Mechanical Stress Relaxation of a Laser Peened and Shot Peened Ni-Based Superalloy

Abstract

Nickel based superalloy IN718 specimens were subjected to laser peening and shot peening. The residual stress and work hardening introduced by laser peening and shot peening were characterized using neutron diffraction method and electron backscattered diffraction (EBSD). A modified set up in reflective mode was utilized during neutron diffraction to optimize the spatial and temporal resolution to perform in-situ residual stress measurements at pre-determined cycle. Residual stress relaxation was only observed in the direction of loading while the residual stress in the transverse direction remained at a similar magnitude. Residual stress relaxation was observed to be most prominent in the first cycle of fatigue at R ratio = 0.1 with little stress relaxation in subsequent fatigue load cycles. Under tensile-tensile loading, stress relaxation occurs when the superposition of tensile residual stress and applied loading exceeds the localized yield strength of the material. Stress relaxation was found to be well correlated with the magnitude of work hardening. Residual stress relaxation as a function of depth and number of cycles were also recorded to illustrate the changes in residual stress during the cyclic loading.