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

A new testing method called punch-through shear test for measurement of Mode 11 fracture toughness, K-11C, 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 50 mm 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 load is applied to punch through the central portion of the core. K-11C 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-11C increases with increasing confining pressure and in the order of limestone, marble and granite. At confining pressures higher than 30 MPa the Mode 11 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-11C should be presented for high confining pressures as large confining pressures enhance the appearance of shear fractures. (C) 2002 Published by Elsevier Science Ltd.