| MadSci Network: Engineering |
Josh,
There really is no such thing as bullet proof glass, although this is what it is always called. This type of glass is actually bullet resistant. Bullet resistant glass comes in different qualities and can become very expensive the higher the quality. The bullet resistant glass in the President’s Limousine windshield is not the same as the bullet resistant glass in the side windows, nor are either the same as glass at a bank.
In order to get regular glass strong enough to withstand a bullet, layers of plastic are placed between layers of glass (laminated glass).The plastic is often polyvinylbutyral, the same material used in automobile windshields. When a bullet hits the glass, the force of impact is absorbed by the layers of glass and plastic. The more layers, the greater the bullet resistance.
Rifle bullets will damage bullet resistant glass more than handgun bullets due to the velocity of the projectile. I have heard of an experiment where a piece of bullet resistant glass was shot with a .308 rifle several times spread out over approximately three or four feet, and the bullets did not penetrate. Now if the bullets all hit in the same location (tight group) it would be more probable for some of the bullets to penetrate the glass. Years ago, a colleague of mine got some bullet resistant glass from a jail made in the 1930’s, shot it with a shot gun (deer slugs), and it did not stop the slug. I’m sure technology has improved 10 fold since then. Lexan [a clear polycarbonate plastic] is also used in some applications but I have no knowledge in this area.
Great Physics question keep up the good work. Dave
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