White etching cracks (WECs) have been identified as a main failure mode of wind turbine gearbox bearings (WTGBs). This study reports an investigation of the destructive sectioning of a failed low speed planetary stage WTGB and the damage found at manganese sulphide (MnS) inclusions. The bearing inner raceway was sectioned through its circumferential and axial directions in order to compare the damage around inclusions in different directions. 112 damage initiating inclusions were catalogued and their properties investigated. WECs were found attached to MnS inclusions of lengths 3-45 µm at depths of up 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 initiated subsurface, by MnS inclusions, but that butterfly cracks with wings propagating at 30-50º 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 inclusions that were closer to parallel to the raceway surface in axially sectioned samples.
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