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Engineering Research: Self-healing spacecraft
Spacecraft that bleed. Aircraft that bruise. Bridges that heal. OK, so it is not quite robots that feel (yet), but the world of smart materials development is creating some amazing possibilities. David Williams investigates
The iconic image is beautifully calm. But the slow, stately progress of a spacecraft drifting across a background of stars belies the extraordinary stresses and strains it has to go through to stay there.
Hurled upwards on the back of a rocket, assaulted by micrometeorites, and alternately heated and cooled as it passes in and out of the earth’s shadow (perhaps once every 90 minutes in a low orbit), spacecraft have to be extraordinarily resilient. Damage increases over time, and if and when the outer skin fails, it is certain catastrophe for those inside.
The European Space Agency
The European Space Agency (ESA) is an organisation that is dedicated to exploring space and making sure that benefits are delivered to people throughout Europe.
But what if the walls of the spacecraft could bleed and clot like human skin when damaged, creating a scab that would close off the wounded area? This was the possibility that intrigued the European Space Agency (ESA), which came to the University of Bristol’s Department of Aerospace Engineering to find out if such a thing were do-able. Dr Ian Bond, a lecturer in aerospace materials at the University of Bristol, and also a member of the Aerospace Composites research group, discusses the situation in hand.
‘I have always been interested in adding new functions to materials,’ he says, ‘and one of the areas we were looking at here was increasing the strength of composite materials by using tiny hollow glass fibres (only 60 micrometres, thousandths of a millimetre, wide) instead of the usual solid filaments. In fact, Bristol is unique in this field as it is the only institution in the UK manufacturing its own hollow fibres.
‘But although this work did not have quite the result we were expecting, it did open up the possibility of filling the hollow fibres with something that might be useful. We realised that if we could fill half the glass fibres with a liquid resin that matched the properties of the composite material and the other half with a curing agent [which causes the resin to become solid], we would be able to create a material that could heal itself.
‘When damage occurred, the glass fibres would be brittle enough to break and bleed out the resin and the curing agent, so flooding the damaged area with new material that would solidify and give the structure back most of the integrity it had lost.’
It was this possibility that excited the European Space Agency and it turned to Bristol to run a one-year feasibility study into creating a self-healing material that could be used in space. Of course, there is no air in space, and Dr Bond and his colleague Dr Richard Task first had to check that their self-healing method would work in a vacuum.
‘One of the problems we first encountered was that the liquid resin wanted to evaporate off in the vacuum chamber,’ he says. ‘In fact, a kind of race took place between the solidification process and the urge to evaporate. In the end what amazed us was how successful the method was.
‘Although we did lose some resin to evaporation, it wasn’t anywhere as much as we had been anticipating. Of course, there is a downside in that the lost gas has to go somewhere. Spacecraft tend to be full of sensitive instruments that might be affected by its presence. Nevertheless, the feasibility study showed that self-healing spacecraft are a real possibility, and ESA continues to be interested in developing the technology even further.’
One of the bars to development is that the team is currently working with off-the-shelf resins rather than one that has been specifically designed to work in a system that can heal itself. However, the chemistry that would be needed to tailor a resin with the specific properties the team needs is not impossibly difficult. Dr Bond expects that, as the technology develops, chemists will come forward with ideas for designing a resin with the right properties.
According to ESA, the promise of self-healing spacecraft has the potential to open up the possibility of longer-duration missions. ‘The benefits are two-fold,’ says ESA. ‘Firstly, doubling the lifetime of a spacecraft in orbit around Earth would roughly halve the cost of the mission.
‘Secondly, doubling spacecraft lifetimes means that mission planners could contemplate missions to far-away destinations in the solar system that are currently too risky. In short, self-healing spacecraft promise a new era of more reliable spacecraft, meaning more data for scientists and more reliable telecommunication possibilities for us all.’
Other uses of self-healing material
However, using a self-healing material in space is just one application of the technology. Safety-critical structures such as aircraft or bridges cannot afford structural failure and so are candidates for using self-healing materials.
For example, the airline industry currently spends a fortune on doing what are essentially visual inspections of aircraft.
This is done to check that tiny amounts of damage caused by fatigue and the minor impact of things such as small stones thrown up on the runway are not compromising the whole structure.
Although this is done in the hangar for most commercial aircraft, pilots of smaller aircraft will still walk round planes simply to check that there is no damage large enough to be seen. This brings in a second possibility of self-healing materials. Not only could they bleed, they can also bruise.
‘This is the second advantage of the system that self-heals,’ says Dr Bond. ‘You can mix an ultraviolet fluorescent dye or a magnetic particulate in with the healing resin. This will illuminate where any healing event has taken place and thereby simplify the inspection process for subsequent permanent repair.’
In order to create an aerospace material that can bleed and bruise, the team will have to integrate its glass fibre system into the carbon fibre composite materials that are used by the aerospace industry because of their strength and stiffness.
Inevitably, integrating a healing material into a second material is going to cause at least some deterioration of the properties of the original material. So balancing these plus and minus factors will be part of the engineering problem the team has to solve.
One of the optimisations they are working on is the need to include a sufficient number of hollow fibres within the material to create the healing effect, but not so many that it destroys the integrity of the original material. So where you put the fibres matters.
Other factors include the sort of damage that is anticipated. If you have the glass fibres nearer the surface, they are more likely to be damaged in an impact, but it may be that it is the deeper damage that is structurally more compromising. The arrangement of the hollow fibres is therefore very dependent on the context.
When it comes to placing the glass fibres in the carbon fibre composite, other factors come into play. Carbon fibre composites are made from sheets of carbon fibres embedded in a tacky resin that hasn’t yet solidified. In a process that it is analogous to pastry making, the sheets are cut up, stacked together and then baked in an oven so that they cure and consolidate. The glass fibre healing ‘veins’ can be inserted between the layers in an arrangement that maximises the healing opportunities but minimises the creation of a plane of weakness.
‘The good thing is that it can almost be achieved,’ says Dr Bond. ‘There is a little bit of a loss of structural integrity when you replace carbon fibre with glass, but what we are finding is that the healing efficiency is quite significant. It is, of course, a commercial decision, but it may be worth a small loss of properties to acquire the ability to heal after a damage event. The penalty isn’t that great and the recovery of property is quite significant.
‘Indeed, there may be something to be gained by the process. At present, there are what are known as “design allowables” in aerospace design. Engineers assume that the structure is already damaged when they design a component for an aircraft.
‘Essentially they build in more capacity than is required and so are not squeezing as much performance from their materials as they might in other situations. If we can demonstrate that healing structures work, it may be that in the future less material could go in to the structure in the first place, so saving weight and, of course, money.’
The final area the team is working on is to include in their system a reservoir of healing fluid that can be pumped to the damaged area from a central source. Instead of using unconnected glass fibres which, once they have bled out, remain empty, the team envisages a vein-like network of fibres delivering as much healing resin to the structure as is needed.
As well as carbon-fibre composite materials, the aerospace industry also utilises so-called sandwich structures, in which a layer of foam sits between two layers of composite material. This creates a structure that is thick, rigid and very light. These sandwich structures are often used for areas such as the floor of aircraft, and they offer the Bristol team the possibility of using the foam core as a place in which the reservoir of healing resin can be stored. An array of small-bore tubing could lie in foam and feed pipes going up to the skin of the structure.
‘One of the failing points of these structures is when the skin comes away from the foam core,’ explains Dr Bond. ‘Our system should be able to stick the skin back on the foam core as it tries to peel away.
‘In fact, we can take a pristine sandwich structure, damage it to the point at which it is beginning to fail in this way, and then demonstrate that we can return it to its normal state, restoring its original properties. Being vascular, the system would be able to heal another event near by.’
David Williams is a freelance journalist reporting on higher education and careers.