Numerical investigation of the progressive fracture mechanisms of four ISRM-suggested specimens for determining the mode I fracture toughness of rocks

The International Society for Rock Mechanics has suggested four methods to measure the mode I fracture toughness of rocks; however, the fracture behaviours of the suggested specimens are not fully understood, and the ideal crack growth hypotheses made in fracture toughness calculations have not been thoroughly assessed. In this study, for the first time, the progressive fracture mechanisms of all ISRM-suggested specimens are numerically investigated. The results indicate that the realistic crack front in the fracture process is significantly curved for specimens with a chevron notch, which violates the adopted straight-through crack assumption in the measuring principle; therefore, the traditional dimensionless stress intensity factors (SIFs) of chevron-notched specimens are likely inaccurate. In particular, the dimensionless SIFs of cracked chevron-notched Brazilian disc (CCNBD) specimens are likely the most inaccurate because the real fracture process of a CCNBD significantly deviates from the idealized situation. The numerical assessments of the progressive fracture mechanism of all ISRM-suggested specimens show that the fracture of the semi-circular bend (SCB) specimen agrees with the measuring principle. Furthermore, the dimensionless SIFs of the chevron-notched samples should be recalibrated using the realistic crack front with a curved shape in future studies. The numerical results indicate the need for additional studies of the progressive fracture mechanisms of the tested specimens to accurately measure the mode I fracture toughness of rocks.