Evolution of residual stress redistribution associated with localized surface microcracking in shot-peened medium-carbon steel during fatigue test

Shot peening (SP) is a conventional method used for improving material properties, especially fatigue strength, through work hardening and the induction of compressive residual stress (CRS) near the surface by plastic deformation. However, CRS is redistributed and relaxed by the occurrence of physical discontinuities such as microcracks. In this study, the effects of residual stress redistribution and relaxation during the fatigue life associated with microdamages on the properties of a material were considered. To this end, annealed medium-carbon steel was treated with SP at three levels of peening intensity to investigate the effects on fatigue life and residual stress distributions. Rotating–bending fatigue tests were carried out to clarify the fatigue life distributions, and X-ray diffraction and scanning electron microscopy were used for residual stress and full-width at half-maximum (FWHM) values, microscopic inspections, respectively. The results indicate that microcracks at the treated surface significantly influenced stress redistribution, depending on the initial residual stress distribution at the surface. Moreover, when the induced CRS was relaxed during mechanical loading, these microcracks caused fatigue life degradation regardless of peening treatment. The effects of surface microcracks on stress redistribution and relaxation were discussed and a valuable range of peening conditions of used material was proposed.