Effect of confining pressure on the damage characteristics and mechanism of granite under ultra-high-frequency impact

Abstract

Assisted ultra-high-frequency impact technology is a promising method for improving hard rock-breaking efficiency in geotechnical and petroleum engineering. However, previous studies did not consider the effects of in situ stress. In this study, triaxial ultra-high-frequency impact tests were conducted at confining pressures ranging from 0 to 20 MPa. Using nuclear magnetic resonance, infrared thermal radiation, crack dye penetrant, and scanning electron microscope tests, the influence of confining pressure on the macro, meso, and micro damage characteristics and mechanism of granite was revealed. The results indicate that with the increasing confining pressure, the porosity increment of small pores continuously decreases from 0.2% to −0.02%; the moderate pores first increase to a peak of around 0.18% before 6 MPa and then gradually decrease to around zero. The large pores began to increase after 8 MPa, increasing to around 0.05% when the confining pressure was 14 MPa. In addition, the failure mode first changed from tensile splitting damage to a relatively intact state with meso-damage, and finally, transverse shear fracture occurred owing to the higher shear stress and low mode II fracture toughness near the crack tips. These findings prove the effectiveness of ultra-high-frequency impact technology in weakening hard rock in deep strata.