How Body Armor Works

CLASSIFICATION OF BULLET RESISTANT JACKETS & TECHNICAL DETAILS

How Body Armor Works
Introduction to How Body Armor Works
Humans have been wearing armor for thousands of years. Ancient tribes
fastened animal hide and plant material around their bodies when they went out
on the hunt, and the warriors of ancient Rome and medieval Europe covered
their torsos in metal plates before going into battle. By the 1400s, armor in the
Western world had become highly sophisticated.
All that changed with the development of cannons and guns in the 1500s. These
weapons hurl projectiles at a high rate of speed, giving them enough energy to
penetrate thin layers of metal. You can increase the thickness of traditional armor
materials, but they soon become too cumbersome and heavy for a person to
wear. It wasn't until the 1960s that engineers developed a reliable bullet-resistant
armor that a person could wear comfortably. Unlike traditional armor, this soft
body armor is not made out of pieces of metal; it is formed from advanced
woven fibers that can be sewn into vests and other soft clothing.

A Progressive Technologies bullet-resistant vest, designed for easy
concealment under normal clothing.

Modern body armor is divided into two main categories: hard body armor and soft
body armor.
Hard body armor, made out of thick ceramic or metal plates, functions basically
the same way as the iron suits worn by medieval knights: It is hard enough that a
bullet or other weapon is deflected. That is, the armor material pushes out on the

A White paper by Sardar Sanjay Matkar 2009
Email: sanjay@matkar.com, CC: sanjaymatkar@hotmail.com


bullet with the same force (or nearly the same force) with which the bullet pushes
in, so the armor is not penetrated.
Typically, hard body armor offers more protection than soft body armor, but it is
much more cumbersome. Police officers and military personnel may wear this
sort of protection when there is high risk of attack, but for everyday use they
generally wear soft body armor, flexible protection that you wear like an ordinary
shirt or jacket.

Ceramics?
Why would body armor be made with ceramic plates?
The ceramic used in body armor is called alumina, with the
chemical formula Al2O3. Sapphires are made of alumina,
and sapphire is a very strong material
You can also find rigid plates made out of the plastic
polyethylene. It is thicker than ceramic and not quite as
strong, but lighter.

Soft Body Armor
Soft body armor is a fairly mystifying concept: How can a soft piece of clothing
stop bullets? The principle at work is actually quite simple. At its heart, a piece of
bullet-proof material is just a very strong net.
To see how this works, think of a soccer goal. The back of the goal consists of a
net formed by many long lengths of tether, interlaced with each other and
fastened to the goal frame. When you kick the soccer ball into the goal, the ball
has a certain amount of energy, in the form of forward inertia. When the ball hits
the net, it pushes back on the tether lines at that particular point. Each tether
extends from one side of the frame to the other, dispersing the energy from the
point of impact over a wide area.
The energy is further dispersed because the tethers are interlaced. When the
ball pushes on a horizontal length of tether, that tether pulls on every interlaced
vertical tether. These tethers in turn pull on all the connected horizontal tethers.
In this way, the whole net works to absorb the ball's inertial energy, no matter
where the ball hits.



In a bullet resistant vest, several layers of bullet-resistant webbing (such as
KEVLAR) are sandwiched between layers of plastic film. These layers are then
woven to the carrier, an outer layer of traditional clothing material.

If you were to put a piece of bulletproof material under a powerful microscope,
you would see a similar structure. Long strands of fiber are interlaced to form a
dense net. A bullet is traveling much faster than a soccer ball, of course, so the
net needs to be made from stronger material. The most famous material used in
body armor is DuPont's KEVLAR fiber. KEVLAR is lightweight, like a traditional
clothing fiber, but it is five times stronger than a piece of steel of the same
weight. When interwoven into a dense net, this material can absorb a great
amount of energy.
In addition to stopping the bullet from reaching your body, a piece of body armor
also has to protect against blunt trauma caused by the force of the bullet. In the
next section, we'll see how soft body armor deals with this energy so that the
wearer doesn't suffer severe injuries.

Beyond Kevlar To Spectra
Kevlar is by far the most common fiber used to make body
armor, but other materials are being developed.
The most readily available alternative fiber is called
Spectra, which is approximately twice as strong as Kevlar.
Spectra is 5 to 10 times stronger than steel.



Another rapidly emerging fiber is spider silk. Yes, spider
silk. Goats have been genetically engineered to produce
the chemical constituents of spider silk, and the resulting
material is called Bio-steel. A strand of Bio-steel can be up
to 20 times stronger than an equivalent strand of steel.
Another candidate is carbon nanotubes, which promise to
be even stronger than spider silk. Carbon nanotube thread
is still rare, and fabric is even rarer. CNet reports the
current price of nanotubes at $500/gram. In time, prices
should fall and make carbon nanotubes a viable fiber for
body armor.

Blunt Trauma and Ranking Resistence
In the last section, we saw that a piece of soft bulletproof material works in the
same basic way as the net in a soccer goal. Like a soccer goal, it has to "give" a
certain amount to absorb the energy of a projectile.
When you kick a ball into a soccer goal, the net is pushed back pretty far, slowing
the ball down gradually. This is a very efficient design for a goal because it keeps
the ball from bouncing out into the field. But bulletproof material can't give this
much because the vest would push too far into the wearer's body at the point of
impact. Focusing the blunt trauma of the impact in a small area can cause
severe internal injuries.
Bulletproof vests have to spread the blunt trauma out over the whole vest so that
the force isn't felt too intensely in any one spot. To do this, the bulletproof
material must have a very tight weave. Typically, the individual fibers are
twisted, increasing their density and their thickness at each point. To make it
even more rigid, the material is coated with a resin substance and sandwiched
between two layers of plastic film.
A person wearing body armor will still feel the energy of a bullet's impact, of
course, but over the whole torso rather than in a specific area. If everything
works correctly, the victim won't be seriously hurt.
Since no one layer can move a good distance, the vest has to slow the bullet
down using many different layers. Each "net" slows the bullet a little bit more,
until the bullet finally stops. The material also causes the bullet to deform at the
point of the impact. Essentially, the bullet spreads out at the tip, in the same way
a piece of clay spreads out if you throw it against a wall. This process, which
further reduces the energy of the bullet, is called "mushrooming."
No bulletproof vest is completely impenetrable, and there is no piece of body
armor that will make you invulnerable to attack. There's actually a wide range of
body armor available today, and the types vary considerably in effectiveness. I



Modern soft body armor consists of several layers of super-strong webbing. This
material disperses the energy of a bullet over a wide area, preventing penetration
and dissipating blunt trauma. This sort of armor, as well as hard armor, ranges
considerably in effectiveness, depending on the materials used as well as the
armor design.
In the United States, body armor levels are certified by the National Institute of
Justice (NIJ), which is an agency of the U.S. Department of Justice. The levels
are I, II-A, II, III-A, III, and IV. Based on extensive laboratory tests, researchers
classify any new body-armor design into one of seven categories: Category I
body armor offers the lowest level of protection, and category IV offers the
highest. The body-armor classes are often described by what sort of weaponry
they guard against. The lowest-level body armor can only be relied on to protect
against bullets with a relatively small caliber (diameter), which tend to have less
force on impact. Some higher-grade body armor can protect against powerful
shotgun fire. Categories I through III-A are soft and concealable. Type III is the
first one to utilize hard or semi-rigid plates.

The front (left) and back (right) of a hard-armor steel plate. The plate
has been shot with several different rifle rounds, all of which were
deflected. The highest caliber round created a slight dent in the back
of the plate, but none of the shots would have caused significant
blunt trauma.
Generally speaking, armor with more layers of bulletproof material offers greater
protection. With some bulletproof vests, you can add layers. One common design
is to fashion pockets on the inside or outside of the vest. When you need extra
protection, you insert metal or ceramic plates into the pockets. When you don't
need as much protection, you can wear the vest as ordinary soft armor.



To determine how effective a particular armor design is, researchers shoot it with
all sorts of bullets, at all sorts of angles and distances. For a piece of armor to be
considered effective against a particular weapon at a particular range, it has to
stop the bullet without causing dangerous blunt trauma. The researchers
determine blunt trauma by molding a layer of clay onto the inside of the armor. If
the clay is deformed more than a certain amount at the point of impact, the armor
is considered ineffective against that weaponry.
Choice of Armor
It may seem odd that a police officer would wear category I body armor, which
will only stop relatively small-caliber bullets, when they could have superior
protection from higher-ranked armor. But there is a very good reason for this
decision. Typically, higher-ranked armor is a lot bulkier and heavier than lower-
ranked armor, which results in several problems:

An officer has reduced flexibility and comfort in bulkier armor, which impedes
police work. You can't chase a criminal very well when you're carrying a
massive weight on your torso. Heavier armor may actually increase the
chances of an officer being severely wounded. An attacker would be more
aware of a heavy armored jacket than a thin concealed vest, and therefore
might aim at an unarmored part of the body, such as the head. The discomfort
of heavier armor makes it more likely that an officer won't wear any protection
at all. Police departments are very careful to select bulletproof vests that are
relatively comfortable in order to encourage officers to actually put them on.
Armor effectiveness and comfort are sure to improve in the future as technology
companies develop lighter, stronger materials. We are certainly a long way from
impenetrable armor; but in 50 years, advanced armor will give police officers a
much greater level of protection when they're walking the beat. Most likely, we
will also see an increase in civilian body armor in the years ahead. There is an
ever-growing market for comfortable soft body armor that can fit under clothes, or
even be worn as an outer jacket. With gun violence on the rise, many citizens
feel as if they're walking onto a battlefield every day, and they want to dress
accordingly!

*****


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