Giving aeroplanes the power of self-healing
The importance of damage tolerance in aircraft was recognised as long as 400 years ago by Leonardo da Vinci. The quest to improve tolerance continues, with much of the focus today on materials. Two EU-funded projects are developing advanced materials with the potential to both improve damage tolerance and the durability of composites.
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The multifunctional composite materials being researched by the projects IASS and HIPOCRATES could revolutionise aircraft structural components and have a major effect on many aspects of the aerospace industry.
Their advantages include enhanced reliability and service life for aeroplanes, which could cut accident rates by more than 60%. In addition, aircraft operating costs could drop to 50% of those of today, as inherent protective and smart abilities could help to significantly extend inspection intervals and reduce need for repair. This would result in less downtime – which could also substantially reduce disruption to air travel. Finally, they could help make aeroplanes lighter, thus reducing fuel consumption and related CO2-emissions.
The IASS approach involves a multifunctional epoxy resin, which the project consortium has endowed with characteristics such as high electrical conductivity, high flame resistance, excellent mechanical properties and self-healing capacities. The idea is to embed conductive nanofillers in the epoxy resin to create a carbon-fibre reinforced composite. Microcapsules containing a self-healing agent are added to the mix, in which a catalyst has already been dispersed.
“When a micro-crack occurs, the capsules break and release the healing agent, which comes into contact with the catalyst,” explains Giuseppina Barra of the University of Salerno (UNISA), which coordinates the IASS project. “The resulting reaction closes the crack. Of course, in order for this to work, the crack cannot be very large, but has to be in the micrometre scale.”
“One very important aspect is that this reaction also occurs at very low temperatures, as they occur outside an aeroplane in flight,” adds UNISA’s Liberata Guadagno, IASS project coordinator. “Moreover, the catalyst doesn’t deactivate at high temperatures, which are needed for the curing [hardening] of the polymer.”
The team’s work is well on track: they have successfully put together all the elements – epoxy, nanofillers, and microcapsules – and created a novel carbon-fibre reinforced composite as planned. However, the production rate of the microcapsules is very slow.
“Therefore, the technology is not economically viable at this stage,” explains Michael Papadopoulos of EASN-TIS, dissemination manager for the IASS project. “This technology could only be implemented if there were a means to produce the microcapsules in a more economically sustainable way, i.e. a greater number of microcapsules in less time.”
But the IASS material already has its uses: UNISA has taken out two patents in Italy and has filed three patent applications in Europe on the multifunctional epoxy formulation that is basically ready for industrial use. It has all the characteristics IASS aimed for, i.e. high electrical conductivity, high flame resistance, excellent mechanical properties, low moisture content, except self-healing, which will require more research.
A threefold challenge
It is in this research into self-healing materials that the HIPOCRATES project, too, is aiming to make headway. The project is based on a resin that is already certified and widely used in the aeronautical industry to reduce certification time and costs further down the line; the consortium is looking to transform the material.
In this, one strand of HIPOCRATES is fundamentally following the same strategy as IASS: to create a composite material that will self-repair tiny cracks autonomously. The difference is that HIPOCRATES is using a different healing agent and catalyst, leading to a different chemical reaction.
“The second approach involves reversible healing polymers,” says HIPOCRATES project coordinator Sonia Flórez of Tecnalia Research and Innovation. “These materials contain internal linkages, which will reverse the damage and close the crack upon delivery of an external stimulus such as heat, radiation or induction.”
In both of these strands, which the teams will later attempt to combine in a single matrix, HIPOCRATES will also make use of nanotechnology to ease the self-healing process and to further enhance the performance and efficiency of the base material itself.
“This nanomaterial can be carbon nanotubes or ferromagnetic particles to name but a few materials that can provide the polymeric matrix with electrical, thermal or magnetic properties,” Flórez explains. “Besides, the nanomaterials allow us to deliver the external stimulus needed in reversible healing polymers to the damage site more effectively.”
About halfway through its runtime now, HIPOCRATES has its work cut out: both strategies have been detailed and the polymer matrices for some of the most commonly used manufacturing processes (e.g. resin transfer moulding, liquid resin infusion) have been identified.
The challenge now is to efficiently integrate the self-healing technology into the resin and reach a maximum percentage of recovery when it comes to repairing damage, while minimising the impact of the newly integrated components on the resin’s manufacturing process, original performance and properties.
It may be decades before such self-healing materials can be put to use in the aerospace industry, mainly due to manufacturing issues. However, both IASS and HIPOCRATES have already substantially improved our understanding of self-healing technology and are continuing their work to take the progress even further.
Project Website Iass
Project Website Hipocrates
Project information on CORDIS Iass
Project information on CORDIS Hipocrates