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.

Theoretical strength of glass fiber is much higher than the experimental values. It is believed that nano-meter size surface cracks, which are developed on the fiber surface during manufacturing and later on handling, deteriorate fiber strength. In the paper, effects of surface crack on the mechanical properties (modulus and strength) of glass fiber are studied through reactive all-atom molecular dynamics (MD) simulations. Surface cracks of different length are created by deleting atoms. Two types of reactive force fields – ReaxFF and Tersoff, are considered to assess their accuracy and computational expense. Simulation results indicate that surface cracks have no effect on glass fiber modulus, however, fiber strength is significantly affected by surface crack. Presence of surface cracks reduces fiber strength. With the increase in crack length, strength decreases and MD derived strength-crack length response is in good agreement with theoretical prediction.