Nuclear Magnetic Resonance and Acoustic Emission on Initial Crack and Fracture Development of Coal with Different Strengths.

The initial fracture and development of fracture under loading conditions of coal are closely related to the stability of surrounding rock and geo-dynamic disasters. In this study, nuclear magnetic resonance (NMR), acoustic emission (AE) and 3D-CAD reduction techniques were employed to analyze the initial fracture distribution, the fracture development and failure mode of coal samples with different strengths under uniaxial loading. Based on NMR and 3D-CAD reduction, the initial fracture rate and fracture distribution of coal samples were obtained. Through uniaxial loading and AE, the distribution of fracture points was analysed. The dominated fracture of the failure state was reconstructed using 3D-CAD. Results show the initial crack rate of hard coal is low, and the ringing accounts and AE energy are small in the initial stage of loading. However, they increase rapidly in the later stage of loading, showing the characteristics of brittleness. In the initial state, the fracture rate of soft coal sample is higher than that of hard coal, and the growth of fracture point is fast. In the later stage, the fracture point number and AE energy are similar to that of hard coal. The initial fracture rate and fracture distribution of coal with different strengths are different, resulting in different occurrence and development of secondary fractures in loading state, which indicates that the deformational behaviour, failure mode and energy release are different for coal with different strengths. This study provides a theoretical basis for the stability analysis and geodynamic disasters prevention of coal with different strengths. [ABSTRACT FROM AUTHOR]