Relaxation of residual stress in shot peened Udimet 720Li under high temperature isothermal fatigue

The extent of residual stress relaxation in turbine disc material Udimet 720Li was measured using laboratory X-rays with the sin2ψ technique, for fatigue samples as a function of temperature and number of fatigue cycles for strain controlled loading to 1.2%. Results showed that extensive relaxation occurs upon the initial fatigue cycle. The maximum compressive residual stress (RS) parallel to the loading direction is found to decrease by 50% for all testing temperatures. The extent of relaxation upon further cycling increased with temperature. In the plastically deformed near surface region, the diffraction peak width decreased with increasing testing temperature and number of fatigue cycles (and exposure time), indicating that the relaxation of cold work is controlled by both thermal and mechanical processes.