Gears are case hardened to produce residual stresses at the surface, which improve wear resistance, bending, and contact fatigue strength. These compressive residual stresses are balanced by tensile stresses in the core. This poses an increased risk of fatigue crack initiation in the material below the surface. Both Tooth Flank Fracture (TFF, also known as Tooth Flank Breakage (TFB)) and Tooth Interior Fatigue Fracture (TIFF) describe a failure mode where a subsurface fatigue crack initiates in the material below the surface, approximately mid-height of the tooth. Previous research has established that the direction in which the crack progresses and the appearance of the associated fracture surface is dependent on the flank loading (i.e. single stage loading versus idler usage). This type of failure can appear at loads below the allowable loading conditions for failure modes, based on international standards (such as ISO 6336). Therefore, understanding of such failure is required at the design stage of geared transmission systems.

The currently proposed approaches for the analysis of TFF and TIFF all have very similar fundamental features, consisting of four stages: calculation of stress history, calculation or specification of residual stresses, calculation of equivalent stresses using a fatigue criterion, and comparison with some initiation thresholds.

MackAldener has shown that an analysis method based on two-dimensional FEA can be utilized to analyze the risk of TIFF in idler gears. However, using general FE packages requires considerable time and computational power to both set-up and run analyses. Therefore, various simplified analytical or empirical methods which reduce these requirements have been proposed in the literature. Unfortunately, these methods introduce limitations on applicability and some compromise on the accuracy of results.

To overcome limitations on applicability and improve the accuracy of the results, the authors previously proposed and validated a methodology where a specialized loaded tooth contact analysis (LTCA) model is utilized to determine load boundary conditions at a selected number of points in the gear tooth mesh cycle. In contrast to a detailed FE analysis, this method allows for quick analysis times, leading to fast optimization.

This study aims to improve the existing understanding of Tooth Interior Fatigue Fracture load capacity and compare calculated load capacity to the allowable loading conditions for bending and pitting fatigue failure, based on the standard calculation procedures. Possible methods that could be used to mitigate TIFF risk are presented, and the effect of these methods on the performance, with respect to the other failure modes, are quantified.