Improvement of fatigue limit by shot peening for high-strength steel containing a crack-like surface defect Influence of stress ratio

Purpose - The purpose of this study is to clarify the influence of stress ratio (R) on the effects of shot peening (SP) on the fatigue limit of high-strength steel containing an artificial small defect. Design/methodology/approach - SP was subjected on the specimens with a semi-circular slit with a depth of a = 0.1, 0.2 and 0.3 mm. Then, bending fatigue tests were carried out under R = 0.4. Findings - The fatigue limits of specimens with a semi-circular slit were improved by SP under R = 0.4. The fatigue limits of the SP specimens with a semi-circular slit under a = 0.2 mm fractured outside the slit, and they had considerably high fatigue limits equal to specimens without a slit. Therefore, a semi-circular slit with a depth of under a = 0.2 mm could be rendered harmless by SP under R = 0.4. Compared to the results of R = 0, the increasing ratios of fatigue limits under R = 0.4 were lower than those under R = 0. However, the size of semi-circular slit that could be rendered harmless by SP was same. In addition, it was found that whether the semi-circular slit is rendered harmless or not is decided by the relationship between the stress intensity factor range of semi-circular cracks and the threshold stress intensity factor regardless of stress ratio. Practical implications - The proposed method can be applied to mechanical parts used in vehicles, aircraft and trains. Originality/value - This is the first paper to investigate the fatigue limits after SP in materials containing a surface defect under positive stress ratio. In this study, the authors investigated the influence of stress ratio on the effects of SP on the fatigue limit containing a surface defects.