GPGPU-parallelized 3D combined finite-discrete element modelling of rock fracture with adaptive contact activation approach

The combined finite–discrete element (FDEM) has become a well-accepted numerical technique to simulate the fracturing process of rocks under different loading conditions. However, the study on three-dimensional (3D) FDEM simulation of rock failure process is highly limited in comparison with that on two-dimensional (2D) FDEM simulations, which is due to the high computational cost of the 3D FDEM. This paper implements an adaptive contact activation approach and a mass scaling technique with critical viscous damping into a GPGPU-parallelized Y-HFDEM 2D/3D IDE code formally developed by the authors to further speed up 3D FDEM simulation besides GPGPU parallelization. It is proved that the 3D FDEM modelling with the adaptive contact activation approach is 10.8 times faster than that with the traditional full contact activation approach, while the obtained results show negligible differences. At least additional 25 times of speedups can be achieved by the mass scaling technique although further speedups are possible with bigger mass scaling coefficients chosen, which, however, will more and less affect the calculated results. Taking the advantage of the drastic speedups of the implemented adaptive contact activation approach and the mass scaling approach, the GPGPU-parallelized Y-HFDEM 3D IDE is then applied to model the fracture process of rocks in triaxial compression tests under various confining pressures. It is concluded that the GPGPU-parallelized Y-HFDEM 3D IDE is able to simulate all important characteristics of the complicated fracturing process in the triaxial compression tests of rocks including the transition from brittle to ductile behaviours of rocks with the increasing confining pressures. After that, some important aspects of the 3D FDEM simulation such as the effects of meshes, loading rates and model sizes are discussed. It is found that the mixed-mode I–II fractures are highly possible, i.e. very reasonable, failure mechanisms when unstructured meshes are used in the 3D FDEM simulation. For modelling rock fracture under quasi-static loading conditions using the 3D FDEM, the loading rate must be small enough, which is recommended to be no more than 0.2 m/s, to avoid its significant effects. Finally, it is concluded that the implementation of the adaptive contact activation approach and the mass scaling technique can further speed up 3D FDEM simulation besides the parallelization and the GPGPU-parallelized 3D IDE code with the further speedup is able to capture the complicated fracturing process of rocks under quasi-static loading conditions.