The effect of loading mode on hydrogen embrittlement
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AND
W. W. GERBERICH
Hydrogen e m b r i t t l e m e n t is shown to occur v e r y e a s i l y in n o t c h e d - r o u n d b a r s under opening mode I (tension) but not u n d e r a n t i p l a n e s h e a r mode III (torsion). The s t r e s s t e n s o r i n v a r i ants u n d e r mode I, II, and III loadings and how these affect i n t e r s t i t i a l diffusion a r e d i s c u s s e d . It is suggested that long r a n g e diffusion of hydrogen down o r t h o g o n a l t r a j e c t o r i e s to the v i c i n i t y of the c r a c k tip, which can o c c u r u n d e r mode I but not mode III, is a key p a r t of any hydrogen e m b r i t t l e m e n t m e c h a n i s m . This p r e m i s e was e v a l u a t e d with AISI 4340 s t e e l heat t r e a t e d to u l t r a h i g h s t r e n g t h l e v e l s . It was found that an i n i t i a l mode I s t r e s s i n t e n s i t y level of 17,000 p s i - i n , l/a p r o d u c e d f a i l u r e in s e v e r a l m i n u t e s . Mode III s t r e s s i n t e n s i t y l e v e l s t h r e e t i m e s this p r o d u c e d no c r a c k i n i t i a t i o n in 300 m i n . F u r t h e r a n a l y s i s of the t i m e - d e p e n d e n t h y d r o g e n c o n c e n t r a t i n g effect utilized a s t r e s s wave e m i s s i o n t e c h nique. This p r o d u c e d p l a u s i b l e c r i t i c a l hydrogen c o n c e n t r a t i o n s even though the p r e s e n t e l a s t i c a n a l y s i s is a f i r s t o r d e r a p p r o x i m a t i o n of the s t r e s s field. T H E p r o b l e m of h y d r o g e n e m b r i t t l e m e n t has g e n e r ated c o n s i d e r a b l e d i s c u s s i o n r e g a r d i n g the exact m e c h a n i s m ( s ) involved, l'a T h e r e does appear to be some a g r e e m e n t however on the r o l e that the l o c a l i z e d s t r e s s fields in the c r a c k tip r e g i o n p l a y in all t h r e e c u r r e n t l y held m e c h a n i s m s ; which is, to c r e a t e h y d r o g e n c o n c e n t r a t i o n s via " u p - h i l l " diffusion that can be c o n s i d e r ably higher than those away f r o m the c r a c k tip. The concept of an e l a s t i c i n t e r a c t i o n b e t w e e n the v o l u m e t r i c c o m p o n e n t of a c r a c k tip s t r e s s t e n s o r and the d i l a tion a s s o c i a t e d with an i n t e r s t i t i a l h y d r o g e n atom has b e e n d i s c u s s e d b e f o r e . 3 More r e c e n t l y , 4 an a n a l y s i s of the h y d r o s t a t i c p r e s s u r e field of a c r a c k under mode I loading coupled with F i c k ' s f i r s t law has yielded steady state solute c o n c e n t r a t i o n g r a d i e n t s s i m i l a r to those obtained d i r e c t l y f r o m t h e r m o d y n a m i c s , s
C = CoeU/kT/(1 -- C O + CoeU/kT) ~- CoeU/kT for C o s m a l l
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where u is the i n t e r a c t i o n e n e r g y b e t w e e n the c r a c k tip p r e s s u r e field and hydrogen atom dilation, and C o is the h o m o g e n e o u s h y d r o g e n solid s o lution c o n c e n t r a t i o n f a r f r o m the c
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