On the use of Brazilian disc specimen for calculating mixed mode I–II fracture toughness of rock materials

The centrally cracked Brazilian disc specimen has been used frequently in the past for investigating mixed mode I–II fracture toughness in rock materials. However, a review of the available test results reveals that the conventional fracture criteria like the maximum tangential stress criterion always underestimate the mixed mode I–II fracture toughness data obtained from the Brazilian disc specimen. In this paper, a generalized maximum tangential stress criterion which takes into account the effects of the three fracture parameters KI, KII and T-stress is used for predicting the mixed mode fracture toughness data available in the literature for several types of rock materials tested with the Brazilian disc specimen. It is shown that the generalized maximum tangential stress criterion provides significantly improved predictions for the experimental results.