Evaluating the fatigue cracking risk of surface strengthened 50CrMnMoVNb spring steel with abnormal life time distribution

Surface strengthening is an important method which can effectively improve the fatigue property of metallic materials. In order to improve the fatigue property, for the first time, a new developed surface spinning strengthening-II (3S-II) method and the traditional shot peening (SP) treatment were applied to modify the microstructure and fatigue performance in the surface layer of 50CrMnMoVNb spring steel. It is interesting to find that the fatigue lives of the samples treated by the 3S-II method show obvious polarization characteristics (either quite higher or slightly lower than those treated by SP method) at the high stress amplitude, which does not follow the traditional life distribution rules and cannot be evaluated by the S-N curve. Moreover, the fatigue lives of the samples with the fatigue crack initiated from the surface defects are extremely lower than those of the samples whose fatigue crack initiated from the subsurface. Therefore, a fatigue risk factor, "Rf", was proposed and developed based on the competition between the applied stress and the material property to evaluate the fatigue cracking risk of the surface strengthened 50CrMnMoVNb spring steel, especially the fatigue risk of the samples influenced by the mixed factors. The calculation results show that the position where Rf max appears is exactly the position of fatigue crack initiation. As a result, "Rf" can be used to quantitatively evaluate the fatigue risk of metallic material under the influence of a variety of factors, including surface strengthening, surface roughness and residual compressive stress and so on.