Microstructural investigation of rolling contact fatigue (RCF) on a failed planetary gear of a windmill gearbox

Abstract

A systematic investigation of failed planetary gear in a windmill gearbox was carried out. The damaged teeth were investigated in order to determine the damage mechanisms that contributed to the failure. During the visual examination, few representative gear teeth were selected with signatures of spalling, micro pitting, cracks following the elliptical path along with fractured surfaces for further investigation. Subsequently, the SEM fractography showed beach marks indicating the fatigue as a operative mechanism for crack propagation. In addition to this, the surface residual stress analysis by x-ray method showed the tensile nature at the tip, while being compressive at the root for damaged gear teeth. Furthermore, the detailed microstructural investigation of damaged gear teeth at a cross-sectional region revealed the nucleation of microcracks at interphase boundaries, dark etch bands along with severe plastic deformation at the core that further transmitted to the elliptical crack path. Finally, it has been established that the failure was initiated due to micropitting, which aggravated the subsurface microcracks leading to spalling, macro crack growth in the elliptical path across the gear teeth leading to final fracture due to the operation of rolling contact fatigue (RCF).