Elucidating the mechanisms of white etching crack failures within wind turbine gearbox bearings

Date
2017
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University of Delaware
Abstract
Operation and maintenance (O&M) costs account for a large percentage of the cost of energy generated by horizontal-axis wind turbines. Of all wind turbine componentry, failures at the gearbox are the second most prevalent, and account for the largest annual downtime. The bearings of the gearbox have been identified as the primary contributors to its failure rate, on average failing at 5-10 years of their 20 year design life. The vast majority of these premature failures are caused by cracks surrounded by nano-grained microstructural alterations, referred to as “White Etching Cracks” (WECs). ☐ Because wind turbine gearbox bearings (WTGBs) have only been investigated post-mortem, specifics as to WEC initiation, such as where the damage is seeded, and the order of formation between the microstructural alterations and the cracking, are unknown. The identification and characterization of these initiation specific mechanisms is critical to understanding the macro-scale gearbox conditions which cause these failures. The first part of this dissertation intends to elucidate the drivers of WECs by employing high resolution X-ray micro-tomography on field bearings in order to locate newly initiated crack networks. This work showed that WECs initiated in the subsurface of bearings, preferentially around inclusions made of a combination of aluminum, manganese, and sulfur. Additionally, it was found that the microstructural alterations form secondary to, and therefore are an artifact of a pre-existing cracking failure. ☐ Equally as important as identifying key drivers, is the development of accelerated benchtop tests capable of recreating failures akin to those within WTGBs. Chapters 6-8 developed a benchtop technique, and identified slip, loading, lubrication condition, and testing time as dominant drivers in the formation of WECs. Chapter 8 identified a method of accurately predicting the formation of WECs, and used this method to strongly suggest that lab generated WECs formed in a similar manner to those documented in the field. ☐ In Chapter 9 the effect of steel cleanliness on the formation of WECs is investigated. It is discovered that if steel from a WTGB is used within benchtop tests, WECs will readily form in conjunction with WTG oils. This marks the first time WECs had ever been formed at a benchtop scale while employing the use of a fully formulated wind turbine lubricant. ☐ Finally, Chapter 10 investigated the effect that a proposed mitigation technique, namely case-carburization, had on the formation of WECs. It is found that employing case-carburization led to a 2.3x elongation in the time until WEC-induced failure. ☐ The work presented within this thesis suggests that rapid changes in the load zone of WTGBs, due to the stochastic nature of wind turbine operation, leads to the initiation of cracking failures around specific inclusions that are prevalent in larger bearings. Then, after a crack has been initiated, continued operation at fluctuating loads and speeds leads to the generation of microstructural alterations. If WECs are to be combated, steps should be taken to mitigate the magnitude of torque reversal within wind turbines, remove the aforementioned detrimental inclusions from bearings steel, and implement heat treatment techniques that lead to compressive stresses in the subsurface of bearings.
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