Optimizing the Stacking Sequence in Dual-Purpose Body Armors

Many police body armor systems are dual purpose, offering both ballistic and knife resistance by combining a flexible ballistic textile pack with a stiffer knife resistant layer. The two types of protection differ in materials and mechanisms such that each individual component may help or interfere with the function of the other. This paper investigates the effect on knife and ballistic penetration resistance when a single thin metal plate was placed at various different positions within an aramid textile armor pack. Two metallic layers were used: aluminum 7075 and commercial purity titanium; these had similar areal densities and were positioned in the front, middle, and back of a 20 layer pack of woven Kevlar^® 49. An instrumented drop weight machine was used to deliver a repeatable knife blade impact at comparable energy levels to those specified in the UK Home Office test standards for knife resistance. Ballistic tests were used to determine the V₅₀ ballistic limit velocity against typical 9 mm and 0.357 Magnum handgun threats. Against a stabbing threat, it was found that positioning the metal plate in the middle of the pack provided the greatest resistance to knife penetration by a factor of almost two, while a plate at the front of the pack provided less resistance and plates positioned at the rear of the pack provided the least resistance to penetration. Against the ballistic threat, the penetration resistance of the textile pack can be significantly improved when a metal plate is at the front of the pack, while for all other positions the effect is negligible. However, this effect is sensitive to both the ammunition type and the metal plate composition. When the metal plate is positioned at the rear of the pack there is a significant decrease in the back-face deformation of the armor pack although, again, this effect is only present for certain ammunition and metal combinations. The overall effect of combining soft and hard elements was that specific performance parameters could be substantially increased by the correct combination. There were no significant negative effects, however, in a number of cases, the combined systems performance was no greater than that of a single element type, despite the added weight.