Knife armour

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 

7 Areal density of UK Home Office approved knife and ballistic-resistant armour systems. HG1/KR42 tested according to the 1993 standard (Parker, 1993), KR1/HG1 tested according to the 1999 standard (Petit and Croft, 1999), and KR1 HG1 SP1 tested to the 2007 standard (Croft and Longhurst, 2007b). 

8 Stages in the defeat of a projectile by a ceramic-faced armour system. 

The absorber materials are in most cases very tough polymer composites. These may consist of aramid fibres in a tough matrix such as heavily plasticised and toughened polymer resin or neoprene. Polyethylene fibre composites are also widely used, although typically these consist of pressure-consoUdated laminates rather than using resin infiltration. Most systems have a covering of fibre composite or fabric to provide impact resistance and to enhance the damage tolerance of the 

9 The effect of projectile type on the areal density and type of ceramic required to provide protection (adapted from Ogorkiewicz, 1996). 

---

Crimes involving knives are increasing in the UK, with the knife being the most commonly used weapon, at 32%. The need for better slash resistant materials is therefore increasing, as the body armours currently worn by security and protective services personnel do not protect the arms, shoulders, neck, and face, as they are very rigid and too heavy to be worn comfortably over long periods of time. 

Bleetman A, Watson CH, Horsfall I, Champion SM. Wounding patterns and human performance in knife attacks optimising the protection provided by knife-resistant body armour. J Clin Forensic Med 2003 10(4) 243-8. 

Examples of stab resistant body armour are often quite different in constractional character. For example, some comprise a matrix of overlapping metal or composite plates located between fabric layers (based on para-aramid or UHMW polyethylene), others are based on more flexible aramid woven textiles which have been coated with silicon carbide particles to blunt the knife point and others incorporate fine tungsten wire within a knitted fabric matrix. Copying the ancient chain mail concept, similar fine mail constructed from stainless steel or titanium wire may be included as a layer. Obviously the overall weight and thickness of the resulting armour is of crucial importance for the comfort of the wearer but this will be determined largely by the magnitude of the threat. 

Rigid armour may defeat a knife by simply resisting perforation or by causing the blade tip to buckle (Fig. 1.6(d)). Thin, sharp blades such as those found on domestic knives (Fig. 1.5(c)) will have a tendency to break or buckle. Heavily constracted blades such as those used for outdoor survival purposes (Fig. 1.5(b)) will be much less likely to buckle, but their large cross-sections will cause large perforations to be produced in the armour. The different behaviour of thin, sharp blades versus large, heavy blades led to the adoption of both types in early armour test standards (Fig. 1.5(ij)) (Parker, 1993). 

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. 

Croft, J. and Longhurst, D., 2007b. HOSDB Body Armour Standards for UK Police (2007) Part 3 Knife and Spike Resistance, St Albans Home Office Scientific Development Branch. 

Armours designed to protect against knife attack usually have a different structure to those designed to protect against bullets or fragments. In general, they consist of a steel mesh (or chain mail) backed by aramid. 

---