Influences of notch width and notch-tip angle on the fracture toughness measurement using the semi-circular bend (SCB) specimen

•The semi-circular bend specimens featuring different notch widths and notch-tip angles are used to investigate the influences of notch geometry on the fracture toughness measurement.•The initial load, peak load, critical crack length, and initial fracture toughness are closely related to the notch geometry of semi-circular bend specimen.•The unstable fracture toughness as the inherent property of rock material is independent of the notch geometry.•The semi-circular bend method is proven to be a favored approach for determining the fracture toughness of rocks. Semi-circular bend (SCB) method has been proposed by the International Society for Rock Mechanics and Rock Engineering (ISRM) as one of the four suggested methods for measuring the mode I fracture toughness of rocks. Due to limitations in the machining precision and the quasi-brittle nature of rocks, there exists inevitable measurement error in the measurement results using this method. Our previous study concluded that the SCB specimen with a sharp notch-tip (sharp-SCB) is the optimal configuration for minimizing the measurement error. This study further investigates the effects of notch width and notch-tip angle on the fracture toughness measurement using the sharp-SCB specimens. Five tip angles (i.e., 30°, 60°, 90°, 120°, and 150°) and three notch widths (i.e., 1 mm, 2 mm, and 3 mm) are considered. Within the theoretical framework of K-resistance (KR) curve, the results indicate that the initial load, peak load, critical crack length, and initial fracture toughness are all significantly dependent on the notch width and the notch-tip angle. As the intrinsic property of the rock material, the unstable fracture toughness is however relatively insensitive to these geometrical parameters. The error analysis of the experimental results shows that the ISRM suggested SCB method is reliable within certain ranges of these geometrical parameters. This study provides useful guidance on applying the ISRM suggested SCB method for quantifying rock fracture toughness.