Influence of punch curvature on the stretching limits of sheet steel
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toughness material (solid lines) shows a marked influence of mean s t r e s s compared with the naturally aged, high toughness alloy. Similarly, an analogous, though considerably smaller, effect observed in pure aluminum-copper alloys %6 is due to the contribution arising from intergranular separation (Fig. 4).
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Stress ratio, R: K m i n / K m e x ....... peakege~l - - - - -- n s t u r a l l y I g K I , R : O- 5 ..... "* -" , R = 0 - 0 5
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1. P. C. Paris: Fatigue, An Interdisciplinary Approach, p. 107, Sagamore, New York, 1964. 2. H. W. Liu: Z BasicEng. (Trans. ASME Series D), 1963, vol. 85, p. 116. 3. K. R. Lehr and H. W. Liu: Int. J. Fraet. Mech., 1969, vol. 5, p. 45. 4. H. W. Liu and Nobu lino: Fracture 1969, Proc. 2nd lnt. Conf. on Fracture, p. 812, Brighton, 1969. 5. G. G. Garrett: Ph.D. Thesis, Department of Metallurgy and Materials Science, Cambridge University, 1973. 6. G. G. Garrett: Internal Report, Department of Metallurgy and Materials Science, Cambridge, 1973. 7. R. O. Ritchie: Ph.D. Thesis, Department of Metallurgy and Materials Science, Cambridge University, 1973. 8. R. O. Ritchie and J. F. Knott: ActaMet, 1973, vol. 21, p. 639. 9. N. E. Frost, L. P. Pook, and K. Denton: Eng. Fract. Mech., 1971, vol. 3, p. 109. 10. S. Pearson: Eng. Fract. Mech., 1972, vol. 4, p. 9.
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Location of Fractogrephs
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Open symbols represent equivalent fracture surfaces.
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F i g . 6 - - A c o m p a r i s o n of t h e f a t i g u e c r a c k g r o w t h c h a r a c t e r i s t i c s of t h e low a n d h i g h t o u g h n e s s m a t e r i a l s , i n d i c a t i n g t h e l o c a t i o n of F r a c t o g r a p h s : F i g s . 1 t h r o u g h 3, t o g e t h e r w i t h t h e i r e q u i v a l e n t s . T h e v a r i a t i o n i n m e a n s t r e s s e f f e c t in t h e s e m a t e r i a l s i s a d e q u a t e l y e x p l a i n e d in t e r m s of t h e d i f f e r i n g r e s p o n s e to ' s t a t i c ' m o d e s of f a i l u r e d u r i n g f a t i g u e .
can conclude that the contribution to local crack advance from the cleaving of brittle particles can account for some of the increase in m above that of ~2 arising from purely striation formation. However, the magnitude of this contribution is unlikely to be affected by changes in matrix m i c r o s t r u c t u r e so that as the toughness d e c r e a s e s an increasing contribution will a r i s e from the component of ductile tearing and void coalescence. Comparison of Figs. 2 and 3, at approximately equal growth r a t e s but in m a terials of markedly different toughness adequately illustrates this point. The tough, naturally-aged m a t e rial (~ 2014-T4) in fact shows little change in f r a c ture appearance with increasing growth rate. Even at high s t r e s s levels inclusions which fracture by cleavage do not give r i s e to any substantial tearing-as marked by the arrow in Fig. 3. The o c c u r r e n c e of these stati
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