This study presents an experimental approach based on partial tooth meshing contact fatigue with constant amplitude and different lengths of load cycles. To achieve this, 20CrH helical gears were considered, limiting the region of interest to one-third of the tooth to accelerate crack initiation and propagation. Different factors, such as number of fatigue cycles, tooth flank morphology, residual stress, hardness, and microstructure evolution under fatigue, were considered. The ultimate goal was to relate these variations with the observed failure modes. The results showed that with increasing cycle number, the residual stress at the same position in the meshing zone of the tooth flank increased first and then decreased. The residual stress values at the lower position of the pitch circle were larger than that of the addendum and dedendum. After cyclic loading, the content of the twin martensite within the microstructure below the pitch circle decreased and the corresponding hardness value of the tooth flank was reduced. After long-term cyclic loading, the precipitated phases aggregated and grew and the discontinuous distribution hindered the grain boundary slip. The dislocation density increased, and the pile-up of dislocation resulted in stress concentration at the grain boundary and the phase boundary, which induced the initiation of fatigue crack of the tooth flank.
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