Browsing by Subject "friction"
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Item Evolution of Frictional Behavior of Punchbowl Fault Gouges Sheared at Seismic Slip Rates and Mechanical and Hydraulic Properties of Nankai Trough Accretionary Prism Sediments Deformed at Different Loading Paths(2012-02-14) Kitajima, HirokoFrictional 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.Item Micro- and nano- scale experimental approach to surface engineer metals(Texas A&M University, 2007-09-17) Asthana, PranayThis thesis includes two parts. The first part reviews the history and fundamentals of surface science and tribology. The second part presents the major research outcomes and contributions. This research explores the aspects of friction, wear, and surface modification for tribological augmentation of surfaces. An effort has been made to study these aspects through gaining insights by fundamental studies leading to specific practical applications in railroads. The basic idea was to surface engineer metals for enhanced surface properties. A micro- and nano- scale experimental approach has been used to achieve these objectives. Novel principles of nano technology are incorporated into the experiments. Friction has the potential to generate sufficient energy to cause surface reactions through high flash temperatures at the interface of two materials moving in relative motion. This allows surface modifications which can be tailored to be tribologically beneficial through a controlled process. The present work developed a novel methodology to generate a functional tribofilm that has combined properties of high hardness and high wear resistance. A novel methodology was implemented to distinguish sliding/rolling contact modes during experiments. Using this method, a super hard high-performance functional tribofilm with ????????????regenerative???????????? properties was formed. The main instrument used in this research for laboratory experiments is a tribometer, using which friction, wear and phase transformation characteristics of railroad tribo-pairs have been experimentally studied. A variety of material characterization techniques have been used to study these characteristics at both micro and nano scale. Various characterization tools used include profilometer, scanning electron microscope, transmission electron microscope, atomic force microscope, X-ray diffractometer, nanoindenter, and X-ray photon spectroscope. The regenerative tribofilms promise exciting applications in areas like gas turbines, automotive industry, compressors, and heavy industrial equipment. The outcome of this technology will be an economical and more productive utilization of resources, and a higher end performance.Item Performance of Polymer Coatings Under Forming Conditions(2012-02-14) Purohit, ZalakPrepainted metal sheets being environment friendly and cost effective as compared to postpainted metal sheets, are widely used in construction, packaging, transportation and automotive industries. One of the key requirements for prepainted coatings is to retain its surface quality and properties during forming process. During forming process, major surface damage occurs when the coated sheet is bent and un-bent around the die corner. To reduce surface damage of coatings, proper control of the parameters during forming and detail study of the surface conditions is required. In the present study, influence of forming parameters such as die radius, lubrication and specimen material are investigated. The influence of these parameters on friction, surface damage and properties of polymer coatings are evaluated. Experiment set-up is built to conduct bending under tension test. This test gives a better way to evaluate coating performance, as it closely simulates the die region of real forming process and considers bending effects. Experimental results show increase in friction and surface damage with decrease in die radius. Moreover, with decrease in die radius hardness of the coating decreases and strain in the specimen increases. Lubrication has some effect on coefficient of friction, but the influence is not as significant as that of die radius. This is attributed to the fact that, the polymer coating itself acts as a solid lubricant in the test. Material effect was studied, polypropylene coating being the softer material compared to PVDF coating shows more surface damage in the form of scratches. Numerical simulations were performed using Finite Element Analysis package (FEA) Abaqus. A 2D model was built, exploiting the plane strain condition for bending under tension test. Numerical simulations indicate that maximum contact pressure and von Mises stress are concentrated at the beginning of the drawing edge. Apart from the location, the value of contact pressure was higher for smaller die radius. Thus, experiments help in studying the effect of forming parameters on coating performance and numerical simulations provide more insight into the critical areas where stresses are high. Numerical simulations also provide a scope to study the effect of material and geometric parameters on performance of coatings without running experiments.Item The Effects of Fluid Flow On Shear Localization and Frictional Strength From Dynamic Models Of Fault Gouge During Earthquakes(2013-12-02) Bianco, RonaldThis thesis explores the effects of fluid flow on shear localization and frictional strength of fault gouge through the use of a coupled 2-phase (pore fluid-grain) Finite Difference-Discrete Element Numerical model. The model simulates slip at earthquake velocities (~1m/s) in a fluid saturated gouge-filled fault. We find three types of shear behavior: (I) distributed shear, (II) random internal localization, and (III) boundary localization. Each shear type is dependent on the applied shear velocity, V, effective confining stress, N, and internal permeability, k. Through quantitative analysis of the positions and magnitude of localized shear bands, we show under which conditions the presence of and transitions between these shear types will occur. During shear, fluid pressure deviations, delta P, are generated by dilation and compaction cycles. The fluid effects on the system are more pronounced in simulations with higher V and lower N and k. Relative to the dry experiments, fluid saturated systems have an increased localization toward the boundaries of the gouge layer (type III), and no occurrence of distributed (type I) shear. Systems with lower N and k show liquefaction events. Liquefaction events originate from increases in fluid pressure, delta P, around force chains between grains. Once delta P, the high pressures weaken the frictional forces between grains and destroy force chains. Shear then occurs at essentially zero friction until a new grain configuration recreates force chains. A reduction in mean friction is seen for systems with large liquefaction events (without inclusion of thermal pressurization), which could introduce a new mechanism in low friction faults. We also find that systems undergoing different types of shear will all trend toward type (III) shear following a liquefaction event.Item The influence of hydrogen gas exposure and low temperature on the tribological characteristics of ti-6al-4v(2009-05-15) Gola, Ryan TravisThis 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.Item Tribological Properties of Ionic Liquids Lubricants Containing Nanoparticles(2014-05-14) Lu, WeiRecently, there has been an increase in research in the application of ionic liquids containing nanoparticles as lubricants due to their properties such as thermally stability, non-volatility and non-flammability. The purpose of this thesis is to describe the tribological and rheological properties of mixtures of nanoparticles (NPs) and ionic liquids (ILs), specifically the mixture of bare SiO_(2) (silica) nanoparticles and ionic liquid 1-butyl-3-methylimidazolium (trifluoromethysulfony)imide and the mixture of SiO_(2) nanoparticles functionalized by octadecyltrichlorosilane (OTS) and ionic liquid1-butyl- 3-methylimidazolium (trifluoromethysulfony)imide. Functionalized SiO_(2) nanoparticles dispersion in ionic liquid was compared to that of the bare SiO_(2) nanoparticles, and shown that functionalized SiO_(2) nanoparticles led to improved colloidal stability. Friction force profiles, friction coefficients, viscosity behavior, wear behavior of these mixtures at various nanoparticles concentrations for a tribo-pair of stainless steel ball and a steel surface were also investigated. It was shown that the friction coefficient of the OTS functionalized SiO_(2) nanoparticles for the optimum concentration (0.1 wt.%) was 36% less than that of the pure ionic liquid, while the friction coefficient of the bare SiO_(2) nanoparticles and the ionic liquid mixture at the optimum concentration (0.05 wt.%) was 23% less than that for the pure ionic liquid. Moreover, friction surfaces of the two kinds of silica nanoparticles at the optimum concentration were examined by scanning electron microscopy (SEM) and friction traces. Eventually, it has been shown that promising tribological properties of ionic liquids can be further enhanced by incorporating bare SiO_(2) nanoparticles into ionic liquids. Moreover, the tribological performance of the mixture of the OTS functionalized SiO_(2) nanoparticles and ionic liquid could be better than that of the mixture of bare SiO_(2) nanoparticles in the same ionic liquid.Item Tribological Properties of Nanoparticle-Based Lubrication Systems(2013-08-02) Kheireddin, BassemNew nanomaterials and nanoparticles are currently under investigation as lubricants or lubricant additives due to their unusual properties compared to traditional materials. One of the objectives of this work is to investigate the tribological properties of these materials in relation to surface topography. Chemical etching and metal evaporation methods were employed to prepare surfaces with various topographies. Surfaces were sheared with the use of a nanotribometer and characterized with an atomic force and scanning electron microscopes. For a system consisting of ZnS nanowires dispersed in dodecane sheared across ductile surfaces, it was found that the geometry of the nanowire relative to the surface topography plays a significant role. Moreover, for brittle surfaces, it was found that beyond a certain roughness the frictional properties remain unchanged. In addition, this work is also intended to explore novel lubricants with nanoparticle additives in efforts to control friction and wear. A system consisting of silica nanoparticles dispersed in ionic liquids was examined at various concentrations. It was found that an optimum concentration of nanoparticles exists and yields the best tribological properties. Such work represents an important step in understanding the tribological properties of nanoparticle lubricant additives in general; one that may ultimately provide the guidelines necessary for designing novel, low-friction, and wear-controlling nanoparticle-based lubrication systems that minimize energy and material losses due to friction.