Formation mechanism of subsurface white etching area and white etching crack of failed bearings in wind turbine gearbox

Bearings of some wind turbine (WT) gearboxes experienced premature failures due to bearing steel microstructural damage under rolling contact fatigue (RCF). However, the formation mechanism of the microstructural damage and its progression to the final bearing failure is still being debated. In this study, subsurface damage sustained in two bearing raceways of a failed gearbox from a field-operated WT is investigated. The study provides evidence of damage initiation and progression from non-metallic inclusions in the subsurface by using actually failed WT bearings as a case study. Damage characterisation has found various microstructural damage in the raceway subsurface including non-metallic inclusions, butterfly wing cracks, White Etching Areas (WEAs), and White Etching Cracks (WECs). Complex interactions between cyclic rolling contact loading with inclusions and cracks are observed. Their roles in the formation of butterfly wing cracks, WEAs and WECs in the raceway subsurface are evaluated. It has been found that the butterfly wing cracks initiated at damaged inclusions is the original cause of the microstructural damage. The propagation of butterfly wing cracks and WEAs to form subsurface crack networks, with WEAs to create WECs, under RCF loading they have accelerated surface flaking and spalling of the raceways, leading to the final failure of the bearings. The formation mechanism of the subsurface initiated WEAs and WECs is evaluated and a damage progression hypothesis leading to the final bearing failure is proposed, including six stages of the microstructural damage development.