How do glass bridges support our weight?

07 March 2017

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Question

What material do they use to make the glass bridges that are currently appearing all over China? They can be hit with a sledgehammer and don’t break - surely normal glass isn't strong enough?

Answer

Chris Smith put this cracking question to University College London materials scientist Anna Ploszajski...

Anna - Well, actually it is glass. But they’ve done two things to treat this glass so that it is safe for you to stand on a piece of glass hundreds of metres up in the air and be perfectly safe.

The two things that they do is they toughen it. Now have your heard of an experiment called a “Prince Rupert’s drop?" This is a very famous experiment done with glass. What you do is you take a drop of molten glass and you plunge it very, very quickly into cold water. Now what this does is it has the effect of putting the outer layer of that glass under, very, very intense mechanical compression. It’s the action of cooling down so quickly that puts the atoms into compression. This means that if you try and put a crack into the surface of that glass, the atoms will just close it up again. It’s very, very difficult to get a physical crack into something that is under such intense compression.

Now as we know, physics requires an equal and opposite reaction to a force, so the middle of the Prince Rupert’s drop is in very, very intense mechanical stress tensile stress. So those atoms are being pulled away from each other very, very strongly as well. So yes, you can hit this glass with a sledgehammer and it will be very unlikely to break.

The second thing that they do is - this is bulletproof glass and it is a glass laminate construction. So this glass is layered and there’s layers of plastic in between the layers of glass. What happens in bullet-proof glass is that when a projectile, or a person's foot, or a sledgehammer tries to hit the glass then that first layer is probably going to smash. That absorbs a lot of the energy in the bullet or in the sledgehammer and then that glass presses down into the plastic layer, and that plastic layer flows like a thick, treacly fudge-type thing and that absorbs a huge amount of the energy and spreads the impact over a very wide surface area. If there’s still any more energy in the sledgehammer after that, the next layer of glass might smash and then spread its energy over the next layer of the plastic. This process continues until the bullet or the sledgehammer has stopped, and that’s why you can’t fall through the glass.

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