Abstract: Crack propagation experiments were conducted on the prefabricated cross-fissured rock specimens to study the crack initiation, propagation, and coalescence processes. The effects of the angle between the primary fissure and the axial load and the angle between the primary and minor fissures on the crack initiation stress and the coalescence stress were analyzed. The experiments were modelled and calculated with a hybrid finite-discrete element program, i.e., 3D Y-HFDEM code parallelized by a graphic processor. The transition of rock damage from continuous to the discontinuous medium was achieved. Crack categories and damage modes were identified and the phenomenon that was difficult to find in the experiments was captured. It is shown that the number of tensile cracks in the crack coalescence zone increases as the angle between the main fissure and the axial load increases. The crack initiation and coalescence stresses are proportional to the angle between the main fissure and the axial load. The damage mode of the rock changes from tensile damage to shear damage as the angle between the primary and minor fissures increases. Cross fissures increase the degree of rock fragmentation. Damage caused by mixed tensile-shear cracks initiating and propagating at the tip of the primary fissure dominates the rock damage and is the main control fissure leading to the loss of bearing capacity of the rock mass. The hybrid finite-discrete element simulation software GPGPU parallelized 3D Y-HFDEM IDE is advantageous in study of rock crack propagation to capture damage and fracture types that are difficult to detect in the laboratory, and can be a powerful tool for study of rock crack propagation.

Key words: rossed fissures, crack extension, 3D hybrid finite-discrete element modelling, coalescence category, crack initiation stress