Analytical Prediction of Shot Peening Effects on Nickel-Based Superalloy Inconel 718 Fatigue Life

Shot peening is a surface treatment largely used in the aerospace industry. This cold working process consists in impinging particles, called shots, at high velocity onto a metallic part surface. The shots plastically deform the treated surface. The process induces surface compressive residual stresses and work hardening. Compressive residual stresses and cold work delay crack initiation and retard short crack propagation at the part surface. The surface is a critical location for crack initiation and shot peening increases service life by pushing crack initiation underneath the surface. Despite its positive effects on fatigue, shot peening also creates dimples that are stress concentration features which promote crack initiation and propagation. Moreover, when impacting edges, the shots induce plastic deformations in the form of burrs. These burrs, called rolled edges, are highly critical for crack initiation and short crack propagation. Shot peening is thus a process that must be optimized. Shot peening at a low intensity will not induce sufficiently compressive residual stresses and cold work to improve the fatigue life and shot peening at a too high intensity will be detrimental to fatigue life due to the formation of severe surface stress concentration features. Optimal shot peening parameters are usually determined by trial and error and shot peening is often not accounted for during the design stages of a component, but rather considered as a safety factor. This thesis aims at developing an analytical tool to predict fatigue life, and crack initiation and propagation mechanisms on shot peened nickel-based superalloy Inconel 718 by accounting for the shot peening and the loading conditions. Such a model could allow, in the future, to reduce the costs associated with the optimization of the shot peening conditions and to finally account for its beneficial effects on fatigue life during design stages. The first step consisted in performing an extensive experimental campaign to characterize, at a microscale and a macroscale, the evolution of peened and unpeened Inconel 718 mechanical properties under cyclic loading. The tests varied from the simplest quasi-static tensile test to the most complex techniques such as the measurement of the residual stresses redistribution, the rupture surfaces observations under a scanning electron microscope or the short cracks propagation monitoring. It was shown that the fatigue life can be increased by a factor of twenty in high cycle fatigue under optimal shot peening conditions. On the other hand, it was observed that shot peening is not beneficial, even detrimental, in low cycle fatigue.