Interior fatigue fracture of gear teeth

A new type of gear tooth fatigue failure is presented and analysed. It is initiated in the interior of the tooth and is given the self-explanatory name tooth interior fatigue fracture (TIFF).
A fractographic examination of TIFF is presented. The fracture surface of TIFF has characteristic features that distinguish it from other gear failures.
A hypothesis for TIFF initiation is presented. The crack-initiating stresses in the interior of the gear tooth are: (i) a residual tensile stress due to case hardening; and (ii) alternating stresses due to idler usage of the gear.
A finite element analysis is used to compute the stress state history of engaging gear teeth throughout the load cycle. The critical plane fatigue criterion of McDiarmid is employed. The analysis shows that the risk of fatigue initiation is high in the root of the tooth and in a large region in the interior of the tooth. These findings agree with test results where both modes of failure occurred.