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Armour plate glass which is more than 60 mm thick and which resists penetration by bullets.
Strictly, Bulletproof glass would be glass that is capable of stopping all manner of bullets fired at it. Such glass cannot currently be made in any usable thickness (if at all), so what is loosely called Bulletproof Glass is, within the industry, referred to as Bullet resistant glass.
Bulletproof Glass is usually constructed using a strong but transparent material such as polycarbonate thermoplastic or by using layers of laminated glass. The desired result is a material with an appearance and light-transmitting behavior of standard glass but offers varying degrees of protection from small arms fire.
The polycarbonate layer, usually consisting of products such as Cyrolon, Lexan and Tuffak, is often sandwiched between layers of regular glass. The use of plastic in the laminate provides impact-resistance, such as physical assault with a hammer, an axe, etc. The plastic provides little in the way of bullet-resistance. The glass, which is much harder than plastic, flattens the bullet and thereby prevents penetration. This type of bullet resistant glass is usually 70-75 mm thick.
Bullet resistant glass constructed of laminated glass layers is built from glass sheets bonded together with polyvinyl butyral or polyurethane. This type of bullet resistant glass has been in regular use on combat vehicles since World War II; it is typically about 100-120 mm thick and is usually extremely heavy.
One-way bullet resistant glass
Advances in bullet resistant glass have led to the invention of one-way bulletproof glass, such as used in some bank armored cars. This glass will resist incoming small arms fire striking the outside of the glass, but will allow those on the other side of the glass, such as guards firing from inside the armored car, to fire through the glass at the exterior threat.
One-way bulletproof glass is usually made up of two layers, a brittle layer on the outside and a flexible one on the inside. When a bullet is fired from the outside it hits the brittle layer first, shattering an area of it. This shattering absorbs some of the bullet's kinetic energy, and spreads it on a larger area. When the slowed bullet hits the flexible layer, it is stopped. However, when a bullet is fired from the inside, it hits the flexible layer first. The bullet penetrates the flexible layer because its energy is focused on a smaller area, the brittle layer then shatters outward due to the flexing of the inner layer and does not hinder the bullet's progress.
A glass's ability to withstand shock is also helped by the annealing process of tempering. When treated at the right temperature, the glass remains harder, which means it takes more force to shatter the surface.
However "One-Way" bullet resistant glass is far from being perfected, there is some evidence that shows this can be done, but in most cases when shooting from the "safe-side" of the glass the intended target would have to be at a "point-blank" distance for the bullet to cause any lethal penetrations.
Recent advances in bullet resistant glass composition
U.S. military researchers are moving quickly to develop a new class of transparent armour incorporating aluminium oxynitride (Trade name: AlON) as the outside "strike plate" layer.It is much lighter and performs much better than traditional glass/polymer laminates. This allows aluminium oxynitride "glass" to defeat threats surpassing .50 caliber armor piercing rounds using material that is not prohibitively heavy.
