MECHANICAL INTERACTIONS BETWEEN RIGID PARTICLES IN A DEFORMING DUCTILE MATRIX - ANALOG EXPERIMENTS IN SIMPLE SHEAR-FLOW

The mechanical interaction between two or more particles is investigated in Newtonian simple shear flow. The experimental models allow observation of both the particle rotation and the deformation pattern around the particles. The finite and instantaneous strain patterns around rigid particles arc strongly heterogeneous and asymmetric, with high finite strain zones aligned to the direction of maximum finite stretch. These correspond to local flow with low vorticity number. Heterogeneous strain patterns around rigid particles spread over a distance of 1-2 times the particle length and this distance increases with increasing bulk strain. Where rigid particles are more concentrated, these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions. In rocks, the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion, which becomes unreliable with high concentrations of rigid particles. Particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length, that is only in very concentrated suspensions of rigid particles. In a composite shape fabric, the development of the sub-fabric corresponding to the smaller minerals will be more disturbed.