Experimental analysis of passive acoustic emission, in particular the Kaiser Effect, under dynamic stress conditions

Abstract

Irreversible damage induced by stress in brittle rock is accompanied by the formation of micro-cracks. The strain energy released during the fracturing process is released in the form of acoustic emission. This thesis applied the non-intrusive method of acoustic emission monitoring to assess the deformation process of brittle rock during cyclical loading and loading to failure in a standard uniaxial compressive strength test set-up. It is confirmed that the stress-strain curve is clearly separated into five phases and that the cumulative hits recorded throughout the failure process correspond with these five phases. Furthermore it is found that there is an obvious rise in trend in the amplitude of the acoustic events as the rock nears failure, but that individual events of high amplitude should not be considered indicative for the damage in the rock. Additionally it is found that high amplitudes characteristic for near failure stress are recorded at 25\% of the failure stress if the rock has been under high stresses, indicating a change in fracture mode. The Kaiser Effect, the phenomenon defined as the absence of detectable acoustic emission events until the load imposed on the material exceeds the previous applied level, was confirmed during uniaxial cyclical loading. Through loading samples from a highly stressed pillar in the Nepheline Syenite Stjernoya Mine in Norway, it is found that the onset of acoustic emission may be indicative of the stress in the pillar. These findings are useful to further develop acoustic emission as a monitoring method in a range of applied earth science applications.