Estimation of the Fatigue Strength of Iron Castings by X-Ray Stress Measurement Technique

Abstract

The relation between the strength of surface layer measured by X-rays under static bending and the fatigue strength was investigated on various iron castings.Fine flake graphite cast iron, malleable cast iron and spheroidal graphite cast iron with ferritic, sorbitic and pearlitic matrix were used for the materials.The nominal bending stress and the X-ray stress of iron castings were related linearly in the early steps of loading. Then passing through some non-linear range, the values of X-ray stress became constant. The nominal bending stress, where the relationship ceased to be linear, were designated the proportional limit of the surface layer σeX, and the nominal stress, where the X-ray stress became constant, was designated the yield stress of the surface layer σsX.The endurance limit σw' of iron castings was confirmed by the rotating bending fatigue test.The yield stress σsM and rotating bending endurance limit σ'wM of steels, which corresponded to the matrix of iron castings, was approximately estimated from the micro-structure and hardness of the matrix.σ'wM/σ'w which represented the ratio of endurance limit of steel and iron casting was defined as fatigue notch factor by graphite βg. σsM/σeX or σsM/σsX corresponded to the ratio of the yield strength of matrix to the statical strength of surface layer of iron casting.The fatigue notch factor by graphite βg showed a linear relation to the ratio σsM/σeX or σsM/σsX and fairly agreed to the latter in value, despite the difference in the micro-structure or hardness of the iron castings tested.Consequently, the endurance limit of iron castings could be obtained from estimation of the following expression.σw'=σ'wM/βg=σ'wM/σsM·σsX.