Theses and Dissertations at Montana State University (MSU)
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Item Intergranular water and permeability of the Lake Vostok accretion ice, Eastern Antarctica(Montana State University - Bozeman, College of Engineering, 2005) Jepsen, Steven Michael; Chairperson, Graduate Committee: Edward E. AdamsThe relative importance of nonhydrostatic stress and lattice-rejected impurities on the phase equilibrium of intergranular liquid water in the Vostok accretion ice, Eastern Antarctica, was examined in this study. In addition, experiments were conducted to examine the influence of intergranular water in ice on the permeability of a Light Non-Aqueous Phase Liquid (LNAPL) hydrocarbon. Sub-grain scale stresses in the Vostok accretion ice were simulated with anisotropic elastic and elastocreep finite element (FE) models and compared to X-ray dislocation topographs. The phase equilibrium conditions were solved separately using stresses simulated by the FE models and ice chemistry data obtained from literature. The permeability of ice to JP-8 aviation fuel, the primary component of drilling fluid used in the Vostok borehole, was tested in three Fuel-Ice (FI) Experiments on unfractured ice in dark conditions near the melting point. The shear stresses simulated by the elastic FE model indicated plastic deformation, via basal glide, in the Vostok accretion ice. This finding was supported by observed dislocation densities exceeding 107 m-2, with higher values reported in literature. The elasto-creep FE model indicated onset of intergranular melt, at scales ₃ 1% the crystal size, in the lower few meters of the westernmost accretion ice. Model predictions of strain rate and internal melt were in reasonable agreement with literature data on polycrystalline ice. Based on an impurity model, which assumed hydrostatic stress, millimeter-size intergranular water veins were predicted in the lower few dekameters of accretion ice. The FI Experiments indicated that these water veins in ice provide conduits for rapid (> 16 cm hr-1) infiltration of JP-8 fuel in dark conditions near the melting point. This transport mechanism, referred to as fuel-tunneling, involved the formation of intergranular tubes, 1-2 mm in diameter, that were absent from experiments using ice grown from distilled water. It was concluded that intergranular water veins in ice near the melting point provide tunneling conduits for LNAPL hydrocarbons. This fuel-tunneling may be accelerated in the basal-most part of the accretion ice due to intergranular melting from both deviatoric stress and mechanical anisotropy. These results have implications for environmentally-clean penetration methods of subglacial lake exploration.Item Fabric tensors and effective properties of granular materials with application to snow(Montana State University - Bozeman, College of Engineering, 2011) Shertzer, Richard Hayden; Chairperson, Graduate Committee: Edward E. AdamsGranular materials e.g., gravel, sand, snow, and metallic powders are important to many engineering analysis and design problems. Such materials are not always randomly arranged, even in a natural environment. For example, applied strain can transform a randomly distributed assembly into a more regular arrangement. Deviations from random arrangements are described via material symmetry. A random collection exhibits textural isotropy whereas regular patterns are anisotropic. Among natural materials, snow is perhaps unique because thermal factors commonly induce microstructural changes, including material symmetry. This process temperature gradient metamorphism produces snow layers that can exhibit anisotropy. To adequately describe the behavior of such layers, mathematical models must account for potential anisotropy. This feature is absent from models specifically developed for snow, and, in most granular models in general. Material symmetry is quantified with fabric tensors in the constitutive models proposed here. Fabric tensors statistically characterize directional features in the microstructure. For example, the collective orientation of intergranular bonds impacts processes like conduction and loading. Anisotropic, microstructural models are analytically developed here for the conductivity, diffusivity, permeability, and stiffness of granular materials. The methodology utilizes homogenization an algorithm linking microscopic and macroscopic scales. Idealized geometries and constitutive assumptions are also applied at the microscopic scale. Fabric tensors tying the granular arrangement to affected material properties are a natural analysis outcome. The proposed conductivity model is compared to measured data. Dry dense snow underwent temperature gradient metamorphism in a lab. Both the measured heat transfer coefficient and a developing ice structure favored the direction of the applied gradient. Periodic tomography was used to calculate microstructural variables required by the conductivity model. Through the fabric tensor, model evolution coincides with measured changes in the heat transfer coefficient. The model also predicts a different conductivity in directions orthogonal to the gradient due to developing anisotropy. Models that do not consider directional microstructural features cannot predict such behavior because they are strictly valid for isotropic materials. The conclusions are that anisotropy in snow can be significant, fabric tensors can characterize such symmetry, and constitutive models incorporating fabric tensors offer a more complete description of material behavior.