Abstract
In this paper, uniaxial cyclic compression and shear test was carried out for lignite samples. The effects of inclination angle (θ) and upper limit of cyclic stress (σmax) on mechanical properties of coal samples were analyzed, and the damage variables of coal samples were studied based on energy dissipation theory. The results show that the uniaxial compressive strength (UCS) of coal samples after uniaxial cyclic compression and shear tests decreases with the increase of the upper limit of cyclic stress and inclination angle. The shear stress component generated by the increase of inclination angle can effectively reduce the UCS and increase the damage degree of coal samples. With the increase of inclination angle, the failure mode of coal samples is changed from tensile failure (θ = 0°), the combined tensile failure and shear failure (θ = 5°) to shear failure (θ = 10°). The peak axial and radial strain of coal samples first increases rapidly and then stagnates. The peak volume strain rapid increases and then stagnates (θ = 0° and θ = 5°). When the inclination angle is 10°, the peak volume strain first decreases rapidly and then stagnates. Even if the upper limit of cyclic stress is lower than its UCS, it will still promote the propagation of micro cracks and the generation of new cracks and increase the internal damage of coal samples. With the increase of the cycle number, damage variables of coal samples after uniaxial cyclic compression and shear tests nonlinearly increase, and the growth rate decreases gradually.
Highlights
In mining engineering, coal is often subjected to the cyclic compression and shear loading [1,2,3]
The study reported that the development of internal micro cracks and the generation of new cracks are still promoted in red sandstone, even if the upper limit of cyclic stress in the test is less than the uniaxial compressive strength (UCS)
The results show that the UCS of the control group coal samples is the largest at the inclination angle of 5◦, which is slightly greater than that at the inclination angle of 0◦
Summary
Coal is often subjected to the cyclic compression and shear loading [1,2,3]. Zhou et al [26], Xu et al [27] and Du et al [28] conducted SHPB tests on inclined cylindrical sandstone samples in the laboratory to study the compression-shear characteristics and failure mechanism of sandstone under the combined dynamic and static loads. He et al [29,30] designed a new type of uniaxial compression and shear equipment, and studied the mechanical properties of igneous rock under the uniaxial compression and shear loads. It was found that the shear stress component accelerates the crack growth and reduces the anti-deformation energy of the crack
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