Effect of binder content on mesoscopic damage mechanical characteristics of gas-bearing coal subjected to shear load

In this study, shear tests are conducted on gas-containing coal with different binder contents using a self-developed meso-shear test equipment. Firstly, mechanical behaviors of coal are investigated, including strength characteristic, crack evolution mode, variation law of crack length, morphology and formation mechanism of mesoscopic crack. Then the effects of binder content on shear mechanical properties, crack macro-meso evolution and damage characteristics are discussed. The results show that shear strength of coal increases linearly with the increase of binder content, and the number and bifurcation of macroscopic cracks also increase, however the proportion of post-peak stable stage decreases. The development of total effective crack length can be divided into three stages: surge, slow increase and steady stage. With the increase of binder content, the type of damage zone between neighboring cracks changes and the mode of formed fracture surface converts from direct coalesces of cracks to bifurcation of main cracks and damage zone connection of the main wing cracks. The mechanism of formed shear mesoscopic crack of gas-containing coal with a binder content of 5.3% is analyzed. It is found that there are several mesoscopic cracks ahead a macroscopic crack, which changes from narrow to wide and approximately distribute along a straight line. Wing cracks connect, which are caused by the continuous appearance of new cracks in the front and the growth of original cracks in the rear. Once the overlap of wing cracks damages, a macroscopic crack occurs and then expands forward step by step. Secondary crack of the main crack wall is composed of several wing cracks arranged as an echelon. Coalescence modes of macro main cracks are connected by wing and anti-wing cracks, which depend on the relative position of adjacent cracks.