Size Matters: Impact Energy Absorption Across Five Decades of Length Scale

Abstract

The Laser-Induced Particle Impact Test (LIPIT) can be used to probe projectile, target, and synergistic projectile-target responses to 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. In addition, a LIPIT apparatus possesses a small laboratory footprint and is suitable for extension to high-throughput testing. Hence, LIPIT experiments have been used to study the dynamic response of many polymers, gels, and metals in different structural forms with various h_t/d_p ratios. These microscopic high-rate deformation behavior and impact energy absorption studies were used 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 (d_p = 3 μm–10 mm) were launched into scaled h_t/d_p amorphous polycarbonate targets of thickness h_t 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 (v_i = 550 m/s and h_t/d_p = 0.25, respectively). Impact energies spanned from hundreds of joules down to nanojoules (eleven decades). The specific impact energy absorption (E_p*), local plastic deformation, and deformation microstructure were compared across all scales. Length scale reduction sets in motion a remarkable 230% amplification in specific energy absorption and a 240% increase in relative impact deformation area. Corresponding numerical impact simulation results emphasize key limitations of current continuum-based material models and indicate potential areas of improvement. These findings demonstrate that material property discoveries made using emerging high-throughput methods (LIPIT, nanoindentation, laser-driven flyers, etc.) may not be directly indicative of macroscopic behavior and performance.