Strain rate effect on the fracture evolution of sandstones under quasi-static loading conditions: Insights from acoustic emission characteristics

To investigate the correlation between sandstone fracture evolution characteristics and strain rate, an acoustic emission (AE) monitoring test was conducted under uniaxial compression. The influence of strain rate on the nucleation mechanism of sandstone micro-fracture agglomeration and the evolution characteristics of multi-type fracture sources were analyzed using AE localization and moment tensor theory. The results indicate that the peak strength, elastic modulus, and peak strain of sandstone specimens exhibit an exponential relationship within the set strain rate range. As the strain rate increases, the three normalized stress thresholds show a slow decreasing trend, with the normalized closure stress threshold decreasing significantly. The AE monitoring results reveal consistent curve evolution trends for the AE count, cumulative count, energy, and cumulative energy of sandstone samples under different strain rates, with a decrease in the cumulative AE count. The proportion of multiple types of fracture sources in sandstone samples varies in different deformation stages. When the sample reaches the stable crack propagation stage, the proportion of shear fracture sources tends to decrease, while the proportion of tensile and compressive fracture sources tends to increase with an increasing loading rate. The relationship between the AE moment magnitude and frequency follows the normal distribution law. As the strain rate increases, the AE b value gradually decreases, and the proportion of largescale fracture events in the sample increases. The strain rate can increase the overall strength of the fracture source inside the sample.