Thermal activation of fatigue damage
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I. INTRODUCTION IT is well known that the mechanical properties of metals, such as tensile strength, fatigue strength, and creep resistance, decrease, often quite dramatically, at elevated temperatures. This limits their application, particularly for lightweight materials such as aluminum and magnesium alloys, and has stimulated the development of composites wherein the alloy is reinforced with higher strength (and higher modulus) ceramic fibers or particles. These reinforcements are very beneficial, particularly as the matrix alloy weakens at high temperature. Nevertheless, the mechanical properties of the composites are still limited ultimately by those of the matrix alloy.[1] The decrease of strength at elevated temperatures is, of course, due to the easier development of plasticity; i.e., lower stresses are required to move dislocations. The ceramic reinforcements in composites are usually so widely spaced that they do not seriously impede dislocation motion. (Their contribution to strength arises from a different mechanism, namely, a preferential sharing of the applied load by virtue of their higher modulus and ultimately by their ability to sustain higher stresses.) Thus, the barriers to dislocation motion in the matrix of a composite are the same as those in the unreinforced alloy, namely, precipitates, and/or other dislocations depending upon the particular microstructure. Consequently, the temperature dependence of the mechanical properties of a composite is dictated by that of the matrix alloy.[1] The effect of temperature on creep in aluminum alloys has been studied very extensively, and rate equations have been developed to describe thermally activated processes with kinetics controlled by diffusion.[2] This article describes an analysis of the temperature dependence of fatigue of an aluminum composite and provides the first evidence that high-temperature fatigue is a stress-assisted thermally activated process. An important consideration is that because of the longevity of a typical fatigue test, the temperature dependence of the fatigue life can arise from two factors: WILLIAM J. BAXTER, Principal Research Scientist, is with the Metallurgy Department, General Motors Research and Development Center, Warren, MI 48090-9055. DONALD R. LESUER, Group Leader, and CHOL K. SYN, Materials Scientist, are with the Materials Technology Department, Lawrence Livermore National Laboratory, Livermore, CA 94551-9900. Manuscript submitted May 24, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
(a) thermally induced changes in the microstructure during the test, and (b) the intrinsic temperature dependence of the mechanism of fatigue damage. We have separated these two effects for the first time and show that when the microstructure is stabilized prior to testing, the rate of development of fatigue damage is thermally activated. Further evidence is provided by fatigue data, taken from the literature[3] for a fully annealed monolithic alloy. II. EXPERIMENTAL Composites of 339 aluminum (12Si, 1Mg, 1Cu, 1Ni, 0.5 Fe wt pct
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