Anisotropy of acoustic emission in coal under the uniaxial loading condition

Abstract

We explore microstructure-related anisotropy of acoustic emission (AE) in coal and investigate connections to time-sequence-related fractal dimension (TRFD) and AE parameters of counts and cumulative energy. We characterize microstructure and anisotropy by imaging via X-ray CT. The anisotropic features of AE in coal are measured during uniaxial compression on a series of coal samples with varying inclinations of the anisotropy (bedding plane) relative to the loading direction (0°, 15°, 30°, 45°, 60°, and 90°). The form of the cumulative absolute AE energy released during loading is U-shaped with respect to increasing anisotropic angle. The maximum cumulative absolute AE energy occurs at an anisotropic angle of 90° (perpendicular to loading) and the minimum magnitude is when loading is at 45°. Cumulative AE counts have a complex structure with bedding plane inclination, decreasing to a minimum at an anisotropic angle of 30°, and peaking at 45° with terminal AE counts at 0° and 90° intermediate between these. The TRFD correlates positively with the cumulative AE counts with a trough at 15° and peak at 45°. The greater value of the TRFD in coal indicates a more uniform distribution of the AE count in time sequence, a smaller difference in the number of AE count between each time interval and less AE energy dissipated during the loading process. A theoretical basis for the observed negative exponential correlation between TRFD and AE energy dissipation during the loading process is developed. In samples with different anisotropic angles, this correlation conforms to the empirical relations that developed based on the space-related fractal dimension (SRFD) and AE energy dissipated by micro-seismic events recorded in situ.