The effects of material hardness on the maximum size of a slit, which was rendered harmless by shot peening, were investigated. Specimens of two types of hardness were made of spring steel (SUP9), and a semi-circular slit was introduced by using an end mill. After the shot-peening process, bending fatigue tests with a stress ratio of R = 0.05 were carried out in air. The maximum depth of slit rendered harmless by shot peening for 270 HV and 350 HV were α0p = 0.2 and 0.3 mm, respectively. Considering the experimental results of the present and previous works, the hardness did not have a large influence on the size of α0p. In order to predict the size of α0p, the range of the stress intensity factors at the semi-circular crack caused by the residual stress and applied stress during the fatigue test were analyzed. The calculated result corresponded well with the experimental result.
