Determining the change in snow microstructure during large deformation processes by the method of quantitative stereology
Edens, Michael Quast
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Modern constitutive theories for snow are now using microstructural parameters in their formulation. In order to improve the theories, more advanced methods of describing the microstructural behavior are needed. This is particularly important since simplifying assumptions must be made in order that the resulting theory is manageable. To confirm understanding of microstructural behavior it is necessary to obtain experimental data pertinent to the density range, deformation range, and deformation rate being modeled. This data is also needed for the evaluation of empirical parameters. The microstructural variables selected to characterize the behavior of snow must be able to represent dominant mechanisms such as pore collapse, bond fracture, and neck and bond growth due to pressure sintering as well as effects of pore pressure, a mechanism to account for a reduction in grain mobility, coupling of deviatoric and volumetric responses, work hardening, and local inertial effects. In this thesis a set microstructural variables that meet these criteria and corresponding mathematical relations from quantitative stereology are reviewed along with relations and techniques required for numerical evaluation. An experimental investigation is carried out to understand the effect changes in these variables have on the behavior of snow subjected to large deformations. Measurements at several stages of deformation are used to understand microstructure changes, dominant mechanisms, and effects on bulk behavior. Microstructure measurements of six snow samples subjected to confined compression tests are presented for precompressed and compressed states corresponding to final loads of 11.2 KN, 22.4 KN, or 44.8 KN. Order of magnitude changes in microstructural parameters are compared with corresponding magnitude changes in stress level for densities ranging from 0.5 g/cc to 0.64 g/cc. Generally, microstructural variables underwent order of 2 changes compared with 16 for the stress level. This experimental investigation presents changes in microstructural variables that result from large deformations and high deformation rate. These results go a long-way toward filling in a gap in the presently available data. Microstructural variables which, to the present, have not been adequately measured and mathematically described are evaluated.