Effects of Portable Pneumatic needle-Peening (PPP) on the bending fatigue limit of type 316 stainless steel welded joint containing an artificial semi-circular slit on weld toe were investigated. PPP were conducted on the weld toe containing a semi-circular slit with a depth of a=1. 0 and 1. 5mm. Then, bending fatigue tests were conducted on the specimens. The fatigue limit of the peened specimen having a semi-circular slit of a=1. 0mm was equal to that of the peened welded test specimen without a slit. Therefore, a semi-circular slit less than a=1. 0mm could be rendered harmless by PPP. In addition, it was found that whether the semi-circular slit in weld toe 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. Furthermore, the bending fatigue limit before and after PPP could be predicted by modified Goodman diagram considering effects of change of residual stress, factor of stress concentration and hardness of weld toe.
