Study on the effect of the lower limit of cyclic stress on the mechanical properties and acoustic emission of sandstone under cyclic loading and unloading

This study is aimed at exploring the effect of the lower limit of cyclic stress (LLCS) on the loading and unloading failure process of sandstone. First, cyclic loading and unloading experiments were performed on sandstone samples under different stress paths. Furthermore, the variations of mechanical properties and acoustic emission (AE) signals of samples during cyclic loading and unloading were analyzed. The results show that the equivalent residual strain produced by a single loading and unloading with a low LLCS is larger than the residual strain produced by that with a high LLCS. Under cyclic loading and unloading, the loading elastic moduli of samples grow as the number of cycles increases. In the loading failure stage, the loading elastic modulus under a high LLCS is greater than that under a low LLCS. In the 2nd and 3rd stages, the AE counts, main frequency signals and high-amplitude signals generated under loading and unloading with a high LLCS all account for smaller proportions than those generated under loading and unloading with a low LLCS. Low-frequency and high-amplitude signals appear in both the third stage and the loading failure stage under a low LLCS, while they only appear in the loading failure stage under a high LLCS. In the 2nd and 3rd stages, the b value first grows and then decreases under a low LLCS, while it keeps rising under a high LLCS. A high LLCS is conducive to suppressing the axial deformation of sandstone and reducing the amount of damage caused during a single loading and unloading. This study boasts theoretical reference value for the monitoring of coal rock stability and the prevention of dynamic disasters.