Fracture mechanics and surface chemistry studies of fatigue crack growth in an aluminum alloy
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THE i n f l u e n c e s of cyclic
load f r e q u e n c y and gas p r e s s u r e on e n v i r o n m e n t a s s i s t e d fatigue c r a c k growth in high s t r e n g t h alloys has been under study for many years.l-14 A c o m p l e t e q u a n t i t a t i v e expianation of this effect and of the d i f f e r e n c e s in r e s p o n s e for different a l l o y - e n v i r o n m e n t c o m b i n a t i o n s is not yet a v a i l a b l e . To a s s i s t in u n d e r s t a n d i n g the i n f l u e n c e s of these v a r i a b l e s , c o o r d i n a t e d f r a c t u r e m e c h a n i c s and s u r f a c e c h e m i s t r y s t u d i e s have b e e n u n d e r t a k e n to identify the rate c o n t r o l i i n g p r o c e s s e s for c r a c k growth in high s t r e n g t h s t e e l s and to a s s e s s the r o l e s of these p r o c e s s e s in d e t e r m i n i n g e n v i r o n m e n t a s s i s t e d fatigue c r a c k growth response.~-17 In the p r e s e n t study, the role of w a t e r v a p o r - m e t a l r e a c t i o n in d e t e r m i n i n g the c r a c k growth r e s p o n s e of a l u m i n u m alloys was e x a m ined. P r e v i o u s s t u d i e s %7 have shown that the r a t e of f a tigue c r a c k growth in a l u m i n u m a l l o y s exposed to water v a p o r (in the a b s e n c e of c a p i l l a r y c o n d e n s a t i o n at the c r a c k tip) was s i g n i f i c a n t l y affected by p r e s s u r e and f r e q u e n c y . Bradshaw and W h e e l e r showed that the r a t e of fatigue c r a c k growth at a given K l e v e l a p p e a r e d to depend on the p r o d u c t of water vapor p r e s s u r e and c y clic load period (1/frequency), and suggested that the o b s e r v e d f r e q u e n c y effect r e s u l t e d f r o m the time a v a i l able for t h e r e a c t i o n of w a t e r v a p o r with the newly c r e ated c r a c k surfaces.6 In d i s t i l l e d water, on the other R. P. WEI is Professor of Mechanics, Department of Mechanical Engineeringand Mechanics, Lehigh University, Bethlehem, PA 18015. P. S. PAO is Scientist, McDonnellDouglas Research Laboratories, St. Louis, MO 63166. R. G. HART and T. W. WEIR are Research Associate and Research Assistant, respectively, Center for Surface and Coatings Research, Lehigh University, Bethlehem, PA 18015. G. W. SIMMONS is Associate Professor of Chemistry, Department of Chemistry, Lehigh University, Bethlehem, PA 18015. Manuscript submitted April 30, 1979. METALLURGICALTRANSACTIONSA
hand, a l u m i n u m alloys exhibited little or no effect of f r e q u e n c y . 1'8 C o m p a r a b l e effects of f r e q u e n c y have b e e n r e p o r t e d for fatigue c r a c k growth in high s t r e n g t h s t e e l s , at K l e v e l s below K i s c c (the s t r e s s c o r r o s i o n c r a c k i n g t h r e s h o l d ) . B a r s o m 11 showed that the rate of fatigue c r a c k growth in a 1 2 N i - S C r - 3 M o steel, t e s t e d in 3.5 pct NaC1 solution at r o o m t e m p e r a t u r e , depended on c y c l i c load f r e q u e n c y , with h
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