Mechanical and fracture behavior of coal under different loading modes: Experimental investigation on anisotropy and mixed-mode fracture toughness

Coalbed methane (CBM) is an important unconventional natural gas resource whose efficient extraction relies heavily on understanding the fracture characteristics of coal reservoirs. This study investigates the mechanical behavior and fracture characteristics of bedded coal under varying loading modes using uniaxial compression, Brazilian splitting, and edge-notched disc bending (ENDB) tests. The results demonstrate that coal strength exhibits strong bedding-angle dependence, with tensile strength following a left-skewed S-shaped trend. ENDB tests reveal size-dependent fracture toughness: smaller specimens show greater resistance to Mode-III (tearing) fracture, while larger specimens favor Mode-I (tensile) resistance. The toughness index Rs increases from 1.38 (Mode-I) to 2.34 (Mode-III), reflecting rougher fracture surfaces and higher energy dissipation during torsional failure. Acoustic emission (AE) analysis further distinguishes tensile-dominated signals in Mode-I from shear-dominated signals in Mode-III loading. Bedding anisotropy strongly controls crack initiation, trajectory, and energy demand, with Mode-I governed by tensile opening and Mode-III involving torsional deformation with higher energy consumption. Despite the apparent Mode-III toughness, failure is still dominated by tensile mechanisms. The experimental results were further analyzed using improved fracture criteria incorporating critical radius and T-stress effects, enabling more accurate prediction of fracture behavior. These findings provide fundamental mechanical parameters and fracture characterization methods essential for optimizing hydraulic fracturing design in CBM reservoirs.