<p dir="ltr">Rolling contact fatigue (RCF) is one of the leading causes of failure in critical tribological components such as rolling element bearings (REBs), gears, cam and followers, etc. This is especially paramount for advanced aerospace applications where REB components need to operate for billions of RCF cycles before routine maintenance or inspection is performed. The rolling motion between the rolling elements and raceway produces RCF, wherein a complex, non-proportional, alternating contract stress is applied over a small material volume. Moreover, the highly localized stress occurs on the same length scale as microstructural features such as carbides, inclusions, grain size, hardness gradients from carburization, surface roughness, thereby amplifying their effect on fatigue performance. Therefore, the objective of this dissertation is to investigate critical microstructural modifications and their effects on RCF performance via experiments and computational modeling.</p><p dir="ltr">Initially, an investigation was undertaken to investigate surface roughness effects on RCF. The surface roughness of various REBs was measured through optical surface profilometry and used to construct rough surface pressure distributions, which were then used in a continuum damage mechanics (CDM) finite element (FE) framework. The results demonstrated that life is reduced as lambda ratio decreases. It was also observed that a 2-parameter Weibull cumulative distribution function can describe the relationship between the near surface orthogonal shear stress concentration and ratio of surface failures.</p><p dir="ltr">Next, the enhancement to RCF life from grain size refinement of through hardened bearing steels was studied. To capture the effects of grain refinement, torsion stress-life data of various grain size were used in the RCF model. A predictive life equation for different grain sizes was constructed based on the exponential trend observed between grain size and life from the simulation data. The life equation was then used to calculate the quotient of RCF at two different grain sizes. This quotient was defined as the life improvement ratio and it was observed that this investigation’s ratios compared well with existing life improvement ratios from RCF experiments.</p><p dir="ltr">Hardness gradient is a common microstructural modification to improve RCF life of tribo-components. Variation of hardness gradients is prevalent in case hardened (i.e. case carburized) bearing materials. Therefore, the CDM-FE RCF model was modified to investigate the effects of various hardness gradient types and depths on fatigue life improvement. The simulation results enabled the identification of potentially optimal gradients aimed to mitigate manufacturing challenges and provided the foundation for the construction of a general fatigue life equation.</p><p dir="ltr">A fundamental study to understand the impact various common RCF failure criteria have on RCF life estimation was then conducted using computational modeling. To capture the variation of a material’s resistance to fatigue, the critical CDM damage parameters were assumed to follow a probabilistic distribution instead of a singular value. The CDM-FE model was modified to consider the shear reversal, the octahedral shear stress, the maximum shear stress, the Fatemi-Socie criteria, and the Dang Van multi-axial fatigue parameter as failure criteria. Simulation life results revealed that the CDM-FE model with shear reversal and Fatemi-Socie criteria best match empirical predictions from well-established RCF life theory. Notably, the Fatemi-Socie exhibited the best agreement over all operating conditions.</p><p dir="ltr">The next investigation focused on the cleanliness of aerospace-quality bearing steels. Torsion fatigue experiments established the stress-life (S-N) relation for three common aerospace quality bearing steels. The S-N data was later used to calibrate the RCF model’s damage equation, which considered the Fatemi-Socie criteria following conclusions from a previous investigation. Simulation results were observed to corroborate well with RCF experiments that were conducted for all three materials, while noting the simulations offered a significant time saving. As a result, a subsequent investigation focused on establishing the stress-life relationship for one of the aerospace quality bearing steels through a combined experimental and analytical approach. Good corroboration was observed between simulations and experiments at three contact pressures. This finding is particularly significant as it strengthens the reliability of computational RCF model as an efficient means to assess the RCF performance of bearing materials.</p><p dir="ltr">Furthermore, the detailed investigation on RCF performance of each critical microstructural modifications and their respective effect greatly improves the state-of-the-art. The findings emanating from the various investigations offer informed fatigue design recommendations that aid in the selection of rolling element bearings for critical tribological and aerospace applications.</p>

This research study investigates the effect of micronized copper azole type C (MCA-C) preservative system on the rolling shear (RS) properties of CLT. In the first part of research, bonding performance of CLT panels treated at two retention levels, 0.96 kg/m3 and 2.5 kg/m3, were evaluated. Three structural adhesive systems, melamine formaldehyde (MF), resorcinol formaldehyde (RF) and one-component polyurethane (1C-PUR) were used to assemble visually graded No. 2 2×6 southern yellow pine (SYP) lumber while manufacturing CLT panels. For treated CLT panels, 1C-PUR provides better bonding performance test results. The RS properties of MCA-C treated CLT panels were studied in the second part of the research. The CLT panels were subjected to out-of-plane loading according to the EN 16351 standard. The mean values of RS strength and modulus of treated CLT were 1.89 MPa and 289.4 MPa respectively.

<p>Lubrication and friction of modern rolling element bearings were investigated to develop a physics-based bearing friction model. A test rig was designed and developed to measure the frictional torque of radially loaded rolling element bearings with oil bath lubrication. Deep groove ball bearings and radial needle roller bearings were studied at various loads, speeds and lubrication conditions. Experimental results indicate that bearing friction models currently used in industry can be inaccurate, especially when predicting bearing fluid drag losses. A separate test rig was designed and developed to investigate the lubrication and friction of rolling element bearing cage pockets, as new cage pocket designs could improve bearing efficiency. Cage pocket oil starvation was observed for certain operating conditions, and the starvation was found to correlate strongly with cage pocket friction. In order to better understand friction and lubrication characteristics of bearings, computational fluid dynamics (CFD) models were developed to compare with the experimental results. Fluid motion inside the rolling element bearings was investigated using CFD to determine fluid drag torque of bearing components. Fluid drag torque obtained from CFD and experimental measurements are in good agreement. Results from the CFD models also included pressure distributions over bearing surfaces and fluid velocity near rolling elements, but were limited to global length scales. At the micro-scale, rolling element bearing lubrication and friction is dictated by elastohydrodynamic lubrication (EHL). The radial needle roller bearings and deep groove ball bearings used in this investigation are characterized by line and elliptical contacts, respectively. EHL modeling was therefore developed for line contacts with a strongly coupled fluid solid interaction (FSI) solver. Solid bodies were modeled with finite element (FE) software to incorporate inhomogeneities such as inclusions and surface features which affect EHL pressure, film thickness and friction. Results were used to investigate lubricant film thickness at lubricated line contacts under various operating conditions. This work was further extended to model EHL circular contacts with an FSI approach, combining CFD and FE software. The newly developed FSI EHL model provided critical insights regarding fluid behavior in and around EHL point contacts and fluid properties within the lubricant film. Given the modeling results at the micro and macro scale within the rolling element bearings, a better understanding of bearing friction and lubrication is developed, and supported by experimental data.</p>

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