Comparison of Moment Tensors and Cluster Formation of Acoustic Emission Events with Macroscopic Failure Process of Salt Rock Specimen under Triaxial Extension

摘要

Acoustic emission (AE) were measured during an triaxial extension test of a cylindrical salt rock specimen (diameter 150 mm, length 300 mm) from the Asse salt mine in Germany. In order to study the failure processes under dilatant loading conditions we analyze the microscopic formation of AE clusters and their source mechanisms using the moment tensor inversion method. The AE signals were detected up to a frequency of 500 kHz using an array of 12 AE wide-band sensors, which was glued to the surface of the specimen. During the experiment (duration 5 hours) about 70,400 events were recorded. Approximately 50% and 25% of these events were locatable using at least 8 and 10 consistent P-and S-wave onsets, respectively. A cluster analysis found a big planar cluster with 142 AE events. We studied source mechanism and moment tensors of these clustered AE events using clear discernible first motion amplitudes of the P wave. The moment tensor analysis pointed out, that the majority of the investigated events show mixed mode mechanism with dominant shear cracking and a small amount of isotropic source component. The tension (T) axes, which are normal to the crack plane of these microcracks, were predominately orientated axially in the cylindrical specimen, perpendicular to the maximum principal stress, which is oriented radially.