DuPont™ Kevlar® fiber may be synonymous with cloth bullet proof vests, but this fiber wasn’t originally intended to be used as a conventional fabric. It was developed to replace the nylon belts reinforcing tires. As it turns out, Kevlar cloth has certain deficits that aren’t apparent when it’s embedded in another material, such as the rubber of a race car tire or the baked resin of a bullet resistant fiberglass panel.
Catching Bullets with Kevlar
Kevlar cloth is woven from synthetic para-aramid fiber (a sort of super-strong cousin to nylon). Para-aramid threads have remarkably high tensile strength: They can put up with an enormous amount of pulling before breaking, and stretch very little in the process. A cloth woven from these fibers stops a bullet much as the backstop at a baseball diamond deflects a foul ball. Obviously, any single strand of steel bailing wire is strong enough to stop a baseball, but a ball striking a single strand of wire will keep right on going (albeit at slightly altered velocity); you need the interlocking structure of the mesh backstop to keep wild pitches from smacking kids in the crowd. Similarly, a hank of Kevlar is strong enough to stop a bullet, but that bullet will just slip between the loose threads. To be effective, Kevlar must be woven into a tight cloth before being integrated into a vest or a fiberglass panel. The resin in the fiberglas panel stiffens the layers of Kevlar cloth–so that it can be used as a building material–making sure that those fibers provide a sturdy backstop for bullets. The resin also protects the Kevlar from exposure to the elements, chemicals, and especially water.
Waterlogged Kevlar Doesn’t Work Well
Kevlar cloth in a vest doesn’t have the benefit of being sealed in resin, and turns out to be especially susceptible to water. When Kevlar vests become waterlogged, the fibers can slip against each other, which compromises the integrity of the bullet-stopping mesh. Many vest wearers never notice this performance loss: The vast majority of rounds used in street crimes are relatively small-caliber, with blunt full-metal jacket or semi-wadcutter bullets. Against these bullets, wet Kevlar performs about as well as dry Kevlar. But things change in the battlefield, where Kevlar vests are worn daily in all conditions, and snipers use powerful rounds with sharp spitzer or soft-point bullets. These sharp-pointed bullets can penetrate further into a wet vest, because they can push between the fibers, which readily slide apart. This can result in dangerous blunt force trauma injuries, because more of the bullet’s momentum is transferred to the soldier’s body. In a worst-case scenario, bullets may even pass all the way through the wet Kevlar mesh.
To address this problem, researchers in Australia have been experimenting with wool-Kevlar blends for bullet proof vests. When waterlogged, wool expands and stiffens, locking the Kevlar fibers into place. Additionally, this blended fabric flexes and breaths naturally, making for a more wearable vest in inhospitable climates.
Weight, Comfort, Stabbing and Blunt Trauma Injury
Battlefield usage introduces a whole slew of Kevlar complications. First and foremost, even in the best conditions, Kevlar is not terribly effective against slashing or stabbing with a narrow blade (which can slip between and slice the woven fibers). Additionally, flexible Kevlar offers little protection from blunt internal trauma: a bullet might be stopped, but could still lead to life-threatening internal bleeding, especially in the case of higher-powered rounds.
All things being equal, conventional Kevlar vests can be remarkably successful defending against conventional ammunition. But these vests begin to struggle when challenged with the more deadly ammunition common to the battlefield. To cope with this greater threat, battlefield body armor usually includes solid ballistic ceramic plates overtop the layers of Kevlar cloth. Conventional all-Kevlar vests are far from comfortable, and these hybrid Kevlar/ceramic shells are considerably more cumbersome.
Advanced Kevlar Technology
One solution to all of these issues is to saturate the Kevlar fabric with a shear-thickening non-Newtonian fluid. Such fluids flow freely in normal conditions, but instantly harden when put under stress. The resulting cloth is pliable and non-restrictive under normal use, but instantly hardens to a shell upon impact–such as when a soldier is shot, stabbed, or blasted with shrapnel. As pressure is released, the fabric relaxes back to pliability. The flexibility of this bullet-resistant cloth also makes it practical to armor the thighs and groin. These are areas with large, vulnerable blood vessels–making them popular sniper targets–but demand a large range of motion if soldiers are to move freely in the field.