The effect of surface dissolution on fatigue crack nucleation in polycrystalline copper
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ACCELERATED fatigue c r a c k i n i t i a t i o n in a g g r e s s i v e aqueous e n v i r o n m e n t s has g e n e r a l l y b e e n a t t r i b u t e d to a n u m b e r of m e c h a n i s m s i n c l u d i n g c o r r o s i o n induced pitting, 1,~ p r e f e r e n t i a l d i s s o l u t i o n of d e f o r m e d metal, 3-s c o r r o s i o n product f i l m r u p t u r e , 6'7 and s u r f a c e e n e r g y r e d u c t i o n . 8-1~ More r e c e n t l y a model of d i s s o l u t i o n a s s i s t e d e n h a n c e d d e f o r m a t i o n was p r o p o s e d b y Duquette and Uhlig to explain the a p p a r e n t i n t e n s i f i c a t i o n of i n t r u s i o n s and e x t r u s i o n s in a low c a r b o n steel. 11 This model r e c e i v e d f u r t h e r s u p p o r t f r o m the work of Pyle et al. on a c t i v a t e d d i s s o l u t i o n at slip steps. '2 Each of t h e s e m e c h a n i s m s and its i n t e r a c t i o n with f u n d a m e n t a l fatigue p r o c e s s e s has b e e n r e c e n t l y r e v i e w e d with the c o n c l u s i o n that l i t t l e is a c t u a l l y known about the f u n d a m e n t a l s of e n v i r o n m e n t / c y c l i c s t r e s s (or s t r a i n ) i n t e r a c t i o n s . 13 This study e x a m i n e s the c h a r a c t e r i s t i c s of fatigue g e n e r a t e d s u r f a c e slip offsets and c r a c k i n i t i a t i o n in l a r g e g r a i n e d copper s p e c i m e n s in a i r and in c o r r o s i v e aqueous s o l u t i o n s . Copper was chosen for t h e s e e x p e r i m e n t s b e c a u s e it is g e n e r a l l y a c c e p t e d that the fatigue b e h a v i o r of p u r e copper is not affected by n e u t r a l aqueous s o l u t i o n s . Acc o r d i n g l y , the d i s s o l u t i o n r a t e of the m e t a l s u r f a c e could be c o n t r o l l e d b y applied anodic c u r r e n t s and the s u b s e q u e n t d e f o r m a t i o n and c r a c k i n i t i a t i o n b e h a v i o r d e t e r m i n e d . Additionally, the fatigue b e h a v i o r of copp e r in n o n a g g r e s s i v e e n v i r o n m e n t s is r e a s o n a b l y well u n d e r s t o o d and could be d i r e c t l y c o m p a r e d with the fatigue b e h a v i o r of the m e t a l u n d e r a c t i v e l y d i s s o l v i n g conditions. E X P E R I M E N T A L PROCEDURES Constant f r e q u e n c y (30 Hz) a x i a l load c o n t r o l l e d t e n s i o n - t e n s i o n fatigue e x p e r i m e n t s w e r e conducted on a n n e a l e d s p e c i m e n s of copper of the following c o m p o s i tion: 0.001 pot Ag, 0.001 pct Fe, b a l a n c e Cu. All other e l e m e n t s w e r e below d e t e c t a b l e l i m i t s b y s p e c t r o -
H. MASUDA,formerly Research Assistant, MaterialsDivision, Rensselaer Polytechnic Institute, Troy, New York 1218l, is now with Toshiba Electronics, Tokyo, Japan.D.J. DUQUETTEis Associate Professor, Materials Division, Rensselaer Polytechnic Institute, Troy, New York 12181. Manuscript submitted September 17, 1973. METALLURGICALTRANSACTIONSA
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