Experimental study on interaction characteristics of explosive cracks under confining pressure and its comparison with free boundary results

The influence of confining pressure on the dynamic fracture mechanical behavior of deep rock masses is crucial in the exploitation of deep resources and the development of underground spaces. In this study, dynamic caustics experiments were conducted using transparent epoxy resin materials to investigate the crack propagation behavior and interaction mechanisms of double cracks under confinement. The experimental results indicate that biaxial confining pressure suppresses crack propagation, interaction between double cracks, and penetration. Without confining pressure, cracks do not penetrate until the relative vertical distance ΔH between crack tips is 20 mm, whereas with confining pressure, cracks cease to penetrate when the relative vertical distance between crack tips is 10 mm; however, interaction between crack tips still exists at this point. During the crack interaction stage, both fracture toughness and propagation velocity of crack tips exhibit significant fluctuations under no confining pressure conditions, whereas under confining pressure, both decrease rapidly. Additionally, under confining pressure conditions, the fluctuation range of crack propagation direction during crack interaction is relatively small. Confining pressure rapidly reduces the degree of stress concentration at crack tips, increases the rate of energy release at crack tips, and enhances the ability of dynamic cracks to resist the influence of other crack tips or crack surfaces. Under confining pressure, continuous Rayleigh waves are typically observed at crack tips due to an imbalance between stored strain energy and the energy release rate. These waves dissipate part of the strain energy, promoting crack deflection and branching.