Macro-mesoscopic perspective damage characteristics and energy-damage constitutive model of coal-rock composite structures subjected to cyclic loading

During deep coal mining processes, periodic mining disturbances cause the neighboring coal strata to bear the effects of cyclic loading and unloading, making it essential to study the mechanical responses and macro-micro failure characteristics of the coal-rock composite structures under different cyclic loads. In this study, three different loading rates were selected to perform uniaxial cyclic compression tests (with simultaneous acoustic emission signal measurement) under two types of cyclic loads, investigating the damage characteristics of coal-rock composites. Based on the principle of energy dissipation, an energy-damage constitutive model for the cyclic loading of composites was constructed and validated with experimental data. The results indicate that the loading rate is directly proportional to the peak strength of the composite specimen, where the peak stress increased by 22.44% and 28.89% for the gradual cyclic loading and unloading path (path I) and the cyclic loading and unloading path (Path II) respectively. The higher the loading rate, the faster the internal crack extension in the specimen, the crushing degree of the coal component in the coal-rock composite specimen is intensified, and the fractal dimension increases subsequently, and the faster the internal crack extension in the specimen becomes. With the increase of the loading rate, the damage along the matrix in the coal fraction increases. The paths with a large span of cyclic gradation (Path I) contribute to stress transfer within the specimen and provide favorable conditions for the development of cracks within the specimen, leading to a higher degree of damage in the corresponding specimen. The consistency between the test curves and the energy-damage constitutive model curves is relatively high, indicating that the proposed energy-damage constitutive model can well describe the deformation behavior of the coal-rock composite specimens during cyclic loading and unloading processes.