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Liquid armor stiffens when threatened.

Personal armor may stop bullets and bombs, but its protection comes at a cost. Today's bulletproof vests are made of Kevlar aramid or other high-strength fabrics. Adequate protection takes multiple layers of fabric, which makes armor bulky and uncomfortable to wear.

One potential solution involves a little-known class of materials called colloidal shear-thickening fluids. During normal use, they flow as easily as conventional liquids. When subjected to sudden stresses that make them flow at higher shear rates, they instantly turn rigid and act like a solid material.

Impregnating conventional aramid fibers with shear-thickening fluid creates a more effective barrier, says armor developer Norman Wagner, a professor of chemical engineering at the University of Delaware's Center for Composite Materials. When he plunges an ice pick through four layers of Kevlar fabric wrapped around a foam block, it goes right through. Four layers of fluid-impregnated Kevlar fabric stop the ice pick cold.

Wagner's demonstration is especially interesting because conventional Kevlar vests usually fail to stop ice picks and knives, although they resist penetration by high-velocity bullets. Wagner's fluid-impregnated fabrics defeat bullets, too, and do so with fewer layers. This leads Wagner to believe that switching to fluid-impregnated fabrics will enable designers to create thinner, more flexible armor.

Wagner began unraveling the secrets of shear-thickening fluids more than 10 years ago. The syrupy colloids consist of submicrometer-size silica particles suspended in polyethylene glycol liquid. Wagner found that shear forces created "jamming clusters" of silica particles that freeze the fluid into a solid. The effect is similar to cement, which grows harder to mix the faster it is stirred.

Wagner originally intended to use his knowledge to improve the manufacture of coated and photographic papers. Instead, he began collaborating on armor with Eric Wetzel of the U.S. Army Research Laboratory's Weapons and Materials Research Directorate.

The University of Delaware recently licensed the technology to Armor Holdings Inc., a leading manufacturer of personal and vehicle armor systems. Wagner sees other non-armor applications as well, ranging from aircraft engines, car doors, tires, and sporting apparel to bomb blankets and paratrooper boots that stiffen on impact to protect a jumper's ankles.

This section was written by Associate Editor Alan S. Brown.

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Title Annotation:bulletproff vests
Author:Brown, Alan S.
Publication:Mechanical Engineering-CIME
Geographic Code:4EUUK
Date:May 1, 2006
Words:365
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