Thermomechanical fatigue of a lead alloy
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I.
INTRODUCTION
THERMOMECHANICAL fatigue (TMF) results from the simultaneous effects of cyclic variations in load, strain, and temperature. It differs from ordinary fatigue in that temperature is a dynamic, rather than static, variable. Also, it is more general than thermal fatigue in that the imposed strains are independent of the thermal expansion. In thermal fatigue, thermal expansion is the source of all of the applied strain. The constraint of thermal expansion either by boundary conditions or through temperature gradients gives rise in thermal fatigue to elastic and, at many times, plastic strains through the compatibility requirement. These strains are the source of thermal fatigue. The presence of additional phases or the anisotropy of grains in a polycrystal may also provide constraints. In the absence of constraints, heating and cooling materials do not seem to cause fatigue. For this reason, one would not expect a reasonably isotropic metal, not known to exhibit thermal fatigue from purely microstructural causes, to undergo a significant aggravation of fatigue when cyclicly cooled during a strain-controlled fatigue cycle where the thermal contraction from cooling was compensated to keep the total mechanical strain constant. But this is the case with 97Pb-3Sn and with a number of other alloys. There exist many instances of materials which undergo altered fatigue damage when temperature cycling is added to strain-controlled fatiguing, it-10] When additional damage occurs as a result of thermal cycling superimposed on fatigue, it has been thought that this is the result of overheating. The effect of the thermal cycle in TMF has been modeled in this way. ttl'121 For this reason, previous work with 97Pb-3Sn showing that
L. LAWSON, Postdoctoral Fellow, and M.E. FINE, Walter P. Murphy Professor Emeritus, are with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208-3108. D. JEANNOTI'E, Senior Engineer, is with IBM Corporation, East Fisbkill Facility, Hopewell Junction, NY 12533. Manuscript submitted April 27, 1990. METALLURGICAL TRANSACTIONS A
a modest cyclic cooling of 55 ~ from 80 ~ to 25 ~ can effect a considerable reduction in life draws attention to the presence of other mechanisms.t131Nor is this effect limited to lead-tin alloys. Much of the work with structural materialstH~ can be interpreted in this way. Were the effects of simultaneous thermal and mechanical cycling merely additive, the phenomenology of TMF could be described in terms of isothermal test experience directly. This does not appear to be the case generally nor for the tests to be reported here. Rather, there appears to be a separate thermomechanical effect associated with the change in fatigue life under TMF conditions. This change may amount to a reduction or increase in the expected number of cycles to failure, depending on frequency and phase, for example, from that determined by any possible averaging of isothermal fatigue lives for the temperatures within the cycle. This paper reports r
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