments) is an important technological problem of long standing. Over the past decade, fracture mechanics techniques (based on linear elasticity) have been increasingly used to assist in determining the mechanisms for crack growth through characterizations of the crack growth kinetics.I-4 In the fracture mechanics techniques, the crack-tip stress-intensityfactor, K, is used to characterize the mechanical driving force for crack growth.2"4 It has been shown that the subcritical crack growth response may be separated into three stages. At low values of K, the rate of crack growth (da/dt) is strongly dependent on K and approaches zero as a "threshold K" value for crack growth is approached. 3,4 This regime of crack growth has been designated as Stage I.4 At intermediate values of K, da/dt becomes essentially independent of K (Stage If). As K approaches the critical value for fracture (Ifc or Kic), da/dt increases rapidly and again becomes strongly dependent on K (Stage HI). The identification of the K-independent Stage II of crack growth has provided an essential link for understanding the mechanisms for environment assisted subcritical crack growth.4 The rate of crack growth is determined by a number of possible sequential processes, with the slow-
e s t one c o n t r o l l i n g the o v e r a l l c r a c k g r o w t h r a t e . Since the growth r a t e is independent of K (the m e c h a n i c a l d r i v i n g f o r c e ) in Stage II, it m u s t be l i m i t e d by the r a t e of a c h e m i c a l r e a c t i o n , o r the t r a n s p o r t of the r e a c t i n g e n v i r o n m e n t , or the diffusion of the d a m a g i n g s p e c i e s . The a p p a r e n t a c t i v a t i o n e n e r g y for Stage II c r a c k growth, t h e r e f o r e , m u s t be d e t e r m i n e d by the activation energy of the underlying rate controlling process. Thus, one might identify the rate limiting process through studies of the temperature dependence of Stage H crack growth kinetics and of the kinetics of various possible rate limiting processes. This basic approach was used in the current study and in many previous studies of environment assisted crack growth .1,5-11 A n u m b e r of s t u d i e s of s u b c r i t i c a l c r a c k growth in h i g h - s t r e n g t h s t e e l s in w a t e r , w a t e r v a p o r and h y d r o gen have been m a d e . 1's-~1 H y d r o g e n was a s s u m e d to be r e s p o n s i b l e f o r the e n h a n c e m e n t of c r a c k g r o w t h in e a c h of t h e s e e n v i r o n m e n t s . The a c t i v a t i o n e n e r g i e s f o r Stage II c r a c k growth in h y d r o g e n and in w a t e r , h o w e v e r , w e r e found to be s i g n i f i c a n t l y d i f f e r e n t ( v i z , a p p r o x i m a t e l y 18 k J / m o l e for h y d r o g e n v s 38 k J / m o l e for w a t e r ) . T h i s d i f f e r e n c e in a c t i v a t i o n e n e r g i e s
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