<p>Human joints are able to withstand millions of loading cycles with loads regularly more than 3 times an individual's body weight in large part due to the unique bearing properties of articular cartilage, a strong, slippery tissue that covers the ends of long bones. PRG4 is a boundary lubricating glycoprotein present on the cartilage surface and in the synovial fluid surrounding it. While evidence that PRG4 lubricates and preserves normal joint function is strong, little is known of its effect on cartilage surface properties, the mechanism by which it lubricates, or its postulated role of preventing wear on joints. The effect of PRG4 on cartilage friction, wear, structure, morphology, and the mechanisms by which it mediates these factors are studied here. Methods to study these parameters at the microscale using atomic force microscopy are also developed. </p><p>Cartilage of mice with the Prg4 gene (which expresses PRG4) deleted is shown to be different in a number of ways from wild type cartilage. The uppermost layer is thicker and less uniform and the surface is rougher and softer. There is also a loss of proteoglycans, structural components of cartilage, from the underlying superficial tissue, and apparent tissue damage in some cases. Wear in the presence of PRG4 in shown to be significantly lower than in its absence, a finding which may have direct implications for prevention and treatment of osteoarthritis. It appears that PRG4 needs to be present in solution, not merely on the cartilage surface to have this effect, indicating that adsorption properties are important for wear prevention.</p> / Dissertation

This study identifies mechanisms controlling interface shearing between pipes and granular materials and develops a predictive jacking force calculation model. The surface roughness of six pipe materials, including Hobas (Centrifugally Cast Fiber Reinforced Polymer Mortar), Polycrete (Polymer Concrete), Permalok Steel (Rolled Steel with a Painted Surface), Wet Cast Concrete, Packerhead Concrete, and Vitrified Clay pipe, have been characterized to determine the role of surface roughness on the soil-pipe interface shearing mechanism. Interface shear tests were performed between pipe materials and two characteristically different granular soils: Ottawa 20/30 sand and Atlanta Blasting sand. Shearing behavior between the sands and the pipe materials was evaluated to determine the mechanisms of shearing on materials with varied roughness values. Interface friction values were established for the pipe materials and soils. A model was developed to jacking forces based on modifications to Terzaghi's Arching Theory for predicting normal stresses and interface friction coefficients developed in the laboratory. Field research on fourteen case histories of microtunneling and pipe jacking projects was presented. Pertinent project details were provided including pipe materials, site geometry, geotechnical information, construction sequencing, lubrication injection, and jacking force records. Jacking force records for each project were separated into isolated segments along the alignment to analyze jacking stresses. Unlubricated segments of the microtunneling drive records were analyzed to compare actual and predicted jacking forces using the proposed model. The predictive model was compared to other models currently available for predicting the frictional component of jacking forces. Lubrication effects on jacking forces were analyzed to determine how the interface friction coefficient changed once lubrication was applied to the pipeline. Two types of lubrication strategies were identified and predicted lubricated jacking forces were shown. A step-by-step guide for using the jacking force predictive model was presented for design applications and estimating lubricated interface friction values.

In this study, a novel technique was used to produce aluminum based in situ composites from powder mixtures of Al and CeO2. This technique has combined hot working nature of friction stir processing (FSP) and exothermic reaction between Al and oxide. Billet of powder mixtures was prepared by the use of conventional pressing and sintering route. The sintered billet was then subjected to multiple passages of friction stir processing (FSP). The microstructure was characterized by the use of TEM, SEM and XRD. The reinforcing phases were identified as Al11Ce3 and £_*-Al2O3. The Al2O3 particles with an average size of ~10 nm are uniformly distributed in the aluminum matrix, which has an average grain size about 390-500 nm. The analysis of TEM indicated that these Al2O3 particles exhibit crystallographic orientation relationship with the aluminum matrix, i.e., (223)£_*-Al2O3//(111)Al and [1-10]£_*-Al2O3 roughly parallel to [1-10]Al. The precipitates of Al2O3 exhibiting crystallographic orientation relationship with the aluminum clearly indicates that they were formed from solid state precipitation. Apparently, significant supersaturation of oxygen in aluminum had been created in FSP, and nanometric Al2O3 particles were then precipitated uniformly in the aluminum matrix. This study shows that both sintering temperature and the tool traversing speed used in FSP have significant influence on the microstructure and mechanical properties of the composite. The composites produced exhibit high strength both at ambient and elevated temperatures. For example, the composite produced by 833K sintering followed by FSP with tool traversing speed of 30 mm/min possesses enhanced modulus (E = 109 GPa) and strength (UTS = 488 MPa) as well as a tensile ductility of ~3%. The major contributions to the high strength of the composite are the submicrometer grain structure of aluminum matrix and the Orowan strengthening caused by the fine dispersion of nanometer size Al2O3 particles inside aluminum grains. In addition, the composite also exhibits high strength at elevated temperatures up to 773 K. The good thermal stability and high temperature strength of the composite may be attributed to the uniform dispersion of nanometric Al2O3 particles, which are very stable at elevated temperatures.

Al-Mg solid solution alloys of various grain sizes were prepared by friction stir processing (FSP). The mechanical properties and micro-structure evolution were studied. The results show that the mechanical properties including tensile strength and ductility are improved by increasing Mg weight fraction. The homogeneous deformation is enhanced by fined slip bands within the grains. On the other hand, Dynamic strain aging or serrated flow stress has been wildly investigated in Al-Mg alloys. Effects of strain rate and magnesium content on dynamic strain aging are also discussed.

In this study, the energy conservation equation in a cylindrical coordinate system and the moving heat source from the tool are used to establish a steady-state three-dimensional heat transfer model for the friction stir welding (FSW). Then, the simplified momentum conservation equation is employed to predict the material flow model for the FSW. Combining the effects of heat transfer and material flow, this numerical model successfully predicts the weld temperature field and the material flow for the FSW. Numerical results show that increasing the welding or translational speed of the tool has the effect of decreasing the magnitude of the temperature within the workpiece, while increasing the rotating speed has the opposite effect. During the feeding process, the material located on the back of the tool pin has higher temperature than that on the front. Moreover, the temperature profile are asymmetrical between the advancing and retreating sides due to the material flow stirred by the tool, and this temperature difference depends on the speed of material flow under the tool shoulder.

This research studies individual and combined effects of hydrogen gas exposure and low temperature on the tribological characteristics of Ti-6Al-4V. Experimental approaches include test system modification and tribological analysis. An existing ballon- disk tribometer was modified to allow liquid nitrogen to be constantly injected into an insulated test chamber to enable testing at low temperature. Twelve 3.8 cm diameter Ti-6Al-4V disks were manufactured and polished, then half were exposed to pure hydrogen gas at elevated temperature and pressure and the remaining disks were untreated. The testing was split in to four groups of three disks based on testing temperature and previous hydrogen exposure. A silicon nitride ball was used for all tests. Each group was tested at two normal loads, 10N and 20N, at the same linear speed. Group 1 was unexposed and tested at room temperature, Group 2 was unexposed and tested at low temperature, Group 3 was exposed and tested at room temperature and Group 4 was exposed and tested at low temperature. Average friction coefficients and the specific wear rate were calculated from the test data. Also high-resolution digital microscope imaging was used to observe and characterize the wear mechanisms of the four groups of samples. Results show that hydrogen exposure facilitated adhesive wear of the surface and that low temperature induced a slip-stick wear mechanism under higher loads, but not at lower loads and regardless of exposure to hydrogen gas. This research opens avenues for future investigation in effects of hydrogen and low temperature embrittlement on the tribological performance of materials. With the increasing interests in hydrogen energy, the present work established a foundation for future study.

The manufacturing process has been a heavily studied area over the past century. The study completed herein has established a foundation for the future of manufacturing research. The next step of this industry is to become proficient at the micro and nano scale levels of manufacturing. In order to accomplish this goal, the modeling of machining system needs to be completely understood throughout the entire process. In effort to attack this problem, this study will focus on the boundaries present in machining systems; and will define and interpret the associated phenomena. This particular focus is selected since nearly all manufacturing related studies concentrate on continuous processes; which by definition considers only one particular operation. There is a need to understand the phenomena corresponding to interactions of multiple processes of manufacturing systems. As a means to this end, the nonlinear phenomena associated in the continuous domains of machining systems will be modeled as linear to ensure the boundary interactions are clearly observed. Interference of additional nonlinearities is not the focus of this research. In this dissertation, the mechanical model for a widely accepted machine-tool system is presented. The state and continuous domains are defined with respect to the boundaries in this system (contact and frictional force acting at the point of tool and work-piece contact). The switching sets defining plane boundaries for the continuous systems of this machine-tool will be defined and studied herein. The forces and force products, at the point of switching from one continuous system to another, govern the pass-ability of the machine-tool through the respective boundary. The forces and force product components at the switching points are derived according to discontinuous systems theory Luo [1]. Mapping definitions and notations are developed through the switching sets for each of the boundaries. A mapping structure and notation for periodic interrupted cutting, non-cutting and chip seizure motions are defined. The interruption of the chip flow for a machining system will be investigated through a range of system parameters. The prediction of interrupted periodic cutting, non-cutting and chip seizure motion will be completed via closed form solutions for this machine-tool. The state of this system is defined to utilize the theory of Luo [1]. This is necessary to properly handle the frictional force boundary at the chip/tool interface, the onset of cutting boundary and the contact boundary between the tool and work-pieces. The predictions by this method will be verified via numerical simulation and comparison to existing research. A goal of this research is to illustrate the effects of the dynamical systems interacting at the frictional force (chip/tool) boundary and the chip onset of growth and vanishing boundary. The parameter space for this machine-tool model is studied through numerical and analytical predictions, which provide limits on the existence of interrupted periodic cutting, non-cutting and chip seizure motions.

This study evaluated the soil properties of clean sand, a silty sand, and a road base that are extensively used as a backfill for full-scale testing at Riverside Campus at Texas A&M University. The three soils were collected at the Riverside Campus and the testing schedule included grain size analysis, hydrometer test, specific gravity, maximum dry density, Atterberg limit, stiffness, direct shear test, triaxial test, and a simple procedure to estimate the maximum and minimum void ratio of the clean sand. Relation between strength/deformation, vertical displacement/shear displacement, and physical properties were evaluated to estimate the frictional resistance and angle of dilation of the clean sand and the silty sand. Numerical simulations of the Direct Shear Test (DST) were conducted on the clean sand using Finite Element Model in the computer program LS-DYNA. The simulations were intended to reproduce the Direct Shear Test (DST) to estimate the frictional resistance and dilatancy effects of the clean sand under different compressive stresses. Field tests were also conducted on the clean sand and the road base. These tests included the in-situ density determination, in-situ water content, and the soil modulus using the Briaud Compaction Device (BCD).

Friction factor data are important for better prediction of leakage and rotordynamic coefficients of gas annular seals. A flat-plate test rig is used to determine friction factor of hole-pattern/honeycomb flat-plate surfaces representing annular seals. Three flat-plates, having a hole-pattern with hole diameter of 12.15 mm and hole depths of 0.9 mm, 1.9 mm, and 2.9 mm, are tested with air as the working medium. Air flow is produced between two surfaces, one having the hole-pattern roughness representing the hole-pattern seal and the other smooth, at the following three clearances of 0.254, 0.381, and 0.635 mm and three inlet pressures of 56, 70, and 84 bar with all possible pressure ratios at each configuration. The friction factor data are presented for all tested configurations, with description of the test rig and theory behind the calculations. The effect of hole diameter, hole depth, clearance, Reynolds number, and inlet pressure are analyzed, and friction factor models based on these parameters are calculated. Friction factor upset (an undesirable phenomenon making the test data non repeatable) is also explained. Dynamic pressure data are presented, measured from dynamic pressure probes located at both the hole-pattern plate and the smooth plates at different locations.

Frictional measurements were made on natural fault gouge at seismic slip rates using a high-speed rotary-shear apparatus to study effects of slip velocity, acceleration, displacement, normal stress, and water content. Thermal-, mechanical-, and fluid-flowcoupled FEM models and microstructure observations were implemented to analyze experimental results. Slightly sheared starting material (Unit 1) and a strongly sheared and foliated gouge (Unit 2) are produced when frictional heating is insignificant and the coefficient of sliding friction is 0.4 to 0.6. A random fabric gouge with rounded prophyroclasts (Unit 3) and an extremely-fine, microfoliated layer (Unit 4) develop when significant frictional heating occurs at greater velocity and normal stress, and the coefficient of sliding friction drops to approximately 0.2. The frictional behavior at coseismic slip can be explained by thermal pressurization and a temperature-dependent constitutive relation, in which the friction coefficient is proportional to 1/T and increases with temperature (temperature-strengthening) at low temperature conditions and decreases with temperature (temperature-weakening) at higher temperature conditions. The friction coefficient, normal stress, pore pressure, and temperature within the gouge layer vary with position (radius) and time, and they depend largely on the frictional heating rate. The critical displacement for dynamic weakening is approximately 10 m or less, and can be understood as the displacement required to form a localized slip zone and achieve a steady-state temperature condition. The temporal and spatial evolution of hydromechanical properties of recovered from the Nankai Trough (IODP NanTroSEIZE Stage 1 Expeditions) have been investigated along different stress paths, which simulate the natural conditions of loading during sedimentation, underthrusting, underplating, overthrusting, and exhumation in subduction systems. Porosity evolution is relatively independent of stress path, and the sediment porosity decreases as the yield surface expands. In contrast, permeability evolution depends on the stress path and the consolidation state, e.g., permeability reduction by shear-enhanced compaction occurs at a greater rate under triaxialcompression relative to uniaxial-strain and isotropic loading. In addition, experimental yielding of sediment is well described by Cam-Clay model of soil mechanics, which is useful to better estimate the in-situ stress, consolidation state, and strength of sediment in nature.