Experimental study on dynamic mechanical behavior of frozen sandstone with different saturations

Water content is one of the critical factors affecting frost damage to rock masses in alpine regions. A dynamic disturbance load further complicates the issue. In this study, the effects of saturation and impact loading on the dynamic behavior of the frozen red sandstone were investigated using a low-temperature split Hopkinson pressure bar (LT-SHPB) experimental system. By combining low-field nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM), the dynamic evolution of the microstructure of the frozen sandstone due to changes in saturation was investigated. The experimental results show that the increase in saturation reshapes the pore structure of the frozen sandstone and promotes the expansion of pores of different sizes during freezing, while the frozen samples at complete saturation are mainly developed with mesopore and macropore. The dynamic strength, elastic modulus and brittleness index of the frozen sandstone under impact loading, which are limited by the critical saturation S_rc, tend to increase and then decrease with saturation increase. In contrast, the ultimate deformation capacity of the frozen sandstone showed an opposite trend with saturation. With increasing impact loading, the dynamic strength, elastic modulus, and peak strain of the frozen sandstone gradually increase, showing an obvious strain-rate enhancement effect; while the brittleness index decreases by 8.1% at full saturation when the impact velocity increases from 4 m/s to 6 m/s, indicating that the dynamic damage mode develops from brittle to ductile. Moreover, the frozen samples changed from tensile damage to composite damage with increasing saturation and impact loading; the distribution of crushing masses remained closely related to their dynamic strength. Based on the experimental results, the mechanism of the effects of saturation variation on the dynamic mechanical behavior of frozen sandstone is discussed.