"Morale Fiber," Input/Output, ME Departments, July 2007

input/output

Morale Fiber

by Michael Abrams

The insurgent wars currently being fought in Iraq and Afghanistan have forced the troops there to keep on the move constantly, a state of affairs that denies them the luxury of protective walls made of sandbags or concrete. Instead they are sheltered in tents built of aluminum frames covered with mere fabric, offering them some protection from the elements and none from mortars—the second most common cause of injury to our soldiers.

But an armor made of a flexible, blast-resistant, fiber composite is putting an end to that.

It's called wood.

"Wood is a natural fiber composite," said Habib Dagher, director of the Advanced Engineered Wood Composites Center at the University of Maine. "We treat it as we would treat other composite materials." And, as Dagher pointed out, many of the requirements for composites were initially developed for wood. But other composite materials don't have what wood has: namely, flexibility.

The challenge of engineering tent wall armor is to make something that is light and cost effective but at the same time capable of withstanding blasts and projectiles—all that while keeping the protective panel from becoming a lethal flying weapon itself. "The tent frames are not designed for very high winds," said Dagher. "Under the blast forces, those pressures are short duration—a few milliseconds. So the trick is to design the system so that the panels don't become more dangerous than the blast itself."

Trial by fire: In independent tests, wood performed better than expected at stopping shell fragments.

It turns out that sandwiching poplar—from Maine, to be specific—with fiberglass and a few other materials makes a panel that is both strong enough and flexible enough to withstand those forces. "Initially we thought it would be very difficult to have the tent withstand the blast forces we are designing for," said Dagher. Combined with the elastic bands that hold the panels to the tent, the armor will now accelerate before the blast force can load the frame in the event of an explosion.

To fuse all those fibers, the cellulose and the manmade, Dagher's team applied a cycle of high pressure and high heat before cooling. "We're not pushing the temperatures to the point that it loses mechanical properties. The process we're using is designed not to take the wood into that temperature regime."

Flexible composite: An experimental blast shielding made largely of poplar and fiberglass can be assembled in about an hour to protect troops that are housed in tents.

To test the armor, an independent lab subjected it to a series of blasts and fired live mortars at it. To everyone's surprise, the wood helped keep the mortar fragments from penetrating the panels. "It was a shock to us that the wood actually improved the ballistic properties," said Dagher. "If I was just to design a ballistic panel, I probably would not be using wood. It was selected for its other structural properties. But the one thing we hear is that if you look at the micromechanics of the penetration problem—how a fragment goes through the outer layers, then the center, then the outer layers—the presence of that layer in the center is allowing more flexibility for the high-performance fiber that we're using."

Thanks to the use of a thermo plastic resin, once the panels are cooled after the heat and pressure cycle, they're ready to go, with handles and holes to attach the armor to the tent frames already in place, with no post processing required. The armor can be quickly assembled and requires no tools or training. Four soldiers can armor the tent in less than an hour. The speed and ease of use means that even soldiers piling sandbags or pouring concrete for a longer stay can use them to provide some protection before the more permanent barriers are complete. Dagher's team also designed a door so no side of the tent would be vulnerable to attack.

The panels, which span 8 feet between poles, fit inside the tent fabric, take up no extra square footage, and are colored white to keep the interior bright. A range of weights are available, the lightest of which is 2.2 pounds per square inch, the heaviest, four.

Already two armor systems are in the field, one in Afghanistan, and one in Iraq. "The troops that we have are being subjected to the kinds of wars the army didn't anticipate. They have to move every two or three weeks," Dagher said. "Now they have the ability to take their armor with them. So it's a major achievement, yes."