Investigation on the instantaneous propagation behavior of mode I/II crack in Beishan granite under quasi-static loading

Abstract

Brittle materials such as granite demonstrate significant rate and stress-state dependencies. Investigating the propagation of mode I/II crack in such materials under varying quasi-static loading conditions is crucial for valuating and predicting the stability of structural components. To this end, Beishan granite was selected as a case study and subjected to quasi-static loading rates to examine the varying propagation rates of failure cracks. By combining a self-designed mixed displacement–strain method and a direct current (DC) voltage fluctuation method, the temporal evolution of the fracture process zone (FPZ) and subcritical cracks under various loading conditions was obtained. Finally, the instantaneous propagation behavior of the failed main crack was revealed from a micro perspective. The results show that the main failure crack in granite evolves through three distinct stages: a crack latent stage, a steady-state propagation stage, and a high-speed crack propagation stage. During the latent stage, the accelerated failure crack propagation rate facilitates the early formation of the FPZ. During the steady-state propagation stage, the length of the FPZ increases rapidly, while subcritical cracks propagate along the crystal boundaries at relatively low instantaneous velocities, leading to a rougher fracture surface. During the high-speed propagation stage, the extremely high instantaneous velocity of the failed main crack allows it to penetrate directly through the high-strength quartz and feldspar crystals, producing a relatively smooth fracture surface. Furthermore, mode II loading produces a flatter fracture surface, longer FPZ, and subcritical cracks compared to mode I loading.