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  • Seeing Below the Surface
Mar 2011
CAMBRIDGE, Mass., March 30, 2011 — A new approach to detecting internal damage in planes and other objects that employs a simple handheld device and heat-sensitive camera promises to replace traditional infrared thermography techniques that require the use of large heaters. 

In recent years, many aircraft manufacturers have started building their planes from advanced high-strength fibers, such as carbon or glass, embedded in a plastic or metal matrix. Such materials are stronger and more lightweight than aluminum but are more difficult to inspect for damage because their surfaces usually don't reveal underlying problems.

Infrared thermographic image of a nanoengineered composite heated via electrical probes (clips can be seen at bottom of image). The scale bar of colors is °C. The MIT logo has been machined into the composite, and the hot and cool spots around the logo are caused by the thermal-electrical interactions of the resistive heating and the logo “damage” to the composite. (Images: MIT News Office)

“With aluminum, if you hit it, there’s a dent there. With a composite, oftentimes if you hit it, there’s no surface damage, even though there may be internal damage,” said Brian L. Wardle, associate professor of aeronautics and astronautics at MIT.

Besides the portable device and heat-sensitive camera, the approach developed by Wardle and his colleagues also requires engineering the composite materials to include carbon nanotubes, which generate the heat necessary for the test.

The inspection technique, which is described in the March 22 online edition of Nanotechnology, could allow airlines to inspect planes much more quickly, Wardle said. The project is part of a multiyear effort funded by the aerospace industry to improve the mechanical properties of existing advanced aerospace-grade composites. The US Air Force and Navy also are interested in the technology, and Wardle is working with them to develop it for use in their aircraft and vessels.

Brian L. Wardle, associate professor of aeronautics and astronautics at MIT.

Advanced composite materials are commonly found not only in aircraft, but also cars, bridges and wind-turbine blades, Wardle said.

One method that inspectors now use to reveal damage in advanced composite materials is infrared thermography, which detects IR radiation emitted when the surface is heated. In an advanced composite material, any cracks or delamination (separation of layers) redirect the flow of heat. That abnormal flow pattern can be seen with a thermographic camera.

This is effective but cumbersome because it requires large heaters to be placed next to the surface, Wardle said. With the new approach, carbon nanotubes incorporated into the composite material heat up when a small electric current is applied to the surface, eliminating the need for any external heat source. The inspector can see the damage with a thermographic camera or goggles.

“It’s a very clever way to utilize the properties of carbon nanotubes to deliver that thermal energy, from the inside out,” said Douglas Adams, associate professor of mechanical engineering at Purdue University in West Lafayette, Ind. Adams, who was not involved in the research, noted that two fundamental challenges remain: developing a practical way to manufacture large quantities of the new material and ensuring that the addition of nanotubes does not detract from the material’s primary function of withstanding heavy loads.

The new carbon nanotube hybrid materials that Wardle is developing have so far shown better mechanical properties, such as strength and toughness, than existing advanced composites.

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