Threshold for fatigue macrocrack propagation in some aluminum alloys
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THE roles of microstructure and
the plastic deformation mechanisms in determining the threshold for fatigue.macrocrack propagation ( A K o ) are not clear. Several theoretical treatments ~,2based on the stress intensity range required to initiate dislocation emission at the crack tip, predict that A K o varies only as Young's modulus (E) with A K o / E being essentially constant. Earlier Harrison 3 observed a constant A K o / E for a number of alloys. That A K o depends on microstructure, deformation mechanism, and environment is now well established. Grain size and strength have been identified as important variables but the dependence of A K o on them is complex. According to Ritchie, 4 in 300M steel (a Cr, Ni, Mo, Si, 0.4 wt pct C steel) A K o decreases from 8.5 to 3.0 M N / m 3a as the yield strength increases from 1185 M N / m 2 (tempered at 650 ~ to 2335 M N / m 2 (tempered at 100 ~ However, Suzuki and McEvily 5 working with dual phase steels found an inverse effect. With a 1018 steel heat treated to form martensite islands in ferrite, the yield stress and A K o were 293 M N / m 2 and 8 M N / m 3/~, respectively, but when the structure consisted of ferrite islands in martensite, yield stress and A K o were 452 M N / m z and 19 M N / m 3/2, respectively. Many investigators have reported an increase in A K o with grain size. For 0.07 wt pct C steel, Masounave and Bailon 6 reported that between grain size, D, of 19 and 153/~m, AK o
= 3.8 + 1.4 •
103D 1/2
[1]
Similar results were obtained by Taira, Tanaka, and Hoshino 7 for 0.20 wt pct C steel heat treated to give ferrite and pearlite, by Priddle s for stainless steels, and Irving and Beevers 9 for some titanium alloys. Carlson and Ritchie, m however, varied austenitic grain size in a
J. MCKITTRICK is Research Associate, International Harvester Company, Science and Technology Division, Hinsdale, IL 60521. P. K. LIAW is Senior Engineer, Westinghouse Electric Corporation, Research & Development Center, Pittsburgh, PA 15235. S. I. KWUN is Associate Professor, Department of Metallurgical Engineering, Korea University, Seoul, Korea. M. E. FINE is Walter P. Murphy Professor of Materials Science and Engineering, Northwestern University, Evanston, IL 60201. Manuscript submitted May 5, 1980. METALLURG ICAL TRANSACTIONS A
quenched and tempered steel and found a small decrease in A K o as the austenitic grain size was increased. The role of environment is also not clear. Cooke, Irving, et a l to observed that the decrease in A K o with increase in R-ratio, (min. stress)/(max, stress) in a quenched and tempered Ni-Cr-Mo steel disappeared if the test is done in a vacuum. Kirby and Beevers jt observed a similar result for 7075-T7351 A1 alloy. Thus there is the possibility that many of the observed effects of change in A K o with heat treatment have an environmental origin. In a previous paper t2 one of the present authors derived a simple equation by suggesting that A K o is determined by the stress, a s, necessary to activate a dislocation source located near the crack
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