| Ice like other crystalline solids deforms with both
an elastic and a plastic component (Petrenko & Whitworth, 1999). If
a small amount of stress is applied to ice for a short period of time, the
ice will elastically deform. When the stress is removed the ice will return
to its initial shape; the strain is completely recoverable. If the stress
exceeds a certain amount the ice will be deformed irreversibly. This is
plastic deformation and the critical stress above which the crystal
deforms permanently is known as the elastic limit. Ice also will
be permanently deformed if stress below the elastic limit is applied for
a prolonged period of time. Such time dependent plastic deformation is known
as creep. |
| Figure 2. 6. 1. The Ice Ih lattice.
a) Perpendicular to the c-axis. The puckered layer of atoms composes
the glide plane, or g-plane, and the vertical bonds accommodating
hydrogen atoms is the shuffle plane, or s-plane. b) Viewed down
the c-axis, a is the unit cell and Burgers vectors parallel
this direction (After Hobbs, 1974). |
| Two mechanisms are known to contribute to creep in
ice. The motion of dislocations allows the relative movement of molecular
planes within the crystal lattice (see diagram) to permit deformation. This
processes in known as glide. A second deformation mechanism involves
the movement of single ions (Nabarro 1948) or dislocations (Herring 1950)
through the volume of a crystal lattice or around the grain boundary (Coble
1963). Diffusional processes act very slowly in ice and are observed when
the deformation regime involves very low stresses. |
