Engineers at the Massachusetts Institute of Technology (MIT) produced aerospace-grade composites with the new technique, which could also speed up production of other large, high-performance composite structures like wind turbine blades.
Modern aeroplane fuselages are made from multiple sheets of different composite materials which are stacked, moulded into shape and then wheeled into warehouse-sized ovens and autoclaves. Heat fuses the layers together before high pressure in the autoclaves squeezes out any pockets of air, producing resilient, aerodynamic shells.
“If you're making a primary structure like a fuselage or wing, you need to build a pressure vessel, or autoclave, the size of a two- or three-storey building, which itself requires time and money to pressurise,” says Brian Wardle, professor of aeronautics and astronautics at MIT. “These things are massive pieces of infrastructure. Now we can make primary structure materials without autoclave pressure, so we can get rid of all that infrastructure.”
Wardle co-authored a paper on the new technique with lead author Jeonyoo Lee and Seth Kessler from Metis Design Corporation. In 2015, Lee and a colleague from Wardle’s laboratory developed a technique that used ‘ultrathin’ films of carbon nanotubes (CNTs) to cure and fuse layers together instead of ovens. Layers were wrapped in the CNTs, which were then heated by having an electric current applied, causing the layers of material to cure and fuse together. The technique was reportedly able to produce composites as strong as those made in conventional ovens using only 1% of the energy.
In the new work, the researchers went one step further by asking if nanoporous networks made of CNTs could replace autoclaves as well. They grew films of vertically-aligned CNTs using a technique they previously developed, then placed the films between layers of materials used for primary aircraft structures. They wrapped the layers in a second film of CNTs, which they heated with electric current. The materials heated and softened in response, and were pulled into the capillaries of the intermediate CNT film.
The resulting composite reportedly lacked air pockets that can weaken materials, similar to aerospace-grade composites produced in autoclaves. Testing showed it was as strong as something treated in an autoclave.
Although the technique produced a strong composite without huge infrastructure, the sample was only a few centimetres wide. The researchers plan to work on new methods to scale-up CNT production, hoping to make much larger pieces to enable manufacturing of entire wings and fuselages.
Wardle also plans to explore different formulations of nanoporous films, engineering capillaries of varying surface energies and geometries to be able to pressurise and bond other high-performance materials. “Beyond airplanes, most of the composite production in the world is composite pipes, for water, gas, oil, all the things that go in and out of our lives,” he said, claiming the new technique could produce composites for those applications without ovens and autoclaves.
This research was supported by Airbus, Ansys, Embraer, Lockheed Martin, Saab, Saertex and Teijin Carbon America through MIT's Nano-Engineered Composite aerospace Structures (NECST) consortium.
The paper was published in Advanced Materials Interfaces.
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