Damage characteristics and laws of micro-crack of underwater electric pulse fracturing coal-rock mass

The purpose of this study is to study the mechanism of crack initiation, the damage characteristics and the propagation law of high-voltage pulse hydraulic fracturing coal so as to evaluate the fracturing effect of coal-rock mass. Based on the mechanism of high-voltage discharge fracturing in water, crack fracture and propagation, the numerical calculation and experiment of high-voltage pulse discharge (3 kV, 5 kV) based on a water pressure (3 MPa) were carried out. The CT scanning technique and PCAS image processing program were used to study the crack initiation and propagation of coal-rock mass. From the results, it is shown that under the action of high-voltage pulse discharge in water, the crack of coal-rock mass is dominated by the I-II composite crack and its damage type is mainly the tensile-shear damage; The periodic oscillation of the shock wave in the water causes the stress at the tip of the coal body to have an oscillating variation. The crack growth rate is related to the Rayleigh wave speed, and the growth of the crack length is unstable; The crack propagation and extension in the coal-rock mass are related to the preset angle of crack, the peak pressure of water shock wave and the direction of the maximum principal stress. Specifically, with the same voltage, the larger the pre-crack angle, the smaller the crack length, and the larger the crack width, area, and cracking rate; with the same crack angle, the higher the discharge voltage, the larger the cracking rate, length, width and area of crack. Compared with the 3 kV high-voltage discharge, the crack propagation pattern is more complicated under the condition of 5 kV discharge. The more the number of cracks, the larger the crack length, width, area and cracking rate, and the better the fracturing effect of coal-rock mass.