Soft and lightweight synthetic rubbers that can absorb large amounts of energy from bullets and shrapnel could help lead to improved armor, researchers say.
Novel armor from the U.S. Naval Research Laboratory incorporating these polymers have already matched the performance of standard military protection while cutting the weight by almost half.
"The initial reaction when people hear about this idea is, ‘No, that can’t work, that’s nonsense,’" Roland says. "But after they hear about all these findings, they say it’s really impressive. It surprised us too, initially, but now we’re completely convinced by all the data."
Throughout history, armor has been made using hard, strong materials that could repel crushing, slicing and piercing. The modern raw materials being used for this end include metals, ceramics and synthetic fibers. Although effective, they can be heavy, and making such armor thicker can grant more protection but add unacceptable weight.
Roland and his colleagues suggest that replacing some of the steel and ceramic in armor with soft rubbers can counter-intuitively provide protection, as well.
"You can put steel armor on a humvee and make it thicker for more protection, but after too much weight, the engine or drivetrain will start breaking down,” says researcher Mike Roland, a polymer physicist at the U.S. Naval Research Laboratory in Washington, D.C. “In the same way, if you make body armor heavier on a soldier, it will perform better, but there will be a price to be paid for that weight.”
Soft, but no joke
Rubber and other soft, elastic materials known as elastomers are relatively light—almost 10 times less dense than steel, Roland says—but they are usually too weak to be of much use in armor.
Still, impacts on elastomers can make the material harden by up to a thousandfold in the span of 10 to 100 microseconds. This is rooted in how pressure waves from these impacts can drive elastomers through a phase change, a bit like how freezing temperatures make water turn to ice.
Putting the material through tests, the researchers found that when the speed at which elastomer molecules moved matched that of an impact’s velocity — about 3,000 feet per second — they could soak up a great deal of the energy.
"It can take the kinetic energy from impacts and absorb it, turn it into heat," Roland says. "We’re not trying to defeat impacts with only the elastomer, as the elastomer alone isn’t enough to stop them, but it can slow bullets and bomb fragments down, so whatever hard material is behind the soft elastomer can handle them."
(An armor-piercing round hits a layer of synthetic rubber. Credit: Mike Roland, Ray Gamache.)
Roland estimates the elastomers they investigated can reduce the kinetic energy of bullets by about 40 percent. About two-thirds to three-quarters of that reduction is due to the elastomer absorbing the energy of a bullet and turning it to heat; the rest comes from how it spreads the impact force over a wider area, he says. After the pressure wave passes through, the elastomer softens again and can go on to absorb more energy.
"Two classes of elastomers we’ve done a lot of work with are polyureas and a butyl rubber," Roland says. "They work well, and happen to be commercially available."
Armor incorporating one of these novel materials has been licensed by ATC Materials in Cleveland to protect missiles being transported on public roads from attack. Researchers are now also exploring the compounds for use in helmets, body armor and military vehicles.
"Not all the potential applications are military-related — we’ve also gotten some funding from the Department of Homeland Security for infrastructure protection," Roland says.
The researchers are now investigating what benefits arise from using alternating layers of hard and soft materials in armor.
"When pressure waves encounter different materials in an object, they can reflect — for instance, if they go from soft to hard, or hard to soft," he says. "We can design armor to make pressure waves jump back and forth, or spread out in space and time, so that when they eventually reach the steel in back of the armor, instead of a single large pressure pulse that can fracture the steel, you have a lot of smaller spatially and temporally dispersed pressure pulses that the steel can handle a lot better."
The scientists will detail their findings March 22 at the American Physical Society conference in Baltimore.
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