Investigation of the region of fatigue crack initiation in a transmission gear

The region of fatigue crack initiation in a cylindrical transmission gear was investigated by mapping the distribution of residual stress and hardness at different distances from the surface. Using these values of residual stress and hardness, the fatigue limits for single tooth bending were obtained at different distances from the surface. Fractographic observation proved that fatigue and fatigue cracks began at the subsurface, about 0.25–0.45 mm below the surface. A rational design of the surface strengthening process can predict and control the region of fatigue crack initiation.