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The p u r p o s e of this i n v e s t i g a t i o n was to p e r f o r m a s e r i e s of e x p e r i m e n t s a i m e d at d i s t i n g u i s h i n g between the s p e c i f i c m o d e l s that could a c c o u n t f o r r e v e r s i b l e hydrogen embrittlement. Basically three mechanisms have b e e n p r o p o s e d to e x p l a i n the o b s e r v e d r e s u l t s . The f i r s t , i n i t i a l l y p r e s e n t e d by Zapffe 4 and modified by T e t e l m a n , 5 d e s c r i b e s the role of h y d r o g e n a s one w h e r e the a t o m i n s o l u t i o n " p r e c i p i t a t e s " a s a gas i n a void. The p r e s s u r e developed in the void is added to the applied s t r e s s to d e c r e a s e the load c a r r y i n g a b i l i t y . The m o d e l d e s c r i b e d by T r o i a n o z'8 f r o m r e s u l t s on h i g h - s t r e n g t h s t e e l s i n d i c a t e s t h a t h y d r o g e n i n the l a t t i c e i s the d a m a g i n g s p e c i e a n d the p r e s e n c e of hydrogen c a n a c t u a l l y r e d u c e the c o h e s i v e s t r e n g t h of the b a s e m e t a l bond. He has f u r t h e r s u g g e s t e d a m o d e l w h e r e b y the h y d r o g e n atom i n t e r a c t s with the 3d e l e c t r o n s h e l l to l o w e r the b i n d i n g e n e r g y . The t h i r d m e c h a n i s m , o r i g i n a l l y p r o p o s e d by Perch, i n volves a l o w e r i n g of the f r a c t u r e e n e r g y by a d s o r p t i o n of the h y d r o g e n . 7 W i l l i a m s and Nelson have applied the s u r f a c e a d s o r p t i o n m o d e l to d e l a y e d f a i l u r e of h i g h - s t r e n g t h s t e e l in an e n v i r o n m e n t of h y d r o g e n gas and r e l a t e d the k i n e t i c s of a d s o r p t i o n to the k i n e t i c s of c r a c k growth. 8 In c a s e s where the s u r f a c e a d s o r p tion or s t r e s s s o r p t i o n m o d e l involves i n t e r a c t i o n of the h y d r o g e n with the 3d e l e c t r o n s h e l l ( c h e m i s o r p tion) at the tip of a c r a c k o r an e m b r y o n i c c r a c k , it is d i r e c t l y c o m p a r a b l e to m a n y a s p e c t s of the T r o i a n o model. The e x p e r i m e n t a l a p p r o a c h i n v o l v e d : 1) An e x a m i n a t i o n of the r e v e r s i b i l i t y of the i n c u b a tion t i m e for c r a c k i n g in h y d r o g e n a t e d h i g h - s t r e n g t h s t e e l notched s p e c i m e n s s u b j e c t e d to s t a t i c - l o a d i n g . 2) A study of e m b r i t t l e m e n t at - 321~ of c a t h o d i c a l l y c h a r g e d s p e c i m e n s w h e r e the c o n t r i b u t i o n of diff u s i o n a l p r o c e s s e s should be n e g l i g i b l e . Each of t h e s e t e s t s was d e s i g n e d to p r o v i d e data which would allow s e p a r a t i o n of the v a r i o u s e m b r i t t l e ment mechanisms. u~pEn cmTic~t. ~NeO~ON

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