Theses and Dissertations from UMD
Permanent URI for this communityhttp://hdl.handle.net/1903/2
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.
Browse
441 results
Search Results
Item New Methodology for Predicting Ultimate Capacity of One-Sided Composite Patch Repaired Aluminum Plate(2019) Hart, Daniel C; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Composite patch repairs are an alternative to traditional weld repair methods to address cracking in aluminum plates. Analytical and numerical design methods use linear elastic fracture mechanics (LEFM) and do not account for elastic-plastic crack tip behavior demonstrated in static tests of one-sided patch repaired ductile panels. This research used digital image correlation (DIC) and three-dimensional finite element analysis (FEA) with first order elements to study crack tip effects due to the one-sided composite patch applied to center crack tension (CCT) specimens loaded monotonically to failure. The measurable effects on crack tip behavior due to the composite patch were ultimate tensile load increase of more than 100% and a total achieved crack opening displacement (COD) increase of 20% over the unpatched behavior. Crack tip fracture behavior was found to be an intrinsic property of the aluminum and directly related to the COD independent of the one-sided composite patch. Increased capacity was related to accumulation of large-strain free surface area and through thickness volume ahead of the crack tip. Test data and numerical predictions correlated with measured load, strain, displacement fields, and J-integral behavior. Correlation of displacement fields with HRR and K fields established a state of small scale yielding prior to failure. Data and predictions indicated critical COD occurs when unpatched and patched large strain area is equivalent, which occurs before crack tip behavior transitions from small scale to large scale yielding and crack growth. Identifying a critical COD for both small and large scale one-sided patch repaired cracked ductile panels results in a predicted failure closer to the ultimate tensile load and 80% greater than predicted with LEFM methods. Observations and predictions demonstrated in this research resulted in three scientific contributions: (1) development of criteria to determine crack growth in cracked ductile panels repaired with a one-sided composite patch using a critical COD, (2) development of a three-dimensional FEA to study development of the plastic zone and evolution of the large-strain region ahead of the crack tip, and (3) development of a numerical methodology to predict ultimate tensile load capacity of cracked ductile panels repaired with a one-sided composite patch.Item EIT BASED PIEZORESISTIVE TACTILE SENSORS: A SIMULATION STUDY(2019) Nankani, Ayush; Smela, Elisabeth; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Electrical impedance tomography (EIT) is an imaging technique that uses voltage measurements to map the internal conductivity distribution of a body by applying current on electrodes attached to the boundary of that body. EIT has many applications, ranging from medical imaging to 3D printing. This imaging method is also being used for tactile sensing using stretchable piezoresistive sensors, mainly for robotic applications. Although prior research has focused on qualitative illustrations of tactile sensing, this thesis focuses on quantitative evaluation. In this thesis different current injection patterns are quantitatively analyzed using performance metrics to understand their effect on the resulting EIT images.Item Investigation of Swirl Assisted Colorless Distributed Combustion (CDC) for Gas Turbine Application(2019) Feser, Joseph Samuel; Gupta, Ashwani K; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Colorless Distributed Combustion (CDC) is a novel method to enhance flame stability and thermal field uniformity, increase combustion efficiency and reduce pollutants emission, including noise. The focus of this thesis is to investigate swirl-assisted distributed combustion at high thermal intensity for gas turbine application. This thesis investigates the impact of fuel enrichment on CDC conditions by using naphthalene as a fuel additive in ethanol to increase the heating value without compromising ultra-low emissions, in addition to investigating how CDC fuel flexibility can mitigate instability associate with hydrogen enriched alternate fuels. To better predict and implement CDC design in future gas turbine combustors a distributed combustion index (DCI) will be developed to determine the impact of heat release intensity, equivalence ratio, preheat temperature and entrainment gas on distributed conditions. Lastly, the impact of flowfield interaction on achieving CDC condition will be examined for enhanced understanding of mixing required for CDC.Item DESIGN AND DEVELOPMENT OF POTASSIUM FORMATE BASED ATMOSPHERIC WATER HARVESTER(2019) Ayyagari, Veeresh; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)With the deteriorating climate and rapid depletion of natural resources, the problem of water scarcity is on the rise and there is a pressing need for sustainable technologies to address this problem. The extraction of water from air has been identified as a potential solution to address the water scarcity problem in arid regions that do not have ready access to seawater. Much of the research in the literature in tackling this problem has been focused on the use of heat pumps and the use of Atmospheric Water Harvesters (AWH) via solid desiccant technologies. Very little work has been done on utilizing the potential of liquid desiccants for the extraction of water using potassium formate. Through this thesis, we have designed and developed a novel and low-cost AWH utilizing (i) an aqueous solution of potassium formate as the medium for absorption of moisture during the night when the relative humidity is high and (ii) solar energy for removal of absorbed moisture from the desiccant solution during the day. A prototype was built and a performance of 1.9 kg/m2/day with a moisture uptake of 0.34 kg of water/kg of salt was recordedItem Moisture Transport through Housing Materials Enclosing Critical Automotive Electronics(2019) Roman, Artur; Han, Bongtae; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In automotive electronics, humidity-sensitive electronics are encapsulated by protective housings that are attached to the car body. Typical housing materials are comprised of polymer composites, through which moisture transport occurs. The objective of this paper is to provide a predictive capability for moisture transport through automotive housings enclosing a cavity with electronic modules. The temperature-dependent moisture properties including moisture diffusivity, solubility, and saturated concentration of three housing material candidates are characterized first. Then, the analogy between heat transfer and the mass transfer is implemented to model the moisture transport into the cavity enclosed by the housing materials. To cope with the transient boundary condition at the housing material and the cavity interface, the effective volume scheme is used, treating the cavity as an imaginary polymer with an extremely large diffusivity and “equivalent solubility.” The prediction is subsequently validated through an experimental setup designed to monitor the in-situ humidity condition inside the cavity sealed by the housing materials. The prediction and experimental results agree well with each other, which corroborates the validity of the FEA modeling and the measured moisture properties.Item HIGH-FORCE ELECTROSTATIC INCHWORM MOTORS FOR MILLIROBOTICS APPLICATIONS(2019) Penskiy, Ivan; Bergbreiter, Sarah; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Due to scaling laws and ease of fabrication, electrostatic actuation offers a promising opportunity for actuation in small-scale robotics. This dissertation presents several novel actuator and motor designs as well as new techniques by which to characterize electrostatic gap closing actuators. A new motor architecture that uses in-plane electrostatic gap-closing actuators along with a flexible driving arm mechanism to improve motor force density is introduced, optimized, manufactured, and tested. This motor operates similarly to other inchworm-based microactuators by accumulating small displacements from the actuators into much larger displacements in the motor. Using an analytical model of the inchworm motor based on the static force equilibrium condition, optimizations of a full motor design were performed to maximize motor force density. In addition, force losses from supporting flexures were included to calculate the theoretical motor efficiency for different motor designs. This force density optimization analysis of the gap-closing actuators and supporting motor structures provided the basis for designing and manufacturing inchworm motors with flexible driving arms and gap-closing actuators. The motor required only a single-mask fabrication and demonstrated robust performance, a maximum speed of 4.8mm/s , and a maximum force on the shuttle of 1.88mN at 110V which corresponds to area force density of 1.38mN/mm2. In addition, instead of estimating motor force based on drawn or measured dimensions which often overestimates force, the demonstrated maximum motor force was measured using calibrated springs. The efficiency of the manufactured motor was measured at 8.75% using capacitance measurements and useful work output. To further increase force output from these motors, several new designs were proposed, analyzed, and tested. Thick film actuators that take advantage of a through-wafer etch offered a promising opportunity to increase force given the linear increase in force with actuator thickness. However, fabrication challenges made this particular approach inoperable with current manufacturing capabilities. New actuator designs with compliant and zipping electrodes did demonstrate significant increases in force, but not the order of magnitude increase promised by modeling and analysis. In order to study and understand this discrepancy, several new techniques were developed to electrically and electromechanically characterize the force output of these new actuator designs. The first technique identifies parameters in an equivalent circuit model of the actuator, including actuator capacitance. By monitoring change in capacitance along the travel range of the motor, electrostatic force in equilibrium can be estimated. Charge transferred to and from the actuator can also provide an estimate of actuator efficiency. The second technique uses a constant rate spike to more thoroughly explore the rapid dynamics of actuator pull-in and zipping. New characterization methods allowed for collecting large amounts of data describing performance of motors with zipping and compliant electrodes. The data was used to back up the main hypothesis of force output discrepancy between theory and practice. Also, it was used to highlight extreme sensitivity of proposed motors toward manufacturing process and its tolerances.Item MAXIMIZING THE FINANCIAL RETURNS OF USING LIDAR SYSTEMS IN WIND FARMS FOR YAW ERROR CORRECTION APPLICATIONS(2019) Bakhshi, Roozbeh; Sandborn, Peter A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Wind energy is an important source of renewable energy with significant untapped potential around the world. However, the cost of wind energy production is high and efforts to lower the cost of energy generation will help enable more widespread use of wind energy. Ideally, wind turbines have to be aligned with wind flow at all times. However, this is not the case and there exists and angle between a wind turbine nacelle’s central axis and the wind flow. This angle is called yaw error. Yaw error lowers the efficiency of turbines as well as lowers the reliability of key components in turbines. LIDAR devices can correct the yaw error; however, they are expensive and there is a trade-off between their costs and benefits. In this dissertation, a stochastic discrete-event simulation is developed that models the operation of a wind farm. By maximizing the Net Present Value (NPV) changes associated with using LIDAR devices in a wind farm, the optimum number of LIDAR devices and their associated turbine stay time will be determined. These optimum values are a function of number of turbines in the wind farm for specific turbine sizes. The outcome of this dissertation will help wind farm owners and operators to make informed decisions about purchasing LIDAR devices for their wind farms.Item USING THE Q-WEIBULL DISTRIBUTION FOR RELIABILITY ENGINEERING MODELING AND APPLICATIONS(2019) Xu, Meng; Herrmann, Jeffrey W.; Droguett, Enrique López; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Modeling and improving system reliability require selecting appropriate probability distributions for describing the uncertainty in failure times. The q-Weibull distribution, which is based on the Tsallis non-extensive entropy, is a generalization of the Weibull distribution in the context of non-extensive statistical mechanics. The q-Weibull distribution can be used to describe complex systems with long-range interactions and long-term memory, can model various behaviors of the hazard rate, including unimodal, bathtub-shaped, monotonic, and constant, and can reproduce both short and long-tailed distributions. Despite its flexibility, the q-Weibull has not been widely used in reliability applications partly because parameter estimation is challenging. This research develops and tests an adaptive hybrid artificial bee colony approach for estimating the parameters of a q-Weibull distribution. This research demonstrates that the q-Weibull distribution has a superior performance over Weibull distribution in the characterization of lifetime data with a non-monotonic hazard rate. Moreover, in terms of system reliability, the q-Weibull distribution can model dependent series systems and can be modified to model dependent parallel systems. This research proposes using the q-Weibull distribution to directly model failure time of a series system composed of dependent components that are described by Clayton copula and discusses the connection between the q-Weibull distribution and the Clayton copula and shows the equivalence in their parameters. This dissertation proposes a Nonhomogeneous Poisson Process (NHPP) with a q-Weibull as underlying time to first failure (TTFF) distribution to model the minimal repair process of a series system composed of multiple dependent components. The proposed NHPP q-Weibull model has the advantage of fewer parameters with smaller uncertainty when used as an approximation to the Clayton copula approach, which in turn needs more information on the assumption for the underlying distributions of components and the exact component cause of system failure. This dissertation also proposes a q-Fréchet distribution, dual distribution to q-Weibull distribution, to model a parallel system with dependent component failure times that are modeled as a Clayton copula. The q-Weibull and q-Fréchet distributions are successfully applied to predict series and parallel system failures, respectively, using data that is characterized by non-monotonic hazard rates.Item INTERPOLATION OF RIGID-BODY MOTION AND GALERKIN METHODS FOR FLEXIBLE MULTIBODY DYNAMICS(2019) Han, Shilei; Bauchau, Olivier A.; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Traditionally, flexible multibody dynamics problems are formulated as initial value problems: initial states of the system are given and solving for the equations of motion yields the dynamic response. Many practical problems, however, are boundary rather than initial value problems; two-point and periodic boundary problems, in particular, are quite common. For instance, the trajectory optimization of robotic arms and spacecrafts is formulated as a two-point boundary value problem; determination of the periodic dynamic response of helicopter and wind turbine blades is formulated as a periodic boundary value problem; the analysis of the stability of these periodic solutions is another important of problem. The objective of this thesis is to develop a unified solution procedure for both initial and boundary value problems. Galerkin methods provide a suitable framework for the development of such solvers. Galerkin methods require interpolation schemes that approximate the unknown rigid-body motion fields. Novel interpolation schemes for rigid-body motions are proposed based on minimization of eighted distance measures of rigid-body motions. Based on the proposed interpolation schemes, a unified continuous/discontinuous Galerkin solver is developed for the formulation of geometrically exact beams, for the determination of solutions of initial and periodic boundary value problems, for the stability analysis of periodic solutions, and for the optimal control/optimization problems of flexible multibody systems.Item INTERFACIAL DEGRADATION OF COPPER WIRE BONDS IN THERMAL AGING AND CYCLING CONDITION(2019) Manoharan, Subramani; McCluskey, Patrick; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Copper (Cu) wire bonds have become the dominant wire material used in microelectronic packages, having replaced gold (Au) in the majority of applications. Cost saving has been the key factor to drive this transition in wire bond material, although there are other advantages to Cu such as better electrical and thermal conductivity, reduced wire sweep during transfer molding and most importantly slower intermetallic compound (IMC) formation with Al (bond pad). Although IMC layers are much thinner than for Au-Al bonded joints, growth of second phase, Cu9Al4, due to exposure to high temperature leads to interfacial separation, which is exacerbated under thermal cycling condition ultimately leading to failure of the joint. Part I of this dissertation aims at addressing the effect of combined loading (thermal aging and cycling) on the reliability of Cu wire bonded devices using a unique long dwell thermal cycling profile that accelerates growth of different IMC phases (CuAl2 and Cu9Al4) and accelerates failure due to CTE mismatch between epoxy mold compound, die and Cu wire bond. Unlike many of the studies presented in literature, the test vehicle in this study are made of commercial off-the-shelf (COTS) parts, where a multitude of factors vary from one another, such as wire diameter, wire bond and bond pad characteristics, etc., the combination of which play a significant role in the life time of these devices and is not fully captured by first-principal models. Hence, a data-based life estimation method is developed, to aid in part selection based on initial bond characteristics. Critical parameters of wire bond that contribute to reliability are identified, the most significant of which is Al bond pad thickness, which controls the growth of IMC and influences time for Cu9Al4 IMC phase formation. Second part of this work is focused entirely on the Al bond pad thickness. Part II-A focuses on the qualitative comparison of pad thickness effect on the quality of initially formed bond through use of bond shear analysis and the effect of bond interface aging on bond shear analysis. Test vehicle consists of three pad thicknesses namely, 0.5 µm, 1 µm and 4 µm, over which Cu wirebonds with four different thermosonic bond recipes are made. Results from Part II-A provide guidelines for bond comparison using bond shear analysis. Part II-B focuses on the effect of bond pad thickness on the reliability of Cu wire bonds under isothermal aging at 175°C and 200°C for 1000 hours and 650 hours respectively. Test vehicle in this study consists of 0.675 µm and 3 µm pad thickness on silicon die in 20 leaded 5x5 QFN package. Wire bonds with one thermosonic bonding recipe are made on all the 90 packages used in the study. Electrical resistance and cross-sectional analysis are used to derive failure times, which is in turn used to build empirical relationship between pad thickness and time to failure. Result from this study shows longer time to failure for wire bonds on 3 µm pad compared to 0.675 µm pad due to delay in Cu9Al4 formation.