Microcracking Characterization in Tensile Failure of Hard Coal: An Experimental and Numerical Approach

Acoustic emission (AE) and particle flow code (PFC^3D) are utilized to characterize microcracking nature in mode-I fracture of coal with strong bursting liability. The microcracks are represented by AE moment tensor (MT), which corresponds to displacement discontinuities involving opening/closing and sliding motions. The microcracking source parameters, including shear/tensile type, volume, orientation, motion, and magnitude, are calculated by minimizing the errors between the analytical and measured displacements, with an imposed constraint. Then, the detailed fracture processes, including microcracking source mechanisms and energy dissipation during coal tensile failure, are analyzed using both experimental and numerical approaches. The results show that the time-varying of crack mode angle and decomposed crack volume suggest a mixed-mode mechanisms locally with both normal and tangential displacements for microcracks. However, the orientations of microcracks with bigger magnitude are almost parallel to the direction of the crack propagation path, and their movement directions are mainly along the direction of the maximum principal stress, which is compatible with the expected fracture mechanism of mode-I opening globally. The dissipated energy and crack opening displacement obtained from AE inversion are approximately consistent with results calculated from energy release rate G and opening displacement measured using DIC, respectively. A new AE MT simulation method, based on particle motion, is developed using PFC^3D to verify the reliability of the above-mentioned microcracking characterization results. The mechanical properties as well as AE responses (including AE rate, magnitude distribution, spatial location, and MT) from PFC^3D numerical simulation are in good agreement with the experimental results, which reinforces the reliability the microcracking characterization results from AE inversion and simulation. Due to the tortuous crack propagation path and the inhomogeneity between grains, the maximum principal stress and resultant displacement coexists both on the normal and tangential of the inclined crack surface. Under the combined action of tensile and shear stress, the normal and tangential motion displacement components along the crack plane are generated, respectively, resulting in mixed-mode microcrack. This research provides a new approach for characterizing the microcracks in loaded rock/coal, which can deepen the understanding of meso-process and mechanism during rock/coal fracture.