Mathematical modelling of deformation mechanisms in ice: Dissertation

Mathematical modelling of deformation mechanisms in polycrystalline isotropic ice over wide temperature and strain rate ranges was investigated in this thesis. The proposed 3-dimensional constitutive equations describe both time-dependent ductile deformation mechanisms and microcracking associated with Hookean deformation showing a brittle nature. A model to describe the high strain rates operative mechanisms, which form the microcrack nucleus, was introduced. It was assumed that the required four independent deformation systems were: basal glide (2 systems) and twinning (2 systems). At very high strain rates, when the dislocation motion on the basal plane is not fast enough, the total permanent response of the icebody causing microcracking may be resulting from twinning. The prominent microcracking (damage) at high strain rates is described by a fourth-order tensor. The damage evolution law is based on the potential energy theorem. The observed micromechanisms in ice are modelled for both the condition for microcrack formation and the effects of microcracks on the mechanical properties of ice. The proposed approach connects continuum damage mechanics with fracture mechanics. The proposed theory reveals the Hall-Petch type of relation, although the frictional stress vanishes. Computed examples satisfactorily gave the measured compressive strength of ice and predicted the ratio of the peak compressive stress to the peak tensile stress as 1.85: 1, which is much lower than the measured one - 8.6 : 1. At elevated temperatures when the strain rate is low, the ductile response of ice is dominant. The following mechanisms are modelled: Hookean deformation, delayed elasticity associated with grain boundary sliding and viscoplaticity associated with dislocation movement. The uniaxial material model is based on the equation proposed by Sinha. However, a more effective approximate time integration method was applied in computing the delayed elastic strain. This work gives a micromechanical representation for the (viscoclastic) delayed elasticity. This was applied during the derivation of the 3-dimensional constitutive SADS-equation, which coincides in uniaxial tension with the Ashby and Duval modification of Sinha's model. A Schapery's nonlinear viscoelasticity theory based time integration method, which is a modification to the procedure applied in the uniaxial case, was proposed for the SADS-equation. Although this method gives realistic predictions during loading it does not allow the material to relax correctly.