Effect of adhesive on ballistic performance of multi-layered steel

•The effect of the number of layers on ballistic performance in glued and unglued multi-layered steels.•Variation of the deformation mechanism with the number and order of layers in armors.•The effect of the adhesive usage at the interface on the ballistic resistance.•Modeling of the adhesive interface with the finite element method. This study investigated the ballistic performances of layered structures for two experimental groups. Group A consists of monolithic, three, and nine-layered of AISI 4340 steel, tempered to 48-50 Rc hardness. In examining the multi-layer effect, group B specimens of AISI 1075 steel, tempered to 47-49 Rc hardness, with 3, 20, and 40-layered were considered. The effect of the adhesive usage at the interface on the ballistic performance was investigated experimentally and numerically in group A. The experiments were carried out using 7.62 mm x 51 (M61) AP bullet at the velocity range of 415 m/s and 844 m/s. Numerical models were developed to estimate ballistic limit velocities and deformations in the specimens in the Ls-Dyna solver by employing the Johnson–Cook constitutive relation and fracture criterion. The experimental results have revealed that the ballistic strengths of monolithic structures (in group A) are superior to those of three and nine-layered combinations, and these specimens are damaged by partial ductile hole formation. Dishing failure played a dominant role as a result of bending in layered structures, and petalling formation was observed on the rear surfaces. It has been found that the application of adhesive in layered specimens relatively improves the ballistic performance by delaying the formation of deformation, especially in the first layers. From the analysis results, it was concluded that the deformation behaviour on the rear surfaces of the specimens could be successfully predicted by the finite element method, and the presence of adhesive bonding could be modelled using the tie-break contact mechanism. As for group B specimens, the ballistic limit velocity decreased with the number of layers from the studies carried out with multi-layered (20 and 40 layers)