| The interaction of the loading machine and rock specimen is studied in this study. Three aspects of rock-machine interaction including the effect of machine stiffness, end platen friction coefficient, and sample aspect ratio will be investigated. The investigation is performed by comparing the results of experimental tests and 2D and 3D numerical models. In the numerical simulation, the rock is modeled as a bonded particle discrete element system, while the machine is discretized using finite elements. It is illustrated that the machine stiffness can drastically change the post-peak stress-strain curve; a soft machine underestimates the slope of the post-peak curve. The results suggest that by increasing the sample aspect ratio, the effect of end platen friction can be reduced. Furthermore, it is observed that end friction is the main reason for the phenomenon of deterministic size effect and this size effect can be effectively removed if the end friction can be reduced to zero. The failure mechanism is also discussed. It is shown that the induced curvature of the steel loading platens is responsible for the internal vertical shear cracks which result in damaged concentric cylindrical shells in the specimen. The generation of these cylindrical thin shells and the induced radial dilation forces produce radial cracks which together with the applied axial stress are responsible for progressive failure of the specimen. |
