Ballistic Limit Velocity and Impact Energy Absorption from Microns to Millimeters

Abstract The Laser-Induced Particle Impact Test (LIPIT) can be used to probe projectile, target, and synergistic projectile-target responses under high strain rate deformation at the microscale. LIPIT’s advantages over other microscale launching techniques include the ability to controllably launch a single microparticle and precisely characterize the projectile momentum and kinetic energy before and after target impact. Hence, LIPIT experiments have been used to study the dynamic response of many polymers, gels, and metals in different structural forms with various target thickness to projectile diameter ratios. These microscopic highrate (>106 s−1) deformation behavior and impact energy absorption studies were used to infer deformation mechanisms, as well as to suggest promising materials for macroscopic applications. Geometric scale, however, can significantly influence dynamic material behavior through scale-induced changes in event time, strain rate, projectile/target material homogeneity, and more. In this study, such geometric-scale effects are intentionally investigated. Noncrystalline alumina spheres ranging five orders of magnitude in diameter (dp = 3 μm to 10 mm) were launched into scaled amorphous polycarbonate targets at normal incidence using either LIPIT or a gas gun, depending on the scale. Projectile impact velocity and the projectile diameter to target thickness ratio were held constant in all experiments (𝑣𝑖 = 550 m/s and ℎ𝑡/𝑑𝑝 = 0.25, respectively). Length scale reduction resulted in a remarkable ∼ 230% amplification in specific energy absorption (E*p) and a ∼ 240% increase in relative impact deformation area. The fraction of target material engaged during impact more than doubled at the smallest scale. Estimated target ballistic limit velocities were roughly three times higher in the LIPIT experiments. This improved performance at smaller scales likely stemmed from rate- and size-induced enhancements of yield and failure stress values, shifting the bulk failure mode from plugging to dishing. These findings demonstrate that material property discoveries made using emerging high-throughput methods are not directly indicative of macroscopic behavior.