Abstract: In order to investigate the failure characteristics and damage law of sandstone under the combined dynamic and static loading conditions, three-dimensional movement combination loading test system with separate Hopkinson compressive bar is applied to perform impact tests on sandstone samples under the condition of multiple combinations of dynamic and static state and different loading rates. CT scan and digital core technology are also used to observe the failure diagrams of different sections in the sandstone sample, and the three-dimensional reconstruction diagram and crack density in the damaged sample are obtained. The failure forms and failure mechanism of sandstone under different stress conditions are studied, and the effects of axial pressure, confining pressure and strain rate on the crack density of sandstone are explored. The test results show that the dynamic failure of sandstone is typical tensile splitting failure under the action of conventional dynamic impact. Under one-dimensional combined dynamic and static loading, the dynamic failure mode of sandstone is typical compression and shear failure, and the interior presents a conjugated double curved compression and shear surface. Under three-dimensional dynamic and static loading, the dynamic failure mode of sandstone is also compression and shear failure, but the internal failure surface is circular (cone) shape. The dynamic failure mechanism of sandstone under different loading conditions is analyzed. Under different loading conditions, the crack density of sandstone increases with the increase of strain rate. The application axial compression and confining pressure restrict the crack generation and growth rate, and the confining pressure has a greater limitation on the crack generation than the axial pressure. It is explained from the perspective of compressive strength and compensation space for crack generation of sandstone. The influence of strain rate on the crack density of sandstone at 0, 200, 400 m and 600 m underground is analyzed quantitatively by using axial compression and confining pressure to simulate the in-situ stress. To produce the same crack density, the strain rate of sandstone at 600 m underground is about 3.4 times that of sandstone without in-situ stress. The quantitative relationship between damage variables and strain rates under different loading conditions is established from the perspective of crack density. The research results can provide a reference for quantification of different explosion stress waves and the development degree of internal crack in rock during blasting mining without facing surface.

Key words: sandstone, combined static and dynamic loads, CT scan, failure mechanism, strain rate, crack density