Characterisation of white etching crack damage in wind turbine gearbox bearings

WECs were found attached to MnS inclusions of lengths 3-45 µm at depths of up to 630 µm from the bearing raceway surface and at a wide range of angles of orientation. Damage at MnS inclusions included internal cracking of the inclusions, separation from the surrounding steel matrix, crack initiation and WEC initiation. Evidence has been found to support the theory that WECs are subsurface-initiated by MnS inclusions, but that butterfly cracks with wings propagating at 30-50° from parallel to the raceway surface are not necessarily the same features as MnS inclusion-initiated WECs. Shorter inclusions were found to initiate longer WECs, as were the inclusions that were closer to parallel to the raceway surface in axially sectioned samples.