Surface Segregation in Several 3 Pct Si Steels
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overloads and non-uniform load histories will be the subject of a forthcoming publication. However, the crack closure behavior described herein is of considerable interest in itself; it has been observed in other aluminum alloys such as 2024-T351 ~and requires full explanation. We have shown for instance that in compact tension specimens, crack closure behavior varies with Kmax with the interesting observation that a rather abrupt transition occurs apparently when the plastic zone extends beyond one full grain. The results do not completely rule out mechanical influences due to specimen compliance which should be more fully explored; in addition they do not rule out the possibility that the transition is influenced by environmental effects since the experiments reported on here were conducted in dry laboratory air. This work was supported by the Air Force Office of Scientific Research under contract AFOSR-77-3246 and was conducted under the direction of Dr. A. H. Rosenstein. The helpful comments of A. H. Rosenstein and P. Paris are gratefully acknowledged. 1. P. Paris and L. Hermann: Proc. Int. Cong. Theor. Appl. Mech., Delft, W. T. Koiter, ed., Pub. North-Holland, Amsterdam, 1977. 2. O. Buck, J. D. Frandsen and H. L. Marcus: Eng. Fract. Mech., 1975, vol. 7, pp. 167-71. 3. M. W. Mahoney and N. E. Pat 9 4th Int. Conf. on Fract., University of Waterloo, Canada, D. M. R. Taplin, ed., pp. 1081-1989, Pergamon Press, 4. A. J. McEvily: Metal Sci., 1977, vol. 11, pp. 274-84. 5. P. E. Irving, J. L. Robinson and C. J. Beevers: Eng. Fract. Mech., 1975, vol. 7, pp. 619-30. 6. W. Elber: Damage Tolerance in Aircraft Structure, ASTM STP 486, pp. 230-42, Philadelphia, PA, 1971. 7. H. Johnson and P. Paris: Eng. Fract. Mech., 1968, vol. 1, pp. 3-45. 8. W. Elber: Eng. Fract. Mech., 1970, vol. 2, pp. 37-45. 9. B. Budiansky and J. W. Hutchinson: J. Appl. Mech., 1978, vol. 45, pp. 267-76. 10. J. Staley: Int. Symp. on Fract. Mech., George Washington Univeristy, Washington, D.C., 1978. 11. L. Hermann, P. Paris and R. J. Asaro: Brown University, unpublished research, 1975-1977. 12. J. Santner and M. E. Fine: Met. Trans. A, 1976, vol. 7A, pp. 583-93. 13. E. Hornbogen and M. Gahr: ActaMet., 1977, vol. 25, pp. 877-81.
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Surface Segregation in Several 3 Pet Si Steels
~Kef f, Z~K(MPo-v/'m ) Fig. 3--Typical crack growth rate data for three different tempers represented in terms of 6 K and A g e f f.
intensity. The ratio then takes on a maximum value of approximately 0.45 and, as this is still significantly larger than the 0.25 characteristic once the transition has occurred, it appears that the transition is at least partially reversible. A much more extensive description of fatigue crack growth in 2048 aluminum, including the effects of METALLURGICAL TRANSACTIONS A
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