Effects of Size on the Dynamics of Dislocations in Ice Single Crystals

Single crystals of ice subjected to primary creep in torsion exhibit a softening behavior: the plastic strain rate increases with time. In a cylindrical sample, the size of the radius affects this response. The smaller the radius of the sample becomes while keeping constant the average shear stress across a section, the softer the response. The size-dependent behavior is interpreted by using a field dislocation theory, in terms of the coupled dynamics of excess screw dislocations gliding in basal planes and statistical dislocations developed through cross slip occurring in prismatic planes. The differences in the results caused by sample height effects and variations in the initial dislocation microstructure are discussed.

Figure 1

Creep strain on outer surface vs time, for various diameter $D$ values.Height and average shear stress are indicated.

Figure 2

Shown on left is the excess screw density on the undeformed mesh,locally resolved to indicate radial ($R$) andcircumferential ($C$) components.At right, the shear stress component ${\sigma }_{xz}$ is shown on the deformedmesh, with displacements scaled by a factor of 5.

Figure 3

Creep curves in forward and reverse torsion from experiments, 1Dand 3D models.

Figure 4

Normalized time at 3.5% strain, experiments and modeling. Experimentaldata from creep tests shown in Fig. 1. Thestraight line suggests the trend.