Myriads of testing methods for measuring fracture toughness of rocks under different loading rates have been proposed in the literature. However, the reason for the measured discrepancy of fracture toughness using different methods has not been thoroughly revealed. In this Keynote, we report our recent numerical and theoretical investigations in this regard. First, progressive fracture mechanisms of typical fracture testing specimens are systematically investigated by numerical modelling. The Fracture Process Zone (FPZ) neglected in Linear Elastic Fracture Mechanics (LEFM) is simulated. The FPZ lengths of the typical specimens and their effects on the KIc measurements are compared. Second, theoretical derivations are conducted to estimate and compare FPZ lengths of these testing specimens for conventional rocks, and their effects on the fracture toughness measurements are assessed using the effective crack model. The FPZs are revealed to be partly responsible for the measuring discrepancy among these methods. Third, our study reveals that, in dynamic fracture tests using split Hopkinson pressure bar (SHPB), even if the dynamic force balance can be achieved by careful pulse shaping, the positions of the critical fracturing profiles remain highly dependent on the loading rates. The loading-rate-dependent cracking profiles of chevron-notched specimens cannot be ignored in order to accurately determine the dynamic rock fracture toughness.
Natural rocks often contain a large number of cracks with sizes ranging from a micro scale to possibly even a few kilometers. On one hand, the sizes, densities and orientations of cracks have influences on mechanical behavior of rocks. On the other hand, the failure/breakage of rocks is often a result of crack initiation and propagation. Thus, cracks play an important role in rock failure processes; it is important to study crack propagation in rocks and especially the resistance of rocks against crack/fracture growth. To this end, rock fracture mechanics have been widely applied to many diverse areas associated with brittle fracture/cracking of rocks.