Abstract
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.
Highlights
High performance polymeric fibers such as Kevlar® and ultra-high molecular weight polyethylene (UHMWPE) are used in ballistic impact applications [1] due to their superior specific tensile stiffness and strength
In Kevlar®, fibril diameter is in the range of 10 to 50 nm for Kevlar® KM2 [2] and the fibrils are connected through a network of hydrogen bonds and van der Waals and coulombic type non-bonded interactions
Nano-fibrils consist of stacks crystallites sizes exists in the fibers, with macro-fibrils consisting of bundles of micro-fibrils, chains interact withare non-bonded van der Waals interactions
Summary
High performance polymeric fibers such as Kevlar® and ultra-high molecular weight polyethylene (UHMWPE) are used in ballistic impact applications [1] due to their superior specific tensile stiffness and strength. Nano-fibrils consist of stacks crystallites sizes exists in the UHMWPE fibers, with macro-fibrils consisting of bundles of micro-fibrils, chains interact withare non-bonded van der Waals (vdW) interactions. Rutledge et al [20] have determined the elastic properties of Kevlar crystal and TC deformation modes on the fiber tensile failure strength has not been studied extensively. In another study strength properties of Kevlar® crystal using reactive force field ReaxFF [27]. Studied the effects of chain ends on the elastic and strength properties of PE crystal using a modified. They reported that the tensile properties were unchanged under small amounts of pre-compression version of reactive force field AIREBO (Adaptive Intermolecular Reactive Empirical Bond Order). Observations and TC on the residual tensile strength of Kevlar® KM2 fibers is summarized
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