Fatigue threshold studies in Fe, Fe-Si, and HSLA steel: Part I. Effect of strength and surface asperities on closure
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I.
INTRODUCTION
THE importance of assessing the origin(s) of fatigue thresholds can hardly be overestimated since existing or developing cracks may dwell near threshold for long periods of time. Ignoring in this paper the important ramifications of short crack phenomena, one still must understand the very large body of important literature associated with long crack thresholds. A number of mechanisms,1 which may be classified as largely environmental, closure, microstructural, or dislocation-related phenomena, have been proposed to explain threshold. As the first and last may be rate dependent, some low-temperature test studies ~'2'3 assessing the near threshold fatigue phenomena have been conducted. In these studies of Fe-binary alloys ~'2 and low-alloy high strength steels, 2'3 it was observed that threshold stress intensity, AK~, just about doubled as the test temperature was decreased from 300 K to 123 K. For example, in l0 p.m grain HSLA, the AKth increases from about 6 to 11 MPa-m ~/2 while in 100/xm grain-size Fe, it increases from about 9 to 15 MPa-m ~/2 over this temperature drop. In a number of other studies, 4-1~mostly at room temperature or above, it has been proposed that closure, whether slip, geometry, or oxide induced, may be largely responsible for observed differences in threshold. Thus, the question arises as to the significance of closure effects in the low-temperature threshold experiments made to date. 1'2'3 Specifically, could trends of AKth vs test temperature be explained entirely in terms of test-temperature dependent closure phenomena? Alternativel 3, could the closure contribution be sufficiently small so that AKeff is very test temperature dependent and hence dependent on some internal structure effect? These questions were alluded to in the W. W. GERBERICH is Professor and Associate Head of the Department of Chemical Engineering and Materials Science, University of Minnesota, 151 Amundson Hall, 421 Washington Avenue, S.E., Minneapolis, MN 55455. W. YU, formerly Research Assistant with the Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN, is now a Postdoctorate with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, Berkeley, CA 94720. K. ESAKLUL, formerly Research Assistant with the Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN, is now an Assistant Professor with the Faculty of Engineering, A1-Fateh University, Tripoli, Libya. Manuscript submitted May 9, 1983. METALLURGICALTRANSACTIONS A
previous paper I where it was suggested that the AK~ dependence could be made up of both extrinsic (e.g., closure) and intrinsic (e.g., dislocation emission) phenomena. The present investigation was undertaken to review all test data, perform extensive fractography, and conduct more sensitive determinations of the closure loads near threshold. This effort has resulted in a two-part investigation where the first part deals entirely with closure phenomena
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