Improving Ballistic Fiber Strength: Insights from Experiment and Simulation

Abstract

The ability to understand and predict failure mechanisms of ballistic fibers through molecular modeling will aid the development of improved next generation fiber material for soft armor systems. By hybridizing tensile test data performed over a range of strain rates with predictions from molecular simulation, the molecular failure mechanisms of neat and carbon-infused para-aramid polymer fibrils (i.e. Kevlar ®) are elucidated and analyzed. To begin, electronic structure calculations are used to generate intermolecular potential functions for polymer monomers and carbon fragments. The carbon/polymer interactions are found to be stronger than the polymer/polymer interactions, indicating that carbon fragments can enhance the internal binding energy of a polymer fibril. Next, using molecular dynamics simulation, the effects of this enhanced interaction energy on the elastic modulus of polymer fibrils is predicted. The predicted elastic modulus of the neat polymer fibril modulus is order-of-magnitude consistent with that measured for fibers during tensile testing. Within the context of the molecular dynamics simulations, carbon/polymer composite fibrils exhibit higher failure stresses than the neat samples. By systematically varying the carbon concentration within the simulations, an optimal carbon concentration that maximizes fibril strength is identified. The competing mechanisms responsible for this optimization process are identified, and a suggested carbon treatment process is proposed. These insights from models can enable exploitation of mechanisms governing each length scale by guiding design and fabrication of an improved ballistic soft armor.