Experimental deformation of polycrystalline H₂O ice at high pressure and low temperature: Preliminary results

Abstract

Interest in the mechanical properties of water ice under the conditions in which it exists in the outer solar system has motivated the development and use of a new high‐pressure, low‐temperature triaxial deformation apparatus. Constant displacement rate tests on 70 samples of pure polycrystalline water ice have been performed at temperatures 77≤T≤258 K, confining pressures 0.1≤P≤350 MPa, and strain rates 3.5×10−6≤ trueε˙ ≤3.5×10−4 s−1. In most cases, the ice polymorph tested was ice Ih. Both brittle and ductile behavior have been observed. Brittle behavior of ice, promoted by lower pressure, lower temperature, and higher strain rate, is analogous to that in rocks, with the important exception that brittle fracture strength becomes independent of confining pressure above 50 MPa pressure and the fracture angle is approximately 45° to the loading direction (i.e., the coefficient of internal friction is approximately zero). Ductile flow, the predominant behavior in our tests at T≥195 K, follows a law of form trueε˙ = Aσn exp (−H*/RT) (σ is stress; R is the gas constant; A, n, H* are material constants). Three sets of material constants are required to fit the data, with changes in sets (or mechanisms) occurring near 243 K and 195 K. The value of n remains near 4 throughout the measured ductile field, but H* drops from 91 to 61 to 31 kJ/mole as temperature decreases. The maximum brittle strength measured was 171 MPa; the maximum ductile strength measured was 91 MPa. At confining pressures near the phase transition pressure of ice Ih → ice II, the ductile strength is observed to drop dramatically. Some overlap with previous work occurs at higher temperatures and lower pressures. Agreement with present work is generally good, both quantitatively in the values of n and H*, and qualitatively in the mechanism of deformation. Although the ductile strengths measured here are somewhat higher than expected on the basis of extrapolations of previous work, the low value of H* at T<195 K indicates that the ice Ih layer on icy bodies in the solar system is much weaker than has generally been predicted.

Grain‐size‐induced weakening of H2O ices I and II and associated anisotropic recrystallization

“…Test specimens were deformed and measured in constantstrain rate experiments at elevated pressure and low temperature, using techniques described previously [Durham et al, 1983]. The testing apparatus is a 1.0 GPa gas deformation apparatus outfitted for cryogenic conditions [Heard et al, 1990].…”

Fracture of multiyear sea ice

“…However, at high stresses and at higher confining pressures, shear fracture shows little or no dependence on confining pressure [Durham et al, 1983;Rist and Murrell, 1994 …”

Effects of dispersed particulates on the rheology of water ice at planetary conditions

“…We have extended our laboratory study of the flow and fracture of water ice I [Durham et al, 1983;Kirby et al, 1987] to include ice + rock mixtures. Water is one of the more important volatile constituents of many of the lowdensity moons of the outer solar system, and since most of these moons have some rocky component, ice + rock mixtures must exist or have existed in at least some regions on these moons.…”

“…Those related studies are (1) a refinement of the flow law for pure ice I, which was necessary to improve the resolution of some of the more subtle effects of particulates; (2) the effect of varying initial grain size in pure ice I; and (3) a microstructural study of textural evolution during the initial stages of deformation. We manufactured the samples by adapting a method we described earlier [Durham et al, 1983], and we briefly outline here. Sieved seed ice grains, made by grinding frozen distilled water in a blender, were packed into stainless steel tube molds to a standard porosity of 40%.…”