EXPERIMENTAL AND NUMERICAL STUDY OF THE RESPONSE OF FABRIC COMPOSITES SUBJECTED TO IMPACT LOADING

Abstract

Fabrics and flexible laminates comprising highly oriented polymers possess high impact resistacne and are often used in flexible armour applications. As these materials are viscoelastic, accurate modelling of their impact and perforation responses requires formulation of constitutive equations representing such behaviour. This study incorporates viscoelasticity in the formation of a model to analyse the impact of small spherical projectiles on two types of flexible materials - cross-woven PPT A poly(p-phenylene-terephthalamide) fabrics and [0°/90°] fibre-reinforced laminates of unidirectional resin-impregnated PE (polyethylene) fibres. These materials are idealised as networks of viscoelastic fibre elements and three-element viscoelastic constitutive models are used to represent fibre properties. Three-element viscoelastic models are simple yet sufficient to account for the effects of intermolecular bonds, intramolecular bonds and viscous slippage of molecular chains on the mechanical properties of oriented polymeric fibres. PPTA fabrics are idealised as a layer of pin-jointed fibre elements and parameters in the constitutive equations are assigned values based on the theoretical strengths of intermolecular and intramolecular bonds. Besides viscoelasticity, the influence of yam crimp (undulations in the yams due to weaving) is also incorporated into the model. Results of the theoretical analysis were compared with data from experimental tests on fabric specimens subjected to projectile impact ranging from 140m/s to 420m/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 yams 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, the effect of crimp was found to be significant for high impact velocities. Flexible laminates are modelled by two fibre network layers bonded to each other at the cross-over points of fibre elements. Bonding is represented by infinitesimal rigid links which break when the inter-ply bond strength is exceeded resulting in delamination between the plies. Values of the parameter in the constitutive equation for PE fibre elements are determined from experimental data on its dynamic mechanical properties. Predictions of residual velocity, energy absorbed by the laminate and delamination area correlates well with results of impact tests. The presence of stretch marks on specimens perforated at low impact velocities is also accounted for by the model. The analysis demonstrates that delamination is an important mechanism for energy dissipation.