Failure characteristics of brittle rock containing two rectangular holes under uniaxial compression and coupled static-dynamic loads

To deeply understand the failure characteristics of defective rock under actual stress condition, impact tests were conducted on prismatic granite containing two rectangular holes with different axial static pre-stresses by a modified split Hopkinson pressure bar (SHPB), and uniaxial compression tests were also carried out for comparison. Combined with digital image correlation (DIC), the dynamic damage and fracture process of specimens were observed by low-speed and high-speed cameras. Moreover, the energy evolution characteristics of specimens were analyzed to further understand the failure mechanism. The results indicate that the pre-stress has dual effects on the dynamic mechanical behavior of rock specimens, and the transition mechanism of the effect of pre-stress can be revealed by the elastic deformation limit. Observations show that the failure of specimens under different loads is caused by the growth of secondary cracks at hole corners. However, with the increase in pre-stress, the crack mode tends to shear and the strain localization tends to concentrate on sidewalls, resulting in severe rock bursting and extensive fracturing. Four coalescence modes around two rectangular holes were summarized: diagonal shear coalescence under static load, no coalescence under dynamic load, shear coalescence inside the middle rock bridge area under the pre-stress of 25-55% UCS, and indirect coalescence outside the rock bridge area under the pre-stress of 75% UCS. The specimen with the pre-stress of 75% UCS releases the internal strain energy during dynamic failure process, while the specimen with lower pre-stress absorbs the external impact energy. Finally, some insights are provided for deep rock engineering based on the test results.