How Effective Are Bulletproof Vests?

Bulletproof vests have certainly saved lives, but exactly how bulletproof are they? Learn what they're made of and how they can fail to protect wearers in this episode of BrainStuff.

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Episode Transcript

Speaker 1 (00:02):
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

(00:22):
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,

(00:44):
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,

(01:07):
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

(01:29):
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

(01:49):
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

(02:09):
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,

(02:33):
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

(02:54):
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

(03:17):
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,

(03:41):
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.