A new method using carbon nanotubes to create aerospace-grade composites without an autoclave has been developed by researchers at the Massachusetts Institute of Technology (MIT).
The new process, which could revolutionise aerospace manufacturing, doesn’t require pressure vessels and huge ovens, which are currently used in the manufacture of airplane primary structures.
The research team included: Brian Wardle, professor of aeronautics and astronautics at MIT; Jeonyoo Lee, a postdoctoral student at the MIT; and Seth Kessler, President and CEO of Metis Design Corporation, a Boston, Massachusetts-based aerospace structural health monitoring company.
Using existing techniques the production of an aeroplane’s main body involves multiple sheets of several composite materials, which are arranged and moulded into the shape of a fuselage and put into gigantic ovens and autoclaves in which those layers are fused together to form a strong and tough aerodynamic shell.
Wardle says, “If you’re making a primary structure like a fuselage or wing, you need to build a pressure vessel, or autoclave sometimes the size of a three-story building, which itself requires time and money to pressurise. These things are massive pieces of infrastructure. We are working towards making primary structure materials without autoclave pressure, so we can get rid of all that infrastructure.”
In 2015, Lee led the team, along with another member of Wardle’s lab, in creating a method to make aerospace-grade composites without requiring an oven to fuse the materials together. Instead of placing layers of material inside an oven to cure, the researchers essentially wrapped the materials in an ultrathin film of carbon nanotubes (CNTs).
When they applied an electric current to the film, the CNTs (picture a nanoscale electric blanket), quickly generated heat, causing the materials within to cure and fuse together.
That method, known as “out-of-oven” (or OoO), produced composites that were as strong as the materials made in conventional aeroplane manufacturing ovens, by using just 1% of the energy.
Removing the need for an autoclave altogether
The team’s next step was to look for ways to manufacture high-performance composites without the use of large, high-pressure autoclaves that create high amount of pressure to press materials together, squeezing out any voids, or air pockets.
Previous research by Spirit AeroSystems working in collaboration with the University of Strathclyde helped develop a manufacturing process that didn’t use an autoclave but instead a multi-zone heated tool with advanced control to tailor the curing cycle time to match individual part geometries.
Other researchers including Wardle’s group have also explored “out-of-autoclave,” or OoA, techniques to manufacture composites without using the huge machines. But most of these techniques produced composites where nearly 1% of the produced materials contained voids, which can compromise a material’s strength and lifetime. In comparison, aerospace-grade composites made in autoclaves are of such high quality that any voids they contain are neglible and not easily measured.
“The problem with these OoA approaches is also that the materials have been specially formulated, and none are qualified for primary structures such as wings and fuselages,” Wardle says. “They’re making some inroads in secondary structures, such as flaps and doors, but they still get voids.”
A new solution to out-of-autoclave
Part of Wardle’s latest work focuses on developing nanoporous networks — ultrathin films made from aligned, microscopic material such as carbon nanotubes, that can be engineered with exceptional properties, including color, strength, and electrical capacity.
The researchers investigated whether the nanoporous films could be used in place of giant autoclaves to squeeze out voids between two material layers, as unlikely as that may seem.
The researchers proposed that if a thin film of carbon nanotubes were sandwiched between two materials, then, as the materials were heated and softened, the capillaries between the carbon nanotubes should have a surface energy and geometry such that they would draw the materials in toward each other, rather than leaving a void between them. Lee calculated that the capillary pressure should be larger than the pressure applied by the autoclaves.
The researchers tested this concept by growing films of vertically aligned carbon nanotubes, then laying the films between layers of materials that are typically used in the autoclave-based manufacturing of primary aircraft structures. The researchers wrapped the layers in a second film of carbon nanotubes, which they applied an electric current to, in order to to heat it up. They observed that as the materials heated and softened in response, they were pulled into the capillaries of the intermediate CNT film.
The resulting composite lacked voids, similar to aerospace-grade composites that are produced in an autoclave. The researchers subjected the composites to strength tests, attempting to push the layers apart, the idea being that voids, if present, would allow the layers to separate more easily.
“In these tests, we found that our out-of-autoclave composite was just as strong as the gold-standard autoclave process composite used for primary aerospace structures,” Wardle says.
The team will next look for ways to scale up the pressure-generating CNT film. In their experiments, they worked with samples measuring several centimeters wide — large enough to demonstrate that nanoporous networks can pressurize materials and prevent voids from forming. To make this process viable for manufacturing entire wings and fuselages, researchers will have to find ways to manufacture CNT and other nanoporous films at a much larger scale.
“There are ways to make really large blankets of this stuff, and there’s continuous production of sheets, yarns, and rolls of material that can be incorporated in the process,” Wardle says.
He plans also to explore different formulations of nanoporous films, engineering capillaries of varying surface energies and geometries, to be able to pressurize and bond other high-performance materials.
“Now we have this new material solution that can provide on-demand pressure where you need it,” Wardle says. “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. This could make making all those things, without the oven and autoclave infrastructure.”
This research was supported, in part, by Airbus, ANSYS, Embraer, Lockheed Martin, Saab AB, Saertex, and Teijin Carbon America through MIT’s Nano-Engineered Composite aerospace Structures (NECST) Consortium.