Theses and Dissertations from UMD
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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM
More information is available at Theses and Dissertations at University of Maryland Libraries.
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Item AUTOMOTIVE DESIGN-TO LIFE-CYCLE CRITERIA FOR LOWERING WARRANTY COSTS AND IMPROVING OWNERSHIP EXPERIENCE THROUGH THE USE OF A NEW "BINARY DECISION MODEL" AND APPLICATION OF A "WARRANTY INDEX"(2009) Ireland, William C.; Christou, Aris; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Financial challenges facing the automotive sector require identification of new opportunities for quality improvement. A new Design-To Life-Cycle-Cost strategy is introduced that applies a unique "Binary Decision Logic Model" that classifies corrective action opportunity into Life-Cycle categories. The intended result is to lower a manufacture's warranty costs and improve ownership experience. This is done by setting Design-To goals in a Life-Cycle way for Reliability and Serviceability. The sample space for data to drive this change of process is found in an existing warranty system with data elements consisting of failure occurrence, failure symptom, mileage, part cost, and labor cost. One can investigate new factors, such as the "Warranty Index," that parses the corrective action in favor of lowering part costs or labor costs found in a typical service event. The data considers opportunities over mileage and time domains to improve vehicle quality over the Life-Cycle.Item Virtual Reality Modeling of a Car Suspension with Active Control Capability(2009) Smoker, Jason James; Baz, Amr M.; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis presents the evolution of a full car model into virtual reality environment to visually demonstrate the dynamics of a car resulting from various inputs controlled both passively and actively. The model is a seven degree of freedom system that can be configured to be excited by either a bump or harmonic input. Active controls available to the system include the well known Linear Quadratic Regulator (LQR) as well as a new Nonlinear Energy Absorber (NEA) which utilizes both nonlinear springs and nonlinear damper. The mathematics of the plant, the kinematics of the system, and the visual specifications of the scene are integrated into a three-dimensional environment where the user can be immersed in the environment and witness in real-time the response of a specific configuration. This project was developed with the mindset that dynamic models of systems can be better understood through visual realization and interaction.Item STUDY OF CONDENSATION OF REFRIGERANTS IN MICRO-CHANNELS FOR DEVELOPMENT OF FUTURE COMPACT MICRO-CHANNEL CONDENSERS(2008) Chowdhury, Sourav; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mini- and micro- channel technology has gained considerable ground in the recent years in industry and is favored due to its several advantages stemming from its high surface to volume ratio and high values of proof pressure it can withstand. Micro-channel technology has paved the way to development of highly compact heat exchangers with low cost and mass penalties. In the present work, the issues related to the sizing of compact micro-channel condensers have been explored. The considered designs encompass both the conventional and MEMS fabrication techniques. In case of MEMS-fabricated micro-channel condenser, wet etching of the micro-channel structures, followed by bonding of two such wafers with silicon nitride layers at the interface was attempted. It was concluded that the silicon nitride bonding requires great care in terms of high degree of surface flatness and absence of roughness and also high degree of surface purity and thus cannot be recommended for mass fabrication. Following this investigation, a carefully prepared experimental setup and test micro-channel with hydraulic diameter 700 microns and aspect ratio 7:1 was fabricated and overall heat transfer and pressure drop aspects of two condensing refrigerants, R134a and R245fa were studied at a variety of test conditions. To the best of author's knowledge, so far no data has been reported in the literature on condensation in such high aspect ratio micro-channels. Most of the published experimental works on condensation of refrigerants are concerning conventional hydraulic diameter channels (> 3mm) and only recently some experimental data has been reported in the sub-millimeter scale channels for which the surface tension and viscosity effects play a dominant role and the effect of gravity is diminished. It is found that both experimental data and empirically-derived correlations tend to under-predict the present data by an average of 25%. The reason for this deviation could be because a high aspect ratio channel tends to collect the condensate in the corners of its cross-section leaving only a thin liquid film on the flat side surfaces for better heat transfer than in circular or low aspect ratio channels.Item Quenching Limits and Materials Degradation of Hydrogen Diffusion Flames(2008-04-25) Morton, Nicholas Ryan; Sunderland, Peter B; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This study examines the types of hydrogen leaks that can support combustion and the effects on various materials of long term hydrogen flame exposure. Experimental and analytical work is presented. Measurements included limits of quenching, blowoff, and pilted ignition for burners with diameters of 0.36 to 1.78 mm. Flow rates of 0.019 to 40 mg/s for hydrogen, 0.12 to 64 mg/s for methane, and 0.03 to 220 mg/s for propane were studied. Materials degradation experiments were conducted on carbon steel, stainless steel, aluminum alloy, and carbon fiber. Noticeable corrosion is present on 304 and 316 stainless steel, galvanized 1006 - 1008 carbon steel. Silicon carbide fibers perform relatively similarly for hydrogen and methane flame exposure.Item Transformation Plans for Optimizing Military Vehicle Testing(2007-05-15) Hoy, Timothy W; Herrmann, Jeffrey W; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The U.S. Army Aberdeen Test Center is a leading Department of Defense developmental test center and test range. A majority of the testing conducted at the Aberdeen Test Center is automotive in nature. Due to recent conflicts around the world, the U.S. Armed Services need to field new armored systems rapidly. The rapid deployment of automotive systems has caused the Department of Defense test community and the Aberdeen Test Center in particular to reevaluate and redefine traditional test plans and practices in order to maximize the amount of valid and pertinent data obtained from shortened test schedules. As a result, this thesis studies new transformation plans to provide ways to optimize military test plans. These transformation plans take into account existing military vehicle data from multiple sources including the Aberdeen Test Center's automotive road courses. These transformation plans are not only useful for shortened military tests, but can also be easily employed in developing test plans for private industry customers as well as long term test projects. The benefits in all cases are the same: an optimized test plan for automotive endurance operations.Item VEHICLE HANDLING, STABILITY, AND BIFURCAITON ANALYSIS FOR NONLINEAR VEHICLE MODELS(2005-12-13) Nguyen, Vincent; Schultz, Gregory A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Vehicle handling, stability, and bifurcation of equilibrium conditions were studied using a state vector approach. The research provided a framework for an improved method of vehicle handling assessment that included non-linear regions of performance and transient behavior. Vehicle models under pure lateral slip, constant velocity, and constant front steer were developed. Four-wheel, two-axle vehicle models were evolved from simpler models and were extended to include vehicle roll dynamics and lateral load transfer effects. Nonlinearities stem from tire force characteristics that include for tire force saturation. Bifurcations were studied by quasi-static variations of vehicle speed and front steer angle. System models were expanded, assessing overall stability, including vehicle behavior outside normal operating ranges. Nonlinear models of understeering, oversteering, and neutral steering vehicles were created and analyzed. Domains of attraction for stable equilibrium were discussed along with physical interpretations of results from the system analysis.Item Development of a Physics of Failure Model and Quantitative Assessment of the Fire Fatality Risk of Compressed Natural Gas Bus Cylinders(2004-07-22) Chamberlain, Samuel Seamore; Modarres, Mohammad; Reliability Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Title of Dissertation: DEVELOPMENT OF A PHYSICS OF FAILURE MODEL AND QUANTITATIVE ASSESSMENT OF THE FIRE FATALITY RISKS OF COMPRESSED NATURAL GAS BUS CYLINDERS The research presented in this dissertation details the development of a new probabilistic fracture mechanics model of corrosion fatigue failure of steel CNG bus cylinders. This model was used to estimate the frequency of leakage or catastrophic rupture, due to the propagation of a micro-crack on the inside, outside or transition surface, of hemispherical and flat-bottom cylinder designs, in assessing the fire and explosion fatality risks associated with a typical CNG bus. Quantitative assessment of the fire and explosion fatality risk was completed by analytically modeling the postulated fire scenarios from initial release of natural gas from a failed cylinder. The frequency of the initiating events, likelihood of subsequent events leading to a fire or explosive event was combined with the consequence of each event in a Probabilistic Risk Assessment (PRA) model to estimate the overall risk. Epistemic and aleatory uncertainties in the approach was evaluated using a combination of parametric modeling, conservative estimation and engineering judgment. Direct computation of the fire fatality risk associated with diesel powered buses is possible because these are mature technologies for which historical performance data are available. Due to the limited experience, fatal incident data for CNG buses fleets are minimal. This study therefore had to rely on analytical modeling of failures, dynamics of fire initiation and propagation along with the subsequent events in this PRA approach. The new methodology provides guidance on performing risk assessment of other novel technologies presently being developed or for which actuarial performance data is not available. This study predicts that the mean fire fatality risk for a typical CNG bus is approximately 23 fatalities per 100-million miles for all persons involved, including bus passengers. Estimated CNG bus passengers mean risk is 14.4 fatalities per 100-million miles or 63% of fire fatalities. Based on historical data, diesel school bus mean fire fatality risk is 0.091 and 0.0007 per 100-million miles for all people and bus-passengers respectively. One can therefore reasonably conclude that CNG school buses are expected to be more prone to fire fatality by 250 times that of diesel buses, with the bus passengers being more at risk by over four orders of magnitude. Explosion due to detonation and deflagration of a flammable vapor cloud within a bus or building, for which there is some historical events, is a major contributor, to this increased risk, a phenomenon not normally associated with diesel fuel. The overall mean fire risk frequency has also been estimated at 2.23 x 10-3 fatalities/bus/year. The 5% and 95% uncertainty bounds are 1.18 x 10-4 and 8.83 x 10-3 respectively. These results provide the foundation for doing comparative analysis of CNG with other technologies by combining the estimated mean fire fatality risk, with the expected health and environmental benefits of using CNG powered buses.Item WAVE PROPAGATION IN RODS, SHELLS, AND ROTATING SHAFTS WITH NON-UNIFORM GEOMETRY(2004-03-16) Toso, Mario; Baz, Amr; Mechanical EngineeringThe propagation of waves in rods, shells and rotating shafts with variable thickness is studied through numerical models and experimental measurements. All numerical models are formulated using the Transfer Matrix approach, which accurately reproduces the dynamic behavior and wave propagation characteristics of the considered structures at each frequency. The numerical predictions show that exponential and linear thickness profiles generate a cut-off frequency, below which waves do not propagate along the structure. Hence, the considered rods and shells are capable of filtering out low frequency and they behave as high-pass mechanical filters. The filtering capabilities of the considered class of rods and shells are investigated for different types of profiles. Furthermore, the effect introduced by using periodicity and changing the material properties of the structure in a functionally graded manner is investigated. The effect of linear profiles is practically evaluated by determining both the frequency and time response for excitations applied at one side of the structure. These results are compared to uniform profiles through the Wavelet Transform (WT), which visualizes the structure vibrational energy simultaneously in both the time and frequency domain. The agreement between the theoretical and experimental results validates the numerical models and demonstrates the effectiveness of the proposed design configurations in attenuating the propagation of waves especially in the low-frequency range. The filtering characteristics are also investigated for rotating shafts with tapered and stepped geometry. It is found out that rotation at a constant speed does not significantly modify the flexural wave propagation characteristics of the system. Also, the interest is extended to studying the Campbell diagrams of tapered and periodically stepped profiles. Experiments on the propagation of vibration from a gearbox through rotating shafts prove that tapered and periodic profiles can effectively redistribute the energy spectrum by confining the propagation to specific frequency bands. Such characteristics become more evident when the shaft is provided with active periodic piezoelectric inserts. The effectiveness of the constant axial loads and feedback control on the shaft performance is determined and compared to the alternative passive periodic treatments.