Bearing failure is a cause of concern in a variety of machinery such as turbines, transmissions, drills, engines, etc. It is often associated with rolling contact fatigue (RCF) triggered from damage initiation at non-metallic inclusions (NMI's). Experimental evidence shows that damage initiation lifetime is highly sensitive to the NMI characteristics and its bonding with the steel matrix. This study numerically investigates the role of NMI features and its bonding with the steel matrix on damage initiation lifetime. NMI characteristics modelled in this study are derived from an experimental investigation of a failed bearing. Simulation results highlight a near to instantaneous debonding at the matrix-inclusion interface followed by accelerated crack initiation. The critical depth for damage initiation shifts towards the surface with the increase in friction coefficient between roller and raceway. The simulations also reveal that larger inclusions show earlier damage initiation, indicating a size effect. The damage hotspots from the simulation results were compared with experimental findings and a hypothesis for crack initiation from a NMI is put forward.

The microstructure of a damaged bearing from the field was characterized in this work with the intention to better understand microstructural features behind formation of White Etching Cracks (WEC) in bearings. Microstructural characterization of the altered white etching area (WEA) involved conventional electron backscattered diffraction (EBSD), followed by transmission electron microscopy (TEM), and transmission Kikuchi diffraction (TKD). In addition, automated crystallographic orientation mapping in TEM was performed on lamellae from selected regions of the WEA extracted via focus ion beam milling. The results revealed that the orientation of detectable grains within WEA is similar to that of the vicinal bulk material. WEA consists of small spherical grains (average 30 nm) and the orientation of the grains varied significantly in the deformed zone, suggesting that recrystallization had occurred. The interface between bulk material and the deformed zone is very sharp. Furthermore, needle-like grains, most likely originating from the zone undergoing only modest levels of severe plastic deformation, occurred in WEA. The occurrence of different grain sizes in WEA and incomplete plastic deformation strongly support the hypothesis of WEC formation via severe plastic deformation followed by recrystallization.

Damage in bearings is closely associated with the presence of microstructural alterations, known as white etching areas (WEAs) and white etching cracks (WECs). One of the main reasons for the creation of these microstructural alterations is the presence of defects in the material, such as non-metallic inclusions. Manganese sulfides and aluminum oxides are widely reported in the literature as the most common types of non-metallic inclusions found in bearing steels. This study classifies 280 non-metallic inclusions in an investigated bearing steel according to several criteria: bonded/debonded with the matrix, size, shape, orientation angle, depth below the raceway surface, and chemical composition. Contrary to the findings in the literature, this investigation reports that the chemical composition of the inclusion (MnS + Al₂O₃) is of secondary importance when considering factors for damage initiation. The orientation of the microstructural alterations is observed to coincide with the high-stress regions, indicating a relation between the formation of butterfly wings and the white etching crack. In our investigation, butterfly wings typically exhibit a 45-degree pattern originating from the non-metallic inclusions. Conversely, the white etching crack starts from the non-metallic inclusion at a shallower angle in correspondence to the raceway. This can be attributed to the stress state, which corresponds to a region where extensive white etching cracks are formed. In conclusion, the microstructural observations demonstrate that the state of non-metallic inclusion—i.e., whether they are bonded or not to the steel matrix—plays an essential role in initiating rolling contact fatigue damage.

Roller element bearings present in the intermediate and high-speed stages of wind turbine gearboxes operate in dynamic working conditions and in some cases may fail within 30% or less of their designed lifetime. Upon investigation, it has been identified that these premature failures happen due to a peculiar failure mode associated with formation of white etching cracks (WEC). This continues to be a great challenge for the wind energy operators as it leads to an increase of maintenance and operation costs in addition to long wind turbine downtime. Therefore, the industry is in dire need of a lifetime prediction methodology that could take in multi-scale inputs ranging from bearing loads at the system level down to the level of bearing material properties at the microscopic level. This work summarizes the overall approach of a project that aims towards an integrated framework which links load data from the bearings and microstructure related non-metallic inclusion statistics from bearing steels, to predict a material based probability of failure. The interlink between both aspects is a numerical rolling contact fatigue (RCF) framework based on finite element analysis, which includes multi-scale data as an input to calculate rolling contact fatigue damage. The outcome will help the wind industry to better predict bearing failures.

Additive manufacturing technologies in general and laser powder bed fusion (L-PBF) in particular have been on the rise in different applications, including biomedical implants. The effects of the various L-PBF process parameters on the microstructure and properties of Ti6Al4V lattice structures have been studied before. However, the relationship between the different modes of laser scanning and the resulting microstructure, internal imperfections, and surface morphology is still underexplored. In this study, the aforementioned parameters and their effect on the compressive mechanical properties and fatigue behaviour of lattice titanium have been studied for both continuous and pulsed laser scanning modes. Moreover, the influence of various combinations of post-processing treatments, such as hot isostatic pressing (HIP), sandblasting, and chemical etching, on the quasi-static mechanical properties and fatigue endurance of the resulting materials were investigated. It was found that continuous laser strategy results in fewer imperfections and higher fatigue resistance, while pulsed laser showed a more homogenous microstructure; likely leading to a more isotropic behaviour. Furthermore, the continuous mode showed larger prior β grains preferentially oriented in the building direction, while pulsed specimens exhibited finer equiaxed grains with no preferred orientations. The highest level of fatigue life was obtained by using an optimized combination of HIP, sandblasting, and chemical etching.