Experimental study on the mechanical and acoustic emission features of frozen single fractured sandstone under drop hammer impact

Mining rock masses in high-altitude cold regions can experience instability due to low temperature environments and dynamic load disturbances. Existing research mostly focuses on the static features of fractured sandstone under different freezing temperatures. Considering the influence of engineering excavation, further research is needed to investigate the mechanical and acoustic emission features of frozen fractured sandstone under dynamic loads. Therefore, a drop hammer impact test is conducted on frozen single fractured sandstone. The mechanical and acoustic emission features of frozen single fractured sandstone are analyzed using acoustic emission monitoring technology. The experimental results show the following points. ① An increase in the inclination angle of the crack will cause an increase in the rebound amplitude of the strain time curve before the peak strain. The crack will change from being distributed on both sides of the crack to being distributed on both ends of the crack. After the drop height of the hammer increases, the strain time curve shows a significant bimodal rebound before the strain peak, and the damage is significantly intensified. A decrease in freezing temperature will lead to an earlier onset of strain peak and an increase in strain peak. ② The propagation of microcracks has stage features, corresponding to strong microcracking activity at the peak strain and accompanied by intense energy release. ③ The activity of microcracking activity increases first and then decreases with the increase of inclination angle of the crack. The drop height of the hammer increases, and the intensity of microcracking activity gradually decreases. The decrease in freezing temperature leads to an earlier occurrence of microcracking activity. ④ Micro cracks are mainly tensile cracks, corresponding to macroscopic failure modes. ⑤ The sharp increase in entropy value is a precursor to sandstone failure and can be used as a warning indicator for dynamic instability of sandstone.