The Friction and Creep of Polycrystalline Ice

  • Barnes P
  • Tabor D
  • Walker J
  • 67

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Abstract

The work described in this paper falls into two parts. The first is concerned with a study of the deformation of polycrystalline ice (crystal size ca. 1 mm) in uniaxial compression and when subjected to indentation. The uniaxial compression experiments covered strain rates from 10^{-9} to 10^{-2} s^{-1} and temperatures from 0 to -48^{\circ}C. It is shown that over the whole range of experimental conditions the secondary creep behaviour can be described by a single relation of the type \dot{\epsilon}_{s} = A(sinhασ)^{n}exp(-Q/RT) where σ is the applied stress, Q an activation energy and A, α and n are suitable constants. This reduces to the more familiar power law over more restricted portions of the experimental curve. Over the whole range n has a value close to 3 but Q has two distinct values: 120 J mol^{-1} above -8^{\circ}C: 78 J mol^{-1} below -8^{\circ}C. The indentation hardness experiments cover loading-times from 10^{-4} to 10^{4}s and a temperature range of 0 to -25^{\circ}C. The hardness behaviour may be linked with the creep properties using the analysis of Atkins, Silverio & Tabor (1966) which assumes that the rate-determining process is the diffusion of the hemispherical elastic-plastic zone surrounding the indenter into the undeformed material ahead. There is very good agreement between the hardness data and the creep parameters determined in the uniaxial compression experiments. In addition the hardness experiments enable experiments to be carried out at much higher compressive stresses. A discussion of these results suggests that at moderate stresses there are three main creep processes. Below -8^{\circ}C the behaviour is dominated by basal glide and not by dislocation climb. At temperatures between -8 and -1^{\circ}C the creep is associated with both a liquid phase at the grain boundaries and grain-boundary sliding. At temperatures very near 0^{\circ}C, if the applied pressure is high enough, pressure melting and regelation may occur. Finally at very high stresses non-basal glide can contribute appreciably to the observed creep. The second part deals with the friction of a cone of polycrystalline ice sliding over a hard flat surface at speeds ranging from 10^{-1} to 10^{-1} m s^{-1}. It is found that under conditions of strong interfacial adhesion (for example on clean granite) the friction is due to the shearing of a thin layer of ice very close to the interface. At speeds above 10^{-3} m s^{-1} frictional heating is sufficient to cause appreciable surface melting and the friction can fall to very low values. Between 10^{-6} and 10^{-4} m s^{-1} the friction is fairly constant and there is cracking and fracture at the interface. At the lowest speeds the sliding is smooth and the friction increases with speed in a manner that suggests creep in the interfacial ice layer. However the creep in the friction experiments is about 100 times faster than in the comparable compression experiments. The difficulty is resolved by single crystal experiments which show that recrystallization occurs over the contact region and that the surface crystallites are preferentially oriented with the basal plane parallel to the plane of sliding. This is an orientation highly favourable to easy shear in a direction tangential to the interface. These results have a direct bearing on the creep of glaciers. They also suggest that similar processes may be involved in the sliding, at elevated temperatures and for protracted periods, of hexagonal metals.

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Authors

  • P. Barnes

  • D. Tabor

  • J. C. F. Walker

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