From fiber to armor

Piecing together the construction of soft body armor


     "When I started in law enforcement in the mid-1970s, the first vest I got looked and felt like a Manhattan Yellow Pages wrapped in a plastic bag," describes Georg Olsen, a long-time veteran of law enforcement and general manager of U.S. Armor, located in Cerritos, California.

     Through the evolution of ballistic fibers and construction methods, today's soft body armor is becoming a more sleek, comfortable, flexible, life-protecting second skin for law enforcement officers. With this evolution comes improved wear rates and more lives saved. "Officers aren't obstinate or stupid," comments Olsen. "They want the protection, but they have to have the flexibility and mobility to do their jobs."

     Part of finding the right protection is making an educated decision when purchasing soft body armor. Officers need to know the types of fibers used, how a vest should properly fit and the advancements in construction methods leading to more comfortable, wearable vests.

Energy's catcher's mitt

     Soft body armor is designed to absorb energy — whether from the impact of a bullet, a vehicle or any other life-threatening source — and disperse it over a wide area. Fibers are the filaments across which this impact energy is transmitted, and how those fibers are created and aligned determines their ability to disperse energy.

     There are two basic types of ballistic fibers used in soft body armor construction: aramid and high-performance polyethylene (HPPE) fibers. Honeywell's Spectra and DSM's Dyneema are HPPE fibers, while Dupont's Kevlar and Teijin Twaron's Twaron are aramid fibers.

     Whether aramid or HPPE, these fibers have a few common traits. They have high strength or tenacity (tenacity being a strength per weight ratio). They also tend to have a high modulus or stiffness and low elongation, which provides for better energy dissipation.

     "The structure of the fiber also imparts some of the high-strength characteristics," explains Dr. Lori Wagner, manager of technology for Honeywell's Advanced Fibers and Composites Group. "It has an orientation and regularity. The order of the molecular chains is aligned along the length of the fiber, which helps with the transmission of energy and gives it the high-strength quality."

     Once aligned on the molecular level, it is important to maintain this orientation in the spinning process. "During the spinning process, being able to orient those chains along the length of the fiber is what imparts the ballistic performance," says Wagner.

     Weaving interlocks the fibers so the strength orientation is directed both vertically and horizontally. But at these intersections of fiber, energy "roadblocks" are created, reducing the rate at which energy can be dissipated.

     Honeywell, located in Colonial Heights, Virginia, has developed a rolled fabric material, called Spectra Shield, that instead of being woven is a laminate. In this process, the fibers are aligned in one direction, spread into a web and adhered with a resin. Then two fiber resin sheets are layered — one in a 0-degree direction and the other in a 90-degree direction — and fused. "This gives you strength in all directions," says Wagner. "At the same time, it doesn't disrupt all of the orientation we have worked so hard to put in the fiber." This construction allows for faster energy dissipation and reduced backface deformation — the energy that was not dissipated by the vest and will impact the body.

     Honeywell discovered this process not only works for HPPE fibers but for aramids as well. "The concept of aligning and positioning fibers to their ultimate performance level extends into any high-performance fiber that can be used in ballistics," she says.

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