Mode II fracture study of rocks using Brazilian disk specimens

The centrally cracked Brazilian disk specimen subjected to diametral compression has been widely used to study mode I, mode II and mixed mode I-II fracture behavior of rocks. The fracture toughness ratio (K-IIc/K-Ic) in this specimen is always significantly higher than the theoretical estimates of available fracture criteria. In this paper, a new and more accurate approach is suggested for predicting mode II fracture toughness of rocks using the maximum tangential stress (MTS) criterion. The new criterion takes into account the first three terms of William's series expansion for elastic stresses near the crack tip in order to provide more accurate estimates for mode II fracture toughness of rocks. Then the fracture toughness ratio is calculated for some rock types using the proposed criterion and their results are compared with the relevant published experimental results obtained from fracture tests using the centrally cracked Brazilian disk specimens. Very good agreement is shown to exist between the theoretical predictions of the new criterion and the reported experimental results. (C) 2011 Elsevier Ltd. All rights reserved.