Strain-Rate Softening of Granular Ice in Brittle Regime: Fact or Artifact?

In the ice-structure interaction, the velocity of ice is known to affect the failure process and to the ice forces. The interaction process is complicated and not fully understood yet. Also in the uniaxial compression tests the strength of ice has been observed to be rate dependent; ice exhibits strain-rate softening in the brittle regime. However, it has been suggested that the softening is due in part to compliant loading frame. The simulation of ice strength in the brittle regime with a three dimensional continuum damage mechanics model based on the sliding crack mechanism is considered in this paper. The model can predict the uniaxial compressive and tensile strength of granular ice as a function of temperature and grain size. The capability of the model is tested against the strain rate dependent brittle strength. The hypothesis is that the velocity dependent coefficient of kinetic friction implemented into the three dimensional sliding crack model can be applied to predict the strain rate softening in brittle regime. The effect of loading frame compliance is also studied in the simulations. The numerical simulations revealed the significance of the stiffness of the loading frame: the softening increases with increasing compliance of the frame. The softening was also found to increase with increasing compressive strength; the observation corresponds to the experimental results found in the literature. When the frame was rigid, the softening was found to be almost negligible.