Jerome B. Johnson

Measuring snow microstructure and hardness using a high resolution penetrometer

Abstract Using a high resolution snow penetrometer we are able to measure snow penetration resistance and snow meso- and microstructure. The variation of the signal is characteristic for different snow types. The penetrometer can be used in the field as well in the laboratory. The range of snow types which can be tested extends from light new snow (50 kg m−3) to very dense snow occurring on ski race tracks (500 kg m−3). The displacement resolution of the penetrometer is better than one mm to detect significant changes in resistance, the force signal is measured every 0.004 mm. The constant penetration velocity can be varied between 6 and 20 mm s−1. A classification is developed to extract textural information of snow from the force signal. The classification is based on experiments with artificially metamorphosed snow and undisturbed snow from the Alps and Alaska. Two weak layers were identified and compared to surface sections. The new instrument makes the measurement of mechanical and stratigraphic features of a snowpack a more objective and easy task than with other methods. The high displacement resolution promises also a better identification of weak layers.

Characterizing the microstructural and micromechanical properties of snow

Abstract A micropenetrometer has been developed that produces snow grain bond ruptures at the microstructural level and provides a unique signal for different snow types. A micromechanical theory of penetration has been developed and used to recover microstructural and micromechanical parameters for different snow types from the penetration force–distance signal. These parameters are the microstructural element dimension, the mean grain size, the critical microstructural deflection at rupture and the microstructural coefficient of elasticity. Additional derived mechanical properties include the compression strength and elastic modulus of microstructural elements and continuum scale volumes of snow. Analysis of the force–distance signal from a Monte Carlo simulation of micropenetration indicates that microstructural and micromechanical parameters may be recovered with a measurement accuracy of better than 5% when spatial and force resolutions are high and the penetrometer tip area is of similar size to the structure dimension.

Thermophysical properties of Alaskan loess: An analog material for the Martian polar layered terrain?

The Martian surface has several regions where thermal inertia measurements indicate a porous ice-free insulating surface, yet are mechanically competent enough to sustain substantial slopes. In support of the interpretation of those regions within the Martian polar layered terrain, we report measurements of thermal conductivity for loess from the field and in the USA CRREL Permafrost Tunnel. Permafrost Tunnel loess is a desiccated material that can form vertical walls, but is of low density (800–1000 kg/m³), modest shear strength (4 kPa), and has a low thermal conductivity (0.1 W/m-K at 1 bar). These properties are similar to the inferred properties of the Martian polar layered terrain. The Birch Hill field sample has a density of 1160 kg/m³ and a conductivity of 0.15 W/m-K. The Chena Spur Road sample has a density of 1360 kg/m³ and a conductivity of 0.7 W/m-K. The relatively high conductivity for the Chena Spur Road is due to the cementation of soil grain contacts, its higher density, coarser grain size, and higher quartz grain content.

Shock response of snow

We conducted gas‐gun impact experiments on snow, with initial densities of 100–520 kgu2009m−3 and at temperatures from −2 to −23u2009°C and stress levels of 2–40 MPa. Carbon stress gauges were embedded in the snow to measure shock stress histories and arrival times. The unsteady and complex nature of the shock stresses necessitated the use of finite element and reverberation analysis techniques to determine the shock pressure‐density (P‐ρ) relationships for snow. Experimental results indicate that variations in initial snow density are reflected in differences in the P‐ρ deformation path. The pressure needed to compact snow to a specific final density increases with lower initial density. Snow deformation was not affected by initial temperature, but was found to be strain rate dependent. Estimated release moduli increased nonlinearly from 50 MPa at a peak compression pressure of about 15 up to 2700 MPa at a peak pressure of about 40 MPa. Calculated stress histories and shock arrival times agreed with measured val...

The response of natural snow to explosive shock waves

Field measurements of shock waves in snow with initial densities from 100 kg/m3 to 555 kg/m3 were made in situ in a natural snow cover. A high amplitude, short duration, uniaxial shock impulse (∼0.6 GPa for 10 μs) was imparted to the snow using sheet explosive, and the shock arrival time and stress histories were measured at depth in the snow. For dry snow (ρ0=250±30 kg/m3), the shock velocity can be described by a power law and decays rapidly with depth, from over 1000 m/s near the snow/explosive interface to 120±20 m/s at 0.20 m. The shock stress attenuation factor at a propagation depth of 0.20 m is about 4×10−3. Tests in which explosive gases were excluded from the snow had higher shock velocities and pressures than tests where the gases penetrated the snow.

SHOCK WAVE STUDIES OF SNOW

Shock-wave studies of snow have been conducted at stress levels of up to 40 MPa. Analysis of embedded gauges and shock-reverberation techniques were used to determine shock pressure-density data for snow with initial densities ranging from 100 kg m{sup {minus}3} to 520 kg m{sup {minus}3} and temperatures ranging from {minus}2{degrees}C to {minus}23{degrees}C. Shock velocities ranged from about 170 m s{sup {minus}1} (for low density snow) to about 280 m s{sup {minus}1} (for high density snow). At constant density and impact velocities, but varying temperatures, there was no variation in shock velocity. This indicates that the internal energy and any temperature dependent strength of ice bonds do not measurably affect shock propagation in snow over the temperature and pressure range of our tests. Our results also indicate that snow is a highly rate sensitive material. 9 refs., 4 figs.

COMMENT ON: EQUATION OF STATE FOR EXTRAPOLATION OF HIGH-PRESSURE SHOCK HUGONIOT DATA J. APPL. PHYS. 65, 3852 (1989)

Oh and Persson [J. Appl. Phys. 65, 3852 (1989)] proposed an equation of state to extrapolate high‐pressure shock Hugoniot data to other high‐pressure and high‐temperature states and compared it to data. The requirement that F=−(∂E/∂V)P/(∂E/∂V)H≊1 (E is specific internal energy, V is specific volume, P is the constant pressure path and H is the constant Hugoniot path) needed to establish the equation of state appears to be in error. I have found F to vary from 0.16 to 3.59 for fifteen common materials of interest to shock physicists. Oh and Persson’s [J. Appl. Phys. 65, 3852 (1989)] comparison of their equation of state to data gives the impression of a better agreement than actually occurs because of possible errors in the transcription of data, and the use of an inappropriate Hugoniot for water. When data are correctly plotted and an appropriate water Hugoniot is used, the comparison of data to theory indicates that the equation of state loses accuracy with increasing pressure or decreasing porous initia...