Anisotropic characteristics and fracture mechanism of direct tensile fractures in layered rock masses

Abstract

Tensile fracturing is a primary failure mode in the initiation and development of rock mass collapses. Investigating the acoustic emission characteristics and fracture evolution mechanisms of layered rocks under tensile failure, and identifying precursor information of instability, is of great significance for monitoring and early warning of rock mass collapses. Direct tensile tests integrating DIC and AE techniques were conducted on specimens with different bedding inclinations. Results indicate that tensile strength increases by 48.2% with bedding inclination. As the bedding inclination increases, the proportion of tensile cracks first decreases and then increases, reaching its lowest point at β = 45°. k-means clustering classified AE signals during tensile fracture into three clusters: Cluster I, Cluster II and Cluster III; in the final loading stage, Cluster III signals densify, indicating main-crack penetration and serving as a critical precursor to rock failure. The evolution cloud map of the E_yy field reveals the complete process of crack initiation, propagation, and coalescence: For β = 0°–30°, cracks propagate along the weak bedding planes; For β = 45°, cracks partially propagate along the bedding planes and partially through the rock matrix; For β = 60°–90°, cracks primarily propagate within the matrix and penetrate the bedding planes. Based on microscopic fracture mechanisms, tensile fracture surfaces can be classified Cleavage Fracture, Intergranular Fracture and Transgranular Fracture. As the bedding inclination increases, the failure mode shifts from predominantly tensile to a tensile-local shear mixed failure; the fracture surface roughness first rises and then falls, the fracture energy grows exponentially, and the dissipated strain energy shows a slow increase.