Quantitative evaluation of effects of non-metallic inclusions on fatigue strength of high strength steels. I: Basic fatigue mechanism and evaluation of correlation between the fatigue fracture stress and the size and location of non-metallic inclusions

Abstract The effects of non-metallic inclusions on the fatigue strength of metals are reviewed and the major factors involved are discussed. It is emphasized that the effects of non-metallic inclusions must be analysed from the viewpoint of small defects or small cracks, because the threshold condition at the fatigue limit is not that for crack initiation but that for the non-propagation of a crack emanating from defects or inclusions. To accommodate this point the equations for the prediction of the threshold stress intensity factor range, ΔK th , and the fatigue limit, σ w , for defects and small cracks were extended to predict those for inclusions contained in high strength steels. It is demonstrated that the square root of the projected area of inclusions and the Vickers hardness, Hv , of the matrix are the crucial parameters in prediction of the fatigue limit of metals containing inclusions. The predictions given by the proposed equations were in very good agreement with the experimental results obtained from the fracture surface showing fish-eye patterns. The reasons why the σ w of high strength steels does not increase linearly with increasing hardness, and the reason why the scatters of fatigue limit are so large, are clarified.