Mechanical responses and fracturing behavior of jointed rock masses with a cavity under different dynamic loads

Dynamic impact tests were conducted on parallel-jointed granite specimens with a cavity to study the effects of joint angle and impact pressure on the mechanical responses, energy evolution characteristics, and fracturing behavior of the specimens. An improved split Hopkinson pressure bar device and a high-speed camera were employed in the experiments. The experimental results indicate that the joint angle and impact pressure have a remarkable influence on the dynamic mechanical parameters. The dynamic compressive strength and elastic modulus exhibit a nonlinear trend of first decreasing and then increasing with increasing joint angle, reaching minimum values at 45 degrees. As the impact pressure increases, the dynamic compressive strength increases, and the elastic modulus increases or decreases, showing distinct nonlinear variation characteristics. With increasing joint angle, the transmitted energy ratio first decreases and then increases, while the reflected and absorbed energy ratios first increase and then decrease, all of which have a critical value. The reflected energy ratio of each sample is relatively small, while the transmitted and absorbed energy ratios are relatively large. With increasing impact pressure, the transmitted and absorbed energy ratios increase, while the reflected energy ratio decreases. Different joint angles and impact pressures lead to some differences in the cracking behavior and failure types of the samples, depending on the induced stress distributions in the samples under different conditions. Overall, the ultimate failure of the specimens is primarily tensile failure, accompanied by local shear failure. In addition, two distinct categories of crack coalescence patterns between the two joints and the cavity are distinguished, namely, coalescence by tensile cracks and shear + tensile cracks. There is no coalescence as a result of pure shear cracks. The influence of the impact pressure may be much greater than that of the joint angle on the type of crack.