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Previous studies have established that, together with the development of a preferred crystal-orientation fabric in ice undergoing creep deformation to high strains, there also develops a tertiary equilibrium crystal size, i.e. the crystal size, rather than affecting the creep rate, is a result of the deformation to large strains.Equilibrium crystal size is considered here as a “balance” between crystal growth with time as a function of temperature and crystal change as a result of temperature dependent deformation. The temperature effects in these two processes (Arrhenius relation) are similar and consideration of the activation energies for the two processes indicates that it may be appropriate to cancel them, yielding a dependence of equilibrium crystal size on stress alone. The results from laboratory experiments of steady-state crystal size plotted as a function of stress support the above proposition.The possibility of using the relationships between steady-state crystal size and deviatoric stress as a polar ice-mass piezometer is discussed.

M.V. Nella Dan was beset in ice near 66°S, 51°Ε from 27 October to 15 December 1985. The scientific investigations planned for the voyage into the sea-ice zone included ice thickness, concentration and extent measurements, aerial photography of the ice, a core drilling project and meteorological observations. Once beset, the programme was expanded to include repeat measurements of a small strain grid and measurements of the sea-ice drift rate. Strongly convergent local conditions had led to the initial besetment of the ship. Analysis of measurements of the strain grid area over an 11 d period shows that, although there is some indication that the ice field may have been divergent, opening by approximately 3.4% over this period, there are large errors in the measurements and some doubt must be placed on the reliability of this estimate. The drift speed and direction were found to be highly dependent on wind speed and direction, the drift rate being approximately 2.7% of the wind speed at an angle of about 29° to the left of the wind direction.

Ice core and borehole studies along flowlines in ice sheets show that the ice crystal structure and flow properties evolve together with strain and rotation along the particle paths. Laboratory studies of initially isotropic polycrystalline ice show that the changes in crystal structure and flow properties of the ice sheets can be simulated by subjecting the ice to the same history of deformation, rotation and stress configuration undergone by the ice in situ. For isotropic ice the minimum creep rate dependence of stress and temperature is given for stresses 0.2–10 bars and temperatures −0.05 to −50°C. Tertiary and anisotropic flow rates are dependent on stress configuration, strain and rotation. Results for uniaxial compression and simple shear are presented. Other factors examined include crystal size, ice density, and the solid and chemical impurity content. Near the base of ice sheets large variations in stress and strain rate can occur. The formation of basal ice crystal structure is discussed, incuding the large crystal multi-maxima fabric ice and its flow properties. Shear rates from boreholes can be complicated by the complex local stress regime caused by bedrock variations. In the interior of ice sheets the relatively cold, stiff upper layers and warmer softer basal layers result in surface velocities which reflect the large scale average of the basal layer shear stress and temperature. An exception occurs approaching the grounding line where basal shear stress and normal stress decrease and sliding becomes dominant over internal deformation.

Systematic knowledge of the mechanical properties of ice is a fundamental requirement for the solution of a wide range of problems in the earth sciences and engineering. Past research has been characterized by specialized studies, involving a variety of motivations and scientific disciplines, without much overall coordination. When the results of this research are synthesized, a reasonably coherent picture emerges, but there are major gaps in knowledge. From the deduced behavior of ice, and from informed speculation where data are lacking, a coherent plan for future research can be developed. This report summarizes existing knowledge and proposes experiments and test programs for a new research thrust. The studies that are proposed can be expected to yield significant benefits in glacier studies, ice engineering, and the mechanics of frozen ground.

Uniaxial compression tests were performed on a series of laboratory-prepared ice samples. The samples initially consisted of randomly-oriented crystals. Each test was carried out under, as near as practicable, the same experimental conditions. The temperature for all tests was −3.0°C and the octahedral shear stress was 2.0 bar. Tests were stopped after different amounts of total strain, thin sections cut, and the ensuing crystal fabric examined. It was found that, between 0.8% and 8% octahedral shear strain, a small circle girdle fabric developed around the compression axis, and that the “strength” of the fabric increased along with a decrease in the girdle half-angle, until a total octahedral shear strain of 32.5%. In extension however, it was found that the girdle half-angle increased with increasing strain. Crystal sizes through the experiments were also monitored and were found to change systematically. The octahedral shear strain rate, after attaining a minimum of approximately 2 × 10−8 s−1 at 0.8% octahedral shear strain, had increased to approximately 6 × 10−8 s−1 by 8% strain, and maintained this value thereafter.

A flow relation for the minimum strain-rates for isotropic ice is presented. This flow relation has been compiled from laboratory tests in shear and compression, and covers the temperature and stress ranges typical of natural ice masses. However, the ice in natural ice masses is mostly anistropic. Three cores drilled at Law Dome, Antarctica, are used as examples to illustrate some of the different anisotropies that develop in ice masses in relation to the stress configurations and accumulated strains. For the shear- or compression-stress configurations, anistropic flow rates can be derived as enhancements of the minimum isotropic flow rates for the particular stress, temperature, and stress configuration. For compression, this enhancement asymptotically approaches 3. For shear, the enhancement may be as high as 8. For combinations of shear and compression, further tests are required.

Three sets of uniaxial compression experiments on polycrystalline ice are described. Five separate ice samples, each with a different crystal size, were represented, and minimum strain rate was attained for all tests. A tertiary creep stage was reached for each set of tests, and for one set this is examined in detail. All sets of tests demonstrated little or no dependence of the minimum ice strain rate on crystal size. In tertiary creep the strain rates were in closer agreement than at minimum. Beyond the minimum strain rate, crystal size was found to change during the development of the tertiary creep stage, such that on termination of the experiment, all crystal sizes were approximately the same. It is concluded, for the range of strain rates covered, that tertiary ice flow may limit crystal growth to an equilibrium size.

A cruise to Antarctic waters from late October to mid December 1985 provided the opportunity to study characteristics of the seasonal sea ice from a time close to that of maximum extent through early spring decay. The area covered by the observations extends from the northern ice limit to the Antarctic coast between long. 50 °E and 80 E. Shipboard observations included ice extent, type and thickness, and snow depth. Ice cores were drilled at several sites, providing data on salinity and structure. The observations verify the highly dynamic and divergent nature of the Antarctic seasonal sea-ice 2one. Floe size and thickness varied greatly at all locations, although generally increasing from north to south. A high percentage of the total ice mass exhibited a frazil crystal structure, indicative of the existence of open water in the vicinity. The ground based observations are compared with observations from satellite sensors. The remote sensing data include the visual channel imagery from NOAA 6, NOAA 9, and Meteor 11. Comparisons are made with the operational ice charts produced (mainly from satellite data) by the Joint Ice Center, and with the analyses available by facsimile from Molodezhnaya.

Uniaxial compression tests were carried out on laboratory-made polycrystalline ice samples with randomly oriented grains. Tests covered the temperature range −5.0°C to −32.5°C and the octahedral stress range 0.5 to 12 bar. Plots on log-log scales of strain rate against time and against total octahedral strain for the experiments, reveal that within these temperature and stress ranges, a minimum strain rate occurs after a time determined by a straight line locus, and at a total octahedral strain of about 0.6%. Some consequences for further studies of minimum flow rates at lower temperatures and stresses are outlined.

An account is given of ice deformation experiments in uniaxial compression. Samples studied include isotropic and anisotropic ice, laboratory prepared and from a core drilled at the summit of Law Dome, Antarctica. There are unexplained differences in the minimum strain rates attained by isotropic ices from the core and from the laboratory. Minimum strain rates for anisotropic ice are higher than for isotropic ice provided that the anisotropy is compatible with the stress configuration. In tertiary creep a constant strain rate is attained, associated with the development of a small circle girdle crystal orientation fabric, and an equilibrium crystal size. It is proposed that this tertiary creep is steady state.

One of the most urgent problems in the study of the mechanical properties of polycrystalline ice is to standardise the properties of ice samples made in the laboratory. With this in mind, this paper describes a technique for preparing fine grained, homogeneous specimens with random c-axis orientations. Specimens are optically clear, bubble free and have a density of 0.917 ± 0.005 Mg m3. Two types of uniaxial compression apparatus are also described; these are used for the measurement of ice strains to ∼6% and to more than 10%, respectively.