Study on the mechanical and fracturing characteristics of parallel slit groove sandstone at different angles under cyclic loading and unloading

In order to explore the influence of the distribution angle of parallel slot seams on various characteristics of sandstone under cyclic loading and unloading, sandstone specimens with two prefabricated parallel slot seams at different angles (the angles with the horizontal direction are α = 0°, 15°, 30°, 45°, 60°, 75°, 90° respectively) were used, and a stepwise loading and unloading mode was adopted to analyze the mechanical, energy dissipation, fragmentation and other characteristics of the sandstone when the angle distributions were different. The results show that the mechanical characteristics of the sandstone are significantly affected by the distribution of α. With the increase of α, the peak failure stress of the sandstone first increases and then decreases, and the number of cycles first decreases and then increases, reaching the minimum when α = 45°. As the number of loading and unloading cycles increases, the loading elastic modulus of the sandstone gradually rises. The peak and valley strains of the sandstone with α = 0° are the largest among all specimens under the same number of cycles. Within the same number of cycles, the accumulated values of elastic energy and plastic energy of the sandstone when α = 45° are smaller than those of other α values. And during the overall change process of the failure mode from tensile failure-tensile failure + shear failure-shear failure, α = 45° is at the turning point of tensile failure + shear failure. At the same time, the penetration mode of the cracks changes from direct penetration to indirect penetration. The particle size distribution range of the fragments after the sandstone with α = 30° and 45° is broken is relatively large. Therefore, considering the mechanical, energy and fragmentation characteristics of the sandstone, it can be seen that the sandstone with α = 45° can fail under stress and with relatively small deformation. Therefore, when carrying out roof slotting operations in engineering, the angle between the parallel slot seams and the horizontal plane can be considered to be 45°.