A Coupled Creep-Plasticity Model for Residual Stress Relaxation of a Shot-Peened Nickel-Base Superalloy

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 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. The original document contains color images. All DTIC reproductions will be in black and white.