Tooth flank fracture in spiral bevel gears - Multiaxial fatigue and material properties

Due to the improvements made in the prediction and prevention of surfaceinitiated gear failure modes like pitting, tooth root breakage, scuffing and micropitting, a transition from surface to subsurface-initiated failures is occurring on spiral bevel gears. The herein presented work continues the efforts of the Improved reliability of thrusters joint industry project - a cooperation between thruster suppliers, gear manufacturers, steel suppliers, forging companies and the maritime classification society - and aims at the prediction and prevention of subsurface initiated fatigue or tooth flank fracture in large maritime spiral bevel gears. In this thesis, the three relevant building blocks for a successful tooth flank fracture prediction, namely a material model for case hardened CrNiMo steel, a numerical stress prediction in the gear tooth’s mean cross-section and a, for rolling contact fatigue applicable, multiaxial fatigue criterion are proposed and verified. Tooth flank fracture is not a new failure mode but has been increasing in severity since the beginning of the 21st century. This thesis represents thereby a continuation of the works done by the FZG and Forschungsvereinigung Antriebstechni (FVA), the International Organization for Standardization (ISO), Det Norske Veritas (DNV) and researchers like MackAldener and Weber, to name a few. It focuses on large maritime bevel gears and extends its applicability to other gear sizes through the derived size and lifetime factors. Numerous uniaxial and shear fatigue tests under alternating and oscillating stresses in the high cycle and very high cycle fatigue regime along with load-controlled bevel gear tests were carried out to derive a material model for case hardened CrNiMo steel and to verify the proposed multiaxial fatigue criterion, the predicted material utilisation and failure origins (i.e. the initiation depths in the studied gear teeth). While the gear failure mode tooth flank fracture was the focus of this academic work, qualitative evidence is presented for the applicability of the developed methodology to other gear failure modes, namely pitting and tooth root breakage.