Using Weibull distribution function to determine the design bounds for carburized 4320 steel shafts subjected to bending dominated fatigue loading

Abstract

The differential pinion shaft in an axle is often heat-treated to high surface hardness to withstand the extreme bending fatigue loading conditions. The carburized pinion shaft made of 4320 steel material was identified as the primary fracture mode when subjected to various fatigue loading conditions. However, minimal data is available to understand the fatigue behavior of carburized 4320 steel shafts. This research proposes a prediction approach to determine the fatigue life of carburized 4320 steel shafts. Since hardness varies rapidly through the carburized specimen's depth, subsurface crack initiations are likely. The fatigue prediction at the shaft subsurface was made possible by creating a set of hardness-based strain-life curves through the depth of the shaft specimen. Since all the carburized shafts in the experiment had electroless nickel plating, this research could capture the unique fatigue behavior by introducing a surface correction factor. Mean stress correction to the strain-life curves was investigated to account for the shaft failure in the case of tensile-dominated loading conditions. The performance of prominent mean stress correction models was compared by correlating with the experimental data. An analytical model was proposed since the standard models were unable to capture the physics completely. This proposed model showed excellent correlation with test data. Finally, this research implemented a Weibull-statistics-based reliability estimation approach to determine the design bounds for the carburized pinion shaft.