Influence of casting surfaces on fatigue strength of ductile cast iron

Abstract

ABSTRACT Quantitatively evaluating the fatigue strength of ductile iron (DI) with casting surfaces involves several complicated factors such as surface roughness, transition of microstructures from surface to interior, several types of defects and residual stresses. Tension–compression fatigue tests have been performed using DI having casting surfaces composed of a ferritic structure, a ferrite‐pearlitic structure and a pearlitic structure. Residual stresses were relieved by annealing in order to separately evaluate each factor. The parameter model was applied for quantitative evaluation of fatigue strength. Surface roughness was considered to be mechanically equivalent to a defect, and the effective defect size due to the interaction between the surface roughness and a defect was defined. The present study proposes a method of evaluating the maximum defect size using statistics of extremes and the lower bound of the scatter of fatigue strength, for practical design.