The influence of cyclic loading-unloading rates on the uniaxial mechanical anisotropy of coal

In this study, the influence of the loading-unloading rate on the uniaxial mechanical anisotropy of coal was investigated via uniaxial compressive tests, acoustic emission (AE) monitoring, digital image correlation (DIC), and a series of coal specimens with five anisotropy angles. The results illustrate that the uniaxial compressive stress (UCS) and the elastic modulus maintain a U-shaped anisotropic feature under different loading-unloading rates. The anisotropic feature of the UCS and the axial strain decreases, while the elastic modulus increases with the increasing loading-unloading rate. The correlation of the UCS, the loading-unloading rate, and the anisotropic angle of coal specimens can be described by a cosine function. The impact of bedding plane on the failure characteristics of coal is more significant at anisotropic angles of 0°, 22.5°, and 45°. Where the penetrating crack mainly propagates along the bedding plane. The increasing loading-unloading rates contribute to a more intact after-failure coal specimen, which makes the anisotropic failure feature of the coal specimen more notable. Meanwhile, the anisotropy of micro-failure characteristics revealed by the AE activity also increases with the loading-unloading rate. The cyclic loading-unloading process generates more damage as specimens with lower UCS value and loading-unloading rate. The damage variable caused by it exhibits a parabolic anisotropy feature at anisotropy angles 0º~90º, with greater values at anisotropy angles 22.5º and 45º. Meanwhile, this anisotropy feature changes to wave-type at the greater loading-unloading rate. In addition, the dissipated energy density generated during the cyclic loading-unloading process also shows a U-shaped anisotropy feature at anisotropy angles 0º~90º, with relatively lower values at 22.5º and 45º. It exhibits a positive exponential correlation with the UCS.