Fracture toughness of stripa granite cores

Abstract

There is a growing interest in the study of fracture parameters of rock material, but to date no standard testing method for measuring the fracture toughness of rock exists. Three round specimen configurations, namely the notched beam, the compact specimen, and the short rod were used in this work and different fracture toughness measures evaluated for Stripa granite. An approximate fracture toughness, K m, obtained without pre-cracking the specimens, was analysed statistically to indicate its dependence on notch length. Then a comparison with an apparent fracture toughness. K Q m , obtained from precracked specimens, showed that low K m values arise because the microcrack growth that precedes crack extension is neglected. Acceptable K Qm values, in the sense that they are good estimates of K Ic , can be obtained from three point bending tests on 40 mm diameter drill cores with notches of about 12 mm depth. The fracture toughness from short rod tests was found to be size independent in this investigation, and the value consistent with K Q m values from the compact tension and the three point bending tests. The ligament width was found to be a more critical size parameter than the crack length during the testing.