Out-of-plane deflection of rigid body armour systems: Digital image correlation techniques and analytical modelling

This paper presents a comparison between experimental results and calculated data on the maximum deflection of experimental ultra-high molecular weight polyethylene (UHMWPE) plates. This deflection is an important parameter in the design of new ballistic helmets, as it determines the risk on having contact between the helmet shell and the skull in the case of an impacting projectile. This considerably increases the risk for traumatic brain injury (TBI). On the experimental side, the rigid armour samples are clamped in a mount and impacted with specified bullets or Fragment Simulating Projectiles (FSP). The dynamic deformation of the backface of the armour is recorded by means of a stereoscopic rig of monochromatic high-speed cameras, providing a full-field set of 3D information. These experimental results are used to validate an analytical model, describing the backface deflection for the impact of fragment simulating projectiles or bullets. This provides information about the necessary spacing between the helmet shell and the head, in order to avoid contact between both upon impact of a projectile. A design based on structural stiffness instead of ballistic resistance could resolve this possible problem.