AN INVESTIGATION OF CYCLIC TRANSIENT-BEHAVIOR AND IMPLICATIONS ON FATIGUE LIFE ESTIMATES

Current research focuses on proportional cyclic hardening and non-Mass ing behaviors. The interaction of these two hardenings can result in t he traditionally observed overall softening, hardening or mixed behavi or exhibited for fully reversed strain controlled fatigue tests. Propo rtional experiments were conducted with five materials, 304 stainless steel, normalized 1070 and 1045 steels, and 7075-T6 and 6061-T6 alumin um alloys. All the materials display similar trends, but the 304 stain less steel shows the most pronounced transient behavior and will be di scussed in detail. Existing algorithms for this behavior are evaluated in light of the recent experiments, and refinements to the Armstrong- Frederick class of incremental plasticity models are proposed. Modific ations implemented are more extensive than the traditional variation o f yield stress, and a traditional strain based memory surface is utili zed to track deformation history. implications of the deformation char acteristics with regard to fatigue life estimation, especially variabl e amplitude loading, will be examined. The high-low step loading is ut ilized to illustrate the effect of transient deformation on fatigue li fe estimation procedures, and their relationship to the observed and m odeled deformation.