Fracture damage characteristics of hard roof with different bedding angles induced by modified soundless cracking agents

Abstract

Soundless cracking agents (SCAs), as an environment-friendly cracking material, are widely used to fracture the hard roof of coal seams. Under the long-term action of geological structure, the bedding characteristics of hard roof are obvious, and the roof bedding will affect the cracking effect of SCA. In order to study the fracture damage characteristics of hard roof with different bedding angles under SCA fracturing, four kinds of similar specimens with different bedding angles (0°, 30°, 45° and 60°) were prepared. The modified SCA fracturing experiments were carried out under the maximum principal stress, and ultrasonic measurement, acoustic emission (AE) monitoring and full-field strain measurement were performed to analyze the dynamic fracture process and damage characteristics of the samples. The results showed that the specimen with 45° (CL-3) had the largest damage and complex fracture morphology, and the 45° bedding angle was conducive to the fracture of the sample. The fracture of the specimens was dominated by tensile damage. With the increase of bedding angle, the cumulative AE energy released by fracture, fractal dimension and fracture density first increased and then decreased, and the distribution of the surface strain concentration area was more closely related to the bedding angle. The specimen fracture was divided into four stages based on the variation of AE parameters, and the percentage of tensile cracks in stage II decreased and that of shear cracks increased. With the increase of bedding angle, the damage degree of the specimen first decreased, then increased and then decreased. The damage value of sample CL-3 is 0.543, which is 7.73%, 29.1% and 12.89% more than that of samples CL-1 (0°), CL-2 (30°) and CL-4 (60°), respectively. As the weak structural surface of the sample, the bedding plane contributes to the generation of cracks under the action of modified SCA.