EFFECT OF MECHANICAL INTERACTIONS ON THE DEVELOPMENT OF SHAPE PREFERRED ORIENTATIONS - A 2-DIMENSIONAL EXPERIMENTAL APPROACH

Modifications in the development of preferred orientations of rigid particles due to mechanical interactions between particles are studied experimentally in two dimensions. Experiments in both pure shear and simple shear were performed and the results analysed using an easy and fast new automatic method. For increased concentration of rigid particles, the rotation of individual particles may be slowed down or even stopped due to collisions or disturbance of the flow in the matrix caused by neighbours. In coaxial flow, the fabric intensity is consequently reduced. In simple shear flow, the fabric evolution is no longer cyclic; the intensity is on average weaker and the fabric axis rotates asymptotically toward the shear plane. In the case of preferred orientations of particles having a low aspect ratio (2.5), the fabric rotates through the shear plane, then undergoes a reverse rotation and rotates toward the shear plane again. In the light of these experimental results, we emphasize that: quantification of finite strain based on such fabrics may lead to significant underestimates, and calculated values should be taken as minimum estimates; the kinematic significance of concentrated rigid particle preferred orientations in rocks is modified by interactions between particles. Techniques for qualitative or quantitative kinematic analysis may become unreliable for simple shear flows. The concentrated population fabrics show an angular behaviour close to that of passive markers: they tend to align parallel to the shear plane in simple shear flow.