Microcrack formation and material softening in rock measured by monitoring acoustic emissions

Brittle deformation in rocks is accompanied by the formation of microcracks which emit elastic energy partly as acoustic emissions. Acoustic emission parameters such as amplitude may be related to geometric parameters such as crack size. We have analyzed catalogues of acoustic emission events recorded during compression tests in rock in terms of the information they give about the accumulated state of damage in a material. We combine this measured damage state with a model for the softening behaviour of cracked solids, and show that reasonable predictions of the mechanical behaviour are possible. Several strategies have to be used to allow for incomplete recording of the acoustic emissions. An independent calibration of the scaling relation between the acoustic emission parameters and the microcrack geometry remains outstanding, although the results here suggest constraints on the scaling relation. We show, however, that this quantitative approach is markedly superior to the more traditional methods of acoustic emission analysis in correlating the acoustic activity with the weakening of the material.