Cyclic Plastic Response and Damage in Materials for High Temperature Applications*

Abstract

The characteristic changes of the cyclic plastic stress-strain response of stainless steel and superalloys derived from the analysis of hysteresis loop at different temperatures are reported. The evolution of the surface relief in strain controlled cycling is documented using high resolution SEM and TEM. The mechanisms leading to cyclic strain localization, formation of surface relief, fatigue crack initiation and early crack growth are discussed. The study of the low cycle fatigue behavior of these materials was concentrated mostly to the analysis of the stress-strain response, internal structures, fatigue life and crack growth (1-3). In recent years, considerable attention has been attracted to the study of the early damage mechanisms in crystalline materials, starting with the cyclic slip localization and resulting finally in the initiation of fatigue cracks and their early growth (4, 5). In this contribution, the methods allowing to study the sources of cyclic stress and early fatigue damage at room and elevated temperatures are presented and used to reveal details of the mechanisms of cyclic plastic straining and fatigue damage evolution in austenitic steels and nickel superalloys. 1. Cyclic Plastic Stress-Strain Response. Cyclic plasticity of materials is usually studied in strain- controlled cycling with constant strain rate, since stress response of the material depends not only on temperature but also on the strain rate. In a standard low cycle fatigue experiment, several specimens are cycled in a symmetrical cycle with constant total or plastic strain amplitudes, and their stress response is recorded and evaluated. Hysteresis loops can be stored in computer memory, and stress, strain and plastic strain amplitudes are evaluated during the fatigue life. The plot of the stress amplitudes vs. number of loading cycles represents the fatigue hardening/softening curves. The plot of the saturated stress amplitude (or stress amplitude at half-life) vs. strain or plastic strain amplitude represents the cyclic stress-strain curve. In order to find the sources of the cyclic stress, more detailed analysis is necessary. Hysteresis loop and the analysis of its shape using the generalized statistical theory (6) can yield information on the sources of the cyclic