The role of strain-induced microstructures on the bulk effective shear viscosity: a numerical study on two-phase aggregates

In this thesis, we investigated the role of a second phase in a polycrystalline aggregate deformed by simple shear with 3-D, high resolution numerical simulations. This second phase is dispersed in a rock matrix with different mechanical properties. We studied how the viscosity contrast, the volume fraction of the second phase and the increasing applied shear strain affect the microstructure and, consequently, the bulk effective shear viscosity of the aggregate. We discuss the shape of the inclusions and its evolution with strain as a simple shear deformation is applied to a cubical volume containing initially spherical inclusions dispersed in a matrix. The resulting microstructure evolves with strain and is a function of the viscosity contrast between the two phases and of the volume fraction. Different microstructures lead to different effective shear viscosities and therefore, the latter are a function of the same parameters too. The viscosity contrast influences the bulk viscosity from the beginning and determines the different behaviour of the inclusions with respect to the matrix, i.e. whether the inclusions elongate (inclusions stronger than the matrix) or flatten out (inclusions weaker than the matrix). It controls the partitioning between the two phases of variables such as pressure, stress, strain, strain rate, vorticity, velocity. The volume fraction of the second phase affects the bulk shear viscosity too and also determines how much the inclusions are interacting with each other. We performed numerical simulations with both linear and power-law rheology. Non-Newtonian regimes emphasise the difference in viscosity. Weak inclusions merge more easily and form S-C structures, strong inclusions are much harder to deform and remain almost undeformed even at low viscosity contrasts.