Ballistic limit predictions of non-identical layered targets perforated in ductile hole formation

The ballistic resistance of metallic targets perforated in ductile hole formation can be accurately calculated for monolithic targets, multi-layered targets with air-gaps between plates, and multi-layered targets of identical, in-contact layers via utilisation of the specific cavitation energy concept. Here we present an extension to those models for application to targets with two in-contact layers of varying material type and thickness. We hypothesise that the perforation of the second (bottom,rear,back) layer is not affected by the resistance of the first (upper,top,front) layer, which has already been perforated. Rather, the second layer acts as an independent monolithic target. Using the ballistic equivalence concept, which has been validated by a comprehensive numerical study with abrasion resistant steel targets (Hardox 400), high-strength aluminium alloy targets (AA6061-T651) and annealed aluminium alloy targets (AA6070-O) perforated by rigid projectiles, we develop a heuristic model in which the second layer is replaced by a ballistically-equivalent layer of material identical to that of the first layer. Utilising finite element simulations, we demonstrate the accuracy of the suggested model for double-layered targets consisting of a Hardox 400 steel layer and a AA6070-O aluminium layer, perforated by a hardened tool steel projectile and find it to be high enough for practical use over a wide range of target configurations.