Bending fatigue strength of carburized gears with an artificial notch at the fillet

The fatigue strength of carburized gears is influenced by the latent defect in the surface layer. To clarify this influence, the bending fatigue strength of carburized gears with an artificial notch is evaluated in this paper. The notches are processed by wire-cut electrical discharge machining (WEDM) and focused ion beam (FIB) etching to obtain the required depth. The fatigue strength of the tooth with the notch over 35 mu m in depth decreases markedly in comparison with the nonnotched tooth. In the case where the notch is shallower than 20 mu m, cracks initiated from the notch are not observed and the strength is not reduced. By assuming a crack of the same length as the notch, the stress intensity factor K-1 is calculated considering the effect of residual stress. The strength expected from the stress intensity factor range is higher than the obtained strength of the gear with the notch.