A microstructural investigation of radiation recrystallized snow layers

Abstract

Radiation recrystallized snow is a pervasive weak layer of snow that, once buried, increases the threat of snow avalanches. While much is known about the conditions required to form radiation recrystallized snow layers, little is understood about the microstructural intricacies that develop resulting in decreased macro-scale mechanical stability. This study utilizes the Subzero Science and Engineering Research Facility at Montana State University to recreate clear daytime meteorological conditions to induce near surface metamorphism in snow. This metamorphic process develops radiation recrystallized layers of faceted crystals in the top 1-2 cm of snow over the course of 12 hours. Mechanical testing is performed before and after recrystallization to compute the relative change in mechanical properties of the recrystallized snow sample. Near surface samples are also extracted and imaged at regular intervals using computed tomography. Imaging results in a 3-D reconstruction of representative snow microstructures recording the temporal evolution of faceted crystal formation. The microstructural data is utilized in two modeling approaches which seek to describe the macro-scale mechanical properties of the snow. A previously developed homogenization approach, which computes macro-scale effective stiffness properties using micromechanical interactions and texture, is enhanced by incorporating measures of individual grain shapes and differing textural measures. Another approach leverages the microstructure directly by simulating the response of macro-scale loads on a geometric mesh of the imaged microstructure using finite element methods. Following recrystallization, physical mechanical testing demonstrated that the metamorphism process forms a stiff and strong sublayer capped by a weaker layer of faceted snow that is 75-80% less stiff in shear and 80-90% less stiff in compression than the strong layer below it. Microstructural analysis revealed multiple fine layers of unique crystal morphologies existing within the faceted region. Homogenization reflected reasonable trends in relative changes of effective stiffness properties but suffered from volumetric scale problems when analyzing the faceted layer. Finite element methods also reasonably computed the relative change in macro-scale effective properties as a result of changes to the microstructural geometry. Additionally, the finite element method estimates changes to effective strength and the location of mechanical failure within the faceted layers.