A numerical investigation into the potential of using additively manufactured composite cellular cores in ballistic helmets

The primary cause of death on the battlefield is due to head injuries. It is therefore critical that adequate protection is provided by combat helmets. It is also known that soldiers endure intense activities, often in hot climates, and that cases of heatstroke and heat exhaustion are common. Cellular structures have the potential to decrease weight, increase airflow and improve the energy absorption offered by conventional helmets. These types of structures can be realised using additive manufacturing processes such as Fused Filament Fabrication (FFF) 3D printing. Composite materials were chosen to increase the stiffness/strength and reduce the weight. This numerical study explores the benefit of utilising an auxetic core to increase the protection offered by a ballistic helmet. Three lattice structures (unit cells shown in Figure 1), each with identical relative density, were sandwiched between an aluminium plate and a model of the human skull. The skull was idealised as a plate, 6.58 mm thick, representative of the frontal male skull [1]. These assemblies were impacted by a 9 mm full metal jacket (FMJ) bullet at 358 m/s, representative of the testing per National Institute of Justice (NIJ) standards (NIJ 0106.01). The von Mises stress in the skull was monitored as the metric to predict skull fracture [2].The use of double arrowhead and hexagonal lattices both reduced the peak skull stress compared to the re-entrant lattice. Use of these lattices therefore result in a lower likelihood of skull fracture occurring and reduced injury. Further work will focus on expanding the geometries under consideration and more closely representing helmet systems, including choice of material and geometry.