Cracking idea

When the Boeing 787 Dreamliner enters service in 2008 it will be lighter and more fuel-efficient than other passenger airliners thanks partly to composite materials, which make up half of its construction. A new method of monitoring such composites could help detect flaws and microscopic cracks, keeping the aircraft airborne and safe.

Composites are being used in a variety of applications because of their lightness, strength and resistance to corrosion, but if a tiny crack develops deep within the layers it is often undetectable until the damage is too great.

That is why two researchers at the University of Delaware put their heads together to find a way of testing the integrity of composite materials in a minimally invasive and effective manner.

Tsu-Wei Chou and Erik Thostenson have discovered that embedding carbon nanotubes as part of a sensor network throughout the composite will help monitor the health of structures.

Carbon nanotubes are known for their uncanny ability to conduct heat and electricity. In the latter case, they are 1,000 times more effective than copper.

Also, their minute size allows them to penetrate areas between bundles of fibres and the various layers that make up composites.

The researchers have found that when these electrically conductive nanotubes are networked throughout the composite material they can act in much the same manner as human nerves. When an electric current is passed through the network, if there is a microcrack, the pathway of the sensors is broken and researchers can measure the response.

Thostenson said cracking is a common occurrence in composites because the material has inherent weaknesses that begin with its construction. 'We make traditional fibre composites with sheets of carbon fibre woven together. We stack the sheets in one direction and then infuse the polymer and the plastic to glue everything together,' he said.

While the fibre component of the composite is rather strong, the glue-like matrix material that holds it all together is weaker. This results in weak spots, said Thostenson, especially in the interface areas in the matrix materials. These can lead to tiny microcracks, which over time can threaten the integrity of the entire composite.

'What's unique about our research is that, not only is the technique very sensitive to detecting the onset of this cracking,' he said, 'but we can distinguish between the matrix micro- cracking in the fibre bundles and also between the layers of the composite.'

He added that testing the health of composite structures is becoming increasingly important because of the growing list of composite applications.

'There have been many techniques [for measuring the health of composite structures] some involving ultrasonics, others using fibre-optic sensors,' he said. 'However, these techniques require expensive equipment and it is very difficult to detect for certain the onset of microcracking.'

Thostenson imagines composite substances infused with carbon nanotubes could be tested in a way that is cheaper, easier and more efficient.

'You could envisage having some sort of system where you place electrodes on an aircraft wing and measure the resistance response between all these different electrodes,' he said.

He said their method for dispersing carbon nanotubes throughout a composite is readily scalable for a variety of applications.

When the researchers fabricate the composites in their laboratory at Delaware's Centre for Composite Materials, they use a vacuum-assisted resin transfer moulding technique to disperse the nanotubes in a plastic — in this case an epoxy resin. They suck the resin through the layers of fibre in a vacuum. The nanotubes comprise only 0.15 per cent of the composite volume.

While the applications for a substance like carbon nanotubes seem endless, Thostenson said that because they are expensive and difficult to obtain in large quantities they are rarely used.

'But as we develop more and more applications for nanotubes the supply will increase as well,' he said. 'There are many future possibilities in the area of nanotechnology, but this particular application is something that could be applied today.'