Experimental study on anisotropy characteristics of fracture toughness and fracture process zone in anthracite coal

In order to determine the influence of anisotropy induced by primary bedding structure on fracture toughness and fracture process zone (FPZ) incubation characteristics of anthracite, the notched semi-circular bending (NSCB) anthracite samples with different bedding angles were subjected to the mode I three-point bending loading test on the MTS hydraulic servo test system. The incubation process of FPZ at crack tip was monitored by the digital image correlation method (DICM) combined with the acoustic emission (AE) localization technique. Based on the Irwin-Bazant model, the Strip-yield uniform traction model and the Strip-yield linear traction model, the fully developed FPZ length of anthracite samples were predicted and compared with the experimental results. The geometric morphology of anisotropic anthracite FPZ and the shear and tensile deformation time series of anthracite FPZ during its incubation at different bedding angles were discussed. The results show that the peak load, peak displacement and fracture toughness of anthracite increase with the increase of bedding inclination. Compared with the coal samples with the bedding angle of 0°, the average fracture toughness of the coal samples with the bedding angle of 22.5°, 45.0°, 67.5° and 90.0° increases by 6.76%, 86.82%, 85.47% and 134.46%, respectively. The FPZ length predicted by the Irwin-Bazant model and the Strip-yield uniform traction model is shorter than the test value, while the prediction of the Strip-yield linear traction model is generally higher than the test value. The estimation of FPZ length of anthracite by the Strip-yield uniform traction model is most close to the experimental value, indicating that the distribution of the cohesive force at the crack tip of the anthracite sample tends to be more uniform. The FPZ length of anthracite sample is proportional to the square of the ratio of fracture toughness and tensile strength: L∝(K_IC/σ_t)². The prefabricated crack tip deformation of the sample is dominated by tensile deformation and assisted by shear. The tensile deformation ranges from 27.71% to 57.26% P_max, and the shear deformation ranges from 72.88% to 92.4% P_max. The time of tensile deformation of the crack tip is superior to that of shear deformation. The strain field distribution on the circumferential measuring lines of the samples with the bedding angle of 0° and 45° is similar to the texture distribution of onion cross section, while the maximum principal strain monitoring value of the samples with the bedding angle of 90° shows a fluctuation and disordered distribution at most measuring angles. The strain gradient of anthracite sample is different on the divergence line of FPZ tip in each direction.