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

Abstract

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 (u) on crack derivation mechanisms is quantified, yielding mathematical relationships between fracture strength (f), crack propagation path angle (b), crack fractal dimension (D), and u. 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 u on critical energy release zones. These findings offer new insights into CCR failure mechanisms for mining safety.

1. Introduction

As shallow coal resources are increasingly depleted, the depth of coal mining in China is rising at a rate of 10–15 m per year, making deep mining an inevitable path to ensure the security of the national energy strategy [1,2]. It is important to note that the complex geological environment at greater depths leads to significant nonlinear characteristics in the mechanical behavior of the coal and rock, particularly the frequent occurrence of composite roof dynamic disasters, which severely restrict the safe and efficient mining of deep resources [3–5]. This engineering context highlights the urgency and theoretical significance of studying the instability mechanisms of deep composite coal-rock (CCR).

Significant progress has been achieved in studying CCR mechanical properties through complementary methodologies, including experimental techniques, theoretical modeling, and computational simulations. These methodological advancements have collectively enhanced the understanding of multiscale failure mechanisms in CCR. Initially, experimental observations evolved from simple to complex stress fields. Under uniaxial compression conditions, the whole process characteristics of progressive failure in CCR were first revealed by He et al. [6] through AE techniques, thereby establishing a foundation for subsequent research. Subsequently, an in-depth energy-based analysis was conducted by Ma et al. [7], who established a structural failure criterion based on strain energy evolution, thus advancing qualitative observations toward quantitative assessment. Moreover, by introducing dynamic fractal theory, Liu et al. [8] elucidated the energy dissipation mechanisms underlying material instability, refining the theoretical framework under uniaxial compression conditions. As research has progressed, the focus has been extended to triaxial stress fields that more closely mimic practical engineering conditions. The mechanical response of stratified rock masses under true triaxial stress was systematically investigated by Zhang et al. [9], and a corresponding failure model was developed.