Study on progressive damage mechanics and fracture mechanism of natural and water-saturated sandstones under dynamic load

In order to study the mechanical characteristics and damage evolution law of natural and water-bearing sandstone under cyclic impact load, cyclic impact test was carried out on sandstone by separating Hopkinson compression bar, the damage evolution law of sandstone in natural and water-saturated state was analyzed, and sandstone damage model was established. It was shown that while the modulus of elasticity and peak stress decreased with the number of cycle impacts, the strain rate of sandstones in both their natural and water-saturated states rose. The peak stresses in the natural water-bearing state and the saturated state were 25.53 MPa and 23.63 MPa, 37.25 MPa and 43.35 MPa, 64.82 MPa and 54.34 MPa, respectively, under the same impact velocity. This clearly demonstrated that the peak stress of sandstone was significantly higher in the natural water-bearing state than it was in the water-saturated state, and that the peak stress increased significantly with the increase in velocity. The damage variables under the peak strain are 0.45 and 0.55, 0.53 and 0.61, 0.62 and 0.69, and the damage of sandstone in the natural state is less than that in the saturated state, and the greater the impact velocity, the greater the damage. A sandstone damage model utilizing statistical damage theory and regression analysis was successfully established, and the overall fit of the model was good. It was shown that water decreases the mechanical strength of sandstone, but increases it when the dynamic load is increased. Under cyclic impact conditions, there is a critical dynamic load situation. Smaller dynamic loads cause an increase, followed by a drop in sandstone mechanical strength, whereas higher loads cause a decreasing trend in sandstone mechanical strength. Finally, the experimental study's findings are summarized, and a prediction model for forecasting the dynamic damage of sandstone samples in various water bearing conditions is offered.