Numerical analysis of mixed mode progressive rock fracture mechanism of cracked chevron notched Brazilian disc specimens

The cracked chevron notched Brazilian disc (CCNBD) method, due to its distinct advantages, has been suggested by ISRM for mode I fracture toughness determination, and later applied extensively to mixed mode fracture researches (pure mode II included). When the CCNBD is applied to mixed mode fracture researches, however, the progressive fracture mechanism and relevant hypotheses have not been thoroughly assessed, and the rationality of mixed mode fracture tests using CCNBD is under suspect. In this paper, the progressive rock fracture mechanism of CCNBD rock samples under mixed mode conditions is first simulated using the meso-scale damage mechanics analysis code. The results show that the fracture initiates not only from the notch tip but also from the saw-cut chevron notch edges, as a result, the crack front is significantly curved, inconsistent with the straight through crack assumption. Furthermore, cracks fail to grow stably to the base of the notch, but depart from the pre-cracked notch plane and develop towards the loading direction, forming a couple of twisty 3D-wing cracks. The results from simulation correspond very well with those from our experiments. Both the fracture pattern and the growing trend of CCNBD under mixed mode conditions are inconsistent with the present hypotheses, which suggests that the application of the CCNBD specimens to the mixed mode (including pure mode Ⅱ) fracture tests is questionable.