The strength of polycrystalline ice

Abstract

The paper contains a general study on the strength of polycrystalline ice. A theory on the fracture of ice is proposed that starts from the observation of the mechanism of failure of monocrystalline ice and is based on the computation of the elastic stability of a grain of polycrystalline ice. It is verified with experimental data obtained from tests carried out at the Ice Mechanics Laboratory of Laval University. For brittle fracture it shows that when a crack is formed in one grain, it propagates immediately to the whole sample in a tensile test and is followed quickly by failure in a crushing test. An equation is derived that shows that the fracture strength depends on the crystal size and on the cohesive strength of the ice, which is only temperature dependent.The maximum strength of polycrystalline ice does not occur in the brittle range, but in the ductile range. Glen's law of creep of polycrystalline ice has been used to obtain, from the data, best fit curves between the yield strength and the strain rate, for each ice type. Strength diagrams are presented for the full range of loading, including both ductile and brittle behaviour. A discussion is made on the transition from one mode of failure to the other and on the failure criteria for complex states of loading.Recommended values for the fracture strength of natural ice are submitted that have a direct application to the design of engineering structures.