Abstract: A novel constitutive model was presented in line with the ideal elasto-plastic rolling and twisting moment and the Mohr-Coulomb-maximum-tensile failure criterion. The model could reflect four failure modes: tensile failure, compression-shear failure, rolling yielding, and twisting yielding. The model was incorporated into PFC^3D with secondary development to set up the equations of macro-and-mesoscopic parameters and to simulate the mesoscopic failure mechanism of Lac du Bonnet (LDB) granite in compression-shear test and tensile test. According to the results, the significant effects of particle rolling and twisting on the macro-cohesion and frictional angle of rock were observed. The macro-cohesion also increased to a constant value with the growth of mesoscopic ultimate moment and torque and meso-cohesion. The macroscopic internal friction angle increased with the increasing of mesoscopic ultimate moment and torque and mesoscopic frictional coefficient. The macroscopic tensile strength increased with the increase of microscopic tensile stress until reaching a constant value. The macroscopic elastic modulus and the Poisson’s ratio were mainly determined by mesoscopic elastic modulus of particles and their stiffness ratio. The numerical simulation result of LDB granite by the presented model was well consistent with indoor experimental results in terms of strength and deformation behaviors, compressive strength to tensile strength ratio as well as crack distribution. Before reaching to the peak value, the mesoscopic tensile cracks were dominant cracks in the LDB granite extending in an “X” shape from the end of specimen towards the middle of specimen under the condition of vertical-low pressure. Under the conditions of vertical pressure close to the peak value or exceeding the peak value, meso-cracks of compression-shear, and meso-failure of rolling and twisting were found. The cracks developed from the bottom to the top of specimen, with dip angles equal to the macro failure angle of LDB granite. And that reveals the mesoscopic mechanism of macro-cracks of LDB granite.

Key words: Lac du Bonnet granite, ideal elasto-plastic rolling-twisting model, Mohr-Coulomb maximum-tensile criterion, discrete element, 3D particle flow, macroscopic and mesoscopic parameters, cracks