Evaluation of Fatigue Strength of Ductile Cast Iron Composed of Ferrite-Pearlite Structures with Casting Surfaces (Eifect of Surface Roughness, Microstructure Transition Layers and Defects on Fatigue Strength)

Abstract

Ductile cast iron is often used for automobile parts as having cast surfaces with shot blast. Influences of surface roughness, microstructure transition layers, a lot of kinds of defects and residual stresses on fatigue strength are very complicated. The microstructure in the cast transition layer contains a work hardening layer due to shot blast and a pearlitic layer or a ferritic layer produced by casting. To clarify the effect of surface roughness, microstructure transition layers and defects on fatigue strength, tension-compression fatigue tests have been carried out on the specimens composed of a ferrite-pearlitic structure. The residual stresses were relieved by annealing to evaluate various factors independently. The fatigue strength has been evaluated using the √area parameter model. The effective defect size √areaeff with the interaction between surface roughness and defects has been defined so as to evaluate the effect of surface roughness on fatigue strength. The evaluation of the maximum defect size by statistics of extreme and the lower bound of the scatter of fatigue strength have been proposed for practical design.