Damage mechanisms in stressed rock from acoustic emission

To better understand the phenomena leading to the failure of rock, unconfined compression experiments on Charcoal granite specimens were performed with the monitoring of acoustic emission (AE). Localization in the form of spalling near the free surface was detected by the concentration of hypocenters. The AE locations, which look random in space and time before localization, actually were clustered and have fractal structure in either space or time. After localization, the fractal dimensions reduced significantly in a certain range of distance and time. The analysis of interaction between spatial and temporal clustering revealed the size of clusters in both space and time. The cluster sizes may be related to the intrinsic properties of the rock. The seismic moment tensor was evaluated through a deconvolution technique to obtain the source mechanism and orientation of each AE event. The dominant mode of failure from AE was sliding on inclined planes, although a significant number of source planes were parallel to the loading axis, while the growth of cracks perpendicular to the loading axis was inhibited. This preferential growth of microcracks is related to a tensorial measure of damage and is used to study stress-induced anisotropy.

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