A dynamic punch-through shear method for determining dynamic Mode II fracture toughness of rocks

The failure of rock bridges between two adjacent discontinuities in rock masses often develops as Mode II fracture (sliding mode). 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. In this method, circular notches are drilled 10 mm deep at both ends of a 50 mm diameter cylinder with 30 mm length to obtain the fracture specimen. A split Hopkinson pressure bar (SHPB) system is used to exert the dynamic load to the rock specimen, which is placed in a specially designed holder. The sample holder allows the punch head to load the sample directly and in combination with momentum-trap technique in SHPB, it 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. The application to an isotropic and fine-grained Fangshan marble demonstrates the flexibility and applicability of the proposed method. The dynamic Mode II fracture pattern and modes of the rock are analyzed using the X-ray Micro-CT technique. The results show that the failure of the rock specimen is shear dominant and the Mode II fracture toughness increases with the loading rate.