Ned Bair PhD Dissertation Defense

The Bren School of Environmental Science & Management
at the University of California, Santa Barbara


PhD Defense

Ned Bair

Ned Bair

Tuesday, Oct. 18, 2011

9:00 a.m.
Bren Hall Pine Room (3526)

"Fracture mechanical and statistical properties of nonpersistent snow crystal avalanches"

Jeff Dozier, Faculty Advisor

Nonpersistent snow crystals are deposited during storms, last for a few days, and are common fracture planes in avalanches. Because persistent crystal avalanches cause more deaths, nonpersistent crystal avalanches receive less study despite being responsible for the majority of avalanche fatalities in some U.S. states, such as California. Nonpersistent crystal avalanches are studied here using thousands of avalanche control records, dozens of crown face profiles, and field work with near-infrared (nIR) imaging and particle tracking. New (24-hr) snow and Extended Column Test propagation (ECTP) are the best predictors of nonpersistent crystal avalanche activity, while changes in temperature and density during storms have no effect. Nonpersistent crystal failure planes are the same hardness as layers immediately above, but are significantly softer than layers immediately below. In one nIR profile, nonpersistent crystals form a thin weak layer, but in others, there is no grain size difference between the fracture plane and adjacent strata. Lack of a weak layer makes fracture in nonpersistent crystals similar to other materials; fractures cut through the bulk without any specific weakness other than a flaw. Nonpersistent crystal failures follow behavior predicted by the anticrack model, a model emphasizing collapse that has only been tested on persistent crystal failures. This suggests nonpersistent and persistent crystals share the same failure mechanism with collapse as a driving force. Critical cut lengths during Propagation Saw Tests (PSTs) vary little with slope angle, consistent with anticrack predictions. A PST on nonpersistent crystals collapsed about 1 mm, half the collapse of a PST on persistent crystals. Easily collapsible planar crystals, such as sectored plates and stellars, are the most common nonpersistent crystals in failure planes. During high instability, fractures are easier to trigger on nonpersistent than on persistent crystals. Sustainability of propagation depends on new snow depth because the slab must withstand bending; otherwise tensile slab fracture occurs. Measurement of displacement during PSTs yield some of the lowest elastic moduli recorded for snow: 0.04-0.41 MPa for slabs with densities from 73-143 kg m-3. The median fracture energy for nonpersistent crystals is 0.09 J m-2, comparable to published values for persistent crystals.