Fatigue crack growth along cleavage planes in an aluminum alloy
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Fatigue Crack Growth Along Cleavage Planes in an Aluminum Alloy K. R. L. THOMPSON AND J. V. CRAIG h LTHOUGH b r i t t l e s t r i a t i o n s have been o b s e r v e d on the fatigue f r a c t u r e s u r f a c e s of s e v e r a l a l u m i n u m a l l o y s ~-4 and a few o t h e r m a t e r i a l s , 2's'6 the p r e c i s e m e c h a n i s m by which they a r e f o r m e d is unknown. F o r s y t h 7 has d e s c r i b e d the b r i t t l e growth s t r i a t i o n s found on the fatigue f r a c t u r e s u r f a c e s of high s t r e n g t h a l u m i n u m a l l o y s a s c l e a v a g e growth i n c r e m e n t s t e r m i n a t e d by p l a s t i c a r r e s t m a r k i n g s ( s t r i a t i o n s ) . Howe v e r , in a r e c e n t r e v i e w a r t i c l e , L a i r d s e x p r e s s e d doubt that c l e a v a g e i s o p e r a t i v e in the f o r m a t i o n of t h e s e " c l e a v a g e s t r i a t i o n s " and s u g g e s t e d the p o s s i b i l i t y of q u a s i c l e a v a g e o c c u r r i n g . L a i r d pointed out that the plane of c r a c k growth o b s e r v e d by F o r s y t h , 1 (100), is not the s a m e as the u s u a l c l e a v a g e plane in fcc m a t e r i a l s , (111). 9'1~ K. R. L. THOMPSON and J. V. CRAIG are Metallurgists, Australian Defence Scientific Service, Department of Supply, Defence Standards Laboratories, Alexandria, N.S.W., Australia. Manuscript submitted September 19, 1969. METALLURGICALTRANSACTIONS
tions by r e l i e v i n g s t r e s s e s at t r i p l e points d u r i n g viscous boundary shear. K a r i m h as a l s o s u g g e s t e d that f r a c t u r e should o c c u r in the n o n r e c r y s t a l l i z e d s e c t i o n w h e r e s t r e s s c o n c e n t r a t i o n has not b e e n r e l i e v e d . H o w e v e r , " f r a c t u r e " in the high r h e n i u m a l l o y s o c c u r s only by ne c king down to >99 pct r e d u c t i o n in a r e a , and m ight b e t t e r be t e r m e d " s e p a r a t i o n " as a r e s u l t of high l o c a l t e n s i l e s t r e s s . The l o c a l s t r a i n s a s s o c i a t e d with n eck i n g induce g r a i n r e f i n e m e n t , p r e s u m a b l y by a r e c r y s t a l l i z a t i o n p r o c e s s . Th u s, the point of s e p a r a tion is a l w a y s a s s o c i a t e d with a f i n e - g r a i n e d e q u i a x e d s t r u c t u r e r a t h e r than the c o a r s e r - g r a i n e d , u n s t r a i n e d regions.
22. M. Garfinkle,W. R. Witzke,and W. D. Klopp: Trans. TMS-A1ME, 1969,vol. 245, p. 303. 23. E. E. Underwood:J. Metals, 1962,vol. 14, p. 914. 24. W. A. Backofen,I. R. Turner, and D. H. Avery:Trans. ASM, 1964, vol. 57, p. 980. 25. D. H. Averyand W. A. Backofen:Tran~ ASM, 1965, vol. 58, p. 551. 26. D. L. Holt and W.A. Backofen: Trans. ASM, 1966, vol. 59, p, 755. 27. W. A. Backofen,F. J. Azzarto,G. S. Murty, and S. W.Zehr: Ductility, p. 279, ASM,MetalsPark, Ohio, 1968. 28. P. E. Rainey: S.M.Thesis,M.I.T.,June 1968. 29. A. Karimand W. A. Backofen:Mater. Sci. Eng., 1968-69, vol. 3, p. 306. 30. D. L. Holt:
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