Designing stronger structures

19 March 2019

Interview with 

Harry Bhadeshia, University of Cambridge

STEEL-SHEET

Steel sheet with drilled holes in regular pattern

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Earlier in the show, Jack Tavener was with blacksmith Magnus Sigurdsson at his forge; he kindly lent us a beautiful Japanese blade he’d made called a katana; Jack took it to the Materials Science Department at the University of Cambridge, to show it to steel specialist Harry Bhadeshia…

Harry - Making something like this takes enormous individual skill. The skills would have been developed for generations really, but this is made from a single material. This would be steel and it would have a carbon concentration, perhaps about 0.58%.

Jack - Is that a lot of carbon?

Harry - It's a lot of carbon compared with say structural steels because this does not need to be joined. If you put too much carbon you can't actually weld the material. This will contain millions and millions of crystals, with carbon being forced into those crystals by this rapid cooling.

Jack - And that's what traps them in?

Harry - That's what traps them in.

Jack - It's only in the modern era that we've come to understand why the techniques the blacksmiths have developed over thousands of years actually work. It’s all to do with how the atoms in the material are arranged and what other atoms are present too. In the case of steel in our sword blade a smattering of carbon atoms in the right places makes the material much stronger than the iron on its own.

But rather than just understanding the properties of a materials we have already, metallurgists like Harry are also trying to design new, even more resilient materials by modelling the all-important crystal structure of the metal in a computer.

Harry - You might use computer modelling to get the best possible way of forging the steel, because forging is used for many critical components. The discs that go into jet aircraft which hold the blades, those are called critical components, that means that if they fail you might bring the aircraft down. You have to be very careful of how you forge it and get the right structure inside that disc in order to avoid that problem. And it's also not to say that computer models always work because the subject is incredibly complicated and we are continuously trying to make better and better methods of modelling materials, but we are not there yet.

Jack - Nevertheless, this approach has enabled Harry and his team to develop some extremely effective new materials. He pointed out a sheet of dark grey metal, roughly the size of a computer monitor that's just 5 mm thick but still capable of stopping a bullet in its tracks. Surprisingly, it's also not a solid sheet; it's covered in a regular pattern of tiny oval-shaped holes and has perforations, and the material itself is a form of steel the team have engineered called super bainite. It's special because the production process means that the metal forms billions of tiny crystals, a tenth of the size found in normal steels and this is key to its strength.

Harry - There are so many boundaries between crystals that if I add up the area of the boundaries it's about a hundred million square metres per cubic metre. That is a very very large density of boundaries between crystals! Now why is that important? Because the boundaries strengthen the material. So this has a strength of about two and a half gigapascals, which is like putting two and a half billion apples in one square metre - it's incredibly strong. When you make things strong they are not particularly tough, that means they can't absorb too much energy. So you know, if you fire a projectile at it, it will defeat the projectile but they'll be too many cracks.

So what we do is we put systematic perforations inside the steel which have two purposes; one is that the edges of those holes, they help to deflect or destroy the projectile because they are sharp edges. The second is that because these perforations are smooth, if a crack forms then it’s blunted by them. If you look at this object, it's had multiple hits and yet the whole piece of armour is still integral and, of course, it leads to a reduction in weight which is a good thing if it's on a vehicle.

Jack - It may seem counterintuitive, but the perforations improve the performance of the material when hit, with a bonus of also making it lighter. The super bainite provides strength, the perforated design provide the toughness to prevent it cracking. In other words, it’s a material is designed that enhances the properties further than what the material can do alone.

Harry then showed me how a change in design of the common steel beams we have in all our buildings could lead to them being built for better fire safety. The cross-section of a beam is what you would see if you were to take a slice anywhere along its length. It is this shape that his team have redesigned.

Harry - Now buildings, the frames are made from beans and normally they're called ‘I’ beams because they're shaped like a capital i.

Jack - The things you see on a construction site, the cranes lifting into position, those sort of beams?

Harry - Exactly. But notice this is not shaped like an ‘I’ because the web at the top is smaller than the web at the bottom.

Jack - If you imagine a capital “I”, the webs of the beam are the flat lines that sit on the top and bottom. Usually they'd be the same size, but in Harry's design the top one is made smaller than the bottom.

Harry - The important thing about this is that the top web is smaller than the lower web. And the reason is we wanted to actually put the floor in between these two, instead of on top.

Jack - So rather than resting the floor on the top web of the ‘I’ beam, in Harry's design the floor slots into spaces on the side of the beam, the vertical part of the ‘I’, meaning the top can be much smaller and lighter. But there's another major advantage - fireproofing. Steel softens when it gets hot so building designers currently have to cram large amounts of insulation into the spaces around the beam to prevent it melting. Harry's design, means you can use the floor itself as the insulator...

Harry - Normally you put a lot of fire protection around these beams so that if the building catches fire you have one hour before it collapses. So if you remember the attacks in the USA...

Jack - The Twin Towers?

Harry - The Twin Towers, they collapsed almost after one hour. So you can remove the fire protection from this area where floor slots in and you save height.

Jack - So it's integrated into one? The floor actually protects the steel from melting?

Harry - Correct. And this new concept of Fire protection actually is effective.

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