Ballistic impact induces multiaxial loading on Kevlar^® and polyethylene fibers used in protective armor systems. The influence of multiaxial loading on fiber failure is not well understood. Experiments show reduction in the tensile strength of these fibers after axial and transverse compression. In this paper, we use molecular dynamics (MD) simulations to explain and develop a fundamental understanding of this experimental observation since the property reduction mechanism evolves from the atomistic level. An all-atom MD method is used where bonded and non-bonded atomic interactions are described through a state-of-the-art reactive force field. Monotonic tension simulations in three principal directions of the models are conducted to determine the anisotropic elastic and strength properties. Then the models are subjected to multi-axial loads—axial compression, followed by axial tension and transverse compression, followed by axial tension. MD simulation results indicate that pre-compression distorts the crystal structure, inducing preloading of the covalent bonds and resulting in lower tensile properties.

This article investigates the failure of ultra-high molecular weight polyethylene Dyneema® SK76 single fibers widely used in protective armor applications. Indenter geometry and the associated stress concentration are known to have a significant effect on the average axial tensile failure strain of Dyneema® SK76 fibers subjected to quasi-static transverse loading experiments by three indenter geometries. In this study, a 3D finite element model is developed to predict the degree of multiaxial loading at the location of fiber failure in these experiments. A failure criterion based on maximum axial tensile strain considering the statistical strength distribution and multiaxial loading (transverse compression and transverse shear) induced degradation effects is applied to predict the fiber failure. The influence of transverse compression on the tensile strength of single fibers is experimentally determined. The average failure strains predicted by the model are found to agree well with the experimental results indicating fiber failure may be initiated based on a gage length dependent maximum axial tensile strain in the fiber.