Arquitectura CAMEL

The tissues of the human body (e.g. skin, skeleton) provide protection to critical organs (e.g. heart, lungs). However, this natural protection is relatively easily overmatched resulting in a penetrating injury that could cause serious injury and/or death. Various types of personal armour exist to help increase protection to the body. Examples include body armour (fragment and bullet resistant torso covering vests / waistcoats), helmets (fragmentation and bullet resistant), face and eye protection (visors, glasses and goggles) and Explosive Ordnance disposal (EOD) suits (UK Ministry of Defence, 2005; Sakaguchi et al., 2012). The coverage provided must not, however, hinder the wearer in such a way that they are not able to carry out their prescribed duties while wearing their body armour. Actions such as sitting, standing and changing between various body positions are made all the more difficult with increased coverage and the extra mass that comes with increased body armour (Brayley, 2011). Other issues that decrease the ergonomics of body armour include unbalanced weight distribution, chafing and overheating (Watson et al., 2010; Carr and Lewis, 2014). For this reason, body armour worn is usually a compromise between the level of protection and extent of body coverage against ease of carrying out routine actions (Brayley, 2011).

There are two mains types of body armour that are used to defeat ballistic threats: soft fabric body armour and hard plate body armour (Chen and Chaudhry, 2005; Tobin and Iremonger, 2006; Brayley, 2011; Prat et al., 2012). Soft panels containing many layers of fabric provide the basis for the majority of all body armour, with hard plates added to increase protection of specific areas (Prat et al., 2012). Hard plate body armour is inflexible; constructed of laminated materials such as ceramics and composites combined with ballistic fabrics (Brayley, 2011; Pinto et al., 2012). Further discussion of hard plates is outside the scope of this research.

2.4.2.1 Soft body armour

Soft body armour is made from multiple layers of lightweight manmade polymeric fibrous materials (Chen and Chaudhry, 2005). These layers of fabric (often referred to as the ballistic panel or panel) are inserted into a ‘carrier’ manufactured from a polyester/cotton or nylon woven fabric to form a soft body armour pack (Pinto et al., 2012). The panel may be encased in a UV- and light-, water-resistant fabric before being

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inserted in the carrier. Soft body armour offers protection from a number of threats; typically armour worn by military personnel is designed to offer protection from fragments, while police are protected from the threat of sharp weapon (knife, spike) and hand-gun attacks (Tobin and Iremonger, 2006). The material(s) and the number of layers present in a pack will alter greatly depending on the protection that is required (Hewins, 2010).

Fibres used in soft armour systems include para-aramids (Kevlar®, DuPont and Twaron®, TeijinAramid), ultra-high molecular weight polyethylene (UHMWPE) (Dyneema®, DSM and Spectra®, Honeywell) and polyamides (Nylon®) (Chen and Chaudhry, 2005; Tobin and Iremonger, 2006). The fibres used can be in woven or nonwoven form (Prat et al., 2012). A woven fabric consists of two sets of yarns interlaced at 90 ° to each other; weft yarns (horizontal yarns) are passed over and under warp yarns (lengthwise yarns) at points called crossovers. Non-woven fabrics are constructed from individual layers of parallel unidirectional fibres, with alternate layers positioned and adhered 90 ° to one another (Tobin and Iremonger, 2006).

2.4.2.1.1 Para-aramids

The term ‘aramid’ is short for ‘aromatic polyamide’. Aramid fibres are man-made synthetic-polymer high performance fibres which contain molecules that are characterised by relatively rigid polymer chains (Teijin Aramid, 2012). When these chains are orientated in a linear direction, strength is increased due to optimising the alignment of the chemical bonds.

Aramid fibres were discovered in 1965 when S. L. Kwolek synthesised a series of para-oriented aromatic polyamides. This led to DuPont de Nemours recognising that a rigid molecular chain and a fibre of ultra-high modulus could be made from a para– oriented symmetrical polymer; this fibre, originally named Fibre B, became known as Kevlar® (Yang, 1993) (Figure 2-10). The result is a combination of properties that include high tensile strength (>3 GPa), high modulus (>60 GPa) and medium strain to failure, hence high toughness (Chang, 2001; Chen and Chaudhry, 2005). With several grades of aramid fibres available, they have many applications e.g. tyres, ropes, space vehicles, boats and body armour (Magat, 1980; Tobin and Iremonger, 2006). Para- aramids were first used to make body armour for the police and military in the early

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1970s. They are now the most common material used in soft body armours (Chen and Chaudhry, 2005). However, increasing competition from other fibre materials such as UHMWPE is challenging this (Tobin and Iremonger, 2006).

Figure 2-10: Molecular arrangement of a para-aramid

2.4.2.1.2 UHMWPE

UHMWPE contains orientated high-molecular weight molecules of polyethylene; its molecular formula is -(C2H4)n- (Figure 2-11), where n is the degree of polymerization; this can be as high as 200,000 (Kurtz, 2009). UHMWPE gives high material stiffness (>3 GPa) and high strength (>40 MPa) (Tobin and Iremonger, 2006). Dyneema® (DSM) has been in production since 1990, and is manufactured by a process of gel-spinning; very long molecules are first dissolved in a volatile solvent and then spun through a spinneret. In the solution, the molecules are disentangled, and remain so after cooling (van Dingenen, 1989). Spectra® fibre, by Honeywell, is another UHMWPE fibre that is produced using a gel-spinning process (Honeywell, 2012). UHMWPE fibres are used for medical sutures, ropes, sailcloth, fishing lines and nets, slings, cut-resistant gloves and apparel, as well as police and military personal and vehicle ballistic protection (DSM, 2012; Honeywell, 2012).

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2.4.2.1.3 Polyamides

Nylon® fibres are a type of polyamide. Developed in the 1930s by a team of scientists working for DuPont, Nylon® is the product of 2 monomers (DuPont, 2012) (Figure 2-12). Utilised up until 1972, Nylon® fibres were originally used to contain metal plates for body armour, before being used as ballistic protection in its own right. With a high degree of crystallinity and low elongation, all-nylon armoured vests started being produced by the end of the Second World War, although they were first used in service in the Korean War. Superior protection is offered by para-aramid and UHMWPE solutions. However, 'Ballistic' Nylon® (Nylon 6,6) is still used today in some armours and helmets as well as in other protective equipment such as motorbike apparel. This is due to its low cost compared to para-aramids and UHMWPEs (Chen and Chaudhry, 2005; Tobin and Iremonger, 2006).

Figure 2-12: Molecular arrangement of Nylon 6,6