Dynamic mechanical properties of fabric armour

Twaron®, a fabric made from aramid fibres and somewhat similar to the commonly known Kevlar®, is also often used in flexible armour applications and hence subjected to high rates of loading. The dynamic mechanical properties of Twaron® 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® is highly strain-rate dependent; the tensile strength and modulus increase with strain rate while the failure strain decreases. Twaron® 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®, 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.