Size effects in mode II brittle fracture of rocks

In this paper, a stress-based criterion is suggested for predicting the mode II fracture resistance of rock materials by considering the effects of specimen size. This criterion is a modified form of the maximum tangential stress criterion which takes into account the second and third terms of Williams series expansion in addition to the singular terms. In order to assess the proposed criterion, a number of modes I and II fracture tests are conducted on the semi-circular bend specimens manufactured from marble. For using the proposed criterion, the constant coefficients of higher order terms are determined for the semi-circular bend specimen with different crack length ratios and various loading span to radius ratios. Finally, it is shown that the proposed criterion can provide good estimates for the mode II fracture resistance of marble from its mode I fracture results while the effects of specimen size are taken into account.