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Title: A study into the origins and significance of white etching cracks in bearing steels
Author: Manieri, Francesco
ISNI:       0000 0004 7969 8607
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2018
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Modern machine elements, such as bearings and gears, are often subjected to harsh operating conditions, arising from increased power densities and reduced lubricant film thickness and, often, unpredictable load histories. Such conditions can lead to excessive rates of premature failures i.e. failure rates that were not predicted by basic life theories. A prominent example of this are the relatively high failure rates of rolling bearings employed in wind turbine gearboxes. Such failed bearings exhibit large crack networks decorated by microstructural alterations that appear white when etched and examined under an optical microscope. Hence, these cracks are known as white etching cracks or simply WECs and their appearance in parallel with premature failures, sometimes as early as 1-20% of expected L10 life, has led many to directly associate the failures with WECs themselves. Given the ever increasing importance of wind in the overall energy generation throughout the world, WECs have received significant attention in the last decade form both industry and academia. However, despite a significant body of recent relevant research, there is yet no agreement on the root causes of WECs, with numerous theories having been proposed in this regard. Furthermore, literature is focused on WECs themselves while their significance to actual failures is not widely discussed or established at all. This study attempts to address some of these deficiencies. The primary objective of this study is to establish the root causes of WEC formation, through reproduction of WECs under controlled lab conditions and consequently, discuss their significance in relation to premature bearing failures. A triple-contact disc machine is used in this research to reproduce and examine WECs in AISI 52100 bearing steel under controlled laboratory conditions. This part of the study focuses on exploring the influence of the potential root-causes of WECs, namely lubricant composition, slide-roll ratio, lambda ratio and nominal contact pressure. , TEM analysis showed that these WECs were representative of those from the field. Hence, pure oils were also used to investigate the influence of additives on WEC formation. WECs were produced with all oils employed here, fully-formulated commercial transmission oil, widely referred to as 'WEC critical oil', a commercial mono-grade diesel engine oil as well as group I mineral base oil and group IV PAO base oil. These results led to the conclusion that, despite contrary suggestions in WEC related literature, oil formulation is not a primary cause of WEC generation in bearing steels. From direct observation of results arisen from this study under rolling contact fatigue conditions, the hypothesis that microstructural changes along the crack path were produced by crack-face rubbing was made, as no WEAs were detected without an associated crack. Hence, a new experiment was set up, consisting of a reciprocating ball on a flat disc, to ascertain whether it was possible to produce WEAs under pure rubbing conditions. Using this set-up, white etching areas were reproduced under pure reciprocating rubbing conditions over a range of strokes, loads and frequencies and under lubricated and non-lubricated conditions. WECs were reproduced over a wide range of slide-roll-ratio and under positive and negative sliding, varying film thickness and contact pressures in line contact configuration where a strong edge effect is present at the beginning of the test, due to the specific geometry of specimens. WECs were not reproduced under elliptical contact configuration, unless a source of stress was conveniently introduced. The above evidence points that applied stress is the main driver of WEC formation and that it is the following specific mechanism that is responsible for WECs formation in present rolling contact fatigue tests: a high stress excursion early in life initiate fatigue cracks; subsequently the contact operates at a lower nominal stress; the early cracks therefore propagate slowly and their faces beat and rub with each other for a prolonged period of time; this rubbing generates microstructural alterations in the vicinity of the crack that exhibit themselves as white etching areas; finally, pitting occurs which can be formed by a crack that is non-white etching as well as a crack that is white etching depending on the location of the crack and its propagation rate. This mechanism also helps to explain the occurrence, or lack of it, of WECs to 'premature' bearing failures: WECs are simply a symptom of a specific stress history, not a cause of eventual component failure. Failure of a component at a time earlier than predicted by current life theories cannot automatically be attributed to presence of WECs. Instead both the 'premature failure and presence of WECs are caused by a third factor: a short lived, high stress event occurring at an early stage of operation followed by operation at a moderate nominal stress.
Supervisor: Kadiric, Amir ; Giuliani, Finn Sponsor: SKF (Firm)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral