Acoustic Emission and Energy Dissipation Characteristics of Gas-Bearing Coal Samples Under Different Cyclic Loading Paths

Cyclic loading widely exists in coal mining activities, and it can significantly change the mechanical and seepage characteristics of coal. In this study, raw gas-bearing coal with different coal rank was mechanically tested under three stress paths: cyclic loading with stepwise increased peak stress (path 1), with step-by-step increased peak stress (path 2) and with crossed peak stress (path 3) using a tri-axial seepage testing machine. The acoustic emission (AE) signals under different loading and unloading paths indicate different mechanical properties of the coal sample. The Kaiser point is not a good indicator of the stress history of coal. The ratios of the quiet effect of the three coal samples under the three stress paths show that loading path 1 can increase defects such as micro-cracks in the coal samples (the AE quiet period decreases), while the other two paths decrease the number of defects (the AE quiet period increases). The cumulative dissipated energy of the coal shows an exponential growth with axial effective stress. The damping coefficient of coal first decreases then increases under cyclic loading. The damage variables can be used to predict the failure of coal samples, regardless of the stress path. Our results provide theoretical support and insight into the permeability increase mechanism and strengthened permeability increase mechanism of coal seams based on cyclic-loading-induced fracturing (repetitive hydraulic fracturing) under multiple protections and gas drainage engineering.