On the fracture mechanism of rock-like materials with interbedded hard-soft layers under Brazilian tests

Abstract

The fracture of layered rocks subjected to tensile stress is involved in various subsurface energy-related activities. Many studies have numerically investigated the fracture evolution behaviour and Brazilian tensile strength (BTS) of layered rock in Brazilian tests. However, the fracture mechanism of layered rocks, especially those with interbedded hard-soft layers, is still not well understood. This is mainly due to the fact that the layered rocks were often modelled as two-phase materials only consisting of either rock matrix with layer interfaces or hard and soft layers without explicitly considering layer interfaces. In this study, rock-like discs interbedded with hard and soft layers are prepared with a dimension of 40 mm in diameter and 15 mm in thickness. The uniaxial compressive strength (UCS), BTS, and Young’s modulus of hard layers are 49 MPa, 6.7 MPa, and 10 GPa, respectively, while those of soft layers are 17 MPa, 3.2 MPa, and 8.0 GPa, respectively. The layered discs are tested with inclination angles between 0° and 90° at 15° interval under Brazilian tests with a loading rate of 0.24 mm/min. Then, three-dimensional layered models comprised of soft layers, hard layers, and layer interfaces are numerically established. Their mechanical behaviours are described by a newly developed constitutive model that can capture the tensile and compressive damage of rock-like materials under various loading scenarios. After validating against experimental results, the effects of Young’s modulus of layer interface and mechanical contrast of hard-to-soft layers on the stress distribution, crack initiation and propagation mechanisms, and peak load of layered discs with various inclination angles are numerically explored. It is observed that the effects of interface stiffness and mechanical contrast ratio on the peak load and fracture mechanism are layer orientation dependent.