[Sound of a traditional craft loom working]
In this research laboratory cutting edge science and technology is getting a helping hand from an old fashioned machine, a traditional craft loom. But instead of weaving cotton for clothing it’s making a completely new type of fabric. It’s a material that will give much better protection from the effects of bomb explosions and severe weather events such as typhoons and hurricanes. The project uses auxetic materials and is led by Professor Ken Evans at the University of Exeter. Ken is a world leader in research in this area.
Professor Ken Evans [KE]
An auxetic material is unusual in that it’s a material which when you pull it does something that you would not normally expect. If you imagine pulling a rubber band two things obviously happen. When you pull it, it gets longer and at the same time it gets thinner and this is a very easy thing to see with a piece of rubber. Auxetic materials do exactly the opposite when you pull them they get longer, but at the same time they get fatter. This particular project is about blast mitigation, it’s about using auxetic materials in the form of a fabric to make a textile which we can then produce a curtain material from which will act to mitigate glass in an explosion situation. And Auxetic materials, we believe, have particular characteristics which mean they will absorb energy much more effectively than the current conventional materials.
The work in this particular area is incredibly new. Although auxetics have been around for 20 years in total, the idea of putting them into a fibre in a fabric is something that’s only been there for the last two or three years.
The hands on work of identifying which materials need to be used, the manufacturing and testing of the yarns and analysing the results of those tests is carried out by Dr Mike Sloan.
Dr Mike Sloan [MS]
The really useful thing about this research is that we’re taking conventional fibres. These are fibrous materials that you can buy off the shelf. You’re talking about stretchy materials, elastomeric materials like polyurethane and then combining them with a higher performance fibre like a dyneema, and also ultra high stiffness carbon fibres and it’s the method in which we combine them that gives them the auxetic effect. So its conventional materials in a helicular arrangement that gives the auxetic effect and there are a number of parameters we can change. We’re looking at the stiffness ratio between the two fibres and also the angle at which the second fibre is wrapped around the middle fibre. It’s a case of identifying what fibres we want to purchase off the shelf, what conventional materials we need to get. We will characterise those in a mechanical testing machine: how strong are they, how much do they stretch. We designed and built a purposely designed spinner here at Exeter and we can load our core fibre onto a single spool feed spool that’s taken up at the end on a take up spool and we just programme in the relative speeds of the actually spools and will manufacture a yarn very accurately controlling what we call the wrap angle: how tightly the wrap fibre is wrapped around the core fibre. Then that yarn will go back to a mechanical testing machine which will characterise its mechanical performance but also it shape change: how auxetic is it when it gets this much longer, how much wider does it get. Then those most promising yarns will go forward onto our loom.
[Sound of a tradition craft loom working]
The loom that we actually use is designed and sold as a craft loom. There’s nothing special that we’ve done to the bit of equipment, there’s no modifications that we have made, the only thing we do is insert our auxetic yarns to make an auxetic textile.
[Sound of a tradition craft loom working]
But we actually take our auxetic yarns, load them onto what we call the shuttles, they are the noisy parts that fly left to right on the loom, and by using the computer controlled software on the loom we can actually change the weave pattern as well.
A crucial part of the design process involves the computer modelling work carried out by Julian Wright, a research fellow at the University.
Julian Wright [JW]
If we want to make enough yarn to make a curtain size piece of fabric for example. that will be several hours worth of time so that’s a very expensive mistake if we don’t get the yarn right at the design stage. A key facet of the computer modelling is to be able to get the yarn right without actually having to spend hours making it and then seeing if it was right. The computer modelling at this stage is concentrated on the yarns, so we are not yet modelling the textiles and we’re certainly not yet modelling the explosions. We’re specifically interested in how will the yarn behave when we stretch it, how fat do we need to make it, how stiff do we need to make one or more of the components, what angle do we have to wrap the helicule component at, and how far can we pull it. One of the major parts of the modelling activity at the moment is to be able to predict the behaviour of the yarn from a knowledge of the two components. We know what polyurethane is like, we know what polyamide is like for example, what we would like to know is if we wrap those two together how will they behave. The major achievement so far of the research, certainly in the context of the computer model, is that we now have very detailed knowledge. We understand very well how to design a helicular auxetic yarn and that’s a major step forward from 12 months ago as it was all based on intuition at that stage.
[Sound of a bomb blast]
The tests that the team has used to put the material through its paces show that the shock wave from a bomb blast travels more than 1,500 mph. Ken and Mike explain what the tests have shown.
The very first thing you see is the light that comes from the blast and then a pressure wave arrives from the explosion and that moves the curtain inwards, as you might expect it to do, and then following on from that you then get destructive damage. So if you have a glass window, the glass shatters and breaks. Now what happens is the curtain is moved by the pressure wave first and because our fabric is auxetic it opens out in a particular way. A curtain at that stage is not damaged by the blast at all and it begins to form almost like a fishing net so the glass fragments then arrive after the pressure wave and are essentially captured by the curtain. At this point the curtain starts returning to its original shape, it stops ballooning out, it moves back in the other direction and in fact what we see is the glass fragments collected and essentially thrown back, almost like a trampoline effect out of the room. So you can see the energy absorbing mechanisms taking place while this process is going on and what’s been particularly useful is to be able to do the tests with very high speed cameras to see exactly what the mechanisms are, so we can understand how the total process works.
We’ve got pressure sensors before and after the curtains and the two things that are of real interest are the peak pressures that we measure and also the time duration. When you take the area under that curve you get what we call the impulse and that’s really the energy that’s experienced as a function of the blast. Our preliminary data has shown the curtains give a 25 per cent reduction in that impulse, so that’s a 25 per cent reduction in the energy that somebody the other side of the window would experience.
As well as support from the Engineering and Physical Sciences Research Council Julian says there are a number of organisations involved in helping to take the research to the next stage.
The industrial partners in the project include the Centre for the Protection of National Infrastructure, the Home Office Scientific Development branch and Molefield Trading Company.
Ken hopes that it won’t be too long before we see the fabric in general use.
I might be being a bit optimistic here but I would say that within five years you could see commercial fabrics on the market providing they meet the promise that we believe they’re going to do. We also have a spin-out company in the University called Auxetics Limited, which has grown out of the research we have been doing for the last ten years or so, which is looking at a range of applications and will also be involved in the commercial development of this particular fabric.
The managing director used to be one of my research students and he was funded on an EPSRC -funded project as a research student. He then moved onto a post doc for two years and after that it looked as if there was sufficient interest commercially that he went on and set up his own company, so that company was largely started by an EPSRC-funded activity.
The research team say there are many potential future uses for this exciting area of work.
The intention is that the fabric should be as transparent as possible in the application areas where you want them for a curtain. Not all the fibres we are currently using are like that, but in principal there’s no problem with them being transparent and having these properties.
We know that these textiles when you stretch them actually open up so you can have a filter that could actually be self cleaning.
Dental floss is an interesting example where if you were flossing your teeth to have a fibre that actually expands when you pull it filling all the gaps and cracks is an interesting idea.
Civil engineering, reinforcement of soils, storm and flood mitigation and there are a number of options for bandages. One’s for wound repair: you can have an auxetic bandage that is actually impregnated with an antibiotic. If a wound becomes infected and swelling occurred that would place the bandage under attention, the pores would open up and it would release the antibiotic thereby becoming a smart self healing bandage.
It’s an amazingly exciting area to work in. It’s particularly exciting when you explain the concept to someone who doesn’t understand it. There’s a moment where the light bulb switches on because it’s a counter intuitive property. Auxetic materials are counter intuitive so you have to start by explaining that the conventional material, like the elastic band, when you stretch the elastic band it gets narrow, most things get narrow when you stretch them and then you say that auexetic structures get fatter when you stretch them. And there’s a moment where it doesn’t quite click when you are explaining and then suddenly you see the light bulb switch on and that’s a really rewarding and stimulating moment.