WEBVTT - How Effective Are Bulletproof Vests?

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<v Speaker 1>Welcome to brain Stuff from how stuff works, Hey, brain Stuff,

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<v Speaker 1>Lorn vogelbom here. Bulletproof vests have saved plenty of lives,

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<v Speaker 1>but this gear isn't perfectly well bulletproof in all situations.

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<v Speaker 1>So exactly how well do they work? And do bulletproof

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<v Speaker 1>vests degrade over time? Can they fail? Let's look at

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<v Speaker 1>a bit of history. Early bulletproof vests were mainly made

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<v Speaker 1>from poly paraphenneline tera thalamide, better known as kevlar. It

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<v Speaker 1>was invented by Stephanique Falek, a chemist working for DuPont

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<v Speaker 1>in the early nineteen sixties. DuPont had already invented nylon

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<v Speaker 1>and spandex and was on the hunt for the next

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<v Speaker 1>generation of high performance fibers. Synthetic fibers are created with polymers,

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<v Speaker 1>large molecules composed of repeating subunits, effectively creating a linked

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<v Speaker 1>chain of molecules that are more than the sum of

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<v Speaker 1>their parts. What foalc discovered would eventually become kevlar and

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<v Speaker 1>would see application and tires, racing sales and bulletproof vests.

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<v Speaker 1>Bulletproof vests today are made from polyethylene fibers, a popular

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<v Speaker 1>plastic polymer seen in nearly everything we use, grocery bags, toys,

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<v Speaker 1>plastic trash bins, and so on. Polyethylene fibers, or pe

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<v Speaker 1>as it's called, are woven in layers to form the

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<v Speaker 1>guts of protective vests that are cheaper and stronger than kevlar.

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<v Speaker 1>In nine, a company called Allied Signal developed a competitor

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<v Speaker 1>for kevlar and called it Spectral Shield. Originally used for sailcloth,

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<v Speaker 1>this polyethylene fiber is now used to make lighter, yet

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<v Speaker 1>stronger material for using bulletproof vests alongside the traditional kevlar.

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<v Speaker 1>According to a manufacturer of bulletproof vests, the layers inside

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<v Speaker 1>a vest are designed to reduce the velocity of a

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<v Speaker 1>round to a point below the speed of sound. At

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<v Speaker 1>roughly this point, the fibers can have time to react

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<v Speaker 1>to the bullet and exert their toughness, their strength, and

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<v Speaker 1>how much they'll bend before they break. The more layers,

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<v Speaker 1>or the greater the density of the weaving of those layers,

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<v Speaker 1>the greater the toughness of the fibers, and the more

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<v Speaker 1>capable they are of stopping a bullet. Pistols fire bullets

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<v Speaker 1>at velocities that are just slightly above the speed of sound.

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<v Speaker 1>A bullet from a nine millimeter, for example, travels around

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<v Speaker 1>nine hundred and eighty feet per second. That's about three

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<v Speaker 1>hundred and eighty meters per second. The speed of sound

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<v Speaker 1>is around one thousand, one hundred and twenty feet or

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<v Speaker 1>three hundred and forty per second. Thus, a good vest

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<v Speaker 1>doesn't need to work too hard to bring a nine

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<v Speaker 1>millimeter bullets velocity down to a point where it can

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<v Speaker 1>make use of the toughness of its fibers, and slugs

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<v Speaker 1>from shotguns travel even slower, so they're relatively easy to

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<v Speaker 1>stop as well. A rifle bullet, however, is another matter,

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<v Speaker 1>entirely depending on the grain and quality of the propellant.

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<v Speaker 1>A bullet fired from an a R fifteen travels roughly

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<v Speaker 1>three thousand, three hundred feet per second. That's about a

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<v Speaker 1>thousand meters per second, which is nearly three times the

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<v Speaker 1>speed of sound. It'll take a far greater density of

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<v Speaker 1>ballistic fiber to stop that bullet. Most vests designed to

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<v Speaker 1>stop rifle bullets are backed up with high density insert

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<v Speaker 1>plates made from ceramics and polyethylene that break up a

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<v Speaker 1>bullet into pieces and capture it. Vests without those plates

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<v Speaker 1>would likely fail to capture a rifle bullet. The material

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<v Speaker 1>of the vests themselves is also prone to break down

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<v Speaker 1>for various reasons. Kevlar, for instance, degrades when exposed to

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<v Speaker 1>salt such as salt and human sweat. The longer a

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<v Speaker 1>person exerts themselves, while ena bulletproof vest made of kevlar

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<v Speaker 1>to the tune of years, the greater the likelihood of failure.

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<v Speaker 1>Vests made from polyethylene will also break down over time

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<v Speaker 1>when exposed to UV light. Manufacturers say that there's about

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<v Speaker 1>a five year lifespan on vests and caution against buying

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<v Speaker 1>used vests unless you know the manufactured date and the

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<v Speaker 1>materials used to make it. As with any product or

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<v Speaker 1>idea or anything, really, we recommend always doing your research,

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<v Speaker 1>especially if your life might be on the line. Today's

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<v Speaker 1>episode was written by Jared w Alexander and produced by

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<v Speaker 1>Tyler Klang. For more on this and lots of other

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<v Speaker 1>tough topics, visit our home planet, how Stuff Works dot com.