Experiments on the penetration of thin long-rod projectiles into thick long-cylindrical borosilicate targets under pressure-free polycarbonate, aluminum and steel confinements

It is well established that confinement pressure inhibits comminution and fragment-flow during projectile penetration of ceramics. Here, a high-pressure gas gun is used to investigate the role of confinement wave impedance on the failure kinetics of ceramics during penetration. Tool-steel rods of fixed lengths and L/D ratios of 12, 16 and 24 impact and penetrate unconfined borosilicate cylinders and those under pressure-free polycarbonate, aluminum and steel confinements. The cylinders are all of the same size with projectile–target diameter ratios lying between 12 and 24, and projectile–target length ratio equal to 8. A stress wave controlling confinement is introduced to approximate an elastic waveguide set-up. Penetration depths into the comminuted borosilicate and the corresponding fragment jet diameters are measured between 168 and 1038 m/s impact velocities with high-speed photography and a witness plate. Expectedly, target resistive pressure increases with confinement impedance but decreases with projectile diameter. However, cylinders confined by steel are less resistive to penetration than those confined by aluminum. This anomalous behavior suggests that comminution increases with dynamic compression and it may be related to densification and the failure wave which occur in silica glasses above certain critical pressures. On this basis, comminution threshold conditions are determined and found to depend strongly on the propagation of stress waves across the target–confinement interface. These results are useful for material selection of impact/penetration-resistant structures with ceramic cores.