Abstract:

Shearing is the most common mode of failure in rocks under the compressive stress states in geomechanical applications, and the failure of rock bridges between two adjacent discontinuities in rock masses mostly develops as shear fracture. Therefore, mode II fracture (sliding mode) is another important failure mechanism in rock engineering, especially in the macroscopic sense. Although a few methods have been proposed to quantify Mode II fracture toughness of rocks under the static loading condition, no method is available to measure dynamic Mode II fracture toughness of rocks. This paper presents a punch-through shear (PTS) method to measure such material parameter of rocks under different loading rates and confinement pressures. In this method, circular notches are drilled 10 mm deep at both ends of a 54 mm diameter cylinder with 30 mm length to obtain the fracture specimen. The lateral confinement pressure is applied to the cylinder specimen by a 54.74 mm diameter Hoek cell. The dynamic load is exerted on the rock specimen placed in the Hoek cell by using a split Hopkinson pressure bar (SHPB) system, which is modified to guarantee that the specimen assembly remains in contact in the Hoek cell during pressurization by applying a static axial-stress. A specimen holder is designed to allow the punch head to load the specimen directly and, in combination with momentum-trap technique in the SHPB, this specimen holder also enables soft recovery of the rock short rod and rock hollow cylinder produced by punching. Pulse shaping technique is used in all dynamic SHPB tests to facilitate dynamic force equilibrium, under which condition finite element method is employed to obtain equations calculating Mode II fracture toughness under different confinement pressures and loading rates. The application to an isotropic and fine-grained Fangshan marble demonstrates the flexibility and applicability of the proposed PTS method. The dynamic Mode II fracture pattern and modes of the rock are analyzed using the X-ray Micro-CT technique. Five groups of marble specimen under confinements of 0, 5,10, 15 and 20 MPa are tested with different loading rates. The results show that the failure of the rock specimen is shear dominant and the Mode II fracture toughness increases with the loading rate and the confining pressure.

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