Low core hardness in a carburized bicycle pedal shaft allows yielding in the core region when the shaft carries a bending load. Quantitatively evaluating the effect of core hardness level requires simulation of the gradient in material properties from the high-hardness thin outer case to the low-hardness core. This study develops a scheme for generating a high-resolution finite element mesh near the outside diameter of the shaft, coupled with a method for specifying elastic-plastic stress-strain curves which vary with depth below the carburized surface. The method enables examination of the stress localization and intensification in the case when yielding occurs in the core. The results show the insufficiency of the linear elastic assumption, and explain failures of shafts with anomalously low core hardness.

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