Making glass bulletproof

24 September 2019

Interview with 

James Perry, University of Cambridge

SMASHED-GLASS

A fractured pane of glass

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We don’t usually think of glass as particularly tough, much the opposite. But we can make our glass tougher, we can even make it bulletproof, but how? Chris Smith was joined in studio by James Perry from the University of Cambridge to explain some tough science...

James - So it turns out that the theoretical maximum strength of glass, if you could decide exactly where you are putting all of those atoms of silica and oxygen, is about a thousand times stronger than an actual lump of glass.

Brittle materials - and glass is a prime example of this - their defamation and their failure are largely driven by flaws, imperfections, and particularly cracks. So as you cool a material down it shrinks, so when you’re cooling your glass it shrinks down and the surface of it looks a little bit like a dried-up pond with cracks all over the place.

So cracks are particularly problematic in tension or if you bend the material. So if you take a plate of glass and you start to bend it, the outer edge is in tension and the inner edge is in compression. The outer edge, as you pull it, a lot of this force - a lot of the stress - gets concentrated at the very tip of those little cracks. So all of that force is concentrated on just one atomic bond, which eventually will pop open at a much lower stress than it would take to expand the whole thing.

Chris - It’s going to undo like a zipper, isn't it? It’s almost going to propagate down into the material. So once it gets started…

James - It doesn’t stop.

Chris - ...it’s going to accelerate.

James - Precisely, yes.

Chris - And you said something interesting, because you said, when it cools down you end up with the outside, it’s going to cool a bit faster than the inside and that's going to have the effect of making those pond wrinkles as you say because you'll get the outside being relatively stretched and the inside squeezed a bit.

Is there any way of intervening to change that then, so that we don't end up with the glass being vulnerable in that way?

James - Yes very much so. So if we go back to the 16th century there's a fun little thing that came across the Royal Society called Prince Rupert's drops. So if you take a blob of hot molten glass and drop it in a pool of water it freezes and it turns into essentially a tadpole. And that tadpole you can wack with a hammer on the head or you can even shoot a lead bullet at it and it won't do anything.

But if you just tweeze the tail with your fingers it will immediately shatter into a million tiny pieces.

Chris - Why?

James - So, skip forward a couple of years to the turn of the last century, and we realised that by cooling very rapidly the material - and for toughened glass now we use high pressure air instead of water - you cool the surface very rapidly and that cools, and then the inner bit cools much more slowly. But as it cools, it also reduces in volume and so it ends up pulling the outside edges in, leaving it in the middle in tension and the outside in compression.

And that stops a lot of those little cracks of the surface from opening up. Which means if you are then loading this toughened glass, then it's got to overcome the compression that it’s already under and then the tension required before it'll fail.

Chris - So why does nipping off the end break it?

James - This toughened glass that is used for things like shelves or anything else you might find in the house, that you don't want it to fall apart at the smallest knock. The problem is it will fail and will fail really quite catastrophically, because as soon as you break through that layer and you manage to tweak the tensile bit in the middle it pings apart.

So in the drop, the tail is so thin that that outside compressive layer is incredibly thin. And so you are firing these cracks right the way through of the tail up to the top. So the same kind of thing happens with toughened glass, in that when it smashes it pulvarises into thousands of tiny little cubes usually. Which in some applications is really useful because you don't end up with big pointy shards you end up with a pile of little ones -

Chris - Like a windscreen, you don't want to impale the driver, you want little bits of glass you can brush off?

James - Precisely. But that's not so useful if for example you want to be able to still keep that barrier intact after that initial impact.

Chris - In other words bulletproof glass, because you what you want the glass so that it will not fail in that catastrophic way across your whole windscreen and then somehow also be there as a defense against the next projectile. Because otherwise you could kill someone by firing two bullets, one to demolish the windscreen and it might fend off the first bullet, but if the whole thing smashes to pieces, you fire the next one and it goes straight through.

So how do we stop that? How do we toughen up glass?

James - Above a certain amount of force or a certain amount of stress, you are going to cause some damage you can't get away from that fact. So the trick then is to build your material such that a single impact of that amount of damage, isn't going to completely catastrophically destroy your material.

So you can do that by building up in layers. The first layer is glass which is very very strong. And so that will help dent the projectile and soften it up. So it goes from a very pointy tip to much flatter tip.

The energy of a bullet is not actually very much, but it's very focused on a small area. So if you can distribute it out then it's easier to slow down. And that's where the second layer comes in. So behind it you put a polycarbonate usually, a plastic, which is very soft but very tough. So it can deform an awful lot and absorb an awful lot of the energy in the process of doing so.

So what's an interesting thing actually to finish on is the fact that you can have one way glass. So if you have a layer of glass followed by a layer of polycarbonate, the glass flattens the bullet, the polycarbonate catches whatever's left. So what's a nice illustration of this is the fact that if you're firing in one direction you have the glass first, that flattens the bullet and then that moves into a polycarbonate layer, which then absorbs the energy. If however you're firing back in the other direction, you first punch straight through the polycarbonate layer, hit the glass layer, which then fails in tension and spawns lots of small fragments of glass.

Chris - So James Bond could  - if he wanted to - shoot some enemies from inside his car and his bullet proof glass windscreen will allow him to do that. But as long as he doesn't shoot first then he's going to fend off any of the enemies bullets!

James - Yes. So long as you put your glass in the window the right way!

Chris - Yeah. And does it really work? Can you really fend off a bullet with glass or is it there really to stop your windscreen demolishing with a stone or something but not a bullet?

James - Yes. So possibly a better description would be bullet resistant plastic with a bit of glass. So for a start these things are pretty thick. You can have them up to a couple of inches thick, and so obviously an awful lot of energy can be absorbed. It's worth pointing out that to use the James Bond analogy, I think it was Tomorrow Never Dies has a car where the bad guys are trying to break into it using hammers and bullets and absolutely nothing happens to the surface of the windows in the car. That is not the case, you will cause some damage relatively easily, but it will take a lot more effort to damage all the way through those multiple layers, in order to get inside.

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