Estimation of Mode I Fracture Toughness of Rock by Semi-Circular Bend Test under Confining Pressure Condition

It is essential to understand the mode I fracture toughness of rock under in situ conditions found underground, such as confining pressure, for assessment of the stability of underground rock structures. Some researchers have performed several types of fracture toughness tests of rocks under confining pressure condition. However, the ordinary fracture toughness tests do not consider the effect of confining pressure in their estimation equations. A modified estimation equation for the mode I fracture toughness of rock under confining pressure condition was suggested in this study. This study was intended for the semi-circular bend (SCB) test, which is one of the ISRM-suggested methods for determining the mode I fracture toughness of rock. The SCB test under various confining pressures from 0 to 10 MPa was performed using Kimachi sandstone. A finite element analysis of the SCB test considering several confining pressure conditions was performed to calculate the stress intensity factor. Based on the numerical simulation results, the modification of the fracture toughness estimation was suggested. Applicability of the suggested estimation was discussed with comparing the mode I fracture toughness of the rock obtained in this study and previous research.