Hardening-damage evolutionary mechanism of sandstone under multi-level cyclic loading

It is significant to investigate the hardening-damage characteristics of the sandstone of the roof under cyclic loading for the prevention and control of roof disasters and safe and efficient coal mining. Therefore, to investigate the hardening-damage effect of sandstone, the multi-level cyclic loading (MLCL) experiments on sandstone were carried out in this research. Firstly, the deformation and acoustic emission (AE) characteristics of sandstone under MLCL were analyzed. Then, the damage characteristics and fatigue life of sandstone were analyzed. Finally, the hardeningdamage competition mechanism of sandstone under MLCL considering cyclic loading and unloading number (CLUN) was explored. The results showed that the elastic modulus, irreversible strain, and elastic strain energy percentage increased with the increasing CLUNs. The AE signals were smaller in the compression-density and elastic stages and larger in the plastic stage. In addition, the percentage of low frequency signals with different CLUNs were 90.887%, 96.443%, and 94.149%, respectively, which indicated that more CLUNs promoted the further fracture development. Meanwhile, the fatigue life prediction was achieved. Taking specimen #2 as an example, when the stress level 6 is reached, only 247 cycles are required to reach the expected fatigue life. Finally, the hardening-damage competition mechanism of sandstone under MLCL was elucidated with the hardening effect dominating in the low stress stage, competing with each other to work together in the medium stress stage, and the damage effect dominating in the high stress stage. On this basis, the disaster-causing mechanism of the roof damage in the upper minedout area under repeated mining was explored. This research aims to effectively prevent damage destruction of the roof and strongly guarantee safe and efficient coal mining in the closed distance coal seam group.