Anthony Kelly: composite materials and carbon fibre

Anthony Kelly introduces the history and science of carbon fibre and other composite materials.

At the time a great drive to find stiffer materials for making aeroplanes’ bodies stiffer than aluminium. And the strongest materials are always, come in the form of fibres. Of course you can’t build an artefact, an engineering artefact, out of a fibre by itself, you’ll get a rope, or the sort of floppy clothes you’re wearing that were woven. What you have to do is put that fibre with something. Now my interest in graphite led me to consulting for the Royal Aircraft Establishment. And quite independently of my thoughts, Willie Watt and two other people there, thought about making a form of graphite in the form of a fibre, and they chose a textile fibre called polyacrylonitrile, which was essentially a fibre used for making carpets. And they carbonised that fibre and produced carbon fibres. Now I happened through my knowledge of what they were doing, and they’d told me about what they were doing, they then had to make what’s called a composite material. I explained that a rope by itself is no good, we need to make a sheet, and so you must have a fibre that is weavable in the same way that it’s woven to make your clothes, and of course a carpet fibre is weavable. So you need to put together the fibre plus something to stick it together, called a matrix. Well my contribution was essentially to be able to deduce some general rules about how the resulting composite would behave and principally to explain why it wasn’t brittle. You asked what a composite material is? You came for this meeting in a motor car perhaps, and your motor car had tyres, and tyres are a good example of a composite material. They’re a fibre stuck together with rubber. Wood is a composite material. But the high performance composite materials, fibrous composite materials, are made by impregnating a carbon fibre like that with some sort of resin, and making from it a piece like that [demonstrates]. Or a bigger piece like that, which has fibres running in this direction and in that direction, and we’ve called it black aluminium because it replaces aluminium and it’s dramatically [demonstrates] not brittle, it doesn’t break. If I throw a piece of glass on the floor [demonstrates] it breaks you see. And now you can see the difference. That one didn’t, this one did. I’m sure you’ve all handled the sort of fibreglass mat that you put in your loft to insulate it. Do you know that? Right. Now if you took something like that and put it in a vessel that was closed, and injected a glue, a resin, so it would go in and spread out all among the fibres, and when that resin had cured, in other words the resin had set, you then open the mould and there would be a solid composite material. So the original thing was a wool-like mass, that’s a very cheap form of composite material. If you’re going to make a piece that you’re then going to make – use it to make bits of aeroplanes, you want to make sure that you are using the strength of the fibre, so that means it’s always going to be stressed along its length. So … this consists of individual fibres. Now first of all, consider running something like this through a glue while the glue is fluid. And then while it is still fluid, laying it on a flat sheet and making a thin tape. And this happens to be a tape but with aluminium matrix, and now you’ve got a thin tape, a very long, thin, tape. Now to make this you’re going to have tapes going in this direction, and tapes going in that direction, of that thickness, and they’re going to be woven, one above the other, and that will all be done while the resin, the glue, is still sticky. It’s called prepregging although it is already pregged. That’s then put in a furnace and the resin cured. And then when you open the furnace that’s what you get out. In the case of steel and to some extent aluminium, a carbon fibre of the type like this used in an aircraft, is corrosion resistant, it doesn’t corrode like fibre aluminium or steel. In the case of steel, of course it’s only between a quarter and a fifth as dense, so it’s five times lighter if you like. Over aluminium, because it is lighter and what is called slightly stiffer. Over steel, because it is very, very, very, very, very much lighter, though it is not particularly stiffer, but it’s also stronger than aluminium and stronger than steel.

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