In rolling bearings the same family of damage appears on raceways as depressions at ball or roller spacing. Almen (1937) studied the phenomenon in bearings subjected to vibration without rotation and named it false brinelling, to distinguish it from true brinelling, the plastic indentation of the raceway by overload. Later authors have drawn a further distinction. Godfrey (2003) and Errichello (2004) describe false brinelling as mild wear occurring under boundary lubrication, leaving polished depressions with dark debris, and fretting corrosion as the more severe damage that follows once lubricant is excluded from the contact, with reddish-brown oxide and rough, pitted surfaces.

Standards use the words differently again. ISO 15243 classifies both under frictional corrosion and reserves fretting corrosion for damage at fit surfaces, such as those between a ring and its shaft or housing, while applying false brinelling to rolling element and raceway contacts (ISO, 2017). An investigation report should therefore state which definitions it follows. In this article fretting corrosion is used in the sense of Godfrey and Errichello: oxidative fretting wear of a raceway contact from which lubricant has been displaced.

Rating life according to ISO 281 assumes continuous rotation and failure by subsurface fatigue (ISO, 2007). Methods for adapting it to oscillating bearings exist and are applied to pitch bearings in the design guideline of Harris et al. (2009), which also warns that for small oscillation angles wear damage and not fatigue may limit life. Schwack et al. (2016) compared the available calculation approaches for bearings under individual pitch control and found that they give markedly different results. A useful descriptor of the motion is the ratio of the distance travelled by the rolling element on the raceway to the width of the Hertzian contact. When it is small, part of the contact is never uncovered and cannot be re-lubricated, and laboratory work has shown that grease properties strongly affect whether damage develops under such conditions (Schwack et al., 2020). Field oscillations are irregular, and counting methods have been proposed to reduce a pitch-angle history to cycles of defined amplitude (Stammler et al., 2018).

The investigation begins away from the bearing. The turbine controller log is exported for as long a period as is retained, with pitch angle, pitch rate, rotor speed, wind speed and operating state. From it the investigator derives how much of the time the blade stood still, in which pitch positions it dwelt, and the distribution of oscillation amplitudes, using a counting method of the kind described by Stammler et al. (2018). Maintenance records supply the grease type, relubrication intervals and quantities, and whether the automatic lubrication system, if fitted, was functioning. Transport and storage history is requested as well, since standstill marks can originate before commissioning.

Before the bearing is removed, grease is sampled from several positions around the circumference and from both rows, with the positions recorded, and the colour and consistency of each sample noted. Where access ports allow, the raceway is viewed by endoscope. Rotation torque is measured over the pitch range if the turbine permits, and the bolted joints to hub and blade are checked for loss of preload, because movement at those joints is itself a possible site of fretting. The pitch angle of the bearing as found is marked on both rings so that damage can later be related to the loaded zone.

After removal the bearing is opened with care not to add marks, and rings, rolling elements, cages or spacers and seals are photographed before cleaning. The raceways are then mapped. For each damaged area the record gives its angular position, its size and shape, its location across the raceway profile and the spacing to its neighbours. Spacing equal to the rolling element pitch indicates damage formed without net rotation. The distribution around the circumference is compared with the load zone expected from blade root bending and with the dwell positions found in Stage 1. Damage on the ring fits and bolt-hole faces is recorded separately from raceway damage.

Laboratory work is aimed at the features that separate one damage mode from another. Marks are examined by optical and scanning electron microscopy and their profiles measured, to establish whether material has been removed by wear or displaced plastically with raised shoulders. Debris from the marks and from the grease is analysed to identify the oxide, since reddish-brown haematite is associated with fretting corrosion and dark magnetite-rich debris with lubricated false brinelling (Godfrey, 2003). Metallographic sections through selected marks show whether there is subsurface cracking or microstructural alteration, and hardness is checked against the specification. The grease samples are analysed for iron content, water, base oil content and consistency, and compared with fresh grease of the same type.

The evidence is then set against each candidate damage mode in turn. Table 1 lists the modes that should be considered and the features that distinguish them. A mode is accepted only if the surface evidence, the debris and the operating history agree. Fretting corrosion of the raceway, in the sense used here, is supported when marks at rolling element pitch show material loss with rough, pitted floors and reddish-brown oxide, when the grease near the marks is depleted of oil or displaced, and when the history shows long periods of small oscillation or standstill under varying load.

Identifying the mode is not the same as identifying the cause. For fretting corrosion the contributing causes to examine are the amplitude of motion relative to the contact width, the ability of the grease to return to the contact, the frequency and adequacy of relubrication, and deformation of the rings by a compliant hub or blade root that alters the load distribution. Each is assessed from the Stage 1 data and the bearing geometry. The outcome is a statement of the most probable cause with the evidence for and against it, and a corrective action that follows from that cause, such as a change of grease, the introduction of periodic larger pitch movements to redistribute lubricant, or a revised relubrication regime.