| Shot peening has been employed in numerous industries for decades to impart beneficial compressive residual stresses on the surface of metal components. Compressive residual stresses retard initiation of surface cracks and therefore improve fatigue resistance and fatigue life. For elastic material behavior, shot-peened residual stresses remain stable under cyclic loading. Under elastic conditions, accurate fatigue life predictions, including credit for residual stresses, are possible for complex geometries with complicated load histories. For inelastic material behavior, shot-peened residual stresses may change continuously under cyclic loading, or elevated temperature static loading, such as thermal exposure and creep. Under inelastic conditions, taking full credit for compressive residual stresses would result in a nonconservative life prediction. As a result, designers are reluctant to incorporate any compressive residual stresses into fatigue life predictions of turbine engine components, subject to elevated temperatures and inelastic loading conditions. Identification and characterization of the underlying rate-controlling deformation mechanism is required for development of a reliable relaxation model for shot-peened materials. Results from this study, as well as the work of others, have shown that prior plastic strain affects creep rate, total creep deformation, and time to rupture. Creep tests on IN100 with room temperature prestrain exhibit a decrease in primary and secondary creep rate with increasing levels of prior plastic strain. Therefore, inclusion of the shot peening deformation (prior plastic strain) into a coupled creep-plasticity model is essential for accurate prediction of creep rate and residual stress relaxation. This research describes a methodical approach for characterizing and modeling residual stress relaxation under elevated temperature loading, near and above the monotonic yield strength of IN100. The approach includes strategic experiments to identify the rate-controlling deformation mechanism in creep of IN100, including the effects of prior plastic strain. The model incorporates the dominant creep deformation mechanism, coupling between the creep and plasticity model, and effects of prior plastic strain. Model predictions correlate well with experimental results on shot-peened dogbone specimens subject to cyclic and creep loading conditions at elevated temperature. |
