Peter W. Jacobs

White Etching Cracking—Simulation in Bearing Rig and Bench Tests

ABSTRACT White etching cracking (WEC) is a contact fatigue bearing failure commonly observed in wind turbine applications. It can lead to fatigue lifetimes more than an order of magnitude shorter than expected lifetimes. Though various mechanical and chemical factors have shown direct or indirect impacts of on WEC failure, the correlations between these factors are yet to be fully understood. The critical intersection among various lubricant- and non-lubricant-related parameters and their influence on hydrogen diffusion and WEC formation are discussed in this article. Experimental results are shown under diverse operating conditions and contact configurations using three test rigs. This study confirms that the mechanical properties of a rolling contact and lubrication parameters alone cannot predict WEC failure. The formation of a tribofilm and accumulation of atomic hydrogen below the contact surface can be essential to explain WEC events. Higher hydrogen concentration in the WEC zone depends on contact area size, the presence of metal-containing additives in lubricants, and higher frictional energy dissipation. Finally, a mechanism of WEC failure has been proposed that intersects the overlap of hydrogen and subsurface shear stress.

Lubricant Effects on White Etching Cracking Failures in Thrust Bearing Rig Tests

Abstract White etching cracking (WEC) is a subsurface bearing failure mechanism influenced by a number of factors, including lubricant composition. Certain metal-containing lubricants have been reported to promote WEC-induced failure; however, the exact mechanisms linking lubricant effects on WEC propensity are still not fully understood. An interesting field that has not been elucidated is the influence of additive concentration and tribofilm growth on WEC initiation, propagation, and failure. The investigations conducted in this work involved two series of oil formulations: one with additives that give rise to WEC (WEC oils) in different combinations and concentrations and another with additives that do not cause WEC (non-WEC oils). A mini traction machine (MTM) in combination with a spacer layer imaging machine (SLIM) was employed to study the growth of tribofilms and their influence on friction response. Insights from the MTM-SLIM study allowed for better interpretation of FE8 bearing tests. When using oils that contribute to WEC formation, the tribofilm-induced WEC mechanism was confirmed, with cracks initiating as early as after 20 h of FE8 testing. Metal-containing additives were found to favor the formation of WECs by generating a high-friction tribofilm and increasing the water content in the lubricant. Furthermore, the source of subsurface H associated with WEC failure is investigated using heavy water (D2O)-saturated oil. A mechanism of water dissociation induced in tribofilm growth (incubation period) is proposed in this article.