Fracture analysis of fisheye failures in the tooth root fillet of high-strength gears made out of ultra-clean gear steels

In shot-peened components, failures can be caused by a crack initiation below the surface at a non-metallic inclusion in the steel matrix. This fracture mechanism is called “fisheye failure”. In addition, this failure type can lead to a decreased endurance fatigue limit for the component. By preventing this crack mechanism, higher bending strength is expected to be achievable. Various research by universities and research institutes has taken place in this field using standard specimen and sometimes special alloy systems. However, in the tooth root of a gear, a complex multiaxial stress condition is present. Furthermore, the most commonly gears used in transmissions are made out of case-hardened wrought steels. Therefore, in the gear industry, there is only limited knowledge available about this failure type and the failure characteristics, so far. The first step to gain more knowledge requires extended and proper characterization of the crack area. Furthermore, in standardized gear calculation methods a consideration of a crack initiation below the surface at non-metallic inclusions is not possible, yet. Therefore, extended model approaches are still necessary. However, the reliability of such model approaches depends strongly on the quality of the input data, which can only be ensured by a proper fracture surface analysis. As a starting point, the aim of this paper is to characterize systematically the fracture area characteristics in the tooth root fillet of case-hardened, shot-peened gears in detail according to a presented approach to further extend a presented extended model approach in the medium term.