Engineers devise new way to inspect advanced materials

Infrared themographic image of a nanoengineered composite heated via electrical probes (clips can be seen at bottom of image). The scalebar of colors is degrees Celsius. The MIT logo has been machined into the composite, and the hot and cool spots around
Aeronautics engineers have unveiled a new method of detecting internal damage when building airplanes by using a simple handheld device and heat sensitive camera.

The team headed by Brian Wardle, associate professor of aeronautics and astronautics at the Massachusetts Institute of Technology (MIT), have devised the new approach which also requires engineering composite materials to include carbon nanotubes, which generate the heat necessary for the test. The project is part of a multiyear, aerospace industry funded effort to improve the mechanical properties of existing advanced aerospace grade composites. Advanced composite materials are commonly found not only in aircraft, but also cars, bridges and wind turbine blades.

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

However, this approach requires large heaters to be placed next to the surface, Wardle noted. With his new approach, carbon nanotubes are incorporated into the composite material. When a small electric current is applied to the surface, the nanotubes heat up, which eliminates 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 utilise 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. Adams, who was not involved in the research, notes 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.

According to Wardle, the new carbon nanotube hybrid materials have so far shown better mechanical properties, such as strength and toughness, than existing advanced composites.

Chris Shaw

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