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Title: Effect of surface traction and non-metallic inclusions on the premature failure of wind turbine gearbox bearings
Author: Al-Tameemi, Hamza Abdulrasool Hussain
ISNI:       0000 0004 7225 8350
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2018
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The reliability of offshore Wind Turbine (WT) operation has been affected by costly maintenance and replacement of critical mechanical components such as bearings in the gearbox which often fail prematurely much earlier than the designed life-time of 20~25 years. It has been observed that irregular microstructural changes known as the White Etching Area (WEA), which could appear as butterfly wings, and the cracks associated with it, termed as White Etching Cracks (WECs), are present in most of the prematurely failed WT bearings. Currently no agreed theories regarding their causes and failure mechanisms, nor an effective solution have been established. In this research, two failed planetary bearings from WT gearboxes were destructively investigated to develop an in-depth understanding on the characteristics of the irregular microstructural changes and damages initiated from defects such as non-metallic Inclusions. One of the investigated bearings had a Black Oxide (BO) coating. The characterisation of the BO coating layer was performed to assess its effectiveness in preventing and delaying the premature WT gearbox bearings failure. To investigate the effect of various tribological parameters on surface and subsurface initiated damage, Rolling Sliding Tests (RSTs) using bearing steel specimens were conducted on a benchtop twin-disc machine. To understand the subsurface initiated damage, the stress distribution around an inclusion was investigated using Finite Element (FE) analysis and the results were correlated to the experimental observations. The investigation of the failed bearings consisted of metallurgical examinations to categorise the key characteristics of surface and subsurface initiated cracks, with a focus on those cracks initiated from non-metallic inclusions. The examinations were conducted using optical microscopy, Scanning Electron Microscope (SEM), Atomic Force Microscopy (AFM) and Energy Dispersive X-ray spectroscopy (EDX). The micro- and Nano-indentation were also used to investigate the mechanical properties of the inclusions, WEA, BO layer and the steel matrix. The RSTs were conducted using the tribometer and rolling contact fatigue testing system (SUROS), the Sheffield University Rolling Sliding twin disc machine; and metallurgical examinations were conducted for the specimens. The FE analysis software ABAQUS was used to model a part of a WT bearing under various loading conditions. The models were 2D with an imbedded Manganese Sulphide inclusion (MnS) of a micro-scale with consideration of the debonding between the inclusion and the steel matrix. It was found that the MnS inclusion was the dominating defect in the uncoated bearing, and those inclusions which are not perfectly bonded to the steel matrix initiated most micro cracks and WEAs. It was also found that the depth of these inclusions from the contact surface was not within the zone of the maximum principal shear stress τ₁(max) which could imply they were affected by surface traction since surface traction and subsurface shear stress are linked. In the BO coated bearing, oxide inclusions initiated most of the subsurface microstructural damage. The examination of both bearings in the axial direction showed a direct relationship between the length of initiating inclusions with the total length of WEA (butterfly wings) or micro-cracks. In addition, surface initiated cracks with similar appearance to the WECs were found to propagate to form surface pitting and spalling of the examined bearings, similar features were observed on some of the RSTs specimens with high surface traction. The RST results confirmed that micro-cracks were initiated by the inclusions and by the effects of non-perfect bonding between the inclusion and the steel matrix. These test results also showed the effect of high surface traction at rotation direction reversed. This work also demonstrated the effect of the sequence of applying different loading conditions on the surface and subsurface initiated damage in certain degrees. The FE modelling results showed that surface traction and the non-perfect bonding between inclusion and steel matrix had a significant effect on initiation of localised ratcheting and tensile stress around an inclusion due to high loads as well as loadingunloading cycles after each roller pass. The stress distribution around inclusion tips showed a similar pattern of butterfly wings at certain surface traction direction. This research identified a number of types of microstructural damage in WT gearbox bearings, some of which had not been reported before. The damage observations were correlated with the findings from the completed RSTs, and the FE modelling showed the effect of non-metallic inclusions and surface traction. Another contribution of this research was the characterisation of the BO coating layer based on hardness measurements, showing that premature WT bearing failure still occurred in spite of employing this coating technique currently recommended by manufacturers.
Supervisor: Long, Hui ; Dwyer-Joyce, Rob Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available