A short-time diffusion correlation for hydrogen-induced crack growth kinetics
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It should be e m p h a s i z e d that this is not a physical model since t h e r e is no f a i l u r e c r i t e r i o n in t e r m s of why a c e r t a i n hydrogen level might be r e q u i r e d to nucleate f r a c t u r e over a specified m i c r o s t r u c t u r a l r e gion. In this s e n s e , it is an extension of the f r a c t u r e m e c h a n i c s concept for t h r e s h o l d s t r e s s intensity. 4 MATERIALS AND P R O C E D U R E S The m a t e r i a l s and p r o c e d u r e s w e r e l a r g e l y identical to those used in a previous investigation. 4 F o r the m a t e r i a l , high strength 4340 s t e e l had been a u s t e n i tized at 850~ (1123 K) for one hour, oil-quenched and t e m p e r e d in the r a n g e of 200 to 500~ (473 to 773 K). C h a r a c t e r i z a t i o n of the r e s u l t i n g strength p r o p e r t i e s , as completed since the l a s t investigation a r e shown in
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IOn W. W. GERBERICH and Y. T. CHEN are Associate Professor and Graduate Student, respectively, in the Department of Chemical Engineering and Materials Science at the University of Minnesota, Minneapolis, MN 55455. C. ST. JOHN, Research Scientist, Centre des Materiaux de l'Ecole des Mines, Corbeil-Essone, France. Y. T. CHEN'S participation is in partial fulfillment of the M.S. Degree at the University of Minnesota. Manuscript submitted October 30, 1974
STRESS tNTENSITY,MPo-m w2 22 44 66 88
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Fig. 1 - M a t e r i a l , e n v i r o n m e n t and stt~ess intensity revel effects on c r a c k growth r a t e (Courtesy of ASM, Ref. 9). VOLUME6A, AUGUST 1975-1485
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Fig. 2-Details of the crack growth measurement technique: (a) compact tension fracture specimen; (b) strip chart of time versus resistance from crack propagation gages; (c) details of crack propagation gage.
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T a b l e I. C o m p a c t t e n s i o n s p e c i m e n s w e r e f i r s t p r e p a r e d b y f a t i g u e p r e - c r a c k i n g at a AK of 10,000 p s i in 1/2 (11 M P a - m l / Z ) . T h e y w e r e then h y d r o g e n c h a r g e d and c a d m i u m p l a t e d until Co ~ 2.6 p p m H, b a s e d upon h y d r o g e n e n t r y into 200,000 p s i (1380 MPa) y i e l d s t r e n g t h 4340 s t e e l . A~ter b a k i n g at 150~ to u n i f o r m l y d i s t r i b u t e the h y d r o g e n , the 19 m m t h i c k s p e c i m e n s w e r e f i t t e d with c r a c k p r o p a g a t i o n g a g e s having 20 g r i d l i n e s a t 250 ~,n i n t e r v a l s . The gage can be c o n s i d e r e d a s a n u m b e r of r e
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