Mechanical behavior of sandstone at various stages under cyclic loading.

This study employed a three-level cyclic loading path to investigate the mechanical behavior of sandstone across different failure stages under constant-amplitude cyclic loading. The loading path encompassed the initial crack closure, elastic deformation, and crack propagation stages. The mechanical behavior at each stage was characterized by analyzing deformation response, energy evolution, and acoustic emission (AE) characteristics. The results indicated that the mechanical response of the rock sample exhibited distinct stage-specific characteristics. In the unstable crack propagation stage, the elastic modulus decreased rapidly, the damage energy increased continuously, and both the number of AE events and the associated energy increased significantly. The average peak stress reduction per unit cycle during the unstable crack propagation stage exceeded that during the elastic deformation stage by more than two orders of magnitude. The elastic modulus was highest in the stable crack propagation stage, followed by the elastic deformation stage, and lowest in the initial crack closure stage. Throughout the crack propagation stage, each stress drop was accompanied by a pronounced surge in AE counts and energy, indicating that these stress reductions resulted from the localized propagation and coalescence of internal cracks.