The effect of specimen polarization on fatigue crack growth rates in 7075-T6 aluminum
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E. DRESTY
AND OWEN
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F. DEVEREUX
METALLURGICAL TRANSACTIONS
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JOHN E. DRESTY, formerly Graduate Assistant, University of Connecticut, is presently Supervisor of Metallurgical Laboratory, United Nuclear Corp., 365 Winchester Ave., New Haven, Conn. 06508. OWEN F. DEVEREUX is Associate Professor, Department of Metallurgy and Institute of Materials Science, University of Connecticut, Storrs, Conn. 06268. Manuscript submitted September 5, 1972.
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I N r e c e n t y e a r s a n u m b e r of a u t h o r s have studied the effect of an e l e c t r o c h e m i c a l e n v i r o n m e n t on fatigue of a l u m i n u m a l l o y s . 1-5 In p a r t i c u l a r , s p e c i m e n p o l a r i z a tion has b e e n shown to have a s i g n i f i c a n t effect on fatigue b e h a v i o r , as shown in s t u d i e s of the alloys 5052H34,1 7079-T651, 4 and 7075-T6. 5 In the l a t t e r c a s e Stoltz and Pelloux studied c r a c k growth r a t e vs s p e c i m e n t p o t e n t i a l in the cited alloy in a 3.5 pct aqueous NaC1 e n v i r o n m e n t finding, in a g r e e m e n t with other cited works, that cathodic p r o t e c t i o n o c c u r r e d , i.e., at a s u f f i c i e n t l y cathodic p o t e n t i a l (E < - 1.3 V SCE) a r e d u c tion in c r a c k growth r a t e was o b s e r v e d . This d i f f e r s q u a n t i t a t i v e l y f r o m r e s u l t s of an independent, but s i m u l t a n e o u s , study by the a u t h o r s in which a continuous v a r i a t i o n of c r a c k growth r a t e in that alloy was s e e n over the potential r a n g e - 1.2 to - 0.7 V SCE in the same environment. The a u t h o r s used sheet s p e c i m e n s 0.051 in. thick by 10 in. long and 2 in. wide cut with the length p a r a l l e l to the r o l l i n g d i r e c t i o n , with a ~ in. d i a m c e n t e r hole and s t r e s s c o n c e n t r a t o r s of 0.005 in. r a d i u s cut with a s p a r k d i s c h a r g e device. C r a c k p o s i t i o n was e s t a b l i s h e d b y m e a n s of f i d u c i a l l i n e s s c r i b e d on e i t h e r side of the hole. T e s t s w e r e p e r f o r m e d on a Satec S F - 1 U fatigue m a c h i n e at 1800 c y c l e s p e r min, u s i n g m a x i m u m and m i n i m u m s t r e s s e s of 6050 and 50 psi, r e s p e c t i v e l y , (R = 0.0083). The t e s t cell c o m p r i s e d a P l e x i g l a s c y l i n d e r with a flexible r u b b e r b a s e through which the s p e c i m e n was i n s e r t e d . An e n v i r o n m e n t of a e r a t e d 3.5 pct r e a g e n t grade NaC1 in d i s t i l l e d w a t e r was used; pH was 6.0 to 6.5. Specimen potential was m a i n t a i n e d by m e a n s of a Wenking model 68 TS-3 p o t e n t i o s t a t , e m ploying a single p l a t i n u m m e s h c o u n t e r e l e c t r o d e to the r e a r of the sheet s p e c i m e n and a s a t u r a t e d c a l o m e l r e f e r e n c e e l e c t r o d e with a Luggin p r o b e a p p r o x i m a t e l y 1 m m f r o m the s p e c i m e n s u r f a c e .
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