Homogeneity-dependent fracture behavior and instability mechanism of composite coal-rock: Insights from three-point bending tests

To investigate the instability mechanisms of heterogeneous geological structures in goaf area roofs, three-point bending tests (TPBT) and numerical simulations are performed on composite coal-rock (CCR). Acoustic emission (AE) monitoring is employed to analyze key parameters, establishing a multi-parameter quantitative system for CCR fracture processes. The impact of lithological homogeneity on fracture evolution and energy migration is examined. Results show that CCR exhibits a three-stage mechanical response: weak contact, strong contact, and post-peak stages, each with distinct crack evolution patterns. A positive correlation is found between lithological homogeneity and tensile crack proportion. No significant correlation is observed between AE average frequency (AF) and AE counts across different lithological CCR; however, peak frequency (PF) displays clear lithology-dependent characteristics. The regulatory effect of the rock homogeneity coefficient ($\phi$) on crack derivation mechanisms is quantified, yielding mathematical relationships between fracture strength ($f$), crack propagation path angle ($\beta$), crack fractal dimension ($D$), and $\phi$. The study highlights how different fracture modes alter energy migration pathways, confirming the coupling effect of grain distribution on mechanical response and crack propagation, and the influence of parameter $\phi$ on critical energy release zones. These findings offer new insights into CCR failure mechanisms for mining safety.