True triaxial experimental study on mechanical characteristics and energy evolution of sandstone under various loading and unloading rates

In this study, the stress change of a rock mass in front of the working face during underground excavation was simulated. The effect of loading and unloading rates on the mechanical characteristics, strain energy, and failure mode evolution of the sandstone were investigated. It was found that the loading rate of vertical stress had little effect on the varying trend of the volume strain, and the volume change of the sandstone was mainly caused by the compression in the vertical direction under low unloading rate. The true triaxial strengths of sandstones under various loading and unloading rates were well characterized by the linear Mogi criterion. For energy evolution, as the loading rate increased, the total absorption energy, elastic strain energy, and dissipation energy increased. Increasing the unloading rate resulted in gradual decrease in the strain energy. The failures of sandstones were more severe at high loading rate as the elastic strain energies were higher in these conditions. For a given loading rate, more cracks were formed by consuming less energy under high unloading rate. Furthermore, compared with the loading rate of vertical stress, the unloading rate of horizontal stress was the main reason for the change in rock failure mode. Based on the change law of the curve of elastic strain energy-strain, the crack damage stress thresholds of the sandstone under different loading and unloading rates were determined, which were found to positively and negatively correlate with the loading and unloading rate, respectively.