ABSTRACT: The bonded particle model is a powerful tool in studying the mechanical behavior of rock. The common practice in simulation of rock failure using this model is to allow brittle fracture of the contacts between particles or at most tensile softening at the contact points ignoring the shear softening of the material. To overcome this shortcoming, a plasticity model that allows both tensile and shear softening of the filling material at the contact points of the particles was implemented in the CA2 computer program. The model was calibrated to mimic the elastic behavior of the Pennsylvania blue sandstone. It is shown that for a more ductile material, there is less scatter of micro-cracking at the peak load. Furthermore, the ductility parameter appears to be a good tool in controlling the ratio of compressive to uniaxial tensile strength of rock. While the ductility of the filling at the contact points of the particles has a drastic effect on the macroscopic post-peak rock behavior in the direct tensile testing, its role in dictating the post-peak rock behavior in compression is negligible and needs further study. The combined effect of ductility and initial micro-cracking on rock strength characteristics was studied as well. The numerical results suggest that the ratio of Brazilian to direct tensile strength of the simulated material is affected by the initial micro-crack intensity; this ratio is around 1 for a material with no initial micro-cracks but it gradually increases as the initial micro-crack intensity is increased.
The Bonded Particle Model (BPM) has found a variety of applications in rock engineering and rock mechanics owing to its simple constituent analogy to circular discs in two dimensions and spheres in three dimensions and its ability to model complex rock and soil behavior using simple contact models for interaction of the particles (Potyondy and Cundall, 2004; Norouzi et al., 2013; Fakhimi and Hemami, 2015; Lisjak and Grasseli, 2014). While the BPM has been considered as a great tool for geotechnical modeling, it has its own shortcomings and drawbacks. One of the challenges in BPM modeling of rock is the problem with obtaining realistic values for friction angle and ratio of uniaxial compressive strength (UCS) to tensile strength; the friction angle and UCS values are normally underestimated by the BPM model using spherical or circular disks (Fakhimi, 2004; Wu and Xu, 2016).