Study on failure characteristics of coal-rock composite specimens with impacttendency under different loading and unloading paths

The failure characteristics of coal and rock mass under mining disturbance in rock burst mine are complex, which seriously affects the monitoring, early warning and prevention of rock burst. It is of great significance to study the failure and acoustic emission characteristics of rock burst prone coal-rock combination under different loading and unloading stress paths. Taking the impact-dangerous coal-rock composite specimens as the research object, the conventional uniaxial loading and different cyclic loading and unloading experiments were carried out by using the uniaxial testing machine of rock mechanics and the acoustic emission monitoring system. The mechanical properties and energy evolution law of coal-rock composite under different stress paths were analyzed, and the relationship between the number of acoustic emission specimens of coal-rock composite and the characteristics of stress paths was studied. Based on this, the evolution trend of b value and entropy value was calculated and studied, and the study of rock burst corresponding to the engineering site under different stress paths was discussed. The results show that the peak strength of coal-rock combination is the largest under conventional loading, followed by variable lower limit loading and unloading, and the peak strength of constant lower limit loading and unloading is the smallest. The coal in the specimen is X-shaped conjugate slope shear failure, and the rock is tensile failure. Under constant lower limit loading and unloading, the damage degree of the specimen is higher, and the specimen is unstable and destroyed after inputting energy in low energy state, and it is unstable and destroyed after inputting more energy in variable lower limit loading and unloading state. The evolution law of acoustic emission events, b value and entropy value and their relationship with failure behavior are obtained. The number of acoustic emission events increases gradually with the increase of loading intensity, and the distribution of acoustic emission events in cyclic loading and unloading shows fluctuation characteristics with the change of stress path, and the cumulative acoustic emission curve increases step by step. Under constant lower limit cyclic loading and unloading, the fluctuation cycle characteristics of b value and entropy with loading and unloading are more obvious, while under variable lower limit cyclic loading and unloading, it shows fluctuation characteristics in cycles with higher stress. The common characteristics of specimens near instability and failure under different stress paths are that B value fluctuates at a low value and entropy value fluctuates at a high level. Higher mining intensity will lead to insufficient self-pressure relief of coal and rock mass in a short time, so the energy released by fracture expansion is greater; The lower mining intensity allows the coal and rock mass enough time for self-pressure relief, and the ground sound energy released by crack expansion is small. According to the failure characteristics of coal and rock mass under different loading and unloading effects and the evolution law of acoustic emission event number, b value and entropy value, the failure behavior of coal and rock mass can be effectively predicted. The lower mining intensity makes the coal and rock mass have enough time to self-decompress, the cracks expand more frequently, and the ground sound energy is released less. The research results provide a basis for reasonable selection of mining methods with different mining intensities, and can provide support for accurate monitoring and prevention and control of rock burst mines in practical engineering applications.