Anisotropic tensile behavior and fracture mechanisms of soft-hard interbedded rocks: Insights from Brazilian splitting tests and multi-interface interactions

Pronounced tensile anisotropy in interbedded rock masses critically affects damage initiation in slopes and tunnels. In this study, Brazilian tests were conducted on soft-hard interbedded rock-like specimens with seven orientation angles (θ: 0°–90°) at a loading rate of 0.2 mm/min to investigate the effect of θ on tensile mechanical properties and fracture behavior, and further examine three loading-axis positions at θ = 90°, with real-time monitoring using acoustic emission (AE) and digital image correlation (DIC). The results indicate that Brazilian tensile strength (BTS), absorbed energy(U), and energy absorption rate decrease, while the degree of anisotropy (γ) increases with θ. BTS is positively correlated with U and negatively with γ. The loading-axis position significantly affects fracture morphology, which is governed by interface effects and the mechanical contrast between soft and hard layers. Fracture typically initiates at specimen ends and evolves in three stages, with failure shifting from central to non-central zones as θ increases. Interfaces play a crucial role in specimen fracture with larger θ promoting activation. Unlike single-interface rocks, interbedded rocks exhibit synergistic interactions among multiple interfaces; Stress tends to concentrate at the interfaces near the ends of the loading zone, triggering competitive crack propagation. Moreover, specimens with fewer interfaces exhibit higher BTS. Beyond 45°, the primary factors governing failure transition from hard matrix thickness and interface number to interface strength, leading to a convergence in tensile strength. These findings provide new insights into the complex anisotropic fracture behavior of soft-hard interbedded rock masses under tensile stress.