Crack propagation characteristics of coal body around the borehole under different loading rates

The loading rate is a crucial factor influencing the deformation and fracture of the coal body around a borehole. The uniaxial compression tests and digital image correlation methods were adopted to scrutinize the mechanical parameters, failure modes, and lateral strains of specimens. A comprehensive analysis of crack propagation characteristics under different loading rates was conducted. Based on the cohesion model, the intrinsic interplay between crack propagation characteristics and loading rates was elucidated. Combined with the actual situation on site, the treatment method of gas anomaly in high recovery rate working face is given. With an increase in loading rate, the peak strength, peak strain, and the initial stress level of cracks gradually increased and tended to stabilize. The elastic modulus exhibited an initial increase followed by a decrease. The failure mode transitioned from shear to tensile, and the fracture process became more intense. Tensile crack propagation was divided into four stages, with the first three stages characterized by "closed-open-closed" patterns. In the fourth stage, lateral strains fluctuated between positive and negative values at high loading rates, exhibiting high-strength nonlinearity, the accumulation of deformation energy density is "rupture -> rupture again". Due to the influence of cohesion, tensile crack propagation at low loading rates was hindered, resulting in K1 = 0 at the tensile crack tip. Under high loading rates, K-1 > 0, leading to extensive tensile failure in the internal load-bearing structure. At the same time, combined with the abnormal situation of gas in the working face with high mining rate, specific solutions are put forward.