Abstract: This study investigates the influence of fracture geometry and interaction mechanisms on the mechanical behavior of coal under static loading conditions. The digital image correlation method is used as a full-field observation technique, and uniaxial compression tests are conducted on three-crack samples of raw coal prefabricated at different positions using an MTS816 rock servo testing machine. The mechanical properties, crack evolution, and failure characteristics of multi-fractured coal under different position distribution conditions are studied by combining particle flow simulations. Based on the fracture mechanics theory, the influence and applicability of T stress on the multi-fracture tip of coal are discussed. The results show that as the fractures transition from parallel collinear distribution to parallel overlapping distribution, the macroscopic strength of coal samples gradually increases. The distribution form of the fracture affects the pressure state of the coal body, where widely spaced fractures create independent stress weak zones that expand and coalesce as stress increases. Conversely, when fractures tend to overlap, stress concentration areas and weak areas form a unified force system that influences each other. When multiple fractures are collinear or offset distributed, shear stress dominates the fracture surface, and the driving mode supplemented by tensile stress leads to tensile-shear composite failure of coal samples. However, when fractures tend to overlap, shear stress on the fracture surface converts to tensile stress in the rock bridge area, resulting in tensile failure dominated by tensile stress. When considering the influence of T stress, the theoretical prediction value of the crack angle containing three T stress components (T_x, T_y and T_xy) is found to be more suitable for studying crack growth angle of multiple fracture tips of finite plate subjected to compression shear stress.

Key words: multi-fissure coal, DIC technology, particle flow simulation, fracture mechanics, mechanical characteristics, extended angle