Our bodies are capable of some amazing things. One of the most amazing, from the viewpoint of a materials scientist, is our body’s ability to heal itself. If for some reason you cut yourself, your body will leap into action, sealing the gap and generating replacement skin that is just as good as new. All without any conscious thought on your part.
Wouldn’t it be great if aircraft could do the same? Instead of having to regularly inspect and repair aircraft structures, both of which cost time and money, we could instead happily fly along, secure in the knowledge that any damage would be fixed, as if by magic.
It’s no surprise then that the announcement by a group of researchers at the University of Bristol that they had developed a ‘self-healing aircraft wing’ made waves. Just some of the places the story was covered include the BBC, the Independent, Popular Science, and Forbes.
The process works by producing tiny capsules filled with a healing agent. These are then embedded into the normal carbon-fibre composite materials used for aircraft structures. Additionally a catalyst is mixed into the composite material. If a crack starts growing in the material, it will break one (or more) of the capsules. This releases the healing agent, which then encounters the catalyst and cures, repairing the crack.
Reading the news coverage, you might think that the Bristol group were the first to come up with this idea. However, this process was actually invented by a group at the University of Illinois, some 14 years ago. Since then there’s been a bunch of research on the topic of self-healing, and a variety of different mechanisms have been devised. This article from the Smithsonian’s Air & Space Magazine has a nice overview.
To figure out what’s special about what the Bristol group have done, the news coverage won’t help you much. Instead you should turn to this recording of a talk given at the recent Catalysis Improving Society meeting at the Royal Society.
While the process developed at Illinois worked well in a lab setting, it wasn’t robust enough to work in a real-world setting. For example, the micro capsules used at Illinois wouldn’t survive the production process required to make carbon-fibre composites. They also required an expensive catalyst.
The Bristol group have managed to produce more robust micro capsules (that will still break when needed to release the healing agent) and have found a much cheaper combination of healing agent and catalyst. These are all important steps towards actually getting these materials in service.
You see, just getting self-healing to work in the lab is the “easy” part. Before you can actually use it on a plane you’re going to have to answer many more questions, such as: Do the micro capsules make your material less stiff or strong, or more prone to cracking in the first place? If so, does the benefit of less maintenance outweigh the cost? Does the self-healing process also work at cruise level, where the temperature is -50C? How about when your plane is sitting in Dubai (black coloured structure can reach 80C or more). How about if it rains while your healing agent is curing? If it does still cure, does the moisture or temperature affect the final strength of the ‘healed’ structure? How much more expensive is self-healing material to make? How long will a crack grow before it hits a micro capsule? Can you guarantee this? Will the healing agent still work after 20 years of flying around? These questions need to be answered before the airworthiness authorities will allow self-healing materials on aircraft, at least if you are going to rely on the self-healing property, rather than inspections, for safety.
None of these things are necessarily things that can’t be overcome, but they will require a lot of research (i.e. time and money) to solve. That’s why I’m rather sceptical of the ‘five years’ some of the news stories are quoting before we see self-healing wings in service. For one thing, neither Airbus nor Boeing are likely to produce a new aircraft design within the next 5 years. Given the cost and effort needed to certify new materials, they’re unlikely to introduce self-healing on any of their current models.
To sum-up, while this is interesting research, the media have rather oversold both how revolutionary it is, and how close it is to reaching service. So sleep soundly aircraft maintainers, you’ll have work for another decade or two yet.