How Does Body Armor 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 vest from the bullet, causing the bullet to deform or "mushroom." Additional energy is absorbed by each successive layer of material in the vest, 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 garment 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 a vest to be constructed from a single ply of material.
Currently, today's modern generation of concealable body armor can provide protection in a variety of levels designed to defeat most common low- and medium-energy handgun rounds. Body armor 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.
Methods of Construction
Typically, concealable body armor 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.
Body armor intended for routine use is most often designed to be worn beneath the normal uniform shirt. Again, manufacturers tend to design different methods of attaching armor 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.
Several manufacturers have been involved in developing and refining materials used in body armor.
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 concealable body armor. 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.
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 flexible, concealable body armor practical for the first time. In 1988, DuPont introduced the second generation of Kevlar fiber, known as Kevlar 129. According to DuPont, 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 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 fiber, manufactured by AlliedSignal, is an ultra-high-strength polyethylene fiber. Ultra high molecular weight polyethylene is dissolved in a solvent and spun through a series of small orifices, called spinnerets. This solution is solidified by cooling, and the cooled fiber has a gel-like appearance. The Spectra fiber is then used to make Spectra Shield composite. A layer of Spectra Shield composite consists of two unidirectional layers of Spectra fiber, arranged to cross each other at 0- and 90-degree angles and held in place by a flexible resin. Both the fiber and resin layers are sealed between two thin sheets of polyethylene film, which is similar in appearance to plastic food wrap. According to AlliedSignal, the resulting nonwoven fabric is incredibly strong, lightweight, and has excellent ballistic protection capabilities. Spectra Shield is made in a variety of styles for use in both concealable and hard armor applications.
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 concealable body armor; and Gold Shield PCR, which is used in the manufacture of hard armor, such as plates and helmets.
Another manufacturer, Akzo Nobel, has developed various forms of its aramid fiber TWARON for body armor. 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.
Akzo Nobel maintains that the use of TWARON in body armor significantly reduces the overall weight of the finished product, thus making vests 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 while affording the same protection.
Another fiber used to manufacture body armor 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.
Technology and Forensic Science History
Police technology, the methods and techniques of, and the equipment available to police agencies and the history behind forensic science innovations.
from a National Institute of Justice Report