Mechanical behaviour of viscoelastic material with changing microstructure

Singh, Sanjay K. (1993) Mechanical behaviour of viscoelastic material with changing microstructure. Doctoral (PhD) thesis, Memorial University of Newfoundland.

[English] PDF (Migrated (PDF/A Conversion) from original format: (application/pdf)) - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (22MB) [English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. (Original Version)

Abstract

The observed response of a crystalline and brittle material to stress is the result of various micromechanical activities inside the material at the grain, or, at the constituent element scale. These activities include microcracking, pore formation and collapse, grain-boundary sliding, and phase change. In this thesis, such microstruc- tural changes, and their effect on the response of viscoelastic materials are presented with reference to the behaviour of ice during its interaction with a structure. -- During ice-structure interaction, zones of high pressure are formed at the structure interface. Extensive microcracking and other microstructural changes such as recrystallization in the ice occur in these zones. When the cracks coalesce, cavities are formed between junctions of weakly connected grains. This finely crushed material is finally extruded from the structure interface. The behaviour of ice and its damage depend on the rate of loading, the degree of confinement, the density of microcracks, grain boundaries, cavities and other microstructures. To further understanding in this area, triaxial tests were carried out on ice at different initial microstructure. -- The process of material modelling is guided by the framework of thermodynamics. The internal variable approach provides a powerful method of incorporating the microstructures into a continuum theory. The changes in microstructures such as cracks and grain boundaries are modelled by a generalized J-integral. while change in the porosity is modelled by an approximate solution for creeping solids. To describe the various changes in the material two theories are developed. In the first theory, solutions for nonlinear elastic media are extended to nonlinear viscoelastic media using a correspondence principle. The second theory for viscoelastic behaviour is based on a mechanical model with nonlinear elements. Three components of deformation, i.e., the elastic, the delayed elastic, and the viscous creep are separately identified, and their changes with the extent of damage are modelled. The first theory is more systematic and requires fewer parameters. Both of these theories provided good predictions for strength tests. The dilatation of the cracking polycrystalline ice and the porous crushed ice is also modelled by the mechanical model. -- A series of plane-strain extrusion tests were analyzed to understand the flow properties of crushed ice. A closed-form solution is presented for the nonlinear and viscous flow of crushed ice, and a finite element solution is also presented for the flow of crushed ice that is also undergoing compaction. These analyses provided a good agreement to the extrusion tests.

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