Microscopic investigation of subsurface initiated damage of wind turbine gearbox bearings

Wind turbine gearbox bearings experience premature failures by White Structure Flaking (WSF), often occurs much earlier than their designed life of 20 to 25 years. This results in increased operational and maintenance costs due to unplanned maintenance and early replacement. The main causes and damage initiation mechanism of this premature failure are not fully understood; despite extensive research and investigation in recent years. In this paper, two planetary bearings from a failed gearbox of a multi-megawatt wind turbine are destructively investigated to characterize the subsurface microstructural damage and to understand damage initiation mechanism leading to surface WSF. The results show that the non-metallic inclusions are not the only initiator of subsurface damage. The microcracks are also initiated in the subsurface to form macrocracks which then propagate or connect to other macrocracks to reach the rolling contact surface causing WSF. The characterization of different forms of subsurface microstructural damage shows a close correlation of the maximum shear stress with the damage initiation. Butterfly wings are found to initiate from the compound type of non-metallic inclusions with low aspect ratio and to be associated with inclusion internal cracking in a direction approximately parallel to the axis of the maximum wing length.