Constitutive behaviour of ice under compressive states of stress and its application to ice-structure interactions

Turner, Joshua (2018) Constitutive behaviour of ice under compressive states of stress and its application to ice-structure interactions. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

In this work the multiaxial constitutive theory for the viscoelastic deformation of ice developed by Jordaan and others is examined and modified. The microstructural changes undergone by the material while under deformation have been modelled via a finite collection of state variables which represent the average “damage” within a region (originally coined to describe the degradation of mechanical components, damage is here used to refer to any change in microstructure). The accumulation of damage causes an enhancement in creep deformation via the process of microcracking at low pressures, or dynamic recrystallization and pressure melting at high pressures. The damage evolution is modelled based on Schapery’s approach, modified to include the effects of low- and high-pressure damage separately. The damage rate is influenced by confining pressure, axial stress, and temperature, with a pressure-temperature shift function introduced to define the relationship between pressure and temperature. An exploratory series of triaxial tests was carried out in the laboratory at Memorial University; a description of the program, sample preparation, testing equipment, and procedure are provided. These tests were designed to investigate the deformation of ice under high shear and confining pressure. The ice samples were found to have an upper limit to their strength, failing at a stress difference (which is equivalent to the von Mises stress for a traditional triaxial test) of 26.0 ± 1.6 MPa. Thin sections of the samples showed the region along the fault to be highly recrystallized. The amount of recrystallization was found to decrease with distance from the fault line, with nearly half of the failed sample being practically undamaged in some cases. The role of numerous properties on ice-structure interactions have been investigated via a numerical scheme and the finite element program ABAQUS. The properties examined include: the effect of elastic damage; the inclusion of power-law breakdown; the implementation of a non-linear damage exponent; the effect of the highshear elastic failure discovered in the above experiments, and; the use of a pressuretemperature shift function for high-pressure damage. Constant elastic properties were found to most closely resemble the results of indentation experiments, particularly with the addition of the non-linear damage exponent. Power-law breakdown was found to be suppressed under confining pressure, and had little effect upon the qualitative behaviour of an ice-structure interaction. Implementing the high-shear elastic limit of 26.0 MPa on the von Mises stress was found to produce plastic deformation, instead of the expected viscoelastic behaviour; the internal stress along the fault-line of a sample is likely higher by a factor of five, leading to the discrepancy in behaviour. Applying the limit via a reduction in elastic modulus led to results more consistent with experimental evidence. A pressure-temperature shift function based on the process of pressure melting was used to enhance or inhibit the accumulation of recrystallization damage; pressures at one temperature were translated to the corresponding pressure at the reference temperature of -10 ◦C by assuming either a constant distance from melting point or a constant homologous temperature. The results were promising, producing the expected differences in loading and layer development. Extension to a reference temperature of -22 ◦C, the temperature limit for pressure melting, is worth consideration.

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