Evaluation of Fatigue Strength of Ductile Cast Iron Composed of Ferritic and Pearlitic Structures with As-cast Surface (Effect of Surface Roughness and Microstructure Transition Layers on Fatigue Strength)

Abstract

For evaluating fatigue strength of ductile cast irons, influences of many complicated factors such as surface roughness, transition of microstructures from surface to interior, defects and residual stresses must be considered separately. In order to clarify the effects of surface roughness and microstructure transition layers on fatigue strength, plane bending fatigue tests have been carried out on the specimens with as-cast surface, composed of ferritic and pearlitic structures. The residual stress introduced by shot blast to surface layers of a specimen was relieved by annealing in order to evaluate the independent influence of various factors separately. The fatigue strength has been evaluated by using the √area parameter model. Surface roughness was regarded to be mechanically equivalent to a defect. The size of the defect has been defined by the projected square root of area, √areaR.The equivalent hardness HV was also defined by considering microstructure transition layers. The evaluation method of the maximum detect size by statistics of extreme and the lower bound of the scatter of the fatigue strength have been proposed for practical design.