Experimental Study on Mechanical Properties and Microcrack Fracture of Coal Specimens under the Coupling of Loading Rate and Compression–Shear Loads

Abstract

Gently inclined and inclined coal pillars are both subjected to the combined action of compression and shear load. Due to the existence of inclination angle, the fracture and instability behaviors of inclined pillars are different from those of near-horizontal pillars, and the difference is closely related to the load level of the pillar. Therefore, it is of great significance to study the influence of different inclination angles and loading rates on mechanical property and fracture behavior of the coal pillar for its strength design. In this paper, a new rock mechanical properties testing system was used with the combination of acoustic emission (AE) technology, and the changes of mechanical properties of coal specimens under different inclination angles and loading rates were obtained. The main conclusions are as follows: (1) the large of inclination angle, the greater of the peak shear stress. When the inclination angle is small (0°, 5°, 10°, and 15°), the peak stress increases first and then decreases with the increase of loading rate, when the inclination angle is large (20° and 25°), the peak stress gradually decreases. When the inclination angle is greater than 15°, the peak stress decreases with the increase of loading rate, and the decreasing amplitude gradually decreases. (3) The failure mode of the coal specimen is changed from tensile-splitting failure (0°–5°) and tension–shear composite failure (10°) to single shear failure (15°–25°). The loading rate has little effect on the failure mode of the specimen, but a significant effect on its failure degree. The crack initiation (CI) threshold and the crack damage (CD) threshold of coal specimens increase first and then decrease with the increase of inclination angle, and the ratios of CI and CD to peak strength are almost constant. With the increase of loading rate, the ratio of CI and CD to peak strength increases first and then decreases in the inclination angle range of 10°. The ratio of CI and CD to peak strength is basically independent of the loading rate. To comprehensively consider the influence of inclination angle and load level on pillar strength, a new parameter of rock strength under the influence of inclination angle and load level is suggested to be introduced into the empirical formula of pillar strength.