Experimental and numerical study on failure characteristics and mechanism of coal under different quasi-static loading rates

In mining engineering, mining rate is a significant factor for determining mining efficiency and safety. In the present work, uniaxial compression tests under different quasi-static loading rates (varying from 0.0005 mm/s to 0.01 mm/s), combined with high-speed digital image correlation (DIC) technique and numerical simulation, were conducted to study the mechanical strength and failure characteristics of coal samples. Experimental results show that strength parameters (including compressive strength, peak strain, elastic strain energy, and elastic modulus) initially increase rapidly, then the rate of increase slows as the loading rate continues to increase. Besides,with increasing loading rate, the failure mode of the coal samples gradually transitions from tension-shear to spalling-ejection failure, while full-field strain transitions from inclined shearing to vertical tensile strain distribution. Furthermore, the particle flow code (PFC) numerical simulation results indicate that the accumulative microcrack number increases and the microcracks inclination angle distribution range decreases approximately from 60° (60° − 120°) to 20° (80° − 100°) with increasing loading rate. Moreover, the micro damage and number of microcracks increase and macrocracks are more difficult to initiate as loading rate increases. As a result, the accumulated energy is less likely to be released and a more severe block ejection will occur for a sample under a high loading rate.