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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Now showing 1 - 5 of 5
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    DEFORMATION MECHANICS OF SOFT MATTER UNDER EXTERNAL STIMULI
    (2019) Cheng, Jian; Li, Teng; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Artificial soft matters are a class of materials which can be easily deformed by external stress, typical examples include foams, colloids, elastomers, and hydrogels. Due to their unprecedented and unique properties, such as large deformability, high resemblance to biological systems, versatile response to multi-physical stimuli, and biological compatibility, soft matters have found applications in fields like soft actuators and robots, soft sensors, bio-mimicking material systems, micro-fluidic system control, biomedical engineering, etc. In these applications, the large deformability of soft matters has taken an enabling role. The deformation theory of polymeric soft matters can date back to 1940s in the early infancy of the statistical mechanics sketch of rubbery materials, with a fast growth in the most recent decade concurring the latest progress in soft matters. However, the mechanical modeling of soft matter leaves many open questions. This doctorate research is devoted to advance the understanding of the deformation mechanics of soft matter, specifically, from the following aspects: (1) how the chemo-mechanical interaction between the solvent molecules and the polymeric network invokes anomalous behaviors of a thin-walled hydrogel structure under internal pressure, in contrast to its polymer counterpart; (2) the application of the dielectric elastomer as sensing medium in soft sensor technology; (3) the development of a novel light-responsive hydrogel material system with the application in bio-mimicking shape transform; (4) and enriching the existing theory to facilitate the mechanistic understanding of the deformational behaviors of a type of fiber-reinforce anisotropic hydrogels. For that, this dissertation (1) reveals the delayed burst of hydrogel thin-shell structures as a new failure mechanism, which is dissimilar from the instantaneous burst of a rubber shell: at a subcritical applied pressure the burst occurs with a delay in time; (2) presents a facile design of capacitive tactile force sensor using a dielectric elastomer subjected to a modest voltage and a pre-stretch; (3) develops a theoretical framework to simulate the light-responsive deformation of the proposed hybrid hydrogel system; and (4) from the perspective of micromechanics, constructs a constitutive model suitable for the microfiber-reinforced anisotropic hydrogel, with large deformation, mass transportation, and the origin of anisotropy are intrinsically captured.
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    Analysis of the Effects of Non-Supine Sleeping Positions on the Stress, Strain, Deformation and Intraocular Pressure of the Human Eye
    (2014) Volpe, Peter; Flatau, Alison B; Aerospace Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This thesis presents analytical models, finite element models and experimental data to investigate the response of the human eye to loads that can be experienced when in a non-supine sleeping position. The hypothesis being investigated is that non-supine sleeping positions can lead to stress, strain and deformation of the eye as well as changes in intraocular pressure (IOP) that may exacerbate vision loss in individuals who have glaucoma. To investigate the quasi-static changes in stress and internal pressure, a Fluid-Structure Interaction simulation was performed on an axisymmetrical model of an eye. Common Aerospace Engineering methods for analyzing pressure vessels and hyperelastic structural walls are applied to developing a suitable model. The quasi-static pressure increase was used in an iterative code to analyze changes in IOP over time.
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    Motion of elastic capsules in microfluidic channels
    (2010) Kuriakose, Shugi; Dimitrakopoulos, Panagiotis; Chemical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    Capsule flow dynamics in microchannels plays a significant role in complex biological phenomena, such as the microcirculation, and in engineering applications, such as in microfluidic devices for drug delivery and cell sorting. In this thesis, we investigate the motion of elastic capsules in wall-bounded flows by extending the Membrane Spectral Boundary Element method developed by Dodson and Dimitrakopoulos for free-suspended flows. First, a validation study of the method is performed for the axisymmetric capsule motion in a cylindrical channel. For a capsule moving along the centerline of a cylindrical channel, our computational model successfully reproduced the parachute shape observed in earlier experimental and computational studies. Next, we investigate the flow dynamics of a strain-hardening Skalak capsule moving along the centerline in a square and a rectangular channel. We examine how the capillary number and capsule size influence the deformation and physical properties of the capsule. For large capsules in a square channel, our investigation reveals that the steady-state capsule shape is non-axisymmetric. The capsule assumes a shape similar to the channel's cross-section i.e. a square shape with rounded edges. Buckling of the capsule's upstream end resulting in a negative edge curvature is observed at higher capillary numbers and for large capsule sizes. For the largest capsules studied, we also observe the development of dimples at the capsule's lateral surface. A comparative study of capsule motion and deformation in cylindrical and square channels shows that the capsule deformation in a cylindrical channel is similar to that in a square channel at a larger capillary number. In a rectangular channel, we observe a three-dimensional (i.e. non-axisymmetric) deformation of the capsule at high capillary numbers resulting in dimpling of the capsule's upstream end at steady state. We also consider the transient motion of a capsule in a converging square microchannel and investigate the influence of viscosity ratio, capillary number and capsule size on the evolution of capsule properties. As the capsule moves through the converging region a fluctuation in the geometric and physical properties of the capsule is observed.
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    Dynamics of Erythrocytes and Microcapsules
    (2008-04-25) Dodson, Walter; Dimitrakopoulos, Panagiotis; Bioengineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    The erythrocytes are the primary carriers of oxygen and carbon dioxide to and from the systemic tissue. The ability of these cells to deform and navigate through the capillary beds is of fundamental importance for proper functioning of the cardiovascular transport system. The erythrocyte is essentially a capsule, and flow-induced erythrocyte deformation involves the interfacial dynamics of a membrane-enclosed fluid volume stressed in a viscous flow. Elastic capsule dynamics is a complicated problem involving the coupling of fluid and membrane forces; it is also found in a variety of scientific and engineering applications. In this work, we investigate the dynamics of elastic capsules and erythrocytes using the Spectral Boundary Element (SBE) method, a high-order / high-accuracy method for capsule and cellular dynamics. For strain-hardening Skalak elastic capsules in an extensional flow, our investigations demonstrate a shape transition in accordance with experimental observations to a cusped conformation at high flow rates, which allows the capsule to withstand the increased hydrodynamic forces. Our computational methodology reveals a region of bifurcation, in which both spindled and cusped steady-state geometries coexist for a single flow rate. The method is also used to investigate the dynamics of strain-softening Neohookean capsules in the same flow pattern. The strain-softening capsules become highly extended at weaker flow rates than strain-hardening capsules, and do not form steady-state cusped shapes. The SBE method has been extended to model the erythrocyte by using a biconcave disc reference geometry and adaptive prestress to enforce area incompressibility. The method accurately reproduces experimental data from erythrocyte ektacytometry, but allows examination of the erythrocyte dynamics beyond the geometric constraints inherent in ektacytometry and other experimental techniques, including observation of the three-dimensional oscillatory behavior over a range of capillary numbers and viscosity ratios. Our results support a prediction by Fischer, Skalak, and coworkers that the erythrocyte shear modulus decreases at small shear deformations. Our work also suggests that cellular deformation is largely independent of the flow pattern, consistent with the findings of experimental investigators.
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    Investigation of Low Temperature Creep Deformation Behavior of a Metastable Beta Titanium-14.8Wt%Vanadium Alloy
    (2004-07-13) Hudson, Candi Monica; Ankem, Sreeramamurthy; Material Science and Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
    This dissertation presents the results of investigated low temperature creep behavior of a metastable beta phase Ti-14.8Weight%V alloy (Ti-14.8V). It is the first such study which relates the activation energy and microstructure with low temperature creep deformation mechanisms in the temperature range of 298K to 800K. A Ti-14.8V alloy with a grain size of 350 m was tensile and creep tested in the temperature range of 298 - 458 K; creep tests were conducted at 95% of the 0.2% yield stress. Activation energies were determined by utilizing strain rate models and resulting least squared Arrhenius plots, which were found to be in the range of 36.6-112.42 kJ/mole for the measured temperature range of 298 - 458K. The resulting activation energies plotted as a function of strain was found to be linear dependent. The determined activation energy values of 36.6 57.55 kJ/mole at the low end of the strain are within the range of activation energy values for dislocation motion. The higher activation energy value of 112.42 kJ/mole is within range of for activation energy value for diffusion of oxygen in beta titanium alloy. These activation energy values are consistent with SEM and TEM observations of deformation mechanisms as dislocations, slip, and stress induced plates (SIP) in the form of twinning were the dominant creep deformation mechanisms for this alloy. The deformation mechanisms changed from predominantly slip to SIP in the form of twins at the higher test temperatures. Further, these findings are consistent with observations, characterization by TEM analysis identified slip dislocations of the 1/2<111> type and twins of the {332}<113> type, which are consistent with time dependent twinning deformation. The results strongly support the mechanism of oxygen controlled time dependent twinning deformation as proposed earlier.