Study on the semi-circular bend method for characterizing the mixed mode I/II fracture toughness of sandstone: A micro-perspective

Abstract

Manufacturing and precracking specimens for semi-circular bend (SCB) tests are simple processes and do not require complicated loading devices. They have been used to conduct mode I, mode II and mixed mode I/II fracture tests on brittle materials. However, characterizing mode I and mode II fractures from a microcosmic and quantitative perspective has never been thoroughly evaluated. In this paper, high-resolution digital image correlation (DIC) and moment tensor inversion are first used to quantitatively measure micron-level displacement and analyse microfailure types. The DIC results indicate that SCB specimens with a notch inclination angle exhibit both opening displacement and sliding displacement, resulting in a mixed mode I/II fracture. A quantitative analysis of three typical classification methods based on moment tensor inversion suggests that SCB specimens with mixed loads are mainly tensile mechanisms. Some tensile sources are not pure tensile cracks, and shear components also play an important role. It is also observed that the moment tensor components, nature of sources and Hudson k value are associated with the notch inclination angle, which sheds new light on the mechanism of mixed mode I/II fractures.