Taking aircraft manufacturing out of the oven
14 April 2015
A new technique, developed by Aerospace engineers at MIT, uses carbon nanotube film to heat and cure composite materials directly, as opposed to using autoclaves.
Composite materials used in aircraft wings and fuselages are typically manufactured in large, industrial-sized ovens. Using this approach, considerable energy is required first to heat the oven, then the gas around it, and finally the actual composite.
Aerospace engineers at MIT have now developed a carbon nanotube (CNT) film that can heat and solidify a composite without the need for massive ovens. When connected to an electrical power source, and wrapped over a multilayer polymer composite, the heated film stimulates the polymer to solidify.
The group tested the film on a common carbon-fibre material used in aircraft components, and found that the film created a composite as strong as that manufactured in conventional ovens — while using only 1 percent of the energy.
The new “out-of-oven” approach may offer a more direct, energy-saving method for manufacturing virtually any industrial composite, says Brian Wardle, an associate professor of aeronautics and astronautics at MIT.
“Typically, if you’re going to cook a fuselage for an Airbus A350 or Boeing 787, you’ve got about a four-story oven that’s tens of millions of dollars in infrastructure that you don’t need. Our technique puts the heat where it is needed, in direct contact with the part being assembled. Think of it as a self-heating pizza: instead of an oven, you just plug the pizza into the wall and it cooks itself.”
Wardle says the carbon nanotube film is also incredibly lightweight: After it has fused the underlying polymer layers, the film itself — a fraction of a human hair’s diameter — meshes with the composite, adding negligible weight.
Wardle and his colleagues have experimented with CNT films in recent years, mainly for de-icing aircraft wings. The team recognized that in addition to their negligible weight, carbon nanotubes heat efficiently when exposed to an electric current.
The group first developed a technique to create a film of aligned carbon nanotubes composed of tiny tubes of crystalline carbon, standing upright like trees in a forest. The researchers used a rod to roll the 'forest' flat, creating a dense film of aligned carbon nanotubes.
In experiments, Wardle and his team integrated the film into aircraft wings via conventional, oven-based curing methods, showing that when voltage was applied, the film generated heat, preventing ice from forming.
In initial experiments, the researchers investigated the film’s potential to fuse two types of aerospace-grade composite typically used in aircraft wings and fuselages. Normally the material, composed of about 16 layers, is solidified, or cross-linked, in a high-temperature industrial oven.
The researchers manufactured a CNT film about the size of a Post-It note, and placed it over a square of Cycom 5320-1. They connected electrodes to the film, then applied a current to heat both the film and the underlying polymer in the Cycom composite layers.
The team measured the energy required to solidify, or cross-link, the polymer and carbon fibre layers, finding that the CNT film used one-hundredth the electricity required for traditional oven-based methods to cure the composite. Both methods generated composites with similar properties, such as cross-linking density.
Wardle says the results pushed the group to test the CNT film further. As different composites require different temperatures in order to fuse, the researchers looked to see whether the CNT film could, quite literally, take the heat.
“At some point, heaters fry out,” Wardle says. “They oxidize, or have different ways in which they fail. What we wanted to see was how hot could this material go.”
To do this, the group tested the film’s ability to generate higher and higher temperatures, and found it topped out at over 540°C. In comparison, some of the highest-temperature aerospace polymers require temperatures up to 400°C in order to solidify.
“We can process at those temperatures, which means there’s no composite we can’t process,” Wardle says. “This really opens up all polymeric materials to this technology.”
The results of this work are published in the journal ACS Applied Materials and Interfaces.
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