Fracture toughness (FT) expresses material resistance to crack propagation leading to its final macroscopic failure. Approximately 20 different laboratory methods are currently used for a mode I and mixed mode I/II FT rock testing. However, it is a well-known fact that, for the same rock, the mode I FT value varies when different specimen types are utilized. Moreover, the preparation of specimens for FT testing may be associated with difficulties in obtaining required notch dimensions. From this point of view, Brazilian tensile strength (BTS) represents an easily and quickly detectable parameter that can be applied for prediction of the mode I FT. In this contribution, the results of investigations of mutual empirical relationships between FT determined using CB and SCB tests, as well as among FT values obtained via these testing methods and BTS are presented. A total of 14 different rock types, mostly sandstones, were used for this research.
The failure process of rocks and similar quasi-brittle materials is the result of complex mechanisms, including microcrack initiation, propagation, and interactions with each other, resulting in crack coalescence. Crack initiation occurs when the stress intensity factor at a microcrack tip reaches its critical value, known as fracture toughness (FT). FT thus expresses the resistance of the material to crack propagation. To date, no uniform, generally valid method for determining the rock FT has been defined. Since the beginning of 1980s, more than 20 different laboratory methods with various experimental arrangements and specimen geometries have been developed especially for a mode I and mixed mode I/II FT rock testing. Four of them, including chevron bend (CB) test and semi-circular bend (SCB) test, were approved as an ISRM suggested methods for FT measurements.
However, it is a well-known fact that, for the same rock sample, the mode I FT varies when different specimen types are used for measurement. This is most likely due to the fact that the measured values of the rock FT are influenced by material heterogeneity, sample dimensions, boundary conditions and asymmetric mechanical behavior of rocks (Vavrik et al. 2021). The discrepancies between the results of FT measurements performed using different methods attracted the interest of many researchers (see, e.g., Funatsu et al. 2015). As for the comparison between SCB and CB methods, it was mostly shown (Chang et al. 2002, Funatsu et al. 2015, Wong et al. 2017) that the FT obtained from this test is much lower than the FT achieved using the CB specimen. On the other hand, some other published results suggest that the FT values from SCB specimens can be comparable with the results from the CB tests (Aliha et al. 2017, Kataoka et al. 2015).
