Now MIT engineers have developed a method to produce aerospace-grade composites without the enormous ovens and pressure vessels. The technique may help to speed up the manufacturing of aeroplanes and other large, high-performance composite structures, such as blades for wind turbines.

The researchers detail their new method in a paper published in the journal Advanced Materials Interfaces.

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-story building, which itself requires time and money to pressurize. 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. Brian Wardle, professor of aeronautics and astronautics at MIT

Wardle’s co-authors on the paper are lead author and MIT postdoc Jeonyoon Lee, and Seth Kessler of Metis Design Corporation, an aerospace structural health monitoring company based in Boston.

Out of the oven, into a blanket

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 them in an ultrathin film of carbon nanotubes (CNTs). When they applied an electric current to the film, the CNTs, like a nanoscale electric blanket, quickly generated heat, causing the materials within to cure and fuse together.

With this out-of-oven, or OoO, technique, the team was able to produce composites as strong as the materials made in conventional aeroplane manufacturing ovens, using only 1 per cent of the energy.

The researchers next looked for ways to make high-performance composites without the use of large, high-pressure autoclaves — building-sized vessels that generate high enough pressures to press materials together, squeezing out any voids, or air pockets, at their interface.

Researchers including Wardle’s group have explored “out-of-autoclave,” or OoA, techniques to manufacture composites without using the huge machines. But most of these techniques have produced composites where nearly 1 per cent of the material contains 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 negligible and not easily measured.

Straw pressure

Part of Wardle’s 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 colour, strength, and electrical capacity. The researchers wondered whether these nanoporous films could be used in place of giant autoclaves to squeeze out voids between two material layers, as unlikely as that may seem.

A thin film of carbon nanotubes is somewhat like a dense forest of trees, and the spaces between the trees can function like thin nanoscale tubes or capillaries. A capillary such as a straw can generate pressure based on its geometry and its surface energy, or the material’s ability to attract liquids or other materials.

The researchers tested their idea in the lab by growing films of vertically aligned carbon nanotubes using a technique they previously developed, then laying the films between layers of materials that are typically used in the autoclave-based manufacturing of primary aircraft structures. They wrapped the layers in a second film of carbon nanotubes, which they applied an electric current 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.

The team will next look for ways to scale up the pressure-generating CNT film. In their experiments, they worked with samples measuring several centimetres 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.

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.

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The new facility will sit alongside the Honeywell operation in Phoenix, AZ and will pod Honeywell’s engines into GKN Aerospace designed and manufactured nacelles for HTF7000 powered aircraft – including Bombardier, Gulfstream and Embraer business jets. GKN Aerospace will then deliver the complete propulsion system, on Honeywell’s behalf, directly to the appropriate airframe end customer assembly line.

The Arizona site will commence operation in the autumn of 2013, becoming fully functional by the end of the year when it is expected to employ up to 50 people, handling the podding of engines for the Bombardier CL300 and CL350, the Gulfstream G280 and the Embraer 450/500 business jets. At capacity, the site will be capable of assembling 28 complete power systems per month.

Phil Swash, CEO, GKN Aerospace – Europe and Special Products explains

We have worked closely with Honeywell since the commencement of their highly successful HTF7000 engine programme in the late 1990s – with our engineering team designing the nacelle to be uniquely adaptable and multi-application. These engines have now been selected to power several different aircraft and so the time is right to establish a more geographically and logistically efficient location for the integration process. Our new operation will allow us to cut transport costs, reduce cycle times and extend the flexibility of the service we offer Honeywell and their aircraft prime customers.

GKN Aerospace has been delivering nacelles for the HTF7000 series of engines for well over a decade and in 2008 gained a lifetime supply contract which included responsibility for the full nacelle/engine integration process. Since then all work has been undertaken at the manufacturing site for the nacelle, either in St Louis, Missouri or Cowes in the UK.

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As production of the new Legacy 500 aircraft gets closer the shipment of the forst empennage from Portugal to Brazil is a significant milestone for the Embraer composites team, Paulo Marchioto, President of Embraer Portugal and Board member at Embraer Compósitos

With close support from Brazil, the team at Embraer Compósitos has demonstrated great skill allowing us, in a very short time since inauguration, to deliver such a sophisticated part.

The empennage is made mainly of carbon fibre parts. It was assembled in a semi-automatic line in one of two Embraer plants in Évora, allowing for higher efficiency in operations like drilling and riveting of primary structures made in composite.

Embraer announced Évora as the chosen location for Embraer Compósitos back in 2008. The new centre was completed and inaugurated in September 2012, together with Embraer Metálicas, also in Évora. With 37,100 and 31,800-square metres respectively, Embraer Metálicas and Embraer Compósitos will reach full production capacity during the second quarter of 2013.

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