Daniel A. Miller

Grain boundary ridge on sintered bonds between ice crystals

Well-sintered snow examined in a scanning electron microscope revealed a newly observed morphological structure that protrudes into the pore space along ice grain boundaries. We have termed this a “grain boundary ridge.” Grain boundary diffusion is a sintering process that occurs at the interface of two crystals, whereby mass migrates from the center of the contact to the surface of the bond. Since mass tends to sublimate from sharp features toward smaller curvature surfaces through vapor diffusion, a ridge developed by grain boundary diffusion will readily sacrifice mass to the surrounding ice surfaces. A mass balance between vapor and grain boundary diffusion based on the observed geometry is considered. This analysis indicates grain boundary diffusion may play a far more significant role than generally acknowledged. While this study was restricted to ice, it may have implications for other crystalline materials.

Ice crystals grown from vapor onto an orientated substrate: application to snow depth-hoar development and gas inclusions in lake ice

A laboratory experiment was conducted in which new ice crystals were nucleated from the vapor phase onto large existing ice crystals obtained from Antarctic lake ice. Flat, smooth ice-crystal surfaces were prepared, with c axes oriented either vertically or horizontally. When these were subjected to a supersaturated vapor environment, multiple individual crystals nucleated onto the substrates adopting the same crystallographic orientation as the parent. A dominant grain-growth scenario for kinetic-growth metamorphism in snow, which in some ways is analogous to the oriented morphologies in lake ice, is hypothesized. In the lake-ice-growth scenario, optimally oriented crystals will grow at the expense of those less preferentially positioned. The proposed dominant grain-growth theory for snow is in agreement with the observed decrease in the number of grains and the proximal similarity of crystal habit in kinetic-growth metamorphism in snow. Similarly, kinetic crystal growth on the interior of gas inclusions in Antarctic lake ice will also acquire the crystallographic orientation of the substrate ice. These small-faceted interior crystals significantly influence light scattering and penetration in the lake-ice cover.

A microstructural dry-snow metamorphism model for kinetic crystal growth

Historically, dry-snow metamorphism has been classified by the thermal environment and thermodynamic processes in a snowpack. Snow experiencing predominantly macroscopic isothermal conditions develops different microstructure than snow subjected to large temperature gradients. As such, much previous research has been categorized by and limited to specific thermal conditions. The current research expands a generalized approach for the movement of heat and mass to include a snow crystal kinetic growth model. An existing spiral defect propagation theory for kinetic growth on simple faceted geometry is utilized. Primary crystal habit as a function of temperature is incorporated. A model of heat and mass transfer through an ice and pore structure is coupled with phase-change thermodynamics during kinetic growth. A kinetic growth microstructure model is developed and integrated into heat and mass transfer representations, which are solved using finite-difference techniques. The kinetic morphology model approximates frequently observed hopper-type crystals. The snow microstructure is allowed to change at every step, resulting in a transient description of kinetic growth metamorphism. Variable kinetic growth rates are demonstrated based on temperature and on crystallographic orientation relative to a temperature gradient. Crystals preferentially aligned with the temperature gradient have significantly higher growth rates, supporting previous observations of predominant crystal habits developing under temperature gradient conditions. Grain-size dispersion increase with time is demonstrated and supported experimentally in the literature. A dominant grain growth theory based on crystallographic orientation that has been previously postulated is supported. A broad range of metamorphic geometric parameters and thermal conditions may now be simulated with a single model.

Nonlinear finite element analysis of composite beams and arches using a large rotation theory

Abstract Todays aerospace industry has advanced to the point of using optimum design techniques in virtually all applications. In structural elements, orthotropic fiber composite materials have emerged as lighter, stronger and a more easily manufactured solution to the material application aspect of design. Composites have the distinct advantage of being designed and built to many different desired specifications by varying materials, amount of matrix/fiber and orientation. As with many high performance applications, the analysis techniques of fiber composites are more complicated than for the simpler counterparts. This research has been directed to capturing large cross-sectional rotation incorporating a geometrically nonlinear finite element composite arch model. The model was derived and simplified from a (two-dimensional) 2-D shell theory that has been demonstrated to be accurate for large displacements and only moderate rotations. The current effort uses a similar potential energy based finite element model with through-the-thickness shear. Large rotation kinematics are derived in a vector format which includes a tangent function in the in-plane displacement relationship. Previously published research (Creaghan and Palazotto, 1994; Palazotto and Dennis, 1992) used a small angle approximation to simplify stiffness expressions, limiting those theories to moderate cross-sectional rotation angles. This tangent function is modeled by a series representation of the angle and thereby preserving the existing degrees of freedom. The approach decomposes the Green strain components into convenient forms for inclusion in the potential energy function which is then extended to a nonlinear finite element solution method. The potential energy is simplified by substituting the new rotation function for the previous rotation angle. Riks and displacement control are used to show solutions to several nonlinear arch problems. Other published analytical and experimental results are compared with the current research. This work is a simple extension of a previously published large displacement/moderate rotation theory (Creaghan and Palazotto, 1994), but the results show significant improvement when large cross sectional bending angles are present.

Preliminary experimental evidence of heating at the running surface of avalanching snow

At the Montana State University Avalanche Research Site, instrumentation has been installed to measure temperatures, flow depth, and velocities during an avalanche. Five thermocouples have been installed along a 30-m section of the avalanche running surface. Temperature time histories were collected during several avalanches at the flow running surface. The flowing snow at the running surface did show a temperature increase as it progressed down the slope, but did not frequently approach the melt temperature. Snow samples were collected before the tests in the release zone and after the avalanches in the debris for microstructural comparison. A computed tomography (CT) X-ray scanner was used to obtain images of the microstructural details of the pretest and debris snow samples. Using the microstructural parameters from the CT images, the growth of the new bonds in the debris was analyzed using a vapor diffusion sintering model. New bonds were shown to grow rapidly at the expense of small high-energy structures that resulted from the avalanche. The analysis showed vapor movement and sintering of new bonds due to surface curvature differences may be a significant debris bonding mechanism in snow that does not approach melt temperatures during an avalanche.

Montana State University's Subzero Science and Engineering Research Facility: New Interdisciplinary Cold Regions Research Laboratories

Montana State University recently completed construction and commissioning of a low temperature research facility with funding from the National Science Foundation, Murdock Charitable Trust and Montana State University. The Subzero Science and Engineering Research Facility (housed in the Department of Civil Engineering) supports a broad range of cold regions research disciplines including structural, soil, asphalt, concrete, geosynthetics, life sciences, ice/snow mechanics, wetlands, streams, transportation, and earth sciences. The simulated environments are applicable from medium/low latitude winter to polar. The facility has several world unique capabilities in environmental simulation and includes eight walk in cold laboratories, three smaller low temperature biological incubators/ environmental chambers, a low temperature epifluorescence microscope, and a temperature controlled computed tomography (CT) scanner. Each of the laboratories, while adaptable for broad application, is designed with a specific scientific focus in mind. Functions of the fully programmable individual cold laboratories include: units that provide simulation of solar and sky radiation providing realistic energy balance, a large scale structural engineering test bed, a class 1000 clean room with class 100 work area, a unit in which the liquid water phase maintains an influential environmental role, room for specimen storage, a teaching laboratory with ample room for student groups and general specimen preparation area. The paper summarizes the specific capabilities of each chamber and describes the ongoing and future research in these facilities.

Nurturing Our Rocket Space Workforce at the United States Air Force Academy

The Space Systems Research Center at the United States Air Force Academy is building a cadre of rocket space professionals “one cadet at a time.” Its motto and aim is for cadets to “Learn Space by Doing Space.” Approximately one half of the cadets majoring in astronautical engineering perform a one year long capstone program of the design, fabrication, test and launch of a sounding rocket (the FalconLAUNCH program). This year’s rocket, FalconLAUNCH-3, launched in April, 2005. This was the preliminary test of the rocket design for FalconLAUNCH-4, scheduled for a 2006 launch from San Nicolas Island, California, carrying a 5-kg payload to 100,000 meters. This program is modeled like any Air Force program, with the cadets acting as the contractor, and the faculty and Air Force funding agencies acting as the Air Force Manager. The program has approximately 20 students with five or six faculty mentors. The program is multi-disciplinary, requiring cadets to use skills learned in physics, electrical engineering, computer science, and management. All of the normal milestones, reviews, presentations, and reports required in an Air Force Program are required of the cadets in the FalconLAUNCH program. The overarching goal is to provide realistic system development and systems engineering experience to future Air Force leaders. While the overall design evolves each year, each class culminates their undergraduate engineering experience with the launch of their rocket. This paper details the development, challenges, and advantages of conducting an undergraduate sounding rocket program and details the design, construction, testing and the April 2005 launch of FalconLAUNCH-3.