MODELING DEFORMATION AND DAMAGE CHARACTERISTICS OF WOVEN FABRIC UNDER SMALL PROJECTILE IMPACT

Fabrics comprising highly oriented polymers possess high impact resistance and are often used in flexible armour applications. As these materials are viscoelastic, accurate modelling of their impact and perforation response requires formulation of constitutive equations representing such behaviour. This study incorporates viscoelasticity into the formulation of a model to analyse the impact of small spherical projectiles on plain-woven PPTA poly(p-phenylene-terephthalamide) fabric. The fabric is idealized as a network of viscoelastic fibre elements and a three-element viscoelastic constitutive model is used to represent polymer behaviour. Viscoelastic parameters are used to reflect intermolecular and intramolecular bond strengths as well as the static mechanical properties of fibres. Results of the theoretical analysis were compared with data from experimental tests on fabric specimens subjected to projectile impact ranging from 140 m/s to 420 m/s. Predictions of the threshold perforation velocity and energy absorbed by the fabric showed good agreement with experimental data. The proposed analysis is able to model deformation development and rupture of the fabric at the impact point. Fraying and unravelling of yarns are also accounted for. The study shows that a knowledge of static mechanical properties alone is insufficient and results in gross underestimation of impact resistance. An important parameter identified is the crimping of yarns. Yarns in woven fabric are not initially straightened out and hence part of the stretching in fabric is due to the straightening of yarns. The effect of crimping was found to be significant for high impact velocities.