Ballistic fabrics

Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. They have a Young s modulus within the range 270-950GPa, a tensile strength of 11-63 GPa,and a density of 1.3-1.4g/cm Their outstanding mechanical properties originate from the covalent bond between the adjacent carbon atoms, which enables this material to be used for the fabrication of the next generation of body armour. 

Zhou et modified the plain woven fabrics by manufacturing honeycomb fabrics and by introducing leno structure and double weft insertions at given intervals. This serves to inaease inter-yam friction, via improved yam gripping, between the warp and weft yams. The stmctures are shown in Figs 6.5 and 6.6. It has been found that such structurally modified fabrics led to better ballistic protection than normal plain woven fabrics. 

The unidirectional (UD) construction was first used in soft body armour for the chest and helmet by air crewmen in the Second World War and was reintroduced by AlliedSignal with the UHMWPE fibre Spectra in the mid-1980s. The unidirectional technology is based on the idea of combining the cross-plied filaments with an elastomeric matrix in a laminated systan. Fig. 6.7 shows a four-plied unidirectional system laminated by two films. Other companies, such as DSM and Park Technologies, also provide similar fabrics for personnel protection. 

The superior performance of UD structure over woven structure has been described in many publications. According to Scott, around 30% improvement in either weight or performance has been observed in both flexible armour and hard composites. Lee et al. ° found that the ballistic limit for angle-phed laminates gives a higher value than the woven fabric-based composites, as the density of the panels increases. It has also been established that a 100% UD fabric panel absorbs 12.5-16.5% more energy than a 100% woven fabric panel.  

When a composite panel is impacted, the projectile tends to exhibit through-the-thickness shear failure on layers near the impact face, forming a plug, while for back layers, the fibre damage mode resembles the tensile failure, as shown in Fig. 6.8. Similar phenomena were also found with dry fabric panels by Chen et al. The fact 

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M.W. Olson G.H- Brice, Bicomponent and Biconstituent Fibers in Ballistic Fabric for Personnel Armor , Rept No USA-NLabs, C/-PLSEL-TR-71-48CE, TS-173, Contract DAA17-70-C-0032, Uniroyal Inc, Wayne (1971)... 

Chitrangad MV. Hybrid ballistic fabric. Wilmington, DE E. I. du Pont de Nemours and Company 1993. 

Nader J, Dagher H. 3D hybrid ballistic fabric testing using a 3D digital image. Exp Tech 2011 35 55-60. 

Chain mail can also be used as a flexible knife-resistant armour. In this case the action of the chain links is to capture the blade tip after a small amount of perforation. It is therefore necessary to use either a very fine chain link diameter or a padding system. Typically the armour systems will consist of a layer of chain mail positioned on the front face of a conventional multilayer ballistic fabric pack. This construction has very good flexibility and provides both knife and ballistic protection at a weight comparable to, although slightly greater than, a laminated fabric solution. It is, however, typically mote flexible than the laminated fabric and has foimd particular use in female armour, which needs this greater flexibility to allow for breast shape. 

For bullets having a KED of above 30 Jmm", ballistic fabric systems become overly bulky. The contact loads tend to be sufficient to cause shear failure of the fibres and capture of the projectile occurs only after numerous layers have been perforated. For a KED of 30 5 Jttmr it is possible to provide protection by use of polymer matrix composites. In essence this is an extension of the tactic used in knife armour, as the projectile is forced to break fibres and radially expand the perforation against significant constraint. It has been shown that at low KED... 

Firing test number 2 degradation of the ballistic fabric. 

Finken, W.S., Robinson, H.K., 1959. Unitary ballistic fabric. US Patent 2,899,987 August 18, 1959 to Centex Corporation, New York, NY. 

Semi-flexible thermotropic LC polyesters and poly(ester-amide)s are used as matrix resins for aramid-based (Twaron ) ballistic fabrics. The ballistic performance of the most successful liquid crystalline polyester system obtained for 20 wt% LC polyester (LCPE)-Twaron ... 

Figure 11.8. Neural network model for Kevlar ballistic fabric [Source Reference 85].

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