ALBERT

Description: Ice sheets and glaciers flow under their own weight and their flow of ice is a major contributor to both global sea-level and climate changes. The macroscopic flow of ice is affected by the properties of the microstructure, which is formed by a small aggregate of individual ice crystals. The deformation of ice is accompanied by recrystallisation, a term which describes mechanisms causing re-orientations of the crystalline lattice, the formation of new crystals or the migration of their boundaries. The ice crystal is marked by a significant viscoplastic anisotropy, which causes a distinctly higher resistance to flow, if the crystalline lattice is unfavourably oriented. With deformation, the ice grains align and develop a crystallographic-preferred orientation within the ice-aggregate, which induces a macroscopic anisotropy. A knowledge of the micro-dynamic deformation and recrystallisation mechanisms and how they affect the properties of the ice aggregate is a key to understand ice sheet dynamics. The objective of this thesis to investigate the deformation and recrystallisation mechanisms in ice and the involved changes in the microstructures of ice- and ice-air aggregates. This is done by means of two-dimensional numerical simulations using the modelling platform Elle, which optimised for modelling interacting micro-dynamic processes. The simulations couple a numerical model for viscoplastic deformation of anisotropic polycrystalline aggregates to implementations of recrystallisation mechanisms in Elle. In particular, an explicit numerical approach to consider secondary phases such as air inclusions in the numerical setup is developed, implemented and used in this thesis for the first time. Additionally, the new approach allows grain-size-reducing mechanisms, which allows the achievement of stable-state microstructures with deformation. In each scientific publication presented in the thesis, qualitative comparisons to natural polar ice accompany the numerical simulations. The results of this thesis show that the deformation and microstructures of ice are generally more heterogeneous than previously thought. Strain localisation is common in ice and related to viscoplastic anisotropy and intensified by the presence of air inclusions. Probably, strain localisation is occurring over a range of scales and has implications for the large-scale flow of ice. The thesis further demonstrates that deformation-induced recrystallisation mechanisms are common in ice and discusses their relation to strain localisation. In particular, the study points out the importance of the dissection of grains by migrating grain boundaries as an additional grain-size-reducing process in polar ice, which was not studied previously. This thesis confirms that the activation of deformation and recrystallisation mechanisms is a function of the deformation conditions such as strain rate, temperature and likely the load of impurities and dust particles. The steady-state numerical-microstructures reflect the prescribed deformation conditions, but appear largely independent from the initial microstructures. These results of this study indicate a high rate of change in crystallographic-preferred orientation and other microstructural properties. Furthermore, the thesis confirms that the development of crystallographic-preferred orientation is a function of strain rather than time or stress.