Damage evolution and failure behavior of coal-rock combination subjected to different cyclic loading paths and loading rates: Insights from energy-driven effects

Abstract

In this study, a uniaxial cyclic compression test is conducted on coal-rock composite structures under two cyclic loads using MTSE45.104 testing apparatus to investigate the macro-mesoscopic deformation, damage behavior, and energy evolution characteristics of these structures under different cyclic stress disturbances. Three loading and unloading rates (LURs) are tested to examine the damage behaviors and energy-driven characteristics of the composites. The findings reveal that the energy-driven behavior, mechanical properties, and macro-micro degradation characteristics of the composites are significantly influenced by the loading rate. Under the gradual cyclic loading and unloading (CLU) path with a constant lower limit (path I) and the CLU path with variable upper and lower boundaries (path II), an increase in LURs from 0.05 to 0.15 mm/min reduces the average loading time by 32.39% and 48.60%, respectively. Consequently, the total number of cracks in the samples increases by 1.66-fold for path I and 1.41-fold for path II. As LURs further increase, the energy storage limit of samples expands, leading to a higher proportion of transmatrix and shear cracks. Under both cyclic loading conditions, a broader cyclic stress range promotes energy dissipation and the formation of internal fractures. Notably, at higher loading rates, cracks tend to propagate along primary weak surfaces, leading to an increased incidence of intermatrix fractures. This behavior indicates a microscopic feature of the failure mechanisms in composite structures. These results provide a theoretical basis for elucidating the damage and failure characteristics of coal-rock composite structures under cyclic stress disturbances.

1. Introduction

During deep coal mining, axial stresses undergo cyclic variations owing to mining disturbances, blasting operations, protective layer excavation, roadway construction, and advancements of the coal mining face. Consequently, both mined coal seams and adjacent rock structures undergo cyclic loading and unloading (CLU) [1, 2]. This CLU process creates concentrated stresses and promotes the storage and release of energy, which serve as primary triggers for dynamic catastrophic events [3].

Compared with conventional quasi-static loads, cyclic loads have greater accuracy in reflecting the complex and diverse mining stresses encountered in practical engineering scenarios. Moreover, cyclic loading generates unique stress patterns in materials, resulting in various changes in their properties. Previous research has shown significant differences in the acoustic emission (AE) characteristics [4], seepage behavior [5], energy-driven patterns [6], and mechanical properties [7] of pure coal and rock under CLU conditions, compared with traditional loading methods. Additionally, cyclic loading tends to induce the collapse of coal and rock bodies under these conditions, resulting in highly complex damage processes [7−9].

In practical engineering settings, both coal seams and surrounding rock layers encounter stress loads, facilitating stress transfer between coal-rock interlayers. This interaction leads to significant differences in the mechanical behavior of mining coal seams compared with pure coal. Therefore, researchers have developed samples that simulate the coal seam-surrounding rock configuration. Numerous studies have investigated the various factors influencing the mechanical characteristics and deformation behaviors of these composite structures. Key factors include the interface inclination angle [10], inte-