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 the logo are caused by the thermal-electrical interactions of the resistive heating and the logo “damage” to the composite. The enhanced thermographic sensing described in the paper works in the same way. Image: Roberto Guzmán de Villoria, MIT
recent years, many airplane manufacturers have started building their
planes from advanced composite materials, which consist of 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
they are also more difficult to inspect for damage, because their
surfaces usually don’t reveal underlying problems.
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,” says Brian L. Wardle, associate professor of
aeronautics and astronautics.
and his colleagues have devised a new way to detect that internal
damage, using a simple handheld device and heat-sensitive camera. Their
approach also requires engineering the composite materials to include
carbon nanotubes, which generate the heat necessary for the test.
Their approach, described in the March 22 online edition of the journal Nanotechnology,
could allow airlines to inspect their planes much more quickly, Wardle
says. This project is part of a multiyear, aerospace-industry-funded
effort to improve the mechanical properties of existing advanced
aerospace-grade composites. The U.S. Air Force and Navy are also
interested in the technology, and Wardle is working with them to develop
it for use in their aircraft and vessels.
composite materials are commonly found not only in aircraft, but also
cars, bridges and wind-turbine blades, Wardle says.
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.
is effective but cumbersome because it requires large heaters to be
placed next to the surface, Wardle says. 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.
a very clever way to utilize the properties of carbon nanotubes to
deliver that thermal energy, from the inside out,” says 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.
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.