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The relationship between discontinuous yielding and cyclic behavior in polycrystalline molybdenum
Authors:P. Beardmore  P. H. Thornton
Affiliation:1. Scientific Laboratory, Ford Motor Company, Dearborn, Mich.
Abstract:The fatigue properties of polycrystalline molybdenum at room temperature have been determined by axial tension-compression tests in both stress and strain control modes to give lives of up to 106 cycles. Under the appropriate conditions, the endurance limit can be considerably reduced,e.g., the stress to cause failure in 106 cycles decreases from ~42,000 psi at 5 cps to ~28,000 psi at 0.5 cps. The fatigue resistance as assessed by either strain control or by stress control is discussed in relation to the tensile and creep behavior, and it is concluded that the reduction in the long life fatigue resistance with decrease in the cyclic frequency is due to the time dependent generation of a mobile dislocation population. The previously accepted value of 0.6 for the ratioFL/UTS for bcc metals can be attributed to this frequency effect. A lower value for this ratio, 0.4, can be derived from the high frequency data by the use of an extrapolation technique. This lower value is confirmed by long life, low frequency tests, in agreement with the value for fcc metals. The importance of obtaining fatigue data under comparable states of mobile dislocation density is emphasized, particularly for materials exhibiting pronounced tensile yield points, in order that meaningful comparisons can be made between different materials.
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