CREEP OF ICE AND MICROSTRUCTURAL CHANGES UNDER CONFINING PRESSURE

摘要

Glaciers are one of the most striking examples of a material undergoing creep. The flow of these large bodies of ice is driven by self-weight and occurs as a result of the very high homologous temperature at which ice exists under atmospheric conditions. Deformation-related motions are in the order of centimetres to metres per year and the hydrostatic pressure at the base of the largest ice caps may be up to 40 MPa. Numerous investigations have been conducted to unravel the physical processes taking place in what may be considered the longest lived, in situ creep experiments available to material scientists. This is in sharp contrast to the dynamically more vigorous events the ice engineering community is confronted with when attempting to estimate the strength and behaviour of ice during its interaction with man-made structures. Examples are: the stresses exerted by an ice sheet on the hull of a ship, the interaction of icebergs with offshore oil production rigs, and loading of bridge piers by river jams during spring thaws. These events may involve significant load fluctuations and hydrostatic pressures exceeding 70 MPa. Displacement rates are in the order of millimetres to metres per second (40 m/sec in the case of ice/propeller interactions). In addition to gravity, water currents and winds acting on floating ice as well as ship inertia are the main driving forces. Mechanical testing and medium-scale field investigations are commonly carried out, either as a complement or as an alternative to the real scale event, to promote a better understanding of the processes involved in the ice during these interactions. This paper is an example of investigations conducted for that purpose. It reports on the creep and microstructural response of laboratory-made polycrystalline ice in compression, at various levels of hydrostatic pressures and up to large strains. These are conditions for which, prior to our testing program, little information was provided in the open literature. An analysis of a brittle and a ductile failure planes produced at low and high confinement is also presented. The implications of these results for the high-speed compressive loading of natural ice by a flat indentor are discussed.