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UD researchers devise liquid body armor technology

When an object is tugged sharply through a sample of shear-thickening fluid, the liquid instantly hardens and prevents further movement.
11:52 a.m., May 24, 2004--Researchers at the University of Delaware, working in collaboration with military scientists, have developed a liquid that hardens on impact and has the potential to improve the protective ability and comfort of body armor worn by soldiers and law enforcement officers.

When fabric that has been impregnated with the liquid, known as shear-thickening fluid, is struck by a knife or bullet, it immediately becomes rigid and prevents the object from penetrating it. The rest of the time, the fabric is flexible and lightweight.

“We believe this can be used to improve on the Kevlar vests that are currently used as body armor, but we also hope it can be used in such things as sleeves and pants to protect a person’s extremities, which of course aren’t covered by a vest,” Norman Wagner, professor of chemical engineering and the leader of the UD research team, said. “Beyond that, it could have applications in aircraft engines, car doors, tires and many other areas.”

Kevlar fabric is used in so-called “bulletproof” vests, worn by the military and by police and corrections officers. The traditional vests can be bulky and uncomfortable, users say, and Wagner said the new technology is expected to improve their wearability.

Wagner’s lab contains numerous samples of Kevlar fabric, some of them wrapped around blocks of foam that he uses to demonstrate the technology. When one of the blocks, covered in four layers of untreated Kevlar, is stabbed with an ice pick, the point goes through the fabric and into the foam. Another block, wrapped in four layers of Kevlar that had been soaked in shear-thickening fluid, blocks the ice pick with no apparent damage to the fabric or the foam underneath.

In other recent tests, Wagner said, arrows that were shot at double layers of treated Kevlar bounced off their targets.

In lab tests when a block of foam covered with untreated Kevlar is stabbed with an ice pick (above right), the point goes through the fabric and into the foam. Another block, wrapped in four layers of Kevlar that had been soaked in shear-thickening fluid, blocks the ice pick with no apparent damage.
Material that has been saturated with shear-thickening fluid is slightly oily to the touch but otherwise feels and drapes like normal fabric. Although researchers have been using Kevlar fabric as their “platform” for the shear-thickening fluid, Wagner said it’s possible that other fabrics also might be useful.

Currently, some types of traditional Kevlar vests protect well against bullets and are used by the military and police officers, while other types are more effective against pointed objects and are worn by prison guards, who are more likely to be stabbed than shot. The new technology is expected to protect well against both types of dangers, Wagner said.

Shear-thickening fluid consists of a polyethylene glycol liquid with tiny, hard particles of silica suspended in it. The combination of the liquid and the nanoparticles results in a thick, syrupy fluid in which Kevlar or other fabric can be soaked. When Wagner pulls an object slowly through a sample of the fluid, he is able to keep it moving in the viscous liquid. But, when he tugs sharply on the object, the liquid instantly hardens and prevents further movement.

“After you stop the stress, it goes back to being a liquid,” Wagner said. “In the same way, after treated fabric becomes rigid when struck by a projectile, it goes back to being flexible. That property makes shear-thickening fluid a very interesting material.”

It also makes it useful in increasing protection to the wearer without adding weight or stiffness that might interfere with the person’s mobility, according to Eric Wetzel, a 1995 UD mechanical engineering graduate who leads the military research team that is working on the project at the U.S. Army Research Laboratory at Aberdeen Proving Ground in Maryland.

“The goal of the technology is to create a new material that is low-cost and lightweight, that offers equivalent or superior ballistic properties as compared to current Kevlar fabric but has more flexibility and less thickness,” Wetzel said. “The technology has a lot of potential.”

UD’s liquid body armor research team (from left): Young Sil Lee, Phil Matthews, Keith Kirkwood, Norman Wagner, Ron Egres and John Kirkwood.
In addition to improved body armor, he said, military uses might include bomb blankets, to cover suspicious packages or other potential explosives, and paratrooper boots, which would stiffen on impact to protect a jumper’s ankles.

The collaborative research project, which has been under way for about three years, was awarded the 2002 Paul A. Siple Award, the Army’s highest award for scientific achievement. The other members of Wagner’s UD team, which works through the College of Engineering’s Center for Composite Materials, include postdoctoral researcher Young Sil Lee, doctoral student Ron Egres, undergraduates John and Keith Kirkwood (twins who will graduate later this month) and junior Phil Matthews.

“We’ve been very fortunate to have such a good team, including very talented undergraduates,” Wagner said. “They’ve all made important contributions to this research. We’ve also been extremely fortunate to partner with Eric Wetzel, whose team has a lot of experience with body armor and ballistics.” He said the two teams meet regularly, have coauthored various papers and have applied jointly for a patent on the technology.

Researchers currently are doing real-world types of testing, Wagner said. Ballistics tests are being performed at the Army lab, while the University lab is conducting other tests, such as those involving sharp instruments. In addition, the treated fabric must undergo environmental testing, to ensure that it performs at extreme temperatures and in a variety of situations.

“We’re now at the stage of deciding what the best uses of this technology will be,” Wagner said. “I think we’ll know within a year, and I think we’ll find a lot of opportunities.”

Article by Ann Manser
Photos by Kathy F. Atkinson

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