Study on the mechanical properties and strain field evolution of gas-bearing coal under cyclic loading

In order to study the mechanical properties and strain field evolution of gas-bearing coal under cyclic loading, the mechanical properties of gas-bearing coal under cyclic loading and unloading conditions were tested with the Rock Mechanics Test (MTS816) system and the independently developed gas-bearing coal gas-solid coupling device. The strength and deformation characteristics of coal samples under cyclic loading and unloading were analyzed, and the strain field evolution of gas-bearing coal was investigated with digital image correlation (DIC) technology. The results indicated that: ① Under the action of cyclic loading and unloading, the loading and unloading curves did not coincide with each other, forming a hysteresis loops. With the increase in the number of cycles, the area of the hysteresis loops gradually increased, and gradually moved towards the direction of increasing strain. Coal samples with varying gas pressures exhibited significant brittle failure under cyclic loading. ② Under the action of cyclic loading and unloading, the peak strength of coal samples decreased, and the loading and unloading deformation modulus increased with the increasing gas pressure. The unloading deformation modulus was always greater than the loading deformation modulus. As the number of cycles increased, the difference in deformation modulus gradually decreased and eventually fell within the range of 0 to 0.1 GPa. ③ Under cyclic loading and unloading conditions, the trend between the irreversible strain of coal samples and the number of cycles was a three-stage pattern of “initial, stable and accelerated expansion”, and the overall curve progressed from an L-shape to a U-shape. The trend between the cumulative irreversible strain and the number of cycles was rapidly increasing, slowly increasing and rapidly increasing, and both irreversible strain and cumulative irreversible strain increased with increasing gas pressure. ④ Under low gas pressure, the strain concentration area of coal samples was mainly a single vertical strain concentration zone. As the gas pressure increased, the local strain concentration zone gradually transitioned from vertical single to disordered and complex. The higher the gas pressure, the more pronounced the strain field fluctuation and the intensity was concentrated in the central region. At high gas pressures, the greater the number of peak points, the greater the strain with increasing gas pressure.