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Abstract
The fatigue behavior of carburized components such as automotive transmission gears is very complex due to hardness and microstructure difference, residual stresses and multi-axial stress states developed between the case and the core. In addition, automotive gears in service, commonly used in helical type, are actually subjected to complex stress conditions such as bending, torsion, and contact stress states. This study presents experimental and analytical results on deformation behavior of carburized steels, widely used in automotive gears, under cyclic stress conditions including axial and torsion loadings. Axial fatigue tests and rotating bending fatigue tests are also included. Predictions of cyclic deformation and fatigue behaviors of the carburized steel with two-layer model are compared with experimental results. The carburized steel investigated in this study exhibited cyclic softening under both axial loading and torsional loading. Predicted results with simple two-layer model for the cyclic deformation and fatigue behaviors were comparatively similar to the experimental data.
Highlights
Carburizing, one of the case hardening heat treatments, has been widely used in automotive parts due to significant improvements in component durability at a comparatively economic cost
Residual stresses developed by case hardening heat treatment and Poisson’s ratio mismatch between the hard elastic case and the soft core usually cause multi-axial stress states even when loaded with uniaxial load
There has been a little research on the deformation and fatigue behavior of carburized steels with case-core sections under complex stress conditions, they have been used in automotive transmission for many years
Summary
Carburizing, one of the case hardening heat treatments, has been widely used in automotive parts due to significant improvements in component durability at a comparatively economic cost. Considering that the shafts have multiple hardness layers with constant cyclic deformation and fatigue properties, prediction for crack nucleation life was made by using Morrow and Smith-Watson-Topper mean stress correction model They showed that predicted life was good agreement with the experimental fatigue lives. Sub-surface failure in the long life regime and surface failure in the short life region occurred, and, the shift from surface to sub-surface nucleation occurred at the intersection of the strain-life curves of the case and the core materials They used both a two-layer model and a four-layer model to predict fatigue life, resulting in a good match between the experimental and calculated results for the carburized specimens except for some points in the very short life region, when applying the upper-bound method. Predictions of cyclic deformation and fatigue behaviors of the carburized steels investigated using a two-layer model are presented and compared with experimental results
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