Fracture evolution and damage model of composite coal-rock under bending: An integrated physical-numerical-analytical approach for rock homogeneity effects

To elucidate the underlying bending and fracture mechanisms inherent in composite coal-rock (CCR) roof structures within deep mining environments, this study introduces a groundbreaking analytical framework. The proposed framework integrates three methodological components: physical modeling through three-point bending tests (TPBT), numerical simulation using the particle flow code (PFC^2D), and analytical damage modeling. This integrated approach is specifically tailored for CCR fracture analysis with detailed characterization of rock layer heterogeneity. Focusing on the strata of the Fuxin Hengda coal mine, we employed a multifaceted monitoring approach, incorporating acoustic emission (AE), digital image correlation (DIC), scanning electron microscopy (SEM), and the 3D profilometry. This comprehensive strategy systematically unveiled the regulatory mechanisms governing how rock heterogeneity influences damage evolution. Our findings reveal that CCR undergoes a distinct four-stage evolutionary pattern under TPBT conditions: “weak contact-strong contact-peak load-post-peak.” This damage progression is intricately linked with DIC strain fields and AE energy release patterns. Notably, sandstone composite coal-rock (CCRS), characterized by its high homogeneity, exhibits localized strain distribution and concentrated energy release, culminating in abrupt brittle fracture. In contrast, sandy conglomerate composite coal-rock (CCR-SC) with its coarse particle structure, fosters multi-level microcrack branching, leading to progressive failure. Furthermore, the developed Weibull damage model quantitatively delineates the interplay between the homogeneity coefficient (φ), coal-rock energy weight factor (w), and fracture rate. Our analysis underscores that damage rates escalate markedly in high-homogeneity rock layers compared to their low-homogeneity counterparts. This observation substantiates the mechanism whereby high-homogeneity rock layers expedite energy release through strain localization. Collectively, these insights offer a robust theoretical foundation for deep coal-rock dynamic disaster prediction and roof stability control.