Simultaneous and sequential multi-site impact response of composite laminates

The unique feature in this study was the investigation of the response of polymer composite material to impact by multiple high velocity projectiles. Energy absorption, new surface creation, and failure mechanisms from both sequential and near-simultaneous multi-site, high velocity impact were compared to assess synergistic and cumulative effects. A single-stage light-gas gun capable of launching three projectiles with controlled impact location and velocity in both near-simultaneous and sequential impact modes was developed to study these effects. Two test programs were conducted to evaluate these impact scenarios on thin S-2 glass/epoxy laminates. In the first program, the effect of laminate thickness was investigated using .30 caliber steel spherical projectiles. The material response near and above the ballistic limit at constant incident velocity was studied with respect to two and three projectile impacts. It was found that specimens subjected to sequential impact absorbed 10.1% more impact energy and exhibited increases of 23.0% (two projectile) and 10.5% (three projectile) in delamination damage over specimens subjected to simultaneous impact. The second test program involved a study assessing projectile mass effects for .50 caliber spherical Al 2O3 (3.94 g), steel (8.38 g), and tungsten carbide (16.08 g) projectiles at constant incident energy. A factor of four increase in projectile mass corresponded to 22.4% (sequential impact) and 12.8% (simultaneous impact) increases in delamination damage. Energy absorption increased 11.9% (sequential impact) and 8.7% simultaneous impact for laminates subjected to tungsten carbide projectiles over Al2O3 projectiles. Energy absorption in laminates subjected to sequential impact was 20.0% higher (average) than those impacted simultaneously. In contrast to the .30 caliber impact study, delamination damage increased 14.6% (average) for specimens subjected to simultaneous impact. In both studies, impact energy absorption increased with increasing cumulative damage. Finite element modeling (LS-DYNA 3D) was pursued to gain insight into failure methods, energy absorption, and damage prediction. New surface creation did not play a significant role as an energy absorption mechanism. However, its influence on compliance dominated the target response.