Predicting Body Armor Back Face Deformation (BFD)

Understanding how personal protection systems deform during an impact event is paramount to their proper use in mitigating injury to the user. Most Kevlar fabric characterization relies on testing fabric suspended in the air; however, in use, these materials may respond differently due to the resistance from the backing material. The current NIJ standard relies on a clay backing material, and measuring the deformation after impact. This does not provide any time of flight information about the deformation response of these materials. In order to characterize the response of the multiple layer Kevlar samples, experimental testing was performed using clear backing materials that simulate human flesh. A clear backing material made of ballistics gelatin was used, allowing images to be obtained of the back of the Kevlar sample during the impact event. From these images, the maximum depth, as well as width and shape profile of the deformation can be tracked as a function of time. In this work a spherical projectile was fired into the Kevlar samples using a gas gun to determine the deformation vs time history. The parameters of the impact velocity, number of layers of Kevlar, and areal density of Kevlar were examined to determine the response on the deformation vs time history. Two types of Kevlar fabrics are used in various multiple layer configurations tested at a range of projectile velocities. It was found that the multi-layer areal density of a sample and the number of layers affect the deformation response significantly and independently. In addition, it was found that shapes of the back face deformations become more rounded as more fabric layers are added.