Progressive fracture mechanism of CCNSCB rock fracture toughness specimens and calibration of wide-range dimensionless stress intensity factors

International Society for Rock Mechanics has suggested four methods for measuring Mode-I fracture toughness (K-IC) of rocks. By combining the suggested semi-circular bend specimen and cracked chevron notched Brazilian disc, the cracked chevron notched semi-circular bend (CCNSCB) specimen is produced, which inherits many merits from previous methods. The CCNSCB method has recently received much attention by researchers for testing KIC of rocks, but has not been numerically assessed. Thus, the method is numerically studied and the progressive fracture process is presented via meso-damage mechanical analysis. Considering different span to diameter ratios., the results show that the real fracture is more aligned with the measuring principle while beta is greater. Therefore, beta = 0.8 is suggested. The minimum dimensionless stress intensity factor Y-min*, which is critical to determine the K-IC value, is calibrated by finite element method (FEM) with a sub-modelling technique for diversified CCNSCB geometries in terms of beta = 0.8. The calibrated Y-min* values are conveniently obtained for other relevant researches. The critical crack corresponding to the peak load simulated by microscopic damage mechanics is quite consistent with that corresponding to the calibrated Y-min* via FEM. It is indicated that the CCNSCB method is appropriate to measure KIC of rocks and the numerical simulations as well as the calibration of Y-min* values are effective.