Acoustic emission response and evolution of precracked coal in the meta-instability stage under graded loading

The fault-slip burst has the features of high frequency, considerable damage and complex mechanisms, because mining activities break the balance of energy in the system of the fault and rock stratum. This disruption of balance causes the sudden activation and slip of the fault with an enormous release of energy and abruptly destroys the excavation space. To identify the mechanical behavior and instability characteristics of the critical stage (meta-instability stage) before fault instability, 5 uniaxial static load experiments are conducted to investigate the count and energy characteristics of acoustic emission (AE), and 15 numerical models are carried out with PFC to analyze the distributional characteristics of the stress, displacement, fracture and force chain fields. The evolutional process of energies is monitored in the models. The results show that the AE signal is the most abundant in the meta-instability stage during the whole loading process, and that the AE count and energy account for 37.27% and 39.18%, respectively, of the whole process. The proportion of cracks matured in the meta-instability stage is negatively correlated with the inclination angle of the preexisting crack and positively correlated with the width of the preexisting crack and the speed of graded loading. The distribution of new cracks changes from the reverse airfoil to the airfoil with an increasing inclination angle of the precrack. During the failure process, energy is primarily released in the form of parallel bond strain energy. In the meta-instability stage, energy accounts for more than 49% of the whole process. The trends for the slip energy and dashpot energy are separated at the beginning of the meta-instability stage, which can be considered as a precursor of failure. The distribution of stress around the precrack is transmitted counterclockwise, forming a deformation pattern in which strain energy is released in the center. The research results provide theoretical support for the identification of the meta-instability stage of fault-slip bursts and are beneficial to research on the risk assessment of coal and rock dynamic disasters.