Micromechanisms of coal fracture: Insights from quantitative AE technique

Abstract

Three-dimensional location and characterization of microcracks in controlled fracture tests of hard coal beams were experimentally conducted through the quantitative acoustic emission (AE) technique. The recorded AE events were physically interpreted as displacement discontinuities (DD), i.e. microcracks, involving only opening/closing and shear-sliding mechanisms to describe the fracture modes directly anticipated within a mechanically loaded material. Based on the AE sensor calibration using a scalar sensitivity parameter and the first motion of P-wave, the moment tensor components were determined through minimizing the errors between the measured and calculated displacements at each sensor location, subjected to a restriction from this “crack-only” source model. Source characterization results were presented in a statistical form, which showed mixed mode mechanisms of microcracks with approximately equal contribution of shear and tensile displacements locally due to the considerably tortuous crack path in the microstructural level of coal material. However, the microcrack volume decomposition under the global coordinate system indicated a mode-I opening micromechanisms dominantly, which was compatible with the expected mechanism from the three-point bending test. Additionally, the orientations of microcrack planes were more or less vertical, with an average angle of 20.4° from the global fracture plane. And around 79.6% of the AE sources with an angle, made by the displacement vectors with horizontal direction, less than 30°. AE magnitudes followed the Gutenberg-Richter relation with a b-value of around 1.3 during coal fracture. Owing to the physical basis of energy dissipation and the linear relationship with scalar moment, the so-called “crack volume” |b|ΔA was used as an index of the absolute AE energy. Energy dissipation region moved forward together with the crack propagation, and most of the energy was dissipated above the crack tip by separating the crack face.