Disputas: Marcin Dabrowski

Prøveforelesning

Bedømmelseskomité

Neil Mancketelow, Geologisches Institut, ETH Zurich, Sveits
Viggo Tverrgård, Institut for Mekanisk teknologi, Danmarks tekniske universitet, Kgs. Lyngby, Danmark
Torgeir B. Andersen, Fysisk institutt, PGP, Universitetet i Oslo

Leder av disputas:  Ørjan G. Martinsen

Veileder:  Daniel W. Schmid, Yuri Yu. Podladchikov

Sammendrag

This PhD project has been carried out at the Department of Physics, University of Oslo in the Physics of Geological Processes Institute, a Norwegian Centre of Excellence. The subject of the thesis is modelling of the mechanical effects related to the microstructure evolution in composite rocks subject to shearing.

The anisotropy development in a two-phase material subject to large deformation and the interaction of rigid inclusions and anisotropic layered host were studied both by numerical and analytical methods. The mechanical interaction in a composite material consisting of numerous inclusions or layers was resolved by running a self-developed optimized flow solver (finite element method) tailored for large numerical resolutions and complex geometries evolving in large strain. The numerical results were complemented by an analytical solution to the mechanical problem of an isolated elliptical inclusion embedded in an anisotropic host and subject to uniform far-field loads. In particular, a differential effective medium (DEM) type of the scheme predicting the overall anisotropy of a two-phase composite consisting of aligned elliptical inclusions was developed and validated numerically.

The results of the numerical simulation show a marked evolution of the effective viscosity in a two-phase composite with an evolving microstructure. Despite the complex non-elliptical shape development, a simplistic model of the shape evolution relying on the isolated elliptical inclusion solution combined with the DEM scheme provide a surprisingly good approximation to the bulk viscosity evolution even for densely packed aggregates. Both the model and numerical simulations show a pronounced structural weakening following a short hardening stage in the simple shear case. The rate of softening is insufficient to explain the localization phenomena occurring in composite rocks by the structural softening alone.

The introduction of a strong inclusion in a layered medium leads to a local flow perturbation in its vicinity. The anisotropy structure in the host evolves due to inclusion motion and may be trapped during its potential growth (e.g. snowball garnet porphyroblasts). These patterns can be used to quantify the strain or even the strain rate in the system. Numerical simulations of the simple shear case show that the inclusion rotation rate is strongly reduced as the structure development progresses in the layered host material. The presence of a coarse host layering as compared to the inclusion size suppresses the structure development. Failing to recognize these effects may result in an underestimated overall strain evaluation in natural shear zones.

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