Acoustic emission study of microfracturing during the cyclic loading of westerly granite

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Abstract
A rectangular prism of Westerly granite was deformed in a uniaxial cycling experiment. Acoustic emission (AE) activity was recorded using eight transducers in real time. Hypocenters of 116 events were determined using a standard least squares approach. Event clustering was definitely evident and could easily be assessed using a stereographic projection of hypocenters. This clustering was observed at volumetric strains well below 0.03%. The distribution of hypocenters changed from cycle to cycle, suggestive of a spatial migration. The cumulative distribution of hypocenters documents the strongly anisotropic development of dilatancy in Westerly granite. Once the sample underwent the initial deformation and a zone of weakness was established, subsequent events clustered in and about the initial zone. Volumetric strain, differential stress, acoustic emission event rate, and cumulative numbers were continously measured during the experiment. The cumulative number of events per cycle generally decreased with the number of cycles. The majority of events occurred upon loading. Those that occured during unloading were few in number and not as systematically related to stress as were those for loading. In general, waveform quality deteriorated with increased cycling and in later cycles waveforms typically displayed low‐frequency components in initial onsets followed by a more typical higher‐frequency arrival.

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