Loading rate dependence of staged damage behaviors of granite under uniaxial compression: Insights from acoustic emission characteristics

A series of laboratory-based uniaxial compression tests combined with acoustic emission (AE) monitoring was conducted to investigate the loading rate dependence of AE characteristics and the damage process of granite. Experimental results showed that the mechanisms of formation and evolution of cracks within the rock during loading can be characterized by AE parameters and real-time spatial AE locations. With increasing loading rate, the cumulative AE count decreased as a negative power function while the cumulative AE energy decreased almost linearly. Decreasing b-values indicate additional large-magnitude AE events and more concentrated distribution modes of fractures under high-speed loading conditions. Moreover, with increasing loading rate, the dominant failure mechanism was observed to transition from shear to tensile. A transition from tensile to shear crack modes during the damage process of each sample was noted to correspond with a decrease in b-values and the onset of crack nucleation, which may serve as a diagnostic precursor to rock failure. Crack nucleation pat-terns also exhibited rate dependence. Under low-speed loading conditions, nucleation was initiated at the un-stable crack growth stage and was characterized by interactions between linked tensile microcrack arrays to form a shear macrorupture zone, whereas crack nucleation advanced to the stable crack growth stage under high-speed loading conditions, resulting in vertical splitting.