Fracture characteristics and failure mechanisms of coal-rock composites under complex true triaxial stress paths

Coal-rock composite structures are common in deep roadway roofs and floors, and their instability is strongly affected by excavation-induced unloading and coal-rock relative thicknesses. To clarify their fracture characteristics and failure mechanisms under realistic stress adjustment paths, laboratory true triaxial loading-unloading tests were conducted on 100 mm cubic specimens with coal-rock ratios of 0:1, 1:2, 1:1, 2:1, and 1:0, combined with acoustic emission (AE) monitoring and PFC3D simulations to investigate their mechanical response, damage evolution and energy characteristics. The results show that with increasing coal-rock ratio, the failure mode gradually transitions from relatively stable splitting-shear failure in rock-dominated specimens to abrupt coal-dominated instability, and composite specimens with intermediate ratios exhibit the most significant interface-controlled X-shaped or semi-X-shaped conjugate shear damage. Due to the mismatch in elastic modulus and Poisson's ratio between coal and mudstone, distinct AE precursor peaks appear during the unloading and stress readjustment stages, which are stronger in composite specimens than in pure coal or pure rock specimens. The core quantitative findings are that the cumulative absorbed energy first increases and then decreases with increasing coal-rock ratio, reaching a maximum at a coal-rock ratio of 67%, whereas the peak strength decreases monotonically. This indicates that rock burst proneness is governed more directly by energy accumulation and release than by strength alone. Numerical simulations well reproduce the fracture process of specimens, and demonstrate that interface bond breakage promotes the conversion of stored strain energy into kinetic energy during final instability. These findings provide a mechanistic basis for evaluating dynamic instability and optimizing support strategies in deep composite strata.