Damage and crack propagation evolution of coal rock under combined voltage discharge in water

To investigate the damage effect and crack propagation evolution of coal rock under combined voltage discharge in water, combined voltage discharge experiments were conducted on coal rock boreholes in water (hydrostatic pressure of 3 MPa). Acoustic emission (AE) and PFC2D numerical simulation were used to analyze the damage characteristics and crack propagation in coal rock subjected to combined voltage discharge, as well as the influence of different factors on the fracturing effect. The research results indicate that: Under single voltage discharge, the maximum AE ring count and the cumulative ring count generally decrease as the number of discharges increases. In contrast,under combined voltage discharge, both the maximum AE ring count and the cumulative ring count increase sharply when the voltage changes, with a 179.6% increase in the maximum ring count and a 124.3% increase in the cumulative ring count. Overall, the total ring count increases at a faster rate compared to single voltage discharge. Under single voltage discharge, after a certain number of discharges, there are almost no new AE events. In constrast, combined voltage discharge results in a significant increase in the number of AE events after voltage changes, with the new event sources exhibiting higher amplitudes and a broader distribution. The increase in events is more pronounced compared to under single voltage discharge. During combined voltage discharge, a large number of new cracks are generated when the voltage changes. The energy released during the discharge is primarily utilized for the expansion of these new cracks. After 12 discharges, more primary cracks are generated compared to single voltage discharge, and the lengths of these cracks are more uniform, resulting in better fracturing effects. In combined voltage discharge, under different voltage transition frequencies and gradients, most crack growth occurs when the voltage changes. After 12 discharges, the number of cracks across different groups becomes similar. As the number of voltage transitions and the voltage gradient increase, less external energy is required; however the crack growth rate accelerates, the number of primary cracks increases, and the length distribution of the cracks becomes more uniform.