Effect of inter-layer toughness in ballistic protection systems on absorption of projectile energy

• An energy audit methodology for ballistic impact of 2-layer systems.
• Demonstration that strong interfacial bonding can raise energy absorption.
• Proposed that this is due to enhanced deformation in the back layer.

This paper concerns absorption of the kinetic energy of a projectile in multi-layer protection systems, with particular focus on the role of inter-layer bonding. Two-layer samples have been produced, composed of a 2 mm thick alumina front plate and a carbon fibre composite back plate with a thickness of about 0.8 mm. These were manufactured under two sets of conditions. The fracture energy of the inter-layer interface was measured for these two types of sample to be 170 and 620 J m−2. Such samples were subjected to impact by spherical projectiles of hardened steel, with a diameter of 8 mm and an impact speed of about 220 m s−1, corresponding to a kinetic energy of about 50 J. Samples composed of the alumina plate alone and of unbonded alumina and composite layers were also tested. It was found that significantly more projectile energy was absorbed by the strongly bonded samples, and that this difference was greater than could be explained solely in terms of the interfacial debonding energy. This is investigated by estimating the magnitudes of all of the identifiable sources of energy absorption, including that of plastic deformation of the projectile. It is concluded that strong inter-layer bonding can promote greater energy absorption in the composite back-plate.