Stress-strain behaviour single crystal

Pure torsion tests were performed on ice single crystals at a constant imposed external shear stress". Softening was evidenced as the creep curves revealed a strain-rate increase, up to a cumulated plastic strain of 7%, see figure 1. Note that such a behaviour was also observed during compression and tension tests. ... 

The creep of polyerystalline ice has been studied by Glen (1955) over a stress range from i to 10 bars at temperatures o to —13 °C and by Jellinek Brill (1956) from 0-3 to 2-5 bars and — 5 to — 15 °C. The behaviour is rather different from that of single crystals in that the creep rate is initially high and then falls with increasing strain, approximately in the form... 

We have now discussed in turn, the stresses required to produce yield, the relationship between stress and plastic strain increment, the structural reorientation occurring as a result of yield, and the relationship between constant strain-rate yield and features of non-linear recoverable creep deformation. Theoretical models to describe the behaviour have ranged from single crystal plasticity through to the oriented continuum ideas of plasticity and viscoelasticity. On many points both the experimental data and the interpretations appear almost contradictory and it is therefore helpful to see if any common ground can be established. 

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