Indiana University-Purdue University Indianapolis (IUPUI) / Articular cartilage (AC) is a strong but flexible connective tissue that covers and protects the end of the long bones. Although cartilage has excellent friction and wear properties that allow smooth joint function during daily activities, these properties are not fully understood. Many material properties of cartilage are anisotropic and vary with anatomic location and the composition of the tissue, but whether this is also true for cartilage friction and wear has not been previously determined. Furthermore, cartilage disease and injury are major health concerns that affect millions of people, but there are few available treatments to prevent the progression of cartilage degeneration. Collagen crosslinking may be a potential treatment to reduce cartilage wear and slow or prevent the progression of cartilage disease. The objectives of this thesis were to investigate the relationships between the friction/wear characteristics of cartilage and the orientation of the preferred fiber direction, the anatomic location of the tissue, the composition of the tissue, and exogenous photochemical crosslinking. In the superficial zone, AC has preferential fiber direction which leads to anisotropic material behavior. Therefore, we hypothesized that AC will show anisotropic behavior between longitudinal and transverse direction in an accelerated, in vitro wear test on bovine cartilage in terms of friction and wear. This hypothesis was proven by the quantification of glycosaminoglycans released from the tissue during the wear test, which showed that more glycosaminoglycans were released when the wear direction was transverse to the direction of the fibers. However, the hydroxyproline released from the tissue during the wear test was not significantly different between the two directions, nor was the coefficient of friction. The material properties of AC can also vary with anatomic location, perhaps due to differences in how the tissue is loaded in vivo. We hypothesized that cartilage from a higher load bearing site will give better wear resistance than cartilage from lower load bearing regions. However, no differences in friction or wear were observed between the different anatomic locations on the bovine femoral condyles. The concentration of collagen, glycosaminoglycans, cells and water in the tissue was also quantified, but no significant differences in tissue composition were found among the locations that were tested. Although wear did not vary with anatomic location, variation in the wear measurements were relatively high. One potential source of variation is the composition of the cartilage. To determine whether cartilage composition influences friction and wear, a correlation analysis was conducted. An accelerated, in vitro wear test was conducted on cartilage from bovine femoral condyles, and the tissue adjacent to the wear test specimens was analyzed for collagen, glycosaminoglycan, cell, and water content. Because wear occurs on the cartilage surface, the superficial zone of the cartilage might play an important role in wear test. Therefore, composition of the adjacent cartilage was determined in both the superficial zone and the full thickness of the tissue. A significant negative correlation was found between wear and collagen content in the full thickness of the tissue, and between the initial coefficient of friction and the collagen content in the superficial zone. This correlation suggests that variation in the collagen content in the full thickness of the cartilage partially explains differences in amount of wear between specimens. The wear resistance of cartilage can be improved with exogenous crosslinking agents, but the use of photochemical crosslinking to improve wear resistance is not well understood. Two photochemical crosslinking protocols were analyzed to improve the wear resistance of the cartilage by using chloro-aluminum phthalocyanine tetrasulfonic acid (CASPc) and 670nm laser light. The cartilage treated with the two crosslinking protocols had lower wear than the non-treated group without changing the friction properties of the cartilage.

The study of defect formation and identification is important to the further application of friction stir welding in industry. To better understand the topic, a systematic study was undertaken to describe material flow effects on the formation of defects, to list the various types of defects encountered across a parameter window, and to identify features in the weld force data that can then be used to recognize defects within the weld without destructive testing. Tracer studies were used to determine the impact of the material flow on defect formation with a determination that proper shoulder contact is necessary to obtain sufficient material flow to fully consolidate the weld. A series of welds across a range of rotational speeds was used to identify mechanisms that led to variations in the mechanical properties of the welded panels. A balance between the x- and yorces on the tool is needed to produce robust welds that were defect free. UMF was shown to identify regions of changing material flow conditions; however, the identification of intermittent defects was not as successful.

Process forces arising during Friction Stir Welding (FSW) have become of interest to investigators interested in obtaining weld quality information from recorded weld data. Successful analysis of process forces require the separation of force signals stemming from material flow mechanisms within the weld from signals influenced by the FSW machine. Three modes of FSW control were characterized for system response: Servo Position (SPC), Electronic Deflection Compensation (EDC), and Constant Load Control (CLC). The gain value of the feedback loops associated with EDC and CLC modes were altered and characterized. SPC mode response to vertical changes in the tool position was also characterized. Machine-specific force signatures associated with the motor transmission assembly and spindle resonance were also identified. Characterization of the influence of machine control modes and other machinespecific frequencies on process force signals will allow future investigators to identify segments of welds during which machine actions influenced recorded force data.

The objective of this work was to develop a new method for establishing the coefficient of friction for the machining conditions found on the rake face of a cutting tool. Critical aspects include the high pressures and high temperatures in the cutting zone. Using the base of an existing High-load High Temperature tribometer an adaptation referred to as a double-sided pin was implemented. The objectives were two-fold to improve the repeatability of the measurements and reduce the setup time by simplifying the alignment problems experienced on the previously used system. The concept of the new setup is to press a double-end spherical pin made of the tool material between two flat surfaces made of workpiece material and rotating it using a string connected to a slider module. The normal load applied, the friction force needed to rotate the pin together with the temperature provided by a welder are then gathered and with the measurements of the imprints the coefficient of friction under specific loads can be established. This work also studied the impact on the tests of pin radius and roughness of the pin surface in order to stablish the measurement limitations and conditions for a successful test. Roughness of the pin had a significant impact on coefficient of friction results. Pins with roughness values around Ra = 0.5 demonstrated half the values of coefficient of friction than pins with Ra = 0.05. This was due the wear and the penetration of the pin into the workpiece sample material. The increase in pin dimensions facilitated testing on softer materials like AISI 1045. It was found to avoid seizure conditions but could not achieve pressures typical of machining difficult to cut materials like Ti-6Al-4V. The increase in pin size also helped in developing an understanding of how to measure the temperature at the contact zone between the pin and disk. Final testing showed a system repeatability of 8% was achieved with setup time reduced by approximately five times. / Thesis / Master of Science in Mechanical Engineering (MSME)

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<p dir="ltr">This thesis presents an investigation into the mechanism of friction generation and lubrication of cages used in modern Deep Groove Ball Bearings (DGBBs). Although cages provide a necessary function, e.g., ensuring proper spacing between rolling elements during assembly and operation, they also serve as an undesirable source of friction to the overall assembly. Cage friction originates primarily from two sources: i) localized cage pocket friction between the balls and the rollers and ii) churning losses from excess lubricant inside the bearing cavity. Localized cage pocket frictional losses were characterized through the development of a novel Bearing Cage Friction Test Rig (BCFTR). This rig was designed and developed to replicate the orientation and relative motion of a fully assembled bearing in steady state operation while measuring cage friction. The BCFTR uses a six-axis load cell to record forces and torques generated due to a rotating ball inside of a rigidly fastened cage segment. The test rig can be set up in two different configurations: i) a load control configuration where a friction coefficient is calculated due to a constant force applied between the ball and the cage segment and ii) a position control configuration where frictional torque is measured for specific positions of the ball relative to the cage. </p><p dir="ltr">In order to gain a deeper understanding of the relationship between cage position, lubrication, and friction, an acrylic cage segment with an exact cage pocket geometry was developed and tested on the BCFTR over a broad range of operating conditions. The clear acrylic cage allowed for the visualization of lubricant flow inside the cage pocket. Videos of oil flow revealed that the quantity of oil inside the pocket correlates closely with the measured frictional torque. Oil volume information from the videos was then used as an input to a cage pocket lubrication model. The model uses the finite difference method to solve the Reynolds and film thickness equations over a spherically defined cage pocket domain. The model was developed primarily to estimate cage pocket friction and corroborate with the results from the BCFTR; however, the model was also used to investigate the pressure distribution and lubricant shear stress in a variety of cage pocket shapes. The finite difference model uses oil volume fraction data to estimate frictional torque and corroborate experimental friction measurements. The results obtained from the model and experiments are in good agreement, proving that the key information required to estimate cage friction is the quantity of oil inside of the cage pocket.</p><p dir="ltr">The main contribution of overall cage friction in DGBBs can be attributed to local drag from inside the cage pocket; however, there remains an appreciable amount of friction and drag losses due to the interaction of the outside of the cage with oil in the bearing cavity. Because DGBB cages reside in the space between the rolling elements and have a significant effect on the churning behavior of the oil, it is paramount to understand how the size and shape of these cages affect the lubricant flow. To achieve this objective, a series of Computational Fluid Dynamics (CFD) models were developed. A full-scale simulation of the inner cavity of a DGBB was developed to observe fluid flow as a function of bearing geometry, operating conditions, and cage shape. Considerable effort was taken to perform optimization studies of the solution method. In addition, an efficient CFD model covering only three rolling elements was also used to investigate fluid flow in a bearing. This model utilized symmetry, periodic boundary conditions, and rotating reference frames to produce equivalent results to the full bearing simulation with a great reduction in computational effort. Results from the model were analyzed both qualitatively and quantitatively through the generation of contour maps of pressure and wall shear stress and the calculation of force and drag coefficient values for each cage.</p><p dir="ltr">The final development presented in this thesis is a high-fidelity Dynamic Bearing Model (DBM) capable of resolving local pocket and external cage lubrication effects of bearings in operation. In this dynamic simulation, the motion of the cage was determined using the finite difference method to solve for the pressure generation and resultant forces inside of each cage pocket at each time step. The computational domain of the finite difference model was designed to reflect the specific cage pocket geometry of four common cage designs. Additional testing on the bearing cage friction test rig was performed to characterize the lubrication state inside of each cage. An inverse distance weighting scheme was utilized to predict starvation parameters for a general ball position inside of the cage pocket. Additionally, the fluid drag losses associated with cage lubrication outside of the cage pocket were included in select dynamic simulations in the form of a drag torque applied to the cage. Results from the dynamic simulation reveal new knowledge on the effect of cage geometry and lubrication on dynamic behavior. Compared to simulations without cage lubrication, results from the new DBM demonstrate a reduction in median ball-cage contact force and improved stability in the trajectory of the center of mass of the cage.</p>

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