Rock fracture toughness testing in Mode II--punch-through shear test

摘要

A new testing method called punch-through shear test for measurement of Mode II fracture toughness, K_(IIC), of rock is introduced. Results from a finite element modelling (FEM) and a series of laboratory tests on limestone, marble and granite are presented. Core samples with 50mm diameter are cut to length equal to diameter. Circular notches are drilled to different depths at both ends leaving an intact portion in the centre of the core. The sample is subjected to different confining pressures up to 70 MPa and an axial Toad is applied to punch through the central portion of the core. K_(IIC) is calculated from a displacement gradient approach using FEM, assuming linear elasticity to be valid according to the fracture breakdown zone model by Cowie and Scholz. Results from testing show that K_(IIC) increases with increasing confining pressure and in the order of limestone, marble and granite. At confining pressures higher than 30 MPa the Mode II fracture toughness approaches a constant value. Fracture initiation and propagation differ among the three rock types where grain size is found to have a strong influence on the mechanism of microfracturing and failure. It is suggested that data of K_(IIC) should be presented for high confining pressures as large confining pressures enhance the appearance of shear fractures.