Effects of Surface Roughness and Residual Stress on Fatigue Strength of Shot Blasted Ductile Cast Irons with Casting Surfaces
Junichiro YAMABE and Motokazu KOBAYASHI
Abstract:Fatigue fractures of actual parts mostly start from the sites of stress concentrations at structural discontinuities. In an early study, the fatigue strength of shot blasted ductile cast irons with casting surfaces, the fracture origins of which were at defects, was quantitatively evaluated by using the ćarea parameter model. However, since some fatigue fractures at the sites of stress concentrations start from surfaces, the quantitative evaluation of the fatigue strength of ductile cast irons whose fatigue origins are at surfaces is also important. In this study, shot blasted ductile cast irons with casting surfaces were employed and plane bending fatigue tests have been conducted for evaluating the fatigue strength of the specimens whose fatigue origins are at surfaces. For quantitatively evaluating the influences of surface roughness and transitional layers at the site around casting surfaces, the defect size ćareaR for the surface roughness and the equivalent hardness HVeq considering the transitional layers were applied. Moreover, the relief of residual stress during fatigue tests was detected. The relieved residual stress was regarded to be mechanically equivalent to the mean stress. By using these parameters, the fatigue strength of shot blasted ductile cast irons whose fatigue origins are at surfaces has been successfully evaluated. Furthermore, fatigue tests using an actual part have been performed and the usefulness of the quantitative evaluation method has been verified. Key Words:Fatigue strength, Ductile cast iron, Surface roughness, Casting defect, Residual stress, Statistics of extremes, The ćarea parameter model