Quantitative Evaluation of Effects of Nonmetallic Inclusions on Fatigue Strength of High Strength Steel

Abstract

First, the effects of nonmetallic inclusions on the fatique strength of metals are reviewed and the influential factors are revealed. Next, it is emphasized that the effects of nonmetallic inclusions must be analyzed from the viewpoint of small defects or small cracks, because the threshold condition at the fatigue limit is not the condition for crack initiation but the condition for the non-propagation of a crack emanating from defects or inclusions. Finally, from this point of view, the equation for the prediction of the threshold stress intensity factor range ΔKth and fatigue limit aw for defects and small cracks was applied 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 Hv of the matrix are the crucial parameters to predict the fatigue limit of metals containing inclusions. The predictions 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 fatigue limit aw of high strength steels does not increase linearly with increasing hardness, and why the scatters of fatigue limit are so large are made clear. © 1988, The Japan Society of Mechanical Engineers. All rights reserved.