PORTLAND, Ore. — Nanotechnology can't relieve airport congestion, but it could be used to repair defects in aircraft wings.

Researchers at Rensselaer Polytechnic Institute (Troy, N.Y.) have demonstrated the ability of carbon nanotubes to enable the self-healing of defects in aircraft wings and other polymer composites. Carbon nanotubes mixed into an epoxy coating that adds just 1 percent to a polymer composite's total weight enabled a matrix of embedded wires to initiate self-healing repairs in structure surfaces.

"The really novel part of our technique is that nanotubes are used to both detect and repair cracks that otherwise might grow to affect the structural integrity of an aircraft wing or other polymer composite," said RPI professor Nikhil Koratkar.

A carbon-nanotube laced epoxy surface treatment was applied over an embedded wire matrix that forms an x-y grid over the entire surface. By scanning through the grid with a signal probe, tiny cracks can be detected. Cracks were sensed as a higher resistance when a probe signal had to travel a longer distance around a crack to complete a circuit.

Once a cracked region is identified, self-healing was initiated by a higher-current pulse through the nanotubes laced into the cracked region. The embedded nanotubes provided the electrical conductivity required to heat that region with an short, but high-current, pulse. Heat-activated healing agents mixed into the epoxy then flowed into the crack to stop its growth and recover 70 percent of the structure's original strength.

Commercially available healing agents can seal and recover most of the strength lost due to nanometer- or even micron-sized cracks in polymer composites. However, identifying the source and location of a crack remains a problem.

RPI's method solves the problem by allowing every square inch of a wing to be scanned for defects before each take off. If cracks are detected, they can be repaired immediately by merely increasing the level of the electrical signal to that region, thereby melting the embedded healing agents. The technique can also detect and repair less common forms of structural damage such as the delamination of a polymer composites layers.

The RPI team is also developing software that can automatically scan structures for cracks, delamination and other defects and repair them in real time using controlled high-current pulses. The team is also characterizing the degree to which the embedded carbon nanotubes--which are 1,000-times stronger that steel--inhibit the development and growth of small cracks in polymeric composites.