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
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Item POLYMER COMPOSITES FOR SENSING AND ACTUATION(2011) Kujawski, Mark Paul; Smela, Elisabeth; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This thesis concerns materials for polymer actuators and mechanical sensors. Polymer actuators are a class of artificial muscle with promising actuation performance; however, they are currently limited by the materials used in their fabrication. The metal-foil type mechanical strain gauges are commercially available and well understood; however, typically have gauge factors less than 5.5 [1], cannot be patterned into custom shapes, and only monitor small areas. New materials provide opportunities to improve the performance of both polymer actuators and mechanical sensors. The aim of this research was to develop, characterize, and implement such materials. Specifically, this thesis describes novel composites of exfoliated graphite (EG) blended with elastomeric hosts. The mechanical and electrical properties of these composites were tailored for two specific applications by modifying the EG loading and the elastomer host: compliant electrodes and strain gauges. Compliant electrodes were demonstrated that had ultimate tensile strains greater than 300% and that could withstand more than 106 strain cycles. Composites fabricated with polydimethylsiloxane (PDMS) exhibited conductivities up to 0.2 S/cm, and having tangent moduli less than 1.4 MPa. This modulus is the lowest reported for loaded elastomers above the percolation threshold. Conductivity was increased to more than 12.5 S/cm by fabricating composites with polyisoprene (latex) elastomers, and the tangent moduli remained less than 5 MPa. Actuation strains of polymer actuators were increased 3 fold using the composites as electrodes, compared to using carbon-grease electrodes. This was due to the composites ability to be spincoated with thin insulating layers of PDMS, allowing 30% higher electric fields to be applied. Strain gauges fabricated with these composites exhibited gauge factors (GFs) > 27,000, to the authors knowledge this is the highest GF ever reported. The effects of humidity, temperature and strain were investigated.Item Self-Contained Hybrid Electro-Hydraulic Actuators using Magnetostrictive and Electrostrictive Materials(2008-12-01) Chaudhuri, Anirban; Wereley, Norman M; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hybrid electro-hydraulic actuators using smart materials along with flow rectification have been widely reported in recent years. The basic operation of these actuators involves high frequency bidirectional operation of an active material that is converted into unidirectional fluid motion by a set of valves. While theoretically attractive, practical constraints limit the efficacy of the solid-fluid hybrid actuation approach. In particular, inertial loads, fluid viscosity and compressibility combine with loss mechanisms inherent in the active material to limit the effective bandwidth of the driving actuator and the total output power. A hybrid actuator was developed by using magnetostrictive TerFeNOL-D as the active driving element and hydraulic oil as the working fluid. Tests, both with and without an external load, were carried out to measure the unidirectional performance of the actuator at different pumping frequencies and operating conditions. The maximum no-load output velocity was 84 mm/s with a 51 mm long rod and 88 mm/s with a 102 mm long rod, both noted around 325 Hz pumping frequency, while the blocked force was close to 89 N. Dynamic tests were performed to analyze the axial vibration characteristics of the Terfenol-D rods and frequency responses of the magnetic circuits. A second prototype actuator employing the same actuation principle was then designed by using the electrostrictive material PMN-32%PT as the driving element. Tests were conducted to measure the actuator performance for varying electrical input conditions and fluid bias pressures. The peak output velocity obtained was 330 mm/s while the blocked force was 63 N. The maximum volume flow rate obtained with the PMN-based actuator was more than double that obtained from the Terfenol-D-based actuator. Theoretical modeling of the dynamics of the coupled structural-hydraulic system is extremely complex and several models have been proposed earlier. At high pumping frequencies, the fluid inertia dominates the viscous effects and the problem becomes unsteady in nature. Due to high pressures inside the actuator and the presence of entrained air, compressibility of the hydraulic fluid is important. A new mathematical model of the hydraulic hybrid actuator was formulated in time-domain to show the basic operational principle under varying operating conditions and to capture the phenomena affecting system performance. Linear induced strain behavior was assumed to model the active material. Governing equations for the moving parts were obtained from force equilibrium considerations, while the coupled inertia-compliance of the fluid passages was represented by a lumped parameter approach to the transmission line model, giving rise to strongly coupled ordinary differential equations. Compressibility of the working fluid was incorporated by using the bulk modulus. The model was then validated using the measured performance of both the magnetostrictive and electrostrictive-based hybrid actuators.Item RESONANCE BASED LOW FREQUENCY SYNTHETIC JET ACTUATOR MODELING, DESIGN, AND TESING(2006-09-01) Gravatt, Lynn Marie; Flatau, Alison; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Synthetic Jet Actuators have been the topic of extensive study in the aerospace industry because of their ability to actively control flow over aerodynamic surfaces without discrete control surfaces such as a flap. One challenge has been to develop a low frequency, lightweight actuator that can provide large displacements. This study will discuss the modeling, design, manufacture, and testing of a bimorph piezo-composite actuator that will provide such displacements at low frequencies. The design employs two opposing benders that provide a piston-type motion. The initial goals of this study were to achieve 30 m/s out of the slot while maintaining the mechanical resonant frequency of the system at about 100 Hz.