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. Traditional solutions for 20th Century military armor, based chiefly on steel and ceramic plates, were really too heavy for soldiers, indeed for many vehicles as well. Composites have increasingly proved to be the answer, being much lighter for the same stopping power and more pliable. Certain polymer composites show, when appropriately engineered, remarkable energy dissipation properties, being able to absorb the kinetic energy from bullets and other high-speed projectiles before these can harm their human targets. They can also protect against knives.

Various mechanisms account for this. Pushing fibers aside against their stiffness and the hold exerted by the composite they 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 projectiles, 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.

This article appeared in the March/April edition of Reinforced Plastics. Log in to your free materialstoday.com profile to access the article.

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