Influence of Notch Geometry on the Rock Fracture Toughness Measurement Using the ISRM Suggested Semi-Circular Bend (SCB) Method

The mode I fracture toughness of rocks plays an important role in the stability assessment of rock structures. Semi-circular bend (SCB) is one of the methods suggested by International Society for Rock Mechanics and Rock Engineering for determining the mode I fracture toughness of rocks. However, due to the limitation in machining precision and quasi-brittle nature of rocks, the notch geometry of the SCB specimen and the fracture process zone (FPZ) may have significant impacts on the measurement accuracy of the fracture toughness. In this study, the SCB specimens made of Fangshan marble with notches featuring different tip shapes and widths were used to quantify the notch geometrical influences on the measurement accuracy. Three types of notch-tip shapes were considered: flat, semicircle and sharp. For each type of notch-tip shape, three notch widths were studied: 1 mm, 2 mm and 3 mm. Digital image correlation method was adopted to measure the FPZ lengths of the SCB specimens. With the FPZ lengths, K-resistance (KR) curves were obtained using the contour integral. Subsequently, the fracture parameters were estimated from the KR curves. The experimental results show that the initial fracture toughness depends on the notch geometry, whereas as an inherent rock property, the unstable fracture toughness is geometry-independent. These results support the geometrical requirements of the SCB specimen by the ISRM suggested method, in which the width/tip of the notch should be as narrow/sharp as possible to achieve accurate fracture toughness value.