Abstract
Acoustic emission (AE) in coal is anisotropic. In this paper, we investigate the microstructure‐related scale effect on the anisotropic AE feature in coal at unconfined loading process. A series of coal specimens were processed with diameters of 25 mm, 38 mm, 50 mm, and 75 mm (height to diameter ratio of 2) and anisotropic angles of 0°, 15°, 30°, 45°, 60°, and 90°. The cumulative AE counts and energy dissipation increase with the specimen size, while the energy dissipation per AE count behaves in the opposite way. This may result from the increasing amount of both preexisting discontinuities and cracks (volume/number) needed for specimen failure and the lower energy dissipation AE counts generated by them. The effect of microstructures on the anisotropies of AE weakens with the increasing specimen size. The TRFD and its anisotropy reduce as the specimen size increases, and the reduction of fractal dimension is most pronounced at the anisotropic angle of 45°. The correlation between TRFD and cumulative AE energy in the specimens with different sizes are separately consistent with the negative exponential equation proposed by Xie and Pariseau. With the specimen size gain, the reduction of the TRFD weakens with the increasing amount of cumulative absolute AE energy.
Highlights
Acoustic emission (AE) is a broadly existing phenomenon regarding brittle fracture [1,2,3,4]. e AE in coal and rock has been studied for both laboratory and field uses [5,6,7,8,9,10,11,12,13,14] due to the wealth information contained in the AE signals
2.19 1.61 0.68 response measurement is conducted under the uniaxial compressive condition, while the microstructural variations in the specimens with different sizes and anisotropic angles are characterized by the X-ray CT imaging
The fractal dimensions of the AE signals of these specimens are calculated by the Grassberger and Procaccia (G-P) algorithm, and scale effect on the correlations between fractal dimension and energy dissipation is investigated. e conclusions are summarized as follows: (a) e cumulative AE counts and absolute AE energy increase with the specimen size, while the average absolute AE energy dissipation per AE count decreases with the specimen size. is may correlate to the increasing amount of both preexisting discontinuities and cracks needed for specimen failure and AE counts with lower energy dissipation generated by them
Summary
Acoustic emission (AE) is a broadly existing phenomenon regarding brittle fracture [1,2,3,4]. e AE in coal and rock has been studied for both laboratory and field uses [5,6,7,8,9,10,11,12,13,14] due to the wealth information contained in the AE signals. E scale effect on the mechanical properties of coal results from the increasing amount of microstructures in a larger specimen [19, 21], while the anisotropic AE features are affected by the directional spatial distribution of these microstructures relative to the loading directions since AE signals are generated by the rapid growth and interaction of microcracks in brittle materials [17]. To understand the scale effect on the anisotropy of AE and investigate the possibility in upscaling usages of the laboratory AE data to analyzing of microseismic in coal mining process, a series of coal specimens are processed with diameters of 25 mm, 38 mm, 50 mm, and 75 mm (height to diameter ratio of 2) and anisotropic angles of 0°, 15°, 30°, 45°, 60°, and 90° (the orientation of bedding plane relative to the loading direction). After the microstructure characterization by X-ray micro-CT, specimens are exposed to destructive unconfined compressive tests with concurrent AE measurement. e experimental data on AE are analyzed in terms of the fractal dimension, and the correlations between the fractal dimension and energy dissipation in different specimen scales are explored
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