How does bullet proof glass work? how is some glass bullet proof and some isn’t? is there different types of glass on a molecular level? (not sure if physics is the accurate flair)
In: Physics
Thinner sheets of glass are laminated together to make a stronger material. The two things to keep in mind is it’s quite thick, and it would be better called “bullet resistant” glass. Because a big enough bullet will punch through it. It works by transferring the energy of the shot into the layers. Once it’s been hit, it quickly loses strength.
[removed]
Bullet proof glass is often laminated of layers of glass and special plastic. The glass layers are hard and act to deform the bullet, while the plastic layers tend to stretch and absorb the energy of the bullet. There are different degrees of “bulletproof-ness,” depending on the number of layers and materials used. Some will stop small caliber pistol bullets, and some will stop large caliber rifle bullets. If you shoot enough bullets at the glass, eventually it will be defeated, but the idea is that it will last long enough for the person behind it to escape or return fire.
Bulletproof (or more accurately bullet resistant) glass is typically made using laminated glass – lots of thin layers of toughened glass with a plastic layer between each of them.
The laminating works by making the impact break through each layer of glass separately – so it hits the first layer and has to expend energy to break that, then the bullet (with reduced energy) has to expend more energy to break the second layer and so on. With enough layers of glass you can burn off the energy of the bullet before it makes it all the way through.
This works better than one very thick layer for two reasons – the main one is that glass is very brittle, so it takes a lot of force to pierce through the pane, but as soon as it breaks it shatters completely. It takes a lot more force to break through multiple thinner panes (and with each pane the bullet has less energy) than it does one thick pane.
Additionally the plastic layers also hold the remains of a broken pane together, so the broken glass stays in place and can still help resist further impacts rather than falling everywhere and becoming a hindrance to everyone nearby.
The other trick is using toughened glass – this is a technique that changes the structure of the glass to make it much harder to break through the surface and cause it to shatter. This is done by cooling the glass in a very specific way – by cooling a molten pane of glass very suddenly the outer surfaces cool quicker than the material in the centre, this means as the centre material cools and shrinks slightly it pulls on the (already cooled and rigid) outer surfaces and stresses them.
This has the effect of making it much harder to overcome those higher stresses and break the glass in the first place, but as soon as you do all the stresses suddenly equalise and the pane completely shatters – this is why toughened phone screens will sometimes shatter pretty suddenly if they take an impact in a direction they are weaker.
Latest Answers