Heat generation during the fatigue of a cellular Al alloy
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INTRODUCTION
CELLULAR metals have been shown to experience fatigue degradation in both tension and compression.[1,2] This degradation is manifested as a rapid increase in strain after a quiescent nucleation period wherein strain accumulation occurs slowly and stably (Figure 1(a)). The consequence is an endurance limit occurring at some fraction of the monotonic flow strength. This limit may be used for design purposes.[2] As fatigue progresses, the effective Young’s modulus decreases (Figure 1(b)), indicative of damage mechanisms operating in the material.[2] The abrupt increase in strain beyond the nucleation stage occurs in discrete bands which have a thickness equal to about one cell size. The mechanisms responsible for localizing the strains into bands and limiting life have received minimal attention. Optical observations have indicated that the cell walls encompassing some of the relatively large cells are susceptible to plastic buckling at the peak stress[1] (Figure 2). These buckled walls experience larger-than-average cyclic strains and submit to a cyclic softening mechanism that induces and spreads a lateral degradation zone, which eventually collapses the material along an entire band. The exaggerated cyclic plastic strains implied by these observations suggest that heat is generated locally around strain concentrators and that the thermal flux be a measure of the plastic work accompanying each strain cycle. The present article provides a demonstration of this local heating effect and connects the flux to the plastic deformation. It also addresses the possibility that fatigue is governed by thermal softening as a result of the heat generated. Heat generation during crack growth and fatigue has previously been studied in the following two situations.[3–6] (1) In metals having low thermal conductivity, particularly amorphous Ti alloys, appreciable heat is generated during rapid crack extension, with local elevations in temperature around the crack reaching about 20 8C.[3,4] (2) In polymers subject to generalized cyclic straining, appreciable heating has been measured, with temperature elevations in PTFE A. RABIEI, Postdoctoral Fellow, and J.W. HUTCHINSON, Professor, are with the Division of Engineering and Applied Sciences, Harvard University, Cambridge, MA 02138. A.G. EVANS, Professor and Director, is with the Materials Institute, Princeton University, Princeton, NJ 08540. Manuscript submitted May 7, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
exceeding 100 8C at frequencies in the 30 Hz range, again because of low its thermal conductivity.[5,6] To the authors’ knowledge, there have been no studies either in metals or polymers of heat generated locally around stationary cracks subject to cyclic loading. II. EXPERIMENTAL METHODS The thermal measurements are performed on a well-characterized, commercially available, closed-cell Al alloy, having the trade name ALPORAS.[1,7–9] Rectangular compression specimens were cut from castings of this material by electrodischarge machining (EDM), as described else
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