Seal of fortune

The high strength-to-weight ratio of composite materials has led to their increasing use in aerospace applications, but the materials still have drawbacks.

One is their susceptibility to damage. Where metals can deform plastically by denting and crumpling, which absorbs the energy of the impact and retains structural integrity, fibre-reinforced composites tend to crack and splinter.

What’s worse is that the damage can sometimes be very difficult to detect, as it tends to occur on the inside — the opposite face of the composite sheet to the impact. This is known as barely visible impact damage (BVID), because visual inspection is likely to miss it completely, making it very dangerous.

Researchers at Bristol University’s aerospace engineering department are working on a system to make it much easier to see damage to composite panels, and effect a temporary repair to keep the component functional until it can be replaced. Working with Hexcel Composites, Ian Bond and colleagues are trying to replicate biological healing mechanisms to devise a ‘bleeding composite’ material. The research could eventually lead to vehicles equipped with their own circulatory systems for repair and cooling, said Bond.

The basic concept of a self-healing composite is simple. Among the reinforcing fibres of the composite are mixed hollow glass fibres filled with one of two materials. One, is a highly conspicuous medium such as a dye that fluoresces under ultraviolet light, while the other is a liquid repair agent, such as a two-part epoxy resin, with the components contained within adjacent fibres.

On impact, the fibres break and the encapsulated materials leak out, marking the damage and spreading the repair agent throughout the break. ‘The results we’ve achieved so far indicate that the system works,’ Bond explained. At the moment its main advantage is that it makes the damage temporarily safe, preventing cracks from spreading.’

The Bristol team is researching two types of composite: glass fibre-reinforced plastic (GFRP), for space applications; and carbon-fibre reinforced plastic (CFRP), for aircraft. Each face different conditions, and have their own set of properties and problems to be overcome.

The cycling of heat and cold which spacecraft undergo can lead to cracking, as the material expands and contracts; plus there is the possibility of damage from orbital debris. CFRP is less likely to suffer from thermal damage, but presents a different problem for the researchers — the mismatch in mechanical properties between the glass and carbon fibres means that the glass fibres cannot act as reinforcement.

‘Eventually, we want to move to an all-carbon reinforcement, because it’s stronger and stiffer, which makes it better for all applications,’ said Bond, ‘but there are advantages to working with glass. You can see what you’re doing, and it’s fairly easy to tailor the fibres — you can decide exactly what force will be necessary to rupture the glass. Carbon fibres fail and fracture in a different way, and it’s more difficult to design them to fail reliably.’

The team is attacking the project from several different fronts. The healing material itself needs to be made stronger and faster setting.

Another aspect is to minimise the ratio of healing fibres to structural reinforcing fibres, to ensure the composite is as strong as possible while still retaining the healing properties.

Because the bleeding composite material works completely autonomously this would make it highly cost-effective, said Bond. But the team’s eventual goal is to develop the system into a complete ‘circulatory system’ for a vehicle. ‘We could use it to carry self-healing medium around the vehicle, or coolant, or any other fluid,’ said Bond. ‘It could be like a living circulatory system.’