Experimental study on fracture mechanism of coal caused by supercritical CO₂ explosion

In this study we carried out a series of experiments on large specimens (1 m×1 m×0.5 m) of coal and concrete using an explosive device and similar material test bench with multi-channel electro hydraulic servo to improve the application of the supercritical CO₂ gas explosion in the low permeability coal seam and study the fracture mechanism of gas explosion. The internal deformation and failure information were recorded using a dynamic strain gauge, and fractures distribution in the blasting hole were observed using an industrial speculum. The gas explosion stress waves and the damage morphology after blasting show that damage areas from near to far are divided into a crushing zone, a cracking zone and a seismic zone. It is the corresponding formation mechanism that the supercritical CO₂ impacts on the medium surrounding explosion hole, thereby forming the spherical wave, whose compressive strength is higher than that of the medium. Under the action of the radial compressive stress, the medium undergoes crushing destruction, and the crushing zone is thus formed. With the stress wave propagating, progressive attenuation of energy is not strong enough to cause the medium's compression failure. Brittle material is only good at resisting compression, but fails under tension. Circumferential stress generated by the stress waves still cause radial cracks. The high pressure CO₂ gas with quasi-static loading action enters into fracture and forms a gas wedge that leads to the fracture's further development, called the forming of the cracking zone. Outside the cracking zone, the medium only vibrates under the low energy stress wave and no obvious damage occurs, and thus it is called the vibration area. The curve of the crack expansion velocity and distance from the gas explosion hole are in accordance with the "S" curve. High speed crack expansion occurs in the crushing zone, while the low velocity expansion occurs in the cracking zone. The farther away from the explosion hole, the smaller the peak strain of the measuring points, and the more complex the jointed fissure in the structure within the same distance; the greater the magnitude of the peak strain that decreases, the more different the strain waves.