Effects of shot peening on the bending fatigue limit of spring steel (SUP9A) containing an artificial semi-circular slit were investigated. Shot peening (SP) and stress shot peening (SSP) were conducted on the specimens containing an artificial semi-circular slit with a depth of a = 0.1, 0.2 and 0.3mm. Then, bending fatigue tests were conducted on the specimens. The fatigue limit was improved by SP and SSP. In the case of SP specimens and SSP specimens, the specimen having a semi-circular slit under a = 0.2mm fractured outside the slit, and they had considerably high fatigue limits almost equal to those of the shot-peened smooth specimens. Therefore, semi-circular slit whose depth of under a = 0.2mm could be made non-damaging by SP or SSP. It was found that the fatigue limit of specimens having a semi-circular slit which received SP or SSP determined by the threshold condition for non-propagation of fatigue cracks emanated outside the slit. The criterion that the semi-circular slit is non-damaging or not is decided by relationship between the stress intensity factor range of semi-circular crack and threshold stress intensity factor range.
