Bomb-proof materials

What can we do to stop explosives from causing injury? Cambridge engineer Graham McShane works on materials designed to fend off bomb blasts by dissipating the energy of the explosion. It turns out, the same ideas may even prove useful in contact sports such as American Football, as he told Chris Smith....

Graham - So, the key thing in developing protective materials is to control the forces that are transmitted to the object that you're trying to protect. So, when a structure or vehicle is hit by a bomb or a blast loading for example, the pressures can get very high. So, those kind of pressures can cause a lot of damage. They can cause high accelerations of the vehicle which can cause the sort of injuries that Bill Proud was just talking about. So the key for a protective material is to mitigate those pressure loads, those forces that are transmitted to the vehicle. Our research is looking at the use of cellular materials to achieve this.

Chris - When you say 'cellular', can you just explain what that means?

Graham - Examples of a cellular material are foams or honeycombs. So the materials that consist of an array of cells with solid cell walls but largely with air gaps in-between. When you crush the cellular material, they're deformed by the buckling of the cell walls. That buckling helps to dissipate those forces. So the structure that you're trying to protect feels a much lower force over a much longer time period which means less damage and less injury.

Chris - I suppose the automotive industry kind of know this already because cars are designed to have crumple zones, so when you run into a wall, the car crumples up and it takes time for that to happen so all of that force and energy is not transmitted straight into the passengers really quickly.

Graham - It's exactly the same principle, but really, in these cellular structures, we're trying to achieve that at a smaller scale.

Chris - You began with metals to do this for boats and things.

Graham - That's right. We were interested in protecting ships against underwater explosions where the pressures are very high. So we need materials that are going to be extremely strong and be able to absorb very large amounts of energy. That's why we were investigating metallic, steel structures. So we're making honeycombs and corrugated structures out of stainless steel by taking plates of the steel and joining them together by welding and brazing. And putting those inside sandwich structures which have solid face sheets outside these cellular materials and then looking at how they can protect the structure against defects of a blast load.

Chris - Do they work?

Graham - They work very well. So, they're able to absorb a lot of energy, but they're also very efficient structures, these sandwich panels. They're very light and very stiff, so they allow you to reduce the weight of your structure as well. The downside is that they're too difficult to manufacture and they're more expensive.

Chris - Can you take the fact that you've worked out these geometries for these materials to dissipate energy in this way and say, "Well, I'm not going to do it in metal. I'm now going to do it in some new material."

Graham - Absolutely. There are wide range of applications that rely on the same principle. So personal protective equipment where you're trying to protect people's heads or bodies against impact injuries, but the regime of loading is very different - the forces are lower and so on. So you might want to use different materials. This is really where 3D printing is coming into its own. So, we're able to use 3D printing to make these cellular materials in very complex shapes such that they can fit around the body or around the head out of plastics and rubbers, and other softer materials. And we can use the same understanding of how these cellular materials buckle but apply them in these new applications.

Chris - And the connection to American Football League?

Graham - Head injury is a big challenge in many sports. American Football is one, rugby is another where people's heads undergo collisions. People get concussions and that can seriously damage their career or put their health at serious risk. So, there's a huge range of potential applications for these materials in sports.

Chris - So you would take what you're learning in terms of how you're going to mitigate blast to the undersides of Land Rovers, how you're going to mitigate damage to individuals; 3D print rubber materials that could go into say, a hat or a helmet or something; and that could benefit a footballer but could equally well I suppose, find a home on the battlefield.

Graham - Absolutely, but there are a lot of challenges in terms of understanding how these shells of cellular materials deform under impact loads.

Chris - I suppose 3D printing must have revolutionised your work because to knock up those metals, it sounds like that was not trivial, it was trying to do that, but if you can 3D print something, you can do lots and lots of different experiments very, very quickly.

Graham - Exactly, right. So 3D printing gives you a huge flexibility to create a wide range of different geometries out of a wide range of different materials and to produce them very quickly. And so, you can produce one off, you can experiment with different geometries and different designs and it gives us real freedom to explore new solutions.

Chris - Well, things like Alzheimer's disease because of repeated head injury are a big problem in contact sports, so there could be a lot of American football players who have a lot to thank you for in the future, Graham. Thanks very much for coming to talk to us about it. That's Graham McShane; he's an engineer from the University of Cambridge.