Failure mechanism of fractured rock and associated acoustic behaviors under different loading rates

•Static loading rate effects on fractured rock are investigated experimentally.•Temporal-spatial varying responses of microcracking are visually observed by AE.•Grain yielding and crack propagating theories for AE generation are established.•Mechanical and acoustic phenomena are interpreted by the proposed fracture theory. Mechanical and associated temporal-spatial microcracking behaviors of fractured sandstones, a hard and stiff rock, were tested and observed under different uniaxial loading rates with the aid of acoustic emission (AE) monitoring. Based on the principle of microcracking nucleation and multiple locked-patches failure theory borrowed from seismology, some experimental phenomena during crack propagation process, AE generated mechanisms as well as the static loading rate effects of loaded fractured rock were illustrated semiquantitatively. Results show that there are significant static loading rate effects on the mechanical, acoustic and microcracking process of fractured hard rock. With the loading rates increase, the dominant failure modes of fractured sandstones change from mode-II shear to mode-I tensile fracture. Acoustic signals gradually increase with the applied axial stress. Especially, in the meta-instable stage, AE counts show repeated “suddenly increasing then quiet” features which can be used for the early-warning of rock catastrophic failure. The failure of rock mass can be considered as the continuous brittle fracture of locked-patches inside. And each locked-patch rupture corresponds to a drop of stress and an extremum value of AE counts.