Evaluation of Fatigue Strength of Ductile Cast Iron Composed of Ferrite-pearlitic Structures with As-cast Surfaces (Effect of Surface Roughness, Microstructure Transition Layers, Defects and Residual Stress on Fatigue Strength)

Abstract

In order to clarify the complex effects of surface roughness, transition of microstructures from surface to interior, defects and residual stress, tension-compression fatigue tests have been conducted by using shot blasted ferrite-pearlitic ductile cast irons with as-cast surfaces. Regardless of applied stress levels, the fracture origins of specimens were mostly at a defect near an as-cast surface, slag or pinhole, at Nf<107. At a relatively lower stress amplitude, some fatigue fractures occurred from an inner defect far from the surface, shrinkage, mostly at Nf>107. The √area parameter model was applied to the quantitative evaluation of fatigue limit. The equivalent hardness considering transitional layers and the effective defect size with the interaction between surface roughness and a defect were defined. Moreover, the relief of residual stress during fatigue tests was detected. The mean value of the relieved residual stress acting on a defect was employed for the quantitative evaluation. By using these parameters, the complex effects of ductile cast irons have been evaluated successfully for practical use.