Study on the Grayscale Characteristics of Borehole Images of Progressive Failure of Coal Bodies with Different Moisture Contents

The failure process of a coal body around a borehole has progressive characteristics. Image characteristics can visually characterize the stress and failure characteristics of the coal body around a borehole during progressive failure. To investigate the effect of the moisture content on the progressive failure of the coal body around the borehole, an image test system for the deformation and fracture of coal rock was used, and progressive failure tests of coal body specimens with different moisture content conditions around boreholes were performed. We acquired images of the deformation field during the entire process of specimen failure. Based on the grayscale image theory, the variation in the grayscale characteristic parameters of the progressive failure process was analyzed. The results show that throughout the progressive failure of coal bodies with different moisture contents around a borehole, the main specimen failure can be divided into six stages: compression density, elastic deformation, crack initiation and stable extension, crack nonstable extension, post-peak softening, and post-peak failure. With increasing moisture content, the sigma(cd)/sigma(f) values of the 20%- and 40%-moisture-content specimens were 5.1% and 11.3% lower than those of the dry specimens, respectively, and the maximum uniaxial compressive strength sigma(f) was 5.1% and 17.4% lower than those of the dry specimens, respectively. The number of cracks that developed decreased. The grayscale histogram had a reduced grayscale peak at each stage, and the surface distortion diminished. The declining grayscale mean curve indicates a lagging development of stress concentration zones. The declining grayscale entropy curve indicates that macroscopic cracks form. The rising grayscale standard deviation curve indicates the delayed development of strain localization zones and weakening of specimen damage. The study explains the deformation and failure characteristics of the coal body around the borehole and the variation in grayness.