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 compress testaments, acoustic emission (AE) monitor and digital image correlation (DIC), and a series of coal specimens with five anisotropy angles. The results illustrate that the uniaxial compressive stress (UCS) and elastic modulus maintain a U-shaped anisotropic feature under different loading-unloading rates. The anisotropic feature of UCS and axial strain decreases, while the elastic modulus ascends with the increasing loading-unloading rate. The UCS, loading-unloading rate, and anisotropic angle of coal specimens are proved 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°, 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 rises with the loading-unloading rate, which accords with the macro-failure characteristics. The cyclic loading-unloading process generates more damage as specimens with lower UCS, the damage variable caused by it exhibits a parabolic anisotropy feature at anisotropic angle 0º~90º, and the value at 22.5º and 45º is greater than other anisotropy angles, though this anisotropy feature of damage variable decreases as the loading-unloading rate gain. In addition, the dissipated energy density generated during cyclic loading-unloading process also shows a U-shaped variation feature at anisotropy angle 0º~90º, with relatively lower value at 22.5º and 45º, and positively exponential correlation with the UCS.