SHOCK COMPRESSION OF SNOW AND ICE

Abstract
Shock wave data have been obtained in snow and ice at stresses up to 450 MPa and temperatures as low as 249 K. The experiments were conducted using a 200 mn diameter, single-stage gas gun. Data were taken in the form of Lagrangian shock profiles measured by carbon piezoresistive foils at several locations in each target. All samples were constructed by assembling successive layers with the gauges placed between the layers. Construction was completed in a cold room, and the samples were aged a minimum of four days at the design test temperature. After removal from the cold room for mounting on the gun, the targets were cooled with cold, dry nitrogen gas flowing through circumferential cooling coils and the temperature was monitored by thermocouple. Snow targets tested to date have all been constructed by recompacting finely crushed ice to the desired density (about 400 kg/m³) in layers in the sample holder. We plan to conduct test on naturally precipitated snow in the winter of 1985-86. Initial results on snow for peak stresses near 10 MPa show that snow loads nearly along a Rayleigh line to densities within a few percent of those reached by quasistatic compression at similar peak stresses. The loading wave is, however, quite dispersive. Release is very stiff compared to loading, but it is much softer that release in dense ice. Preliminary analysis of ice shocked to about 450 MPa at 249 K indicates that the elastic wave (about 3700 m/s) may be as large as 400 MPa. If Poisson's ratio is equal to its zero-pressure value, the shear stress for a uniaxial strain wave of that amplitude is about 210 MPa or about 7.5 percent of c₄₄. This is only slightly larger than the 6 percent value at 263 K derived by Gaffney (1) from Larsen's data (2). The small difference between experiments conducted above and below the minimum melting point of the ice phase diagram (251 K) supports the contention that melting is not involved at the elastic limit, even at 263 K. Rather, yielding at the Hugoniot elastic limit (HEL) is probably related to fundamental loss of strength by the lattice. Above the HEL, further deformation is accompanied first by conversion to a hydrostatic state and later by partial transition to some high pressure phase.