Initial and unstable fracture toughness of quartz-diorite rock under mixed mode I/II loading: Experimental and numerical investigation

Abstract

The initiation and propagation of crack in rock bodies is a complex process because the rock is a typical quasi-brittle material. In this study, the fracture process of the specimens under four angles of crack inclination (α = 0°, 15°, 30°, and 46°) was examined using the digital image correlation technique, and, based on the double-K fracture model and linear superposition principle, the initial fracture toughness and unstable fracture toughness of the specimens under different fracture modes were obtained. The crack is in the elastic state when the load is less than the crack initiation load; when the load exceeds the crack initiation load, the crack propagates stably; and when the load reaches the peak load, the crack propagation becomes unstable and uncontrollable, which coincides the principle of the double-K fracture model. The initial fracture toughness (KI,IIini) and crack propagation angle (θ0) of the specimens are consistent with the predictions of the MTS criterion. Furthermore, the fracture of the specimens will transform into pure mode I with the extension of crack though they are initially loaded in the mixed mode I/II or pure mode II conditions. Accordingly, the rock fracture toughness at the peak load, i.e., the unstable fracture toughness (KI,IIun), is controlled by the mode I component (KIun) and shows less dependence on the fracture mode. Finally, it was shown that the MTS criterion coupled with the initial fracture toughness is applicable for simulating the fracture of rock specimens under mixed mode I/II loading.