Bulletproof glass
Last updated: May 12, 2008.
If you're in the front line, in the danger zone,
ducking bullets from all directions, you need all the help you can get.
No-one can move fast enough to dodge a sniper's bullet, when they can't even see it coming.
There's only one way to protect yourself: place a barrier in front of your body that
will dissipate a bullet's energy. That's the basic idea behind bulletproof glass.
Let's take a look at how it works.
Photo: There's no such thing as totally bulletproof glass.
In this controlled test carried out by the US Air Force, a very powerful assault rifle
has shattered a piece of armored glass to smithereens.
Picture by Gary Emery courtesy of the
US Air Force.
Why ordinary glass shatters
If you've ever caught a fast-moving baseball or cricket ball, you'll
know the trick is to move your hand back and slow the ball down
gradually so you reduce its energy very slowly. That reduces the force
on your hand so the catch hurts less. (Putting it more scientifically,
the force is equal to the rate at which the ball's momentum changes. So
if you change its momentum slowly, by bringing it to a halt gradually,
the force you feel is reduced.) Unlike your hand, a piece of
glass can't move. If someone fires a bullet at an ordinary piece of
glass, the glass can't bend and absorb the energy.
So the glass shatters and the bullet carries on through with hardly any loss
of momentum. That's why ordinary glass offers no protection against bullets.
Photo: Normal glass offers little protection against bullets.
When a bullet strikes a pane of glass, the bullet's energy pushes against the glass, making fractures radiate out from the point where the bullet hit along lines of weakness. This is what makes glass shatter into huge shards.
Glass damaged like this adds an extra element of danger: if the bullet doesn't kill you, the glass just might.
Picture by Bennie J. Davis III courtesy of the US Air Force.
How bulletproof glass works
"Bulletproof" glass is very different. More correctly called
bullet-resistant glass (because no glass is totally bulletproof), it's made from
layers of tough plastic called polycarbonate sandwiched in between
pieces of toughened glass. This sandwich of layers is called a
laminate.
It can be up to ten times thicker than a single pane of ordinary
glass and it's usually very heavy. When a bullet strikes bulletproof
glass, its energy spreads out sideways
through the layers. Because the energy is divided between
a number of different pieces of glass and plastic,
and spread over a large area, it is quickly absorbed.
The bullet slows down so much that it no longer has enough energy to
pierce through—or to do much damage if it does so.
Although the glass panes do break, the plastic layers stop them flying
apart. Think of bulletproof glass as "energy-absorbing" glass and
you'll have a good idea how it works.
Photo: This bulletproof armor withstood the impact of
a .30 caliber armor-piercing bullet fired from 23 m (25 yards) away
using a Russian M-44 sniper rifle. Picture courtesy of US Air Force.
Where is bulletproof glass used?
Bulletproof glass comes in all shapes and sizes to give different levels of protection in different situations.
You're most likely to find it places like banks, where the tellers typically sit behind thick bulletproof windows and use bulletproof drawers to exchange paperwork and money with customers.
Generally speaking, the thicker the glass and the more layers it has, the more energy it can absorb and the more protection it will give. Basic bulletproof glass ranges from about 3cm (1.185 in) to 4cm (1.59 in) thick, but it can be made
twice this thick if necessary.
The only problem is, the thicker you make bulletproof glass the heavier and darker it becomes.
That's may not be a problem in a bank, but it's certainly a consideration when you're trying to bulletproof
a president's car or a
"Popemobile".
Making bulletproof glass thicker also makes it more opaque, because light struggles to get through all those extra layers. That can cause difficulties if it impairs the driver's visibility.
Rap artist Buster Rhymes ran into problems
in 2007 when police stopped his SUV (with its 5cm/2in-thick bulletproof glass) "for having excessively tinted windows"!
Further reading
You might be interested in reading these other articles on our site:
- Glass: How ordinary glass works
- Protective materials: Other kinds of protective materials, including aluminum firefighter suits and tough, protective composites.
