Mechanical properties and acoustic emission activity characteristics of different coal-rock ratios under uniaxial cyclic loading

Under the influence of mining and excavation, the coal-rock combinations (CRCS) will bear multiple load disturbances. It is of great significance to study its mechanical properties under cyclic loading. Currently, there are relatively many studies on CRCS with equal ratios. Under different geological conditions, different thicknesses coal seams combined with roof or floor will form diverse CRCS. In this paper, cyclic loading tests were conducted on three coal-rock ratios with different CRCS. The connection among "stress-AE activity-fractal dimension-damage" of CRCS was established, the relationship between b-value and crack development was analyzed, and the failure mechanism of CRCS was studied. The research results show that the mechanical properties of CRCS decrease as the proportion of coal specimens increases, while the change in fractal dimension of cracks is the opposite. The combinations with high coal specimen ratio, it has the smallest b-value before failure, suffers the most serious damage, has the largest fractal dimension, and is the most prone to failure. The failure of combinations starts with the generation of microcracks inside the coal specimens or near the coal-rock bonding surface, and then these microcracks penetrate each other, resulting in the overall failure of the specimens.