Micromechanical Study of fabric evolution in quasi-static deformation of granular materials

In micromechanical studies of granular materials, relations are investigated between macro-level, continuum characteristics and micro-level, (inter) particle characteristics. For quasi-static deformation of granular materials, the fabric tensor is an important micromechanical characteristic that describes the orientational distribution of contacts between particles. The evolution of the fabric tensor during quasi-static deformation has been investigated, using two-dimensional Discrete Element Method simulations of constant-pressure tests of a dense and a loose isotropic initial assembly. Invariants of the fabric tensor are the coordination number, i.e. the average number of contacts per particle, and the fabric anisotropy, i.e. the difference between the principal values of the fabric tensor. At large strains, both coordination number and fabric anisotropy become constant. This constant value is the same for the dense and the loose initial assembly, i.e. a critical fabric state is attained. The evolution towards the critical state value for coordination number is much faster than for fabric anisotropy. The three micromechanical mechanisms through which the fabric tensor can change are contact creation, contact disruption and contact reorientation. Contact disruption is the dominant mechanism (except at the steady state) that is strongly anisotropic. The rate of contact disruption decreases rapidly with increasing imposed strain. Contact creation is approximately an isotropic mechanism. The rate of contact creation is fairly constant over the range of imposed strains. Fabric change due to contact reorientation is small, except at the steady state. An evolution relation is proposed for the changes in the fabric tensor as a function of the fabric tensor and the strain increment tensor. With its coefficients obtained by fitting with the data for the fabric tensor from the Discrete Element Method simulations, good agreement is observed between results from these simulations and from the proposed fabric-evolution relation