Evaluation of fatigue limit and harmless crack size of needle peened offshore structure steel F690

In this study, a 0.4-1.6 mm deep semicircular crack was machined in DNV F690 offshore structural steel with an aspect ratio (As) of 1.0. After needle peening (NP), four-point bending fatigue tests were conducted. The fatigue limit estimated using the equation proposed by Ando et al. decreased with increasing crack size, but agreed with that obtained in the four-point bending fatigue test. The compressive residual stress increased the fatigue limit of the NP smooth and crack specimens. The fatigue limit of the NP crack specimen with a crack depth a = 0.8 mm was over 95 % of that of the smooth specimen. Semicircular cracks with crack depths up to a = 0.8 mm could be rendered harmless by NP, however, this was not the case for the NP crack specimen having a = 1.2 mm. It was estimated that the harmless crack depth of the NP crack specimen was a = 1.14 mm