Super-Strong Steel Armour

Ben -   Another potential application for a very strong metal is in armour, protecting people and vehicles from threats like bullets and bombs.  At the Defence Science and Technology and Laboratory or DSTL in Porton Down, researchers including Professor Peter Brown work on finding the right balance of material properties for the perfect armour.

Peter -   People have been looking for good armours for centuries and the trouble with armour is that as soon as you create a good one, somebody creates a bigger, better weapon to defeat it.  So there's no end point you get here and go, "aha!  That's the perfect armour."  The other thing I always find quite intimidating about armour is you're up against Leonardo da Vinci because he invented the bullet.  In terms of opposition, a da Vinci design is not easy to defeat.  He's been dead 500 years and is still causing trouble.  And because the rules keep changing, as soon as you feel as though you've got a grasp on what you might need for an armour, the threat changes and all those rules go out the window. 

Generally speaking, it should be hard.  We find the best high performance armour steels are also hard and the very, very best armours, those that provide the highest levels of protection, are ceramics.  They're unbelievably hard. But you would then have thought that something like toughness is a good thing.  If you're going to give your armour a whacking great clap from a fast moving projectile, something that's tough, common sense would tell you, it is a good idea.  But you look at ceramics they are as brittle as anything.  If you drop a ceramic plate, it will shatter.  So, common sense would tell you toughness is a good thing.  So you start using common sense to help you out in armour design and you would probably come a cropper quite quickly.

Ben -   I assume they can't just be a one-size fits all armour as well, if you are putting something on the outside of an armoured vehicle, that must be very different from the body armour that a soldier would wear.

Peter -   That's the trouble. An armour that's good for one threat, say, armour piercing 762 ammunition, that solution probably won't work for your improvised explosive devices.  So, not only is the armour properties dependent on a whole range of properties, it's also dependent on what the threat coming in is as well.  That gets you to the point of trying to understand what happens at the ballistic event as the threat and the projectile, whatever it might be, hits your armour, whatever that might be. That moment of impact is an unbelievably violent event, any sensor you're putting there, by definition, it's destroyed as the bullet hits it, so it's really very difficult event to get to. 

We're getting closer and closer with modern characterisation techniques, but we're not there yet.  Until we actually understand the ballistic event itself, I think we're going to struggle to design armours in the sense that buildings are designed.  That's thoroughly understood and if you design a structure, or a building, and the computer says "yes" you don't build a model you just build the building!  Materials modelling is not at that stage yet.

Ben -   How do you test the material?  Presumably, you are just simulating these impacts by actually firing projectiles at a block of material.

Peter -   Yup!  Absolutely right.  There's no substitute for putting it on the range and shooting at it because at the end of the day, you're asking people to put their lives on this technology.

Ben -   So how did you come to be testing super bainite as a potential armour?

Peter -   Well, it's a steel.  That's a good start.  The thing about steel as general comment is that it's often the solution to several questions in one material, without that much change in its compositional processing.  So in terms of how well super bainite performs, bearing in mind that most things have no useful usable ballistic properties, super bainite is right out there with the very best armour steels have ever been invented.  I think to be honest, armour steels for the foreseeable future are probably at their limit of ballistic performance.  But yeah, super bainite is no better than the best, but as good as the very best.  And trust me, that's saying something.

Ben -   So how do you now see this being deployed?  Is this going into body armour or is this going to line the next generation of armoured cars?  Where do you see super bainite turning up?

Peter -   The principal application will be for the protection of land vehicles - armoured fighting vehicles that go in the battlefield, through to diplomatic limousines.  It has commercial uses as well.  You can use it for steel toe caps and hacksaw blades.  You can keep producing super bainite for non-defence applications day in and day out, and you can sell as much of that stuff as you can make.  And then when there's a defence requirement, you just put in an order.  The standard super bainite composition is slightly tweaked for defence applications.   And that's where partnering with an industrial company has proved very useful.  Not only are they're prepared to make the alloy, they may have to prepare to make it in a range of different product types.  You can buy in the solid plate or you can buy as perforated plate which has an array of holes in it that reduce the weight and increases the ballistic performance - although traditionally putting holes in the armour is the enemy's job.  It turns out that if you do it right, it's now our job.

Ben -   Why would putting holes in armour make it better?

Peter -   Yes, so if you're feeling confused at this point, join the rest of the world.  That is an entirely rational and common-sense based feeling, but I've warned about this common sense earlier on.  In fact, we're not putting holes into the armour, we're putting in edges. If you drill out half the steel and replace it with holes or fresh air, the bullet will always land near an edge, and the material will fail in an uneven way and cause the bullet to topple over, and it stops becoming a fast moving sharpened projectile pointed in the right direction and turns into a blunt toppling fragment.  It still has a lot of energy, but at least a sharp pointed bit isn't heading in your direction anymore. 

We've got means of energy absorption and that toppling mechanism is a way of defeating the sharpened projectile concept that da Vinci originally came up.  The thing about holes is, if they're too big, then all the bullets just go through the holes.  So, there's a ratio of hole size to the threat size and if you've got a big threat, you need bigger holes.  But that means that the smaller threats, which are still very dangerous, can get through.  So, you have to move away from the circular hole to a slot that's quite narrow, but it still has the same amount of edge length.  That slotted perforated design is the one that we are probably going to go forward with and commercially exploit fairly shortly.

Ben -   Professor Peter Brown, explaining why you should throw common sense out of the window and trust that holey super bainite will make good armour.