The aerospace industry in USA has introduced yet another use for carbon nanotubes. The innovative idea uses nanotubes to build a self-healing layer into composite structures such as aircraft wings. Just like metal, composites can be subject to tiny surface cracks which lead to catastrophic failures. Majority of failures in any engineered structure are due to fatigue-induced micro cracks that spread to dangerous proportions and eventually jeopardize the structure�s integrity.

Researchers at Rensselaer Polytechnic Institute in New York State have come up with a solution to this. The idea is to embed a fine grid of wires in a given composite surface, covered in an epoxy matrix full of nanotubes. Carbon nano tubes are randomly dispersed in an epoxy resin, which can be moulded into different structures. By infusing the polymer with electrically conductive carbon nano tubes and monitoring the electrical resistance at different points in the structure the location and length of even the tiniest stress-induced crack can be detected. By sending pulses down wires, it becomes possible to know as soon as a crack appears because the electrical resistance of the nanotube filled epoxy changes. Once a crack is located, one can send a short electrical charge to the area to heat up the carbon nano tubes and in turn melt an embedded healing agent that will flow into and seal the crack.
The fact that the wires are in a transverse grid also furnishes an exact location. As a step forward, the team of scientists thinks that if a higher-energy current is send down the wires, it will cause the nanotubes to heat up. When the nanotubes heat up, it will lead to melting of ingredients in the epoxy so as to fill up and re-bond the crack automatically, as soon as it appears. This process could occur automatically, as soon as a crack was detected, controlled perhaps by an aircraft's computers. By this method, almost 70 % of the material's original strength can be recovered/retained, preventing catastrophic failures. Tests reported that the self-repair coating weighed only 1% of what the uncoated structure did.

This new system for detecting cracks can eventually be integrated into the built-in computer system of a fighter jet or large piece of equipment. The system will allow the operator to monitor a structure�s integrity in real time, and any micro cracks or delamination will become obvious by provoking a change in electrical resistance at some point in the structure.
Adding a small amount of carbon nano tubes can go a long way toward enhancing the strength, integrity and safety of plastic materials widely used in engineering applications. Real-time detection and repair of fatigue-induced damage will greatly enhance the performance, reliability and safety of structural components in a variety of engineering systems. This new crack detection method will eventually be more cost effective and more convenient than ultrasonic sensors commonly used today, as this sensor system can also be used in real time as a device or component is in use. On the other hand, the sonic sensors are external units that require a great deal of time to scan the entire surface area of a stationary structure.
The system should help increase the lifetime, safety and cost effectiveness of polymer structures, which are commonly used in place of metal when weight is a factor.