Characterizing brittle failure through quantitative acoustic emission.

摘要

Quantitative acoustic emission is related to the characterization of source events in terms of their orientations and type of mechanism that generated them. The proposed source model is based on the representation of acoustic emission sources in terms of force dipoles, which constitute the components of the seismic moment tensor that characterizes the event. Displacement discontinuities are assumed to be the only type of source mechanism and a constraint is imposed to guarantee that the form of the seismic moment tensor is compatible with this assumption. The solution is obtained by minimizing the errors between the normal displacements generated by a seismic moment at the source and the measured displacements at each transducer location, estimated through calibration of the transducers. Several calibration tests were performed with the objective of evaluating the validity of using a scalar sensitivity parameter to relate normal displacements to output signals, and therefore avoiding the deconvolution process to obtain a full transfer function. It was shown that it is possible to use a single equation to represent the dependency of rise time sensitivities on the frequency of the recorded signals which is valid for all material types. The amplitude sensitivities can be approximated as a linear function of the signal frequencies which is strongly dependent on the coupling between the transducers and the specimen surface. Therefore, calibration curves should be generated for each transducer just before the actual experiments. Visualization tests were performed on PMMA, quartz and glass specimens with the objective of verifying source characterization results obtained using the proposed model. Two test configurations were used, three-point-bending and indentation, and the experimental results showed very good agreement with the event orientations estimated from the model. The proposed methods were then applied to the characterization of microcracking in rocks. Analysis of four different tests with sandstones indicated that there may be a change in source mechanisms during a test associated with localization of acoustic emission hypocenters, which can be interpreted as a precursor to failure.