Reinforced Plastics Volume 61, Number 2 March/April 2017 www.reinforcedplastics.com FEATURE Ballistic composites – protecting the protectors George Marsh It seems scarcely credible that thin polymer fibers, bound together in resin, can stop projectiles ranging from a hand-gun bullet to a high-power rifle round, but they can and have done so, saving many lives in the process. Composite body armor protects a wide range of civilians from security guards to police officers and from bailiffs to VIPs. But of course, it is military forces who are the leading user group. degraded. Second is a slowing phase and third is the catching of Traditional solutions for 20th Century military armor, based the round so that it is retained within the protective garment. chiefly on steel and ceramic plates, were really too heavy for Laminates are designed to maximize the effectiveness of these soldiers, indeed for many vehicles as well. Composites have in- stages. Outer layers that provide controlled delamination on creasingly proved to be the answer, being much lighter for the impact are effective in deforming the tip of a projectile, thereby same stopping power and more pliable. Certain polymer compo- blunting it. Certain non-polymer composites, for instance those sites show, when appropriately engineered, remarkable energy that are ceramic based, may be harder than polymer composites dissipation properties, being able to absorb the kinetic energy and therefore more effective in this phase. Even so, reinforced from bullets and other high-speed projectiles before these can plastics often provide the best balance of weight, anti-ballistic harm their human targets. They can also protect against knives. performance and cost. Various mechanisms account for this. Pushing fibers aside Underlying composite layers subsequently absorb kinetic ener- against their stiffness and the hold exerted by the composite they gy progressively as more and more fibers are displaced and are part of, absorbs energy and the greater the number of fibers encountered, the stronger is the effect. Still more energy is absorbed as fibers become stretched during contact with projec- tiles, elongation-before-break being an important variable for armor designers. A third mechanism is that of delamination, whereby energy is absorbed in parting fibers from their resin containment medium. Yet another mechanism occurs in woven fabrics where the woven intersections slow down the shock waves propagated along the fibers from the impact point, absorbing energy as they do so. This does, however, increase the strain within the material and when this exceeds what the material can tolerate, penetration can occur. Hence whether to use wovens or non-wovens is a matter for careful consideration by protection designers (Figs. 1–3). According to ballistic specialists with Swiss-headquartered com- posites firm Gurit AG, stopping a bullet has three distinct stages. FIG. 1 First is the blunting of the projectile so that its penetrating power is Flack jackets based on Kevlar or other proprietary aramid can be worn under normal clothing by both men and women. Image licensed by E-mail address: [email protected]. Shutterstock. 0034-3617/ß 2015 Elsevier Ltd. All rights reserved. 96 http://dx.doi.org/10.1016/j.repl.2015.10.003 Reinforced Plastics Volume 61, Number 2 March/April 2017 FEATURE pockets incorporated in the garment. Such inserts are typical of protection that is rifle ballistic rated since fabrics alone cannot normally stop a rifle round traveling at 800 m/s whereas they can prevent penetration by a hand gun bullet traveling at half that speed. Although body armor designed to defeat rifle fire is inevi- tably more rigid than fabric-only garments, keeping the armor inserts separated ensures an adequate level of garment flexibility for most purposes. Nylon start An early example of the transition from metals to composites for personnel protection was the British Army’s GS Mk 6 combat helmet, issued during the 1960s. This had an outer shell of ballistic nylon impregnated with a 50:50 mix of phenoformaldehyde and FIG. 2 polyvinyl butyral (PVB) resins, this matrix accounting for 20% of FEATURE Kevlar helmet with camouflage cover and protective goggles. Image the composite’s weight. The shell comprised 23 layers of plain- licensed by Shutterstock. 2 weave 290 g/m nylon, weighing 1 kg for the medium size of helmet. An inner impact absorbing layer of high-density polyeth- stretched. Composites used in body armor, where structural ylene (HDPE) foam brought this weight up to 1.3 kg. Fiberglass, strength is less of a requirement than in vehicle protection, and even combined nylon and fiberglass, were also tried during generally have very high fiber content, up to 80% or more, and those early years, providing some benefits over nylon. specialized polymer fibers are used. Even in armor that has a During the 1970s, the US Army was to adopt para-aramid fibers ceramic outer face, this is generally backed by composite laminate for head protection, albeit in resins similar to those used by the to meet the slowing and catching requirements, including of British. PVB-based resins were readily Beta-staged into a prepreg penetrative fragments that are expelled from improvised explosive that could then be hot-molded to the tight complex curvatures devices (IEDs) and fragmentation rounds (such fragments are required for combat helmets. Aramids, the best known of which known as spall). are DuPont’s Kevlar and Twaron from Teijin (the Netherlands and IEDs deliver blast, fragments and fire so that combined protec- Japan), subsequently spread from helmets into body protection. tion against these is desirable. Materials that mitigate these effects The Kevlar ‘flak jacket’, for instance, became almost a generic, as well as being environmentally tolerant – to moisture, heat, etc. – initially for ballistic vests that protect against fragments from are in demand. Another required attribute is that they should be shells, grenades and other munitions, but also later for garments able to resist not just single hits but also repeated shots, as from a that are substantially bullet-proof. machine gun. Para-aramid fibers have a rigid rod-like molecular structure that Body armor is typically based on woven and non-woven fabrics provides high-tensile strength, high elongation-to-break and good that are flexible enough to provide wearers with freedom of damage tolerance. They are also inherently non-flammable. Today movement while still possessing high anti-ballistic properties. they tend to be embedded in more contemporary matrices, typi- Many garments incorporate extra protection in strategic areas cally epoxy or phenolic. The resulting composites are heavily fiber such as over the heart; this can take the form of plates of compos- dominated. Like other specialists in ballistic fibers, DuPont Pro- ite, ceramic, metal or hybrid material inserted into containment tection Technologies has worked hard to optimize its base material FIG. 3 From police officers to soldiers, Dyneema Force Multiplier Technology is protecting personnel around the world. 97 FEATURE Reinforced Plastics Volume 61, Number 2 March/April 2017 for ballistic use. Refined molecular structures in products like Dyneema is a gel-spun multiple-filament UHDPE fiber that Kevlar XP and Kevlar M5 combine high strength with superior combines high strength, high modulus in the fiber direction thermal and flame resistance. According to global business director and resistance to most chemicals. Gel spinning involves melting Jeroen B. Jacobs, Kevlar is, weight for weight, five times stronger the PE polymer but also dissolving it in a solvent so that molecular than steel and, in its various forms optimized for different envir- chains become realigned, the resulting high alignment enhancing onments and user groups, has saved thousands of lives. fiber strength. The solvent is then extracted. Low diameter fibers Despite the dominance of Kevlar and other aramids, glass still are produced in a range of strengths and densities to suit different survives as an armor fiber. There is a place for the higher-perfor- ballistic applications. mance fibers, such as S-glass used in preference to the more Another polymer fiber, polyphenylene-benzobisoxiazole (PBO) general E-glass. According to Gurit, this raises ballistic tolerance that emerged in the 1990s, notably under the trade name Zylon by 10–20%. Although this company’s recently launched PF700 (Toyoba, Japan), has 1.6 times the tensile strength of aramid, but ballistic prepreg is mainly intended as a vehicle armor rather than has a high cost. An all-PBO ballistic vest would probably be very FEATURE for personnel protection applications, it is worthy of mention here expensive but the fact that such a garment could be made much because it exemplifies once again the sheer versatility of glass fiber thinner for equivalent performance could be a considerable draw if and its unsuspected utility in meeting tough anti-ballistic require- costs could be reduced. A US company started using PBO in ments. PF700 uses the fibers in combination with phenolic resin, protection for police officers in 1998, but five years later withdrew the latter being preferred over epoxy due to its flame resistance, the product after an officer died despite having been wearing a PBO even though epoxy may be stronger. vest when shot. Degradation in properties over time proved to be Carbon and boron have been experimented with but the brittle the issue. nature of these fibers has restricted their use in anti-ballistic applications. Ballistic optimization There are so many variables ballistic garment designers must Later fibers juggle – material amount, fiber volume fraction, fiber type and Latterly there has been penetration of the market by high perform- diameter, woven/non-woven, fabric geometry, fiber sizing, ply ing fibers based on ultra high-density polyethylene (UHDPE). count, fiber/resin bond strength, resin type and process parame- These include Dyneema from DSM Dyneema, part of Dutch con- ters, and so on – that optimization can be a considerable challenge, cern Royal DSM, and Spectra from Honeywell in the United States.