How does a bullet proof vest work?

When a handgun bullet strikes body armor, it is caught in a "web" of very strong fibers. These fibers absorb and disperse the impact energy that is transmitted to the bullet proof vest from the bullet, causing the bullet to deform or "mushroom." Additional energy is absorbed by each successive layer of material in bullet proof vests, until such time as the bullet has been stopped.  

Because the fibers work together both in the individual layer and with other layers of material in the vest, a large area of the bullet proof vest becomes involved in preventing the bullet from penetrating. This also helps in dissipating the forces which can cause nonpenetrating injuries (what is commonly referred to as "blunt trauma") to internal organs. Unfortunately, at this time no material exists that would allow body armor to be constructed from a single ply of material.

Currently, today's modern generation of concealable bullet proof vests can provide protection in a variety of levels designed to defeat most common low- and medium-energy handgun rounds. Bullet proof vests designed to defeat rifle fire is of either semirigid or rigid construction, typically incorporating hard materials such as ceramics and metals. Because of its weight and bulkiness, it is impractical for routine use by uniformed patrol officers and is reserved for use in tactical situations where it is worn externally for short periods of time when confronted with higher level threats.

Below you can find a small movie which shows the impact of a bullet. This show has been broadcasted by Discovery Channel Europe recently. Macromedia Flash Player 8 is required to be able to see the movie. You can download the plug-in here.

Methods of Construction

Typically, a concealable bullet proof vest is constructed of multiple layers of ballistic fabric or other ballistic resistant materials, assembled into the "ballistic panel." The ballistic panel is then inserted into the "carrier," which is constructed of conventional garment fabrics such as nylon or cotton. The ballistic panel may be permanently sewn into the carrier or may be removable. Although the overall finished product looks relatively simple in construction, the ballistic panel is very complex.

Ballistic fabric is available from a number of manufacturers in various styles and compositions, each type having unique ballistic resistant properties. The body armor manufacturer may construct a given model of ballistic panel from a single fabric style or from two or more styles in combination. The location and number of layers of each style within the multiple-layer ballistic panel influence the overall ballistic performance of the panel. In addition, some manufacturers coat the ballistic fabric with various materials. For example, the manufacturer may add a layer of nonballistic material for the sole purpose of increasing blunt trauma protection. Even composites of two or more different ballistic materials are available. As a consequence, it is impossible to compare one product with another based solely on the number of fabric layers in the ballistic panel.

The manner in which the ballistic panels are assembled into a single unit also differs from one manufacturer to another. In some cases, the multiple layers are bias stitched around the entire edge of the panel; in others, the layers are tack stitched together at several locations. Some manufacturers assemble the fabrics with a number of rows of vertical or horizontal stitching; some may even quilt the entire ballistic panel. No evidence exists that stitching impairs the ballistic resistant properties of a panel. Instead, stitching tends to improve the overall performance, especially in cases of blunt trauma, depending upon the type of fabric used.

Bulletproof vests intended for routine use are most often designed to be worn beneath the normal uniform shirt. Again, manufacturers tend to design different methods of attaching a bullet proof vest to the body. Hook-and-pile fasteners are common, as are "D" ring tightening straps. With the exception of metal fasteners of any type (which can deflect a bullet on impact and pose a hazard), the method of attachment is a matter of personal preference.

Materials Used

Several manufacturers have been involved in developing and refining materials used in a bullet proof vest.

DuPont has developed law enforcement protection products for more than 25 years. Its Kevlar brand fiber, first developed in 1965, was the first material identified for use in the modern generation of a concealable bullet proof vest. Kevlar is a manmade organic fiber, with a combination of properties allowing for high strength with low weight, high chemical resistance, and high cut resistance. Kevlar is also flame resistant; does not melt, soften, or flow; and the fiber is unaffected by immersion in water.


Figure: Kevlar fabric

Kevlar 29, introduced in the early 1970s, was the first generation of bullet resistant fibers developed by DuPont and helped to make the production of a flexible, concealable bullet proof vest practical for the first time. In 1988, DuPont introduced the second generation of Kevlar fiber, known as Kevlar 129. According to DuPont, a bullet proof vest with this fabric offered increased ballistic protection capabilities against high energy rounds such as the 9mm FMJ. In 1995, Kevlar Correctional was introduced, which provides puncture resistant technology to both law enforcement and correctional officers against puncture type threats.

The newest addition to the Kevlar line is Kevlar Protera, which DuPont made available in 1996. DuPont contends that the Kevlar Protera is a high-performance fabric that allows lighter weight, more flexibility, and greater ballistic protection in a bullet proof vest design due to the molecular structure of the fiber. Its tensile strength and energy-absorbing capabilities have been increased by the development of a new spinning process.

Spectra Shield® products have been protecting military and law enforcement personnel for the past 20 years, and that history of innovation continues with the release of the Spectra Shield® II product line.

SR-3124, a Spectra Shield® II hard armor product, incorporates high-strength Spectra® 3000 fiber into hard armor shield construction, to provide the highest levels of protection for breast plate, helmet, and vehicle applications.

SA-3118 and SA-3113 are the Spectra Shield® II soft armor offerings, providing the best known ballistic protection for military and law enforcement vest applications. SA-3118 maximizes ballistic performance, while SA-3113 was designed for added flexibility and comfort, while maintaining a high level of performance.

SR-1214 is the standard Spectra Shield® hard armor product, used in plate, helmet and vehicle programs throughout the world.

SA-1211, continues to be used in some of the most demanding vest applications, and is designed to meet the comfort and flexibility demands of the user, while maintaining strong ballistic performance and reducing the effects of blunt trauma.

Figure: Spectra shield fabric

AlliedSignal also uses the Shield Technology process to manufacture another type of shield composite called Gold Shield. Gold Shield is manufactured using aramid fibers in place of the Spectra fiber. Gold Shield is currently made in three types: Gold Shield LCR and GoldFlex, which are used in a concealable bullet proof vest and Gold Shield PCR, which is used in the manufacture of hard armor, such as plates and helmets.


Figure: GoldFlex fabric

Another manufacturer, Akzo Nobel, has developed various forms of its aramid fiber TWARON for bullet proof vests . According to Akzo Nobel, this fiber uses 1,000 or more finely spun single filaments that act as an energy sponge, absorbing a bullet's impact and quickly dissipating its energy through engaged and adjacent fibers. Because more filaments are used, the impact is dispersed more quickly. Akzo claims their patented Microfilament technology allows maximum energy absorption at minimum weights while enhancing comfort and flexibility.


Figure: Twaron fabric

Akzo Nobel maintains that the use of twaron in a bullet proof vest significantly reduces the overall weight of the finished product, thus making a bullet proof vest more comfortable. Akzo also contends that stitching panels made from layers of TWARON is largely unnecessary, and that the lack of stitching contributes to a lighter weight and softer feel of the bullet proof vest while affording the same protection.

Another fiber used to manufacture a bullet proof vest is DYNEEMA. Originated in the Netherlands, Dyneema has an extremely high strength-to-weight ratio (a 1-mm-diameter rope of Dyneema can bear up to a 240-kg load), is light enough that it can float on water, and has high energy absorption characteristics. Dyneema is a polyethylene fiber.


Figure: Dyneema fabric

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Extracts from a National Institute of Justice Report

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