A not so often seen failure mode for gears is tooth flank fracture. It is observed on cylindrical and bevel gears, especially but not exclusively on case carburized gears. The characteristic of the damage is a crack initiation in the region of the active contact area and inside the material, with a crack propagation to the surface of the loaded flank and in the direction of the root area on the other side of the tooth. Most of the cases mentioned in literature occurred on gears with a larger module. The main influence parameters are the local Hertzian contact stress and the hardness profile. The pressure on the flank causes crack initiation below the surface, typically at a preexisting material fault. The crack grows in two directions, into the core of the tooth and towards the surface of the flank. Secondary cracks might occur that are oriented in an angle to the original crack. Typically as soon as the crack is visible the tooth breaks. This is a difference to the root fracture, where the crack initiates at the surface and special ink can be used to find cracks before the gear totally fails. For tooth flank fracture a new standard is in progress, ISO DTS 19042 [3]. It is in the very early stages, however, the outline of the method itself is already quite fixed. It is based on the work from Witzig [6], which itself goes back to Annast [4] and Bruckmeier [5]. This paper refers to the version from 18th of February, 2015. Since changes are currently very frequent, some of the observations below might already be outdated. The method as far as it is developed now is based on a local analysis of the Hertzian contact stress and the hardness profile at arbitrary points of the active area of the tooth flank. Therefore the integration of the calculation in the gear contact analysis is a natural way of implementing it. Along with the functions to vary parameters this is a suitable platform to gain first experiences with the new method and to perform sensitivity studies. The paper presents the first results obtained from a systematic variation of gear parameters, to show the influence of these parameters as modeled by the method.
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