Dynamic mechanical behavior and damage constitutive model of shales with different bedding under compressive impact loading

Considering the structural complexity of rock mass, shale and other unconventional energy mining problems exist widely. The dynamic mechanical behavior and the constitutive relationship of shales with different bedding (0°, 30°, 45°, 60°, 90°) were comprehensively investigated by split Hopkinson pressure bar (SHPB) technology. Basic dynamic mechanical properties of shales, such as the dynamic compressive strength, the failure strain, the dynamic elastic modulus, and the absorbed energy of shales, were analyzed. The high-speed photography was conducted to capture the dynamic failure patterns of shale samples. The test results showed that the dynamic mechanical parameters of shale materials with different bedding are affected by strain rates significantly. The failure patterns of shale at the high strain rate levels could be divided into three major types: splitting tension failure, shear failure, and mixed failure of splitting tension and shear. The energy absorption characteristics were similar to the strength characteristics and had a positive correlation with the degree of failure. In addition, the establishment of improved constitutive equation of shales was based on the ZWT viscoelastic constitutive theory, which simplified the low-frequency term and took into account material damage characteristics.