Abstract:

Understanding the mechanical anisotropy and damage evolution of layered sandstone under cyclic loading is crucial for evaluating the dynamic stability of underground rock engineering. In this study, seven rectangular sandstone specimens with different bedding dip angles were subjected to staged cyclic loading tests. By integrating strain monitoring in different orientations, energy decomposition, and acoustic emission signal monitoring techniques, the mechanical anisotropy, energy evolution, and acoustic emission characteristics of layered sandstone under staged cyclic loading were analyzed. The results show that: 1) The bedding dip angle significantly affects the anisotropic response characteristics of sandstone strength and deformation. At low and high bedding dip angles, the sandstone exhibits relatively high strength, a low strain anisotropy coefficient (γ), and predominantly tensile failure. In the intermediate dip angle range (30º, 45º, 60º, 75º), γ increases, and shear failure becomes dominant. For a given bedding dip angle, the longitudinal strain anisotropy coefficient (γ_A) is greater than the transverse strain anisotropy coefficient (γ_L). 2) The total input energy and elastic energy calculated from the longitudinal strain on the strike bedding plane (ε_SA) show a U-shaped relationship with bedding dip angle, whereas those calculated from the longitudinal strain on the dip bedding plane (ε_DA) exhibit a Λ-shaped relationship. This finding indicates that energy release is closely associated with bedding-plane orientation. 3) The damage variable constructed based on cumulative ringing counts reveals remarkable anisotropy in damage evolution. The rise-time/amplitude ratio (RA) and average frequency AF (RA-AF combined analysis) can accurately distinguish crack types. The proportion of shear cracks exhibits a Λ-shaped distribution, peaking at a dip angle of 75º, which corresponds to the U-shaped distribution of macroscopic strength. Acoustic emission location technology can effectively characterize the spatial distribution of final damage. Therefore, this study investigates the effects of bedding dip angle on sandstone mechanical anisotropy, bidirectional energy distribution, and damage evolution characterized by multiple acoustic emission parameters. The results are expected to provide theoretical support for dynamic stability evaluation and disaster early warning of layered surrounding rock.

Key words: layered sandstone, cyclic loading and unloading, acoustic emission, anisotropy, bedding dip angle