Gray Characteristics Analysis of Strain Field of Coal and Rock Bodies Around Boreholes During Progressive Damage Based on Digital Image

Gas extraction borehole is one of the important means to solve the mine gas disaster. However, the deformation and damage of coal and rock bodies around borehole will significantly affect the gas extraction effect. The main purpose of this research is to explore the effect of the strength of sealing grouting material on the deformation and damage characteristics of coal and rock bodies around borehole. For this purpose, the digital image correlation (DIC) experimental platform for the deformation and damage of coal and rock bodies was used to carry out the surface deformation observation test of coal and rock bodies around boreholes during progressive damage. The Matlab was used to convert digital images into gray images and calculate the corresponding gray histograms. Three gray characteristic parameters including mean, standard deviation, and entropy were used to quantify the influence of the strength of the grouting material on the gray evolution law of the strain field of the coal and rock bodies around boreholes during progressive damage that support the following results: (1) As the strength of the grouting material increased, the compressive strength of the specimens tended to rise, and the width of the cracks on the upper and lower sides around boreholes decreased significantly. (2) The gray clouds of strain field can be used to characterize the damage state and crack propagation characteristics of coal and rock bodies around boreholes. During progressive damage, the gray frequency of the surface strain field of the coal and rock bodies around boreholes shows a trend of “increasing and then decreasing”, and the shape of gray histogram changes from “pyknic” to “lanky”. The variation of gray frequency and the gray histogram decreases with the strength of the grouting material increases. (3) The evolution process of gray characteristic parameters further reflects the crack propagation law of coal and rock bodies around boreholes during progressive damage. According to the stress threshold values, the evolution process of gray characteristic parameters can be divided into five stages, in which the appearance of mutation segment of gray characteristic parameters is closely related to crack initiation and propagation. By studying the gray images and gray characteristic parameters of the coal and rock bodies around boreholes before and after grouting sealing during progressive damage, the crack propagation law was analyzed from the perspective of digital