Damage control

A team of UK researchers has been awarded a £1.2m research grant to develop and integrate crack-arresting and self-healing capabilities into composite structures.

Engineers from Bristol University and Imperial College London will collaborate on the ambitious CRASHCOMPS (crack arrest and self-healing in composite structures) project to explore approaches to controlling and ’curing’ compression fractures in carbon fibre epoxy composites.

The project aims to benefit military and commercial aircraft as well as maritime applications — areas where composite structures are becoming more prevalent.

The use of composites, however, is restricted by design constraints that have led to heavier components being used. This is to compensate for their sensitivity to low-energy impacts caused by hailstorms, runway debris, dropped tools and, in some cases, catering trucks bumping into commercial airliners during turnaround.

The CRASHCOMPS project starts from the premise that defects in a composite structure should not hinder design. instead, they should be anticipated and compensated for via defect arrest and healing.

’Ours is a new approach,’ said Dr Ian Bond of Bristol’s Aerospace Materials department. ’Damage will happen, but we aim to steer it where we want it and then do something about it once it is isolated within a structure.’

The team plans to design a preferential growth path into the local material. This area would be shaped like a picture frame, with two vertical sides as the redirecting paths and the top and bottom as the self-healing areas so that when the crack gets redirected into the healing areas, a resin will be released into the composite to fill the damage.

In its work on damage tolerance, the Bristol team will investigate, among other ideas, the use of piezoelectric materials and a biomimetic ’vascular’ network to arrest, redirect and repair compression fractures.

With the piezoelectric design, materials become stiff when voltage is applied to them. In this case, the crack could possibly trigger some sort of capacitance in the structure that would then trigger the voltage and direct the crack along a defined path.

In the vascular design, a network of arteries can be created within a laminate structure that carries a circulating resin through the structure. When damage occurs the resin moves to the damaged area to repair it.

According to Imperial’s Dr Emile Greenhalgh, the healed crack is not quite as good as when pristine, but does come close.

’An aircraft structure is designed to fail at the design ultimate load but generally you fly it at about 66 per cent of that level. The work from Bristol does indicate that it is almost achieving the original pristine properties,’ he said.

Another part of Bristol’s work is to optimise the self-healing resin so that it will be liquid enough to rapidly run back along the crack but also remain structurally strong. A challenge of using this resin, however, is the high volume it occupies in the structure, which is why Imperial is also looking at other less volume-consuming methods

’One of the exploratory themes is to use something called polyHIPEs (high internal phase emulsions) which seals using foam.’ said Greenhalgh. ’The idea is to have a reservoir of this material and then when damage is introduced to the region, the presence of air causes a chemical reaction — and the material foams and fills up that volume [of the crack].’

The project’s ultimate goal is to arrest a crack, which researchers believe would be more difficult than redirecting one. To this end, Imperial plans to explore novel techniques, such as modifying the carbon fibres.

Greenhalgh said: ’One of the things we’re looking at is exploiting hairy fibres, where carbon nanotubes are grown off the carbon fibres. If we have nanotubes growing from the fibres, the nanotubes will extend into the resin and locally stiffen the resin and stop cracks growing into it.’

Using this method, the interface gains a much higher surface area and there is a higher load transfer between matrix and the fibre.

’We acknowledge that this is far-reaching stuff and quite adventurous,’ admitted Bond.

Anh Nguyen and Jason Ford