Rate dependency on quasi-dynamic behaviours of cracked sandstone under uniaxial compression: mechanical, fracturing and acoustic emission characteristics

Abstract Investigating the rate-dependent behaviours of cracked rocks under quasi-dynamic loads is crucial to understand the instability and failure mechanisms of rock engineering threatened by dynamic disturbances. To carry out the quasi-dynamic tests under uniaxial compression, a self-devised loading apparatus was employed in this study, and the quasi-dynamic behaviours of cracked sandstone were investigated by using digital image correlation method and acoustic emission (AE) technique, including the mechanical, fracturing and AE characteristics. Based on the experimental results, the quasi-dynamic tests are transient, and the testing time dramatically decreases from several seconds to several milliseconds as the loading velocity increases from 0.1 to 100 mm/s. The uniaxial compressive strength of cracked sandstone under quasi-dynamic loads shows rate dependency, and the increasing crack angle would lead to an increase in the quasi-dynamic strength as well. By analysing the distribution and evolution of strain localization zones (SLZs) around the crack, the increasing strain rate would result in the occurrence of SLZ bifurcation phenomena, implying that the fracturing and failure behaviours of cracked rocks become more violent. In contrast, the rising crack angle would lead to the difficulty in forming the SLZs and bifurcation phenomena. In terms of the AE characteristics, the increase in strain rate induces the decreasing number of AE signals recorded in the quasi-dynamic tests, resulting in the decreasing total AE counts. By dividing the AE characteristics with the number of AE signals, the positive correlations were identified between the strain rate and AE behaviours per signal (i.e., AE counts per signal, AE amplitude per signal and AE energy per signal), which might be the reason why the sample failure become more violent with the increasing strain rate.