Last line of defence - protective combat clothing
Modern personal protective combat clothing has evolved through impact simulation, and improved material strength and behaviour. Professor Ian Horsfall and Dr Debra Carr at the Impact and Armour Group at Cranfield University, Shrivenham, UK, outline the developments and challenges.
Most people know Kevlar is used in body armour, but few will appreciate the full range of materials that are used in the manufacture and testing of military and police systems. Body armour typically contains multiple layers of woven fabric manufactured from para aramid fibre, of which Kevlar is the best-known brand. Within Europe, the fabric is as likely to be made from another para-aramid, such as Twaron, or ultra-high molecular weight polyethylene fibre, such as Dyneema. Despite the use of advanced fibre body armour is still under developed; the majority of systems are of simple construction and are created almost entirely using empirical tests. These fail to address the full complexity of the operational environment.
The hierarchical natures of body armour which encompasses the fibre, yarn, fabric and garment properties alongside the mechanisms within the fabric, such as high rate deformation, inter- and intra-yarn friction and inter- and intra-layer interactions, exceeds the capability of computer processing power.
At the sharp end
Despite these challenges, body armour has advanced over recent years, with progressively lighter and more durable systems being fielded, and additional features, such as stab resistance, being incorporated. Within the UK, the main threat to police officers is not guns, but knives.
Conventional bullet resistant fabric systems are designed to work against soft nosed easily deformed pistol bullets and offer only limited protection against sharp-edged knives. The knife threat has led to a range of systems, including polymer-stabilised fabrics, which mechanically stabilise the yarns, or stainless steel chainmail to provide supplementary penetration resistance. A more novel approach is to use plasma coating to create a highly abrasive protection layer, which enhances the fabric's ability to trap a knife blade.
The body armour designer has to also combine performance criteria with physiological and biomechanical requirements, such as comfort and trauma reduction. These factors broaden the research into areas of ergonomics, trauma biomechanics and biofidelic simulants.
Recent work within the Impact and Armour Group has included the development and validation of test systems that enable quantitative analysis of the blunt trauma in the abdominal region caused when armour is struck by a bullet. These simulant systems use silicone rubber panels of various geometries that mimic the behaviour of the body’s thoracic and abdominal portions.
Clay-based test blocks are traditionally used to provide a record of the deformation behind armour, but these have limited scientific background and their relevance is regularly brought into question. More advanced methods examine the deformation rate as a means to determine likely thoracic injuries.
Data from real incidents has shown that damage may be more likely over the softer portions of the body, such as the abdomen, and therefore simulation of this area has been attempted. Real-time measuring systems, it now appears, can discriminate between different injury types and may offer a more quantitative method to assess armour performance. This should lead to armour systems that provide protection better matched to user requirements, and rely on an improved understanding of the interaction between the threat, armour and the human body.
All in the head
Military combat helmets are primarily designed to provide protection from fragmentation, but also from non-ballistic impacts (see image left).
Fragmentation is recognised as being the major cause of injury in warfare originating from traditional munitions, such as artillery shells, or from improvised explosive devices. Typical fragment size, shape and velocity varies widely. Therefore, fragment protective body armour and helmets are tested using standard pieces, usually a chisel-nosed fragment simulating projectile.
A typical military helmet has a mass of around 1.4kg for a medium size. Such helmets are comprised of a composite shell manufactured using (typically) plain woven fabric. Para-aramids are most commonly used in modern helmets, but nylon 6,6 and ultra-high molecular weight polyethylene can be used.
Most countries now include a liner in their helmets. The main image at the top of this article shows a helmet, combat, general service MkVI, which is a UK tri-service helmet dating from the mid-1980s. The MkVI is no longer a frontline helmet, but is a good example of a typical modern military helmet. Inside the nylon 6,6/phenolic-PVB shell there is a closed cell high-density polyethylene foam. The liner and shell offer a high level of non-ballistic (or bump) protection. The helmet also includes height and size adjustment, and a retention system.
The helmet only provides protection if it is located securely on the wearer's head during normal operational duties. Helmet security is thus ensured by the retention system, which is usually manufactured from cotton webbing and is typically a 19mm wide, two ply, plain woven with binders. The tensile properties, at a range of test speeds, of the webbing and retention system are important.
Webbing tensile properties are usually measured at a test speed of 100mm/min using a conventional universal testing machine. The nominal force-atbreak for the webbing at this test speed is 1.5kN.
The Impact and Armour Group has developed a method to measure the webbing’s tensile properties at test speeds up to 16m/s. An Imatek accelerated drop-tower is used to drive a tuning-fork impactor onto the jig’s lower grip. The specimen (100mm gauge length, tabbed with aluminium) is mounted between a fixed upper grip and a floating lower grip of the high-speed jig, which is located under the impactor (see image below). Force is measured immediately above the fixed grip, and distance is measured via the movement of the machine carriage.
Statistical analysis of force-, strain- and energy at- break suggests that only the fastest test speeds (16 m/s – 20 m/s) affects the webbing’s mechanical properties and results in increased brittle behaviour. Ongoing research includes extending the work to consider the affect of degradation by ultraviolet radiation and perspiration on webbing tensile properties, and investigating the high-speed properties of webbing specimens that include the types of joins typically used in military chin-strap manufacture.
Dr Debra Carr, CEng FIMMM, Impact and Armour Group, Department of Engineering and Applied Science, Cranfield University, Defence Academy of the United Kingdom, Shrivenham, Wiltshire, SN6 8LA, UK. Tel: + 44 (0)1793 785 654 Email: firstname.lastname@example.org