Dynamic response of thin targets under ballistic impact

The dynamic behaviour of thin targets upon impact is a topic of increasing interest, as the requirement for weight reduction is critical in many industries that involve such phenomena, such as the aerospace and defence industries. In contrast to intermediate or semi-infinite targets, impact-induced stress waves may propagate for distances equal to multiple target thicknesses in thin targets until full projectile penetration, causing perturbations within the target and interaction with the projectile. For this reason, the global response of such constructs is an important energy mechanism for dissipating the kinetic energy of the impacting projectile. With that said, the presented research focusses on the ballistic performance of thin targets under ideal and non-ideal impact conditions from the perspective of its energy constituents, with the aim of considering intricacies in the target configuration and projectile geometry that may affect the ballistic response.

Firstly, alterations in the response of a thin metallic layer were considered when the target configuration was altered from monolithic to layered (in-contact, not bonded), by adding a polyurea as a frontal layer. For that reason, ideal impact (normal impact with spherical projectile) was considered, to contain the complexities in the target, and the effect of applying a polyurea coating on the dynamic response of a thin target substrate was examined through numerical simulations where a material model for polyurea was developed, validated, and utilised. This is the first time that (a) polyurea was modelled by a complete behaviour accounting for energy dissipating characteristics (viscoplasticity) in a ballistic setting and (b) the effect of adding polymeric layer on the response of thin target (energy dissipating mechanisms translating to ballistic limit and residual velocity results) was considered. It was found that even though a 7.4 (m/s)/mm rise in the ballistic limit of the polyurea/aluminium bilayer with increasing polyurea frontal layer thickness was observed at higher than 4 mm coating thicknesses, this trend was not observed at lower polyurea thicknesses, and the ballistic limit of bilayer targets with a polyurea coating lower than 4 mm reduced from the one observed by the monolithic aluminium target. Based on the energy analysis on the bilayer target, it was concluded that the energy components, associated with a global response, present in the monolithic Al target impact case were limited in the bilayer, and the change in trend of the ballistic limit was attributed to competition between energy dissipation mechanisms.

The next step of the research was to expand to non-ideal impact cases by simplifying the target and introducing ballistic and projectile geometrical complexities. Thin plate-like projectiles, and the effect of their nose-geometry on the dynamic response of a thin aluminium target upon normal and oblique impact conditions, were considered for that purpose. This is the first-time impact by thin, plate-like projectiles is considered in detail to delineate the consequence of projectile geometrical parameters and analyse their effects on the thin target ballistic response. To do so, extensive experimental work and numerical simulations were performed, with statistical analysis conducted on the recorded results. It was concluded, that for the cases examined, the impact-induced kinematic response of the target reached up to approx. 20% of the projectile’s critical kinetic energy (close to the ballistic limit) and was associated approx. linearly to the energy dissipated by plastic deformation in the target. In that case, approx. 3/4 of the projectile’s energy dissipated by the non-local internal energy within the target and the remaining 1/4 by local erosion. While maintaining equal projectile/target mass ratios and material properties, this critical energy dissipation mechanism was proven sensitive to the nose geometry, obliquity, and impact velocity.