Fracture Propagation Patterns and Multifractal Characteristics of Rock–Coal–Rock Composite Structures with Different Interface Inclinations

Rock–coal–rock (RCR) composite structures in inclined coal seams exhibit asymmetric deformation and inclination-dependent failure. This study investigates how coal–rock interface inclination affects mechanical response and fracture development using uniaxial compression tests, CT imaging, three-dimensional reconstruction, and multifractal analysis. As the inclination increased from 0° to 40°, load-bearing capacity decreased and the dominant failure mode changed from splitting through mixed tensile–shear failure to interface-controlled shear failure. Crack volume and surface area decreased with inclination; the crack-to-rock/coal volume ratio was 0.04 at 10°, while the crack specific surface area was 4.345 and reached a maximum of 4.432 at 40°. A pseudo-color enhancement workflow improved the identification of fine cracks that were difficult to capture by conventional binary processing. Multifractal parameters quantified crack-network complexity and revealed pronounced anisotropy among specimens and spatial heterogeneity within each specimen. Coal-bearing regions generally sustained greater damage, whereas rock regions exhibited more heterogeneous crack distributions. Correlation analysis linked the mechanical response to fracture geometry and multifractal characteristics, showing that interface inclination progressively promotes weak-plane-controlled instability. These findings clarify the fracture mechanism of inclined RCR composites and provide a basis for stability assessment in inclined coal-seam mining.