Strength deviation mechanism in simulating rock by the parallel bond model of PFC^2D

The Parallel Bond Model (PBM), a popular contact model in the early phases of the two-dimensional particle flow program (PFC2D), was later shown to represent pebbles that would result in strength deviations, primarily in internal friction angle and tensile strength. In order to reveal the rupture evolution mechanism of PBM-simulated rock strength deviation, PBM and Flat Joint Model (FJM) that had the same calibration reference were employed, were employed to conduct numerically direct tension tests and triaxial compression tests. On the basis of them, rupture evolutions, force chain distributions and particle displacement responses were analyzed to explore the evolutionary process mechanism of simulated rock samples. The findings demonstrate that, whether it is the ability to resist tensile failure or compressive shear failure, the PBM-simulated rock samples exhibit a higher peak strength response compared to the FJM-simulated rock samples, and the deviation is mainly attributed to the mechanism differences in the microcrack evolution process. Compared to FJM-simulated rock samples, PBM-simulated rock samples exhibit a later microcrack initiation but a higher degree of fracture localization, leading to a more concentrated microcrack development. This results in a higher degree of concentration of energy accumulation before the peak stress state and manifests as a more rapid power-law acceleration of cracking. Ultimately, a relatively more uniform macroscopic main fracture zone is generated, resulting in a higher tensile strength and shear resistance produced by the PBM-simulated rock samples. Furthermore, due to the high localization during the evolution of microcracks, PBM-simulated rock samples exhibit a lower cohesion and a higher internal friction angle, with a correspondingly larger value of the Hoek-Brown strength parameter m_i. By analyzing the whole process of the evolution mechanism causing strength deviation in PBM-simulated rock samples，this research is able to provide a reference basis for future studies to analyze and interpret a large number of early PBM simulation results.