Constant strain- and stress-rate compressive strength of columnar-grained ice

Abstract

Unconfined compressive strength of transversely isotropic columnar-grained ice has been investigated for loads applied normal to the longitudinal axis of the columns at the high homologous temperature of 0.96 T m (T m is the melting temperature) under truly constant strain and stress rates. A closed-loop, servo-hydraulic test system inside a cold room was used. Both the strain- and stress-rate dependences of upper yield stress can be expressed in terms of power laws. The observed strain-rate dependence of strength was found to be numerically the same as the dependence of viscous-flow rate on stress in constant stress creep tests at the same temperature. It is shown that the strain-rate sensitivity of yield strength compares well with previous results (obtained under constant cross-head rates using a conventional machine) only if the average strain rate to yield is used as the independent variable instead of the conventional nominal strain rate. The paper also discusses the strain and time aspects of the tests. It shows interdependence among values for compressive yield strength, strain rate, failure strain and time very similar to the interdependence among the corresponding values in tensile creep failures in metals, alloys and other polycrystalline materials at high temperatures. It is emphasized that the splitting type of brittle-like premature failure depends on the stiffness of the test system and should not be considered to be a fundamental material property. The concept of failure modulus is proposed for examining the ductile to brittle transition.