New Interference Approach for Ballistic Impact into Stacked Flexible Composite Body Armor
In this study a new interference model is developed to investigate the ballistic response of a hybrid, two-layered, flexible body armor. Because the stacking order of layers, which have distinctly different mechanical properties, has a large effect on the V 50 limit velocity for penetration, we apply the analysis to a two-layer system consisting of fabrics of aramid (Kevlar29®) fiber and of ultrahigh molecular weight and drawn polyethylene (Dyneema@) fibers. Using right-circular cylindrical projectiles, previous experimental results of Cunniff ("An Analysis of the System Effects in Woven Fabrics Under Ballistic Impact," Textile Research Journal, Vol. 62,1992, pp. 495―509) [using a similar fiber, Spectra@ instead of Dyneema®] showed nearly a factor of 2 difference in the V 50 velocity in the two possible stacking orders, that is, Spectra@ versus Kevlar29® as the strike face. This new model extends our previous fundamental work ("A New Membrane Model for the Ballistic Impact Response and V50 Performance of Multiply Fibrous System," International Journal of Solids and Structures, Vol. 40, 2003, pp. 6723―6765) by addressing interference factors in terms of instantaneous material inflow to the impact cone using a convolution over current local strain around the projectile edge during its deceleration. Our previous multilayer model ("Modeling of System Effects in Ballistic Impact into Multi-Layered Fibrous Structures for Soft Body Armors," International Jourual of Fracture, Vol. 135, 2005, pp. 219―251) ignored the interference aspect though a subsequent work ("Effects of Layer Stacking Order on the V50 Velocity of Two-Layered Hybrid Armor System," Journal of Mechanics of Materials and Structures, Vol. 3, 2008, pp. 627-639) approximated its effects but without instantaneous response of material inflow to local tensile strain. The current model is much more complete and produces velocity, strain, and deformation histories either to perforation or to halting the projectile. In the new model, V 50 velocity differences with stacking order and with layer bonding versus no bonding are even larger. However, backface deflections turn out to be minimally affected. Calculations are very fast and postprocessing figures can be obtained in less than a minute using MATLAB with a laptop (2 MHz Core 2 Duo CPU 2 Gb RAM).