Incidentally, another class of fibers exists, known as PBO fibers, marketed by the Japanese company Toyobo. The trade name for this fiber is Zylon, and while it is known to be very effective ballistically and lightweight, Zylon vests have given rise to concerns regarding how well they retain their ballistic integrity over time. Since these issues are broader than the scope of this article, Zylon will not be discussed here.
Early vests were woven, in a process similar to other fabrics, then multiple layers of fabric were sandwiched together to form vest panels. To raise the stopping power of these early vests, more layers were added. Of course, that made them heavier and hotter.
Current technology is different. Instead of a weave pattern, the fibers are laid out in parallel fashion. Think of a bunch of pencils laying side by side on a table, so that their long sides are touching all along their length, like the construction of a wooden raft made of logs. The second layer of fibers is laid atop the first layer, with the fibers running at right angles to those in the first layer. The next layer is also laid at right angles to the second layer, and so on. While simplistically put, this image conveys the idea of a more effective fiber arrangement.
When a projectile strikes ballistic fabric, it is important that the energy be able to flow away from the point of impact in order to dissipate the energy over a larger area. The faster the energy can flow away from the point of impact, the more the energy is spread across a larger area; thus less energy is deposited into the impact point, and less "shock" energy is transmitted through the fabric to cause backface deformation against the underlying tissue.
Backface deformation, an indentation on the wearer's side of the armor, is what causes blunt trauma, or the heavy bruising of the wearer. Weaving the fibers is less effective at energy dissipation than the previously described layering. When one weaves fibers into a sheet of ballistic fabric, the points where the fibers cross each other form something akin to energy roadblocks. Energy will flow past these junctions, but will be slowed along the way.
On the other hand, when the fibers are laid parallel to each other, the energy is allowed to flow unimpeded, and therefore more quickly and efficiently, away from the impact point. Less trapped energy near the impact site means less energy moving forward into the panels and into the body. Ergo, less injury.
This change in assembly techniques also increased the effectiveness of the fabrics at stopping rounds, since there is an energy dissipation benefit in that regard as well. One of the primary benefits of this evolution is increased capability at stopping multiple hits.
Corrections versus Patrol
In the corrections environment, protection against bullets is not the problem, instead, protection from slashing, cutting and stabbing is. Many vests intended for street law enforcement are just not suitable for work in a correctional setting. While ballistic panels offer some protection, an ice pick or similar instrument can sometimes penetrate them.
Enter armor specially designed for corrections officers. Frequently worn over the uniform as an outer garment, this armor is often thicker. Additionally, the actual construction of the panels is different. The yarns or filaments in the fiber are finer, so they are smaller and more densely packed. This helps to reduce the likelihood that a shank or an ice pick will penetrate. One option for corrections officers is a multi-threat vest, designed for both projectile resistance as well as correctional threats.
Vests Wear Out
Ballistic fibers can gradually lose their effectiveness and efficiency over time, especially if they are routinely exposed to heat and moisture, as they are when worn daily (especially in hot climates). Vests have a life expectancy of five years or so. Check manufacturer's recommendations. That doesn't mean that they suddenly "switch off" their effectiveness when they expire, but the expiration date is the date specified by the manufacturer as the last date that they expect the vest to still be within specifications taking into account normal wear and tear.