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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Subject: search.filters.subject.Finite Element Analysis

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    PERFORMANCE OF LINK SLAB USING ECC AND UHPC
    (2019) Li, Naiyi; Fu, Chung C.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Conventional bridge design has been challenged by durability and reliability issues due to expansion joints. Chloride-contaminated infiltration and debris accumulation in traditional expansion joints affect the bridge performance negatively. Link slab has emerging as an effective alternative in bridge rehabilitation and design by eliminating expansion joints while the bridge structure remains simply supported. Engineered Cementitious Composites (ECC) and Ultra-High Performance Concrete (UHPC) are desired material in link slab application due to their high strength and tensile ductility. Presented in this thesis is a research project conducted by the BEST center at the University of Maryland to study link slab application with UHPC and ECC. Two bridge models were generated for finite element analysis. In addition, a field test was prepared. Experiments were carried out to study material properties of ECC and UHPC for a preliminary assessment of link slab performance using these materials. Results found that ECC and UHPC are adequate for link slab for their strength, high tensile ductility and fine cracks development.
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    Microstructural Evolution and the Resultant Mechanical Behavior of Duplex Stainless Steels
    (2018) Schwarm, Samuel Christian; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    As the current generation of commercial light water nuclear reactors approach initial design life specifications (40-50 years), the plausibility of extending the operational life of duplex stainless steel piping to 80 years has become an important research focus. Successful evaluation of this potential requires an improved understanding of microstructural evolution and corresponding changes in mechanical behavior that occur during continuous operation at temperatures up to 320 °C, which notably results in aging embrittlement in these systems. This investigation characterizes the effects of thermal aging on the mechanical properties of cast CF–3 and CF–8 stainless steels at operational (280 °C, 320 °C) and accelerated temperatures (360 °C, 400 °C) by a variety of test methods. Bulk mechanical tests have been performed to measure changes in properties such as tensile strength, impact energy, and ductility during aging embrittlement. The results show an increase in strength and decrease in ductility and impact energy after aging to 17,200 h. The phase structure is investigated by electron microscopy and correlated to the mechanical properties and aging conditions in order to form a comprehensive understanding of the progression of embrittlement and elucidate trends. Smaller length scale tests, such as instrumented nanoindentation, reveal the effects of aging on local properties of the constituent ferrite and austenite phases. The resulting data are utilized to evaluate the influence of local microstructural changes, such as spinodal decomposition, on thermal aging embrittlement of the steels. Finite element method (FEM) models have been developed based on the real microstructure and local properties of the steels in order to analyze the micromechanical relationships between phases at different stages in the aging process. This research combines mechanical, microstructural, and computational characterization methods to build a comprehensive evaluation of the effects of thermal aging on structure-property relationships of these important structural stainless steels.
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    MULTI-SCALE MECHANICS OF COMPOSITE SANDWICH STRUCTURES WITH BIOLOGICALLY INSPIRED FIBER REINFORCED FOAM CORES: A POTENTIAL TEMPLATE FOR DEVELOPING MULTIFUNCTIONAL STRUCTURES
    (2013) Haldar, Sandip; Bruck, Hugh A; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This dissertation describes a novel multi-scale characterization and modeling approach for developing bio-inspired composite sandwich structures. The source of bio-inspiration was chosen to be Palmetto wood, a naturally occurring porous composite material with macrofiber reinforcement. Characterization of Palmetto Wood at multiple length scales revealed that the mechanical behavior is dominated by the stronger and stiffer macrofibers, while the porous cellulose matrix controls load transfer and failure between macrofibers. Shear dominated debonding and pore collapse mechanisms have been identified as the leading modes of failure mechanism. The role of macrofiber volume fraction and strain rate on macroscale response and damage evolution has been evaluated through experiments. It is seen that increase in macrofiber concentration increases the stiffness of the Palmetto wood, leading to a higher concentration of macrofiber in the outer region of the wood by evolution. A damage model has been developed to decouple the effect of the plastic strain and pore collapse on damage evolution. Using Palmetto wood as a template, prototype bioinspired sandwich composite structures have been fabricated using carbon fiber reinforcement in the foam core to translate the mechanics principles of Palmetto wood. The sandwich composite structures with bioinspired foam core and standard foam core have been characterized under quasi-static and dynamic three-point bending load. The model developed to study damage evolution in Palmetto wood has been applied to the behavior of bioinspired sandwich to quantify the parameters. The enhancement in mechanical behavior has been achieved by reinforcement of the carbon rods in the core like the macrofibers in the Palmetto wood. An increase in macroscale reinforcement in the core led to the behavior that tunes the material response to a better combination the flexural stiffness, energy absorbance and damage evolution characteristics. A Finite Element Analysis (FEA) model has been developed to numerically study the effects of reinforcement in the foam core on its flexural behavior as observed in the experimental characterization. The simulations performed using homogenized, isotropic properties from simulations of the bioinspired core affirm the experimental observations. The viability of developing multifunctional sandwich structures from the multiscale characterization and modeling of the bioinspired foam cores has also been investigated. Prototype sandwich battery structures were fabricated using copper coated fiberglass and Zn plate facesheet, a carbon foam core, and an adhesive of NH4Cl and ZnCl2 bound by HTPB and epoxy polymers. Very low power generation was demonstrated using the prototype batteries, however it was determined that the mechanical strength and energy absorbing capability were compromised, as expected from the model, indicating that the use of macrofiber reinforcement could potentially enhance multifunctional behavior.