Fatigue crack tip deformation

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INTRODUCTION

W H E N fatigue crack growth occurs in type 304 stainless steel, the plastically deformed material at the tip of the fatigue crack can undergo a strain-induced transformation from austenite to martensite (a '). As shown in Figure l, it has been found that the extent of this transformation is greater in vacuum than in air, m and the present article is directed at providing an explanation for this difference. The environment, of course, has long been recognized to influence fatigue crack growth behavior, and it is well known that for metals and alloys, the rate of fatigue crack growth in air is higher than in vacuum. For example, Figure 2 shows the effect of an air vs a vacuum environment on the fatigue crack growth rate of type 304 stainless steel at room temperature. Ill For both steels 12j and aluminum alloys, 131 at room temperature, it has been shown that the moisture content of air, rather than oxidation, is primarily responsible for this effect. However, it is a difficult matter to be specific about the mechanistic processes by which moisture exerts this influence. Presumably, a reaction of water vapor with the exposed metal at a crack tip is involved with attendant release of hydrogen into the metal. There are several possible ways by which this hydrogen can influence fatigue crack growth, and among these, the process of hydrogen-induced softening and strain localization t4 7] is of particular interest. Some evidence for strain localization in air during fatigue crack growth is provided by the fact that the slip process at a fatigue crack tip is less uniformly distributed in air than in vacuum, tSl In the present investigation, consideration was given as to whether or not strain localization in air also influenced the extent of the strain-induced transformation by comparing crack tip deformation behavior in air with that in vacuum. A.J. McEVILY, Professor, is with the Metallurgy Department and Institute of Materials Science, University of Connecticut, Storrs, CT 06269. J.L. GONZALEZ VELAZQUEZ, formerly Graduate Student, University of Connecticut, is Professor, lnstituto Politecnico Nacional, Mexico City, Mexico. Manuscript submitted July 1, 1991. METALLURGICAL TRANSACTIONS A

The research program to be described focused primarily on the behavior of type 304 stainless steel, with auxiliary tests of type 316 stainless steel and the aluminum alloy 2219 added for purposes of comparison. The strain-induced transformation in the 304 alloy is accompanied by a volume expansion of about 2 to 3 pct, I91 and a number of authors have indicated that this transformation leads to a reduction in the rate of fatigue crack growth3 l~ This effect of the transformation on the growth rate has been ascribed to a perturbation of the stress field at the fatigue crack tip. tl21 In the present investigation, we have directed attention to the shape of the crack tip in both the plane stress surface regions as well as the plane strain interior regions of a specimen under load. The results confirm the finding that the ex

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