Damage Evolution and Fractal Characteristics of Lignite During Uniaxial Compression Using Nuclear Magnetic Resonance

In order to study the evolution law of the pore–fissure and seepage channels during the failure of lignite under load and and reveal the damage evolution law of lignite pore fracture structure. A uniaxial compression test was conducted on lignite by using a nuclear magnetic resonance tester equipped with a special gripper. The T₂ spectrum at each stage of the coal loading process was tested in situ, and magnetic resonance imaging (MRI) was performed. On the basis of fractal theory, the fractal dimension of the T₂ spectrum and MRI grayscale map was calculated. Results showed that the change in the pore structure was mainly due to the seepage pores, and the change in the adsorption pores was small. The damage development and destruction of coal mass are more concentrated in the later stage during the bearing failure process. The fractal dimension of the pore structure represented the connectivity degree of the pore–fissure network of the coal body, and the fractal dimension of the grayscale map (calculated by box dimension) represented the development degree of the seepage channel. During coal loading, the fractal dimension of the pore structure presented a convex shape, and the fractal dimension of the grayscale map exhibited a concave one. In the process of deterioration and improvement of pore network connectivity, the seepage channels show an evolutionary trend of first compression and then development. When the seepage channels are reduced by pressure, the fractal dimension of the pores shows an exponential decrease trend, whereas when the damage zone develops gradually, the fractal dimension of the pores shows an exponential increase trend. The experiment revealed the evolution law of coal pores and cracks during the process of bearing failure, which is helpful to quantitatively characterize the damage process of coal pores and cracks, and provide help to predict and control the development of pores and cracks.