Bulletproof glass
From Free net encyclopedia
Bulletproof glass is glass that is capable of stopping all manner of bullets fired at it. Such glass does not currently exist and so within the industry is referred to as Bullet resistant glass. It 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 behaviour 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. A bullet will then pierce the exterior glass layer with ease, but will be stopped by the strong polycarbonate layer before it can pierce the inner layer of glass. This type of bullet resistant glass is usually 70-75mm thick.
Bullet resistant glass constructed of laminated glass layers is built from glass sheets glued together using liquid rubber. This type of bullet resistant glass has been in regular use on combat vehicles since World War II, it is typically about 100-120mm thick and is usually extremely heavy.
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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.
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 (Air Force Research Lab article). 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.
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Categories: Glass | Armor