UMD Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/3

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 given thesis/dissertation in DRUM.

More information is available at Theses and Dissertations at University of Maryland Libraries.

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2015 - 20193

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    Dynamic Control of Fiber Orientation for Additive Manufacturing via a Soft-Actuating Nozzle
    (2019) Armstrong, Connor; Bigio, David I; Sochol, Ryan D; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Recently, additive manufacturing of fiber-reinforced composite hydrogels has been used to create self-assembling and self-folding structures through hydration-triggered shape change. Additive manufacturing of shape-changing structures has applications in spatially-limited environments such as in-vivo biological implants and components for space travel. Fiber orientation in composite hydrogels dictates the degree of anisotropic swelling deformation of hydrated structures. This thesis explores the impact of extrusion channel geometry on fiber orientation as well as the relationship between fiber orientation and swelling deformation of composite hydrogels. To study the impact of fiber orientation on swelling deformation, fiber orientation in composite hydrogels was varied using diverging extrusion dies of increasing divergence angles. It was found that increasing channel divergence angle reduced the number of fibers oriented in the direction of flow, which led to increasingly isotropic swelling deformations. To create a gradient of fiber orientations in extruded structures, an extrusion nozzle utilizing soft actuators to alter its divergence angle in real-time was developed. Hydrogels extruded through the soft-actuated dynamic nozzle exhibited similar fiber orientation and swelling behavior to those extruded through the fixed divergence angles. Spatially-varied swelling deformation characteristics promise to improve additive manufacturing of self-assembling and self-folding structures by increasing the complexity of controllable shape change geometries achievable in extruded composite polymer structures.
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    Biomimetic polymer based composites with 1-D titania fillers for dental applications
    (2018) Mallu, Rashmi Reddy; Lloyd, Isabel K; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The aim of this study was to develop acrylic matrix composites reinforced with one-dimensional (1-D) titanium dioxide (TiO2) micro and nano fillers that mimic the structure of enamel. To accomplish this, 1-D TiO2 was synthesized without surfactants or templates using a sol-gel assisted hydrothermal process. Two different approaches were investigated. One used titanium metal powder and yielded TiO2 rutile microrods. The other used titanium tetraisopropoxide (TTIP) and created TiO2 anatase nanorods. TiO2 morphology (size, aspect ratio and state of agglomeration) was affected by glycolic acid concentration and phosphate ion concentration for the titanium metal-based powders, and NaOH concentration for TTIP based powders. Composites were made with silanized TiO2 micro- and nano-rods in a 50:50 BisGMA:TEGDMA matrix. Organized composites made by injection molding or centrifuging and settling had more uniform mechanical properties (hardness, strength, Young’s modulus and toughness) than unorganized composites. Curing the composites under pressure reduced porosity enhancing mechanical behavior.
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    Pyrolysis of Reinforced Polymer Composites: Parameterizing a Model for Multiple Compositions
    (2015) Martin, Geraldine Ellen; Stoliarov, Stanislav I; Fire Protection Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    A single set of material properties was developed to describe the pyrolysis of fiberglass reinforced polyester composites at multiple composition ratios. Milligram-scale testing was performed on the unsaturated polyester (UP) resin using thermogravimetric analysis (TGA) coupled with differential scanning calorimetry (DSC) to establish and characterize an effective semi-global reaction mechanism, of three consecutive first-order reactions. Radiation-driven gasification experiments were conducted on UP resin and the fiberglass composites at compositions ranging from 41 to 54 wt% resin at external heat fluxes from 30 to 70 kW m-2. The back surface temperature was recorded with an infrared camera and used as the target for inverse analysis to determine the thermal conductivity of the systematically isolated constituent species. Manual iterations were performed in a comprehensive pyrolysis model, ThermaKin. The complete set of properties was validated for the ability to reproduce the mass loss rate during gasification testing.