Experimental study on dynamic strength and crack evolution of rocks under different strain rates

The study on the dynamic mechanical characteristics of rocks under impact loading holds significant theoretical guiding significance for achieving controlled blasting during roof cutting blasting. Based on a split Hopkinson pressure bar (SHPB) test system, sandstone from the roof of Dingji Coal Mine in Huainan city was tested. By varying impact velocities to achieve different strain rates, the stress-strain relationship of sandstone under impact loading at different strain rates was obtained. The strength characteristics of sandstone specimens under different strain rates were analyzed and internal crack evolution and fractal dimension changes were investigated by CT scanning. The results indicate that stress-strain curves under varying strain rates comprise four stages, namely: the primary fracture compaction stage with concave sections, the approximately elastic linear deformation stage with different slopes, the yield stage with nonlinear increase of stress, and the softening failure stage with significant decrease of stress as deformation increases. The dynamic peak stress, dynamic elastic modulus and strain rate show a significant positive correlation. The increase of strain rate can enhance the ultimate bearing capacity of the specimen. The dynamic strength growth coefficient of the specimen increases with the increase of strain rate, but there exists a critical strain rate state. Under the action of impact dynamic loading, the cracks in sandstone specimens expand from the outside to the inside, tangential to radial, and interpenetrate each other, presenting the characteristics of tensile splitting failure evolution. With the increase in the number of impacts, the specimen as a whole shows an increase in the number of cracks, an increase in the volume and area of cracks, and a slow to significant increase trend in the volume and area of cracks, while the fractal dimension shows a significant to slow increase trend.