Dynamic mechanical properties of fabric armour

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