A novel specimen for mixed-mode fracture testing of anisotropic rocks

Abstract

This study introduces a novel Semi-Circular Bend (SCB) specimen to overcome the inherent coupling between geometric constraints and material response in traditional fracture tests on layered rocks. The novel design fixes both the support span and bedding orientation while independently varying the prefabricated crack inclination angle, enabling controlled mixed-mode I/II fracture conditions. This approach enables, for the first time, a precise investigation of crack trajectory influence under constant geometric constraints. A quantitative analysis was performed by combining parametric finite element analysis and three-point bending experiments on slate samples. The results demonstrate that the fracture toughness of the layered rock exhibits a nonlinear evolution with the increasing crack angle β. The dominant fracture mechanism shifts from tensile to shear failure as β increases. While existing anisotropic fracture criteria like the Maximum Tangential Stress (MTS) and Minimum Strain Energy Density (SED) are accurate under tensile loading, they significantly underestimate the material's toughness in shear-dominated regimes. We further propose and validate a Tangential Strain Energy Density (TSED) criterion, which shows superior accuracy, especially under Mode II conditions. This work provides a robust experimental-theoretical framework for assessing fracture behavior in bedded rock masses with natural cracks, with direct implications for tunnel and slope stability.