Evaluation of fatigue strength and inclusion size for internal crack initiation of Cr-Ni-W steel

In view of the interior fatigue failure induced by the non-metallic inclusion for Cr-Ni-W steel in the ultra-long life regime beyond 107 cycles, four statistical methods, such as statistics of extreme values (SEV), generalized extreme values (GEV), generalized Pareto distribution (GPD) and exponential generalized Pareto distribution (EXPGPD), were adopted to evaluate the characteristic inclusion size, and then to predict fatigue strength in combination with the Murakami equation. As a result, the evaluated inclusion sizes by using SEV and EXPGPD methods monotonically increase with increasing of steel volume, whereas those by using GPD and GEV methods first tend to increase, and then gradually reach a constant value. Among them, the upper and lower limit values for inclusion size are obtained by using the EXPGPD and GPD methods, respectively. Compared with the experimental fatigue strength corresponding to the fatigue life of 108 cycles, the predicted fatigue strength by using the EXPGPD method is somewhat lower, whereas that by using the other methods is higher. From the viewpoint of anti-fatigue, the evaluation of characteristic inclusion size and prediction of fatigue strength by using the EXPGPD method is suitable for Cr-Ni-W steel with the volume larger than 1 mm3.