Dynamic mechanical properties of fabric armour

Twaron(R), a fabric made from aramid fibres and somewhat similar to the commonly known Kevlar(R), is also often used in flexible armour applications and hence subjected to high rates of loading. The dynamic mechanical properties of Twaron(R) fabric are examined via high-speed tensile tests on specimens using a split Hopkinson bar. The load-deformation and failure characteristics at different rates of stretching are determined, from which constitutive equations representing its viscoelasticity and strain-rate dependence are formulated. This facilitates modelling of the material response to impact and perforation. Experimental results indicate that Twaron(R) is highly strain-rate dependent; the tensile strength and modulus increase with strain rate while the failure strain decreases. Twaron(R) specimens are also observed to fail in a more brittle fashion as the strain rate increases; this phenomenon significantly reduces the amount of energy absorbed at high strain rates. An analysis based on the morphology and fracture mechanisms of poly(p-phenylene-terephthalamide) (PPTA) fibres, the main constituent of Twaron(R), is formulated to account for the experimental observations. The proposed constitutive equation, based on a three-element linear viscoelastic model is able to describe reasonably accurately the experimental stress-strain response over a range of strain rates. (C) 2000 Elsevier Science Ltd. All rights reserved.