Undstanding Rock Cracking Behavior from Microseismicity
The effects of loading mode:
We investigated the effects of loading condition on the cracking behavior of a medium-grained granite under mixed-mode I-II loadings. We found that the macrocrack paths near the pre-existing notch tip are closely related to the microcracking behavior illuminated by AEs. Most of the macrocrack paths traverse the region with high AE event density. For the AE event density contours corresponding to the fully-developed fracture process zones (FD-FPZs), their shape is sub-circular and symmetric with respect to the pre-existing notch under mode I loading, while those under the mixed-mode I-II loadings are irregular and asymmetric. By conducting the moment tensor inversion, we found that the presence of mode II loading component alters the AE source mechanisms with regard to the temporal evolution features of event-type ratios preceding the unstable propagation of macrocracks.
AE event density contour of FD-FPZs preceding the initiation of macrocracks of SCB specimens under model I (left) and mix-mode I-II loadings (right).
align="center">The typical temporal changes of axial load and event-type ratios of SCB specimens under model I (left) and mix-mode I-II loadings (right).
The effects of rock texture:
Rocks are natural materials with diverse microstructures which control their mechanical behavior. This study investigated the effects of texture of three lithologies-granite marble, and sandstone-on their microcracking mechanisms. Results indicate that rock texture and pre-existing microcracks are the primary factors controlling the microcracking mechanisms and the associated AE features [2]. This study highlights the importance of considering microscopic features when interpreting AE or micro-seismic data related to rock cracking.
Spatial-temporal evolution of AE events (right) in SCB granite specimens (left) under mode I loading.
Spatial-temporal evolution of AE events (right) in SCB marble specimens (left) under mode I loading.
Spatial-temporal evolution of AE events in SCB sandstone specimens under mode I loading.
The location of thin-section samples in SCB specimens.
Microscopic view beside the macrocrack (left) and the AE event scatter of FPZ preceding the initiation of macrocracks (right) in a SCB granite specimen.
Microscopic view beside the macrocrack (left) and the AE event scatter of FPZ preceding the initiation of macrocracks (right) in a SCB marble specimen.
Microscopic view beside the macrocrack (left) and the AE event scatter of FPZ preceding the initiation of macrocracks (right) in a SCB sandstone specimen.
References:
[1] Guo T.Y., & Wong L.N.Y.* (2021). Cracking mechanisms of a medium-grained granite under mixed-mode I-II loading illuminated by acoustic emission. International Journal of Rock Mechanics and Mining Sciences. 145, 104852. https://doi.org/10.1016/j.ijrmms.2021.104852
[2] Guo T.Y., & Zhao Q.* (2022). Acoustic Emission Characteristics During the Microcracking Processes of Granite, Marble and Sandstone Under Mode I Loading. Rock Mechanics and Rock Engineering. 55, 5467-5489. https://doi.org/10.1007/s00603-022-02937-1