Effect of cyclic loading-unloading on the mechanical anisotropy of coal under uniaxial compressive condition

To understand the mechanical anisotropy of coal disturbed by repeated excavations during the mining process, the cyclic loading-unloading process was implemented during the uniaxial compressive testaments; fundamental parameters were monitored and compared with that obtained under the conventional uniaxial compressive loading condition, based on a series of specimens with anisotropy angles of 0°, 22.5°, 45°, 67.5°, and 90°. The results indicate that the uniaxial compressive strength (UCS), elastic modulus, and peak strain exhibit similar anisotropy features under conventional and cyclic loading-unloading conditions. The UCS and elastic modulus are generally U-shaped versus the anisotropy angle (0 ~ 90°), while the peak strain exhibits first reducing and then increasing features, respectively obtaining the minimum and maximum value at the anisotropy angle of 22.5° and 90°. The cyclic loading-unloading strengthens the UCS and elastic modulus and increases the anisotropy feature of UCS, elastic modulus, and peak strain. The percentage of shear failure generated during the coal failure process exhibits a parabolic character versus the anisotropy angle (0 ~ 90°). It reduces as specimens are cyclically loaded and unloaded with the maximum value at the anisotropy angle of 45°, lower than that obtained at 22.5° under conventional loading conditions. The cumulative dissipated energy density and residual strain increase with the cyclic loading-unloading cycles; accordingly, an exponential equation was proposed and verified applicable in describing the correlation between them.