Normal projectile penetration and perforation of layered targets

The response to normal impact of hard-steel blunt and conically-nosed projectiles on multi-layered plates of soft aluminum, both adjacent and spaced, as well as on adjacent disks of thin aluminum and polycarbonate, including sandwich arrangements, was studied experimentally in the subordnance velocity regime in the vicinity of the ballistic limit. A comparison of the penetration resistance of these targets with corresponding monolithic plates of aluminum or polycarbonate has been effected on the basis of aereal density. The ballistic resistance of the metallic monolithic target was found to be greater than that of several adjoining plates of the same thickness; this was considered to be due to the greater bending resistance of the former. On the other hand, a 15% greater ballistic resistance of the polycarbonate target was observed than for the corresponding aluminum disk. In several cases where an adjacent aluminum layer preceded the polymer, a terminal deflection of the aluminum plate in the direction opposite to that of the striker travel was observed, which was found to be due to the change in the penetration mode of the metal from petalling to extrusion. Elastic energy stored in the polycarbonate was returned to the metal cover, causing not only this rearward deflection, but also vibrations. When this occured at the ballistic limit, the period of these oscillations was found to correspond closely to that of a plate with a distributed mass with the projectile mass concentrated at its center. Wave propagation effects were not considered in the analysis, but also contribute to this reversed deflection. The experimental ballistic limits for monolithic targets of the two materials were compared to predictions of other investigators for both the blunt and conically-tipped strikers, derived from simple models of the process. The expected ballistic limit for combined targets was calculated from an energy approach with the aid of relations for this parameter based on the above models for a single material. Satisfactory correspondence was obtained between the test results and calculated values.