Cracking mechanisms of a medium-grained granite under mixed-mode I-II loading illuminated by acoustic emission

Cracks at various scales in rock masses are often subjected to mixed-mode I-II loadings in Earth's dynamic processes and geotechnical engineering works. Insightful understanding of the macroscopic and microscopic cracking mechanisms of rocks under this loading condition is of far-reaching significance. To investigate the effect of the loading condition and granite heterogeneity on the cracking mechanism, we conduct three-point bending tests on pre-notched semi-circular medium-grained granite specimens. For comparison, identical tests are also conducted on polymethyl methacrylate (PMMA) specimens. The results show that the initiation angles of the macrocracks in granite are less influenced by the loading condition as compared with the more homogeneous PMMA. Based on the monitoring of the acoustic emissions (AEs), we find that the microcracks under mixed-mode I-II loading do not nucleate as easily as compared with that in mode I case. For the AE event density contours characterizing the fully-developed fracture process zones, their shape is sub-circular and symmetric with respect to the pre-existing notch under mode I loading, while those under mixed-mode I-II loadings are irregular and asymmetric. Most of the macrocrack paths traverse the high AE event density region. As the mode II stress intensity factor (KII) increases, the AE events with higher energy tend to concentrate beside the crack paths in front of the notch tips. The moment tensor inversion suggests that the presence of mode II loading component alters the AE source mechanisms in terms of the temporal evolution characteristics of event-type ratios preceding the unstable propagation of macrocracks.

Automated summary

Research shows that under mixed-mode I-II loading conditions, the initiation angles of macrocracks in granite are less influenced compared to more homogeneous materials like PMMA. Acoustic emissions monitoring reveals that microcracks under mixed-mode I-II loading conditions do not nucleate as easily as under mode I loading. Additionally, the presence of mode II loading alters the acoustic emissions source mechanisms, affecting the unstable propagation of macrocracks.