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ARLINGTON, Va. — The military’s new Interceptor body armor has proved to be one of the major success stories of the campaign in Iraq, with everyone from combat medics to Defense Secretary Donald Rumsfeld singing its praises.

Most soldiers know that the key to the armor’s bulletproof qualities are the so-called “SAPI” plates, for Small Arms Protective Inserts.

The two four-pound plates, which fit into pockets sewn into the front and back of the outer vest, can each stop shrapnel from mortar and grenade explosions, as well as 7.62 mm AK-47 rifle rounds.

But surprisingly, the quarter-inch layer of white backing on the plates, which looks like plastic to an untrained eye, is as much responsible for the SAPI’s stopping power as the ceramic composite it backs.

That backing isn’t really plastic at all. It’s Spectra Shield, a patented “carbon-chain polyethylene that is the lightest, strongest fiber in the world,” according to Lori Wagner, manager of Spectra Technology Performance Products, part of Honeywell’s Specialty Materials division in Colonial Heights, Va.

Pound for pound, Spectra is 10 times stronger than steel, Wagner, who has a doctorate in chemical engineering, said in a recent interview with Stripes.

The SAPI’s ceramic composite and Spectra backing work in tandem.

First, the ceramic absorbs the initial impact of an incoming bullet or fragment.

“But all the ceramic does is provide a broad surface that can break, deform or turn the bullet,” Wagner said.

So that’s where the backing comes in. The Spectra absorbs the projectile’s energy and spreads it along the surface of the shield, preventing fragments from penetrating the soldier’s body, she said.

The material works the way it does largely because it’s made of carbon, which has a simple molecular structure, Wagner said.

“There’s nothing hanging on the outside” of the carbon molecule, Wagner said. “Simplest is best, in this case.”

After engineers form carbon into long chains, using a process called “gel spinning,” the strands can be packed and stacked with virtually no space in between, Wagner said — in effect, like a “net” with no holes for anything, such as a bullet, to sneak through.

Honeywell engineers are now working on the next generation of Spectra, which they hope will be “a quantum leap forward” in stopping power, Wagner said.

Meanwhile, Honeywell is working to develop other military applications for Spectra, Tim Swinger, Spectra’s global marketing manager for armor, said during the interview.

The Marine Corps is working with Honeywell on using Spectra as armor for its Expeditionary Fighting Vehicle, and the Army is evaluating Spectra as armor for Humvees, Swinger said.

The Army already uses the material aboard some of its Apache helicopters, and may expand onto the Comanche.

The Air Force and Navy, meanwhile, are interested in making lightweight blast-protection containers out of Spectra, so ordnance can be transported more safely, Swinger said.

But for now, knowing that their material is saving lives in Iraq offers Spectra engineers a tremendous sense of satisfaction, Swinger said.

“It makes you want to go in to work every day,” he said. “My wife wonders why I work so much, but it’s because you know you’re helping save someone’s life.”

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