Fatigue threshold studies in Fe, Fe-Si, and HSLA steel: Part II. thermally activated behavior of the effective stress in
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I.
INTRODUCTION
IN a previous study on the fatigue crack propagation of Fe and Fe-Si alloys, a strong correlation between the observed fatigue threshold and the thermal component of yield stress was revealed. ~It was pointed out that there could be at least two explanations for this behavior. The first was intrinsic* *See Part I for definition.
and the second was an effective stress intensity argument. The latter involved large possible crack closure effects at low temperature which could reduce the effective stress intensity at the crack tip. The intrinsic aspect of this observation involved the dislocation structure, through either substructure formation or dislocation dynamics. In either case, the mechanism might be thermally activated and thus produce the observed correlation. With a goal toward isolating the effect of closure, the threshold fatigue cracking experiments were duplicated with increased amplification to improve the resolution at the lower end of the compliance curves. The results produced more accurately determined closure values, or more precisely, the crack opening loads. The results and discussion were reported in the preceding paper. 2 In that paper, it was proposed that the closure can be a thermally controlled process. The temperature effect might be introduced through either change in the strength of the material or through a change in surface morphology. The 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, AI-Fateh University, Tripoli, Libya. 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. Manuscript submitted May 9, 1983. METALLURGICALTRANSACTIONS A
low temperature creep of the asperities behind the crack tip might also play a role in the crack closure. Nevertheless, when adjusting for closure and considering only the effective stress intensity range applied at threshold, the correlation to the thermal component of the flow stress was even stronger (Figure 1). This suggested that the low temperature effect on higher flow stress might explain the increasing trend in effective stress intensity. For this reason, the intrinsic thermally activated properties of the materials of this study were examined.
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