Evolution of fracture process zone and variation of crack propagation velocity in sandstone

To solve the safe containment and recovery efficiencies of gas in rock masses, a study on fracture process zone (FPZ) and crack propagation is conducted. By using digital image correlation technology, the displacement of three-point bending specimens was measured. By analyzing the distributions of displacement at different loading stages, a specific region between the pre-crack tip and the loading point was divided into three zones: the intact zone, the crack propagation zone, and the FPZ. The length and the migration velocity of FPZ were determined, and the crack propagation velocity was also measured. The microstructures in FPZ were investigated through optical microscopy, x-ray diffraction analysis, and x-ray photoelectron spectroscopy. The results show that (1) FPZ length slightly varies during crack propagation and the FPZ is fully formed at the peak load; (2) the average value of the bond energy (446.7 eV) in the grains is greater than that (296.7 eV) in the matrix, thus the microdamage appears in the matrix around grain boundaries in FPZ; (3) the mean FPZ length varies from 4.09 to 8.42 mm for all tested specimens during crack propagation; (4) the propagation of the crack and the migration of FPZ proceed simultaneously in the loading process, and both velocities of crack propagation and FPZ migration are almost the same and with the same trend; (5) the peak velocity of crack propagation appears after the peak load, and the crack propagation progress was intermittent due to fracture energy accumulation, fracture energy release, and FPZ's shielding effect.