Welcome to brain Stuff from how stuff works, Hey, brain Stuff, Lorn vogelbom here. Bulletproof vests have saved plenty of lives, but this gear isn't perfectly well bulletproof in all situations. So exactly how well do they work? And do bulletproof vests degrade over time? Can they fail? Let's look at a bit of history. Early bulletproof vests were mainly made from poly paraphenneline tera thalamide, better known as kevlar. It was invented by Stephanique Falek, a chemist working for DuPont
in the early nineteen sixties. DuPont had already invented nylon and spandex and was on the hunt for the next generation of high performance fibers. Synthetic fibers are created with polymers, large molecules composed of repeating subunits, effectively creating a linked chain of molecules that are more than the sum of their parts. What foalc discovered would eventually become kevlar and would see application and tires, racing sales and bulletproof vests.
Bulletproof vests today are made from polyethylene fibers, a popular plastic polymer seen in nearly everything we use, grocery bags, toys, plastic trash bins, and so on. Polyethylene fibers, or pe as it's called, are woven in layers to form the guts of protective vests that are cheaper and stronger than kevlar. In nine, a company called Allied Signal developed a competitor
for kevlar and called it Spectral Shield. Originally used for sailcloth, this polyethylene fiber is now used to make lighter, yet stronger material for using bulletproof vests alongside the traditional kevlar. According to a manufacturer of bulletproof vests, the layers inside a vest are designed to reduce the velocity of a round to a point below the speed of sound. At roughly this point, the fibers can have time to react to the bullet and exert their toughness, their strength, and
how much they'll bend before they break. The more layers, or the greater the density of the weaving of those layers, the greater the toughness of the fibers, and the more capable they are of stopping a bullet. Pistols fire bullets at velocities that are just slightly above the speed of sound. A bullet from a nine millimeter, for example, travels around nine hundred and eighty feet per second. That's about three
hundred and eighty meters per second. The speed of sound is around one thousand, one hundred and twenty feet or three hundred and forty per second. Thus, a good vest doesn't need to work too hard to bring a nine millimeter bullets velocity down to a point where it can make use of the toughness of its fibers, and slugs from shotguns travel even slower, so they're relatively easy to
stop as well. A rifle bullet, however, is another matter, entirely depending on the grain and quality of the propellant. A bullet fired from an a R fifteen travels roughly three thousand, three hundred feet per second. That's about a thousand meters per second, which is nearly three times the speed of sound. It'll take a far greater density of
ballistic fiber to stop that bullet. Most vests designed to stop rifle bullets are backed up with high density insert plates made from ceramics and polyethylene that break up a bullet into pieces and capture it. Vests without those plates would likely fail to capture a rifle bullet. The material of the vests themselves is also prone to break down for various reasons. Kevlar, for instance, degrades when exposed to
salt such as salt and human sweat. The longer a person exerts themselves, while ena bulletproof vest made of kevlar to the tune of years, the greater the likelihood of failure. Vests made from polyethylene will also break down over time when exposed to UV light. Manufacturers say that there's about a five year lifespan on vests and caution against buying used vests unless you know the manufactured date and the
materials used to make it. As with any product or idea or anything, really, we recommend always doing your research, especially if your life might be on the line. Today's episode was written by Jared w Alexander and produced by Tyler Klang. For more on this and lots of other tough topics, visit our home planet, how Stuff Works dot com.