Statistical analysis of high cycle fatigue life and inclusion size distribution in shot peened 300M steel

In order to fully exploit the benefits of shot peening process in terms of fatigue performance, the proper choice of peening conditions is indispensable, which is largely material specific. In this study, the influence of 3 different peening conditions on the axial fatigue life of high strength 300M steel (modified AISI 4340) was investigated in the high cycle fatigue regime. A statistical estimate of 90% survival life at 95% confidence interval was made to rigorously compare the fatigue performance of the material with different surface conditions. The peening induced roughness, compressive residual stresses and their relaxation during cyclic loading were quantified. It was however revealed, that shot peening had no significant effect on this material when tested at 55% of yield strength (σy,0.2). The fatigue life was dominated by the larger non-metallic inclusions within the bulk material. An empirical relationship between inclusion size and fatigue life was proposed based on this observation. Furthermore, the inclusion size distribution was well described using Gumbel extreme value distribution allowing the largest inclusion to be predicted in a given volume of steel. The prediction results were in good agreement with the experimentally observed value of the largest inclusion size observed at a fatigue crack initiation site.