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 project uses a woven wood structure with fibreglass pods that depend from the ceiling which transforms from a large meeting space into a smaller one.

Transformable structures often require expensive and complex electromechanical systems to create movement. This research explores an alternative approach utilising transformable woven structures that can smoothly transform with lightweight and soft and materials/mechanisms. A series of prototypes were built at different sizes to demonstrate articulating woven structures for various applications.

Transformable Meeting Spaces are aimed at re-imagining interior office or building environments. There are two predominant approaches to office design – open spaces versus fixed offices. Open office plans have been shown to decrease productivity due to noise and privacy challenges yet they provide flexibility and collaborative opportunities. Fixed offices offer privacy and quite environments but restrict the type of working spaces available and occupy more square footage.

This research proposes an alternative whereby structures can easily transform between private phone booths, lounge spaces or other quiet meeting spaces into open flexible areas. By utilising woven and transformable materials these meeting spaces can expand and contract to create a meeting room for 6–8 people or morph into the ceiling leaving a clear and open area below.

The MIT School of Architecture’s Self-Assembly Lab has teamed up with Google to create Transformable Meeting Spaces, a project that utilizes woven structure research in wood and fiberglass pods that descend from the ceiling, transforming a large space into a smaller one. Designed as a small-scale intervention for reconfiguring open office plans—which “have been shown to decrease productivity due to noise and privacy challenges”—the pods require no electromechanical systems to function, but rather employ a flexible skeleton and counterweight to change shape.

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But composite materials are also surprisingly vulnerable: While aluminium can withstand relatively large impacts before cracking, the many layers in composites can break apart due to relatively small impacts — a drawback that is considered the material’s Achilles’ heel.

MIT aerospace engineers have found a way to bond composite layers in such a way that the resulting material is substantially stronger and more resistant to damage than other advanced composites.

The researchers fastened the layers of composite materials together using carbon nanotubes — atom-thin rolls of carbon that, despite their microscopic stature, are incredibly strong. They embedded tiny “forests” of carbon nanotubes within a glue-like polymer matrix, then pressed the matrix between layers of carbon fibre composites. The nanotubes, resembling tiny, vertically aligned stitches, worked themselves within the crevices of each composite layer, serving as a scaffold to hold the layers together.

In experiments to test the material’s strength, the team found that, compared with existing composite materials, the stitched composites were 30% stronger, withstanding greater forces before breaking apart.

Roberto Guzman, who led the work as an MIT postdoc in the Department of Aeronautics and Astronautics (AeroAstro), says the improvement may lead to stronger, lighter airplane parts — particularly those that require nails or bolts, which can crack conventional composites.

More work needs to be done, but we are really positive that this will lead to stronger, lighter planes. That means a lot of fuel saved, which is great for the environment and for our pockets.

Today’s composite materials are composed of layers, or plies, of horizontal carbon fibres, held together by a polymer glue, which Wardle describes as “a very, very weak, problematic area.” Attempts to strengthen this glue region include Z-pinning and 3-D weaving — methods that involve pinning or weaving bundles of carbon fibres through composite layers, similar to pushing nails through plywood, or thread through fabric.

A stitch or nail is thousands of times bigger than carbon fibres. So when you drive them through the composite, you break thousands of carbon fibres and damage the composite.

Carbon nanotubes, by contrast, are about 10 nanometers in diameter — nearly a million times smaller than the carbon fibres. Researchers we’re able to put these nanotubes in without disturbing the larger carbon fibres, and that’s what maintains the composite’s strength.

Guzman and Wardle came up with a technique to integrate a scaffold of carbon nanotubes within the polymer glue. They first grew a forest of vertically aligned carbon nanotubes, following a procedure that Wardle’s group previously developed. They then transferred the forest onto a sticky, uncured composite layer and repeated the process to generate a stack of 16 composite plies — a typical composite laminate makeup — with carbon nanotubes glued between each layer.

To test the material’s strength, the team performed a tension-bearing test — a standard test used to size aerospace parts — where the researchers put a bolt through a hole in the composite, then ripped it out. While existing composites typically break under such tension, the team found the stitched composites were stronger, able to withstand 30 percent more force before cracking.

The researchers also performed an open-hole compression test, applying force to squeeze the bolt hole shut. In that case, the stitched composite withstood 14 percent more force before breaking, compared to existing composites.

The strength enhancements suggest this material will be more resistant to any type of damaging events or features. And since the majority of the newest planes are more than 50% composite by weight, improving these state-of-the art composites has very positive implications for aircraft structural performance.

This work was supported by Airbus Group, Boeing, Embraer, Lockheed Martin, Saab AB, Spirit AeroSystems Inc., Textron Systems, ANSYS, Hexcel, and TohoTenax through MIT’s Nano-Engineered Composite aerospace STructures (NECST) Consortium and, in part, by the U.S. Army.

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Adapting an old trick used for centuries by both metal-smiths and pastry makers, a team of researchers at MIT has found a way to efficiently create composite materials containing hundreds of layers that are just atoms thick but span the full width of the material.

Materials such as graphene, a two-dimensional form of pure carbon, and carbon nanotubes, tiny cylinders that are essentially rolled-up graphene, are some of the strongest, hardest materials available, because their atoms are held together entirely by carbon-carbon bonds, which are the strongest nature gives us for chemical bonds to work with. So, researchers have been searching for ways of using these nano-materials to add great strength to composite materials, much the way steel bars are used to reinforce concrete.

The biggest obstacle has been finding ways to embed these materials within a matrix of another material in an orderly way. These tiny sheets and tubes have a strong tendency to clump together, so just stirring them into a batch of liquid resin before it sets doesn’t work at all. The MIT team’s insight was in finding a way to create large numbers of layers, stacked in a perfectly orderly way, without having to stack each layer individually.

Although the process is more complex than it sounds, at the heart of it is a technique similar to that used to make ultra-strong steel sword blades, as well as the puff pastry that’s in baklava and napoleons. A layer of material made of either steel, dough, or graphene is spread out flat. Then, the material is doubled over on itself, pounded or rolled out, and then doubled over again, and again, and again.

With each fold, the number of layers doubles, thus producing an exponential increase in the layering. Just 20 simple folds would produce more than a million perfectly aligned layers.

Now, it doesn’t work out exactly that way on the nanoscale. In this research, rather than folding the material, the team cut the whole block — itself consisting of alternating layers of graphene and the composite material — into quarters, and then slid one-quarter on top of another, quadrupling the number of layers, and then repeating the process. But the result was the same: a uniform stack of layers, quickly produced, and already embedded in the matrix material, in this case polycarbonate, to form a composite.

In their proof-of-concept tests, the MIT team produced composites with up to 320 layers of graphene embedded in them. They were able to demonstrate that even though the total amount of the graphene added to the material was minuscule — less than 1/10 of a percent by weight — it led to a clear-cut improvement in overall strength.

The team also found a way to make structured fibres from graphene, potentially enabling the creation of yarns and fabrics with embedded electronic functions, as well as yet another class of composites. The method uses a shearing mechanism, somewhat like a cheese slicer, to peel off layers of graphene in a way that causes them to roll up into a scroll-like shape, technically known as an Archimedean spiral.

That could overcome one of the biggest drawbacks of graphene and nanotubes, in terms of their ability to be woven into long fibres: their extreme slipperiness. Because they are so perfectly smooth, strands slip past each other instead of sticking together in a bundle. And the new scrolled strands not only overcome that problem, they are also extremely stretchy, unlike other super-strong materials such as Kevlar. That means they might lend themselves to being woven into protective materials that could “give” without breaking.

One unexpected feature of the new layered composites is that the graphene layers, which are extremely electrically conductive, maintain their continuity all the way across their composite sample without any short-circuiting to the adjacent layers. So, for example, simply inserting an electrical probe into the stack to a certain precise depth would make it possible to uniquely address any one of the hundreds of layers. This could ultimately lead to new kinds of complex multilayered electronics.

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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% of the energy.

Brian L. Wardle, an associate professor of aeronautics and astronautics at MIT said;

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.

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. The team, including MIT graduate students Jeonyoon Lee and Itai Stein and Seth Kessler of the Metis Design Corporation, has published its results in the journal ACS Applied Materials and Interfaces.

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Wardle and his colleagues have experimented with CNT films in recent years, mainly for deicing airplane wings. The team recognised that in addition to their negligible weight, carbon nanotubes heat efficiently when exposed to an electric current.

The research team 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, they integrated the film into airplane wings via conventional, oven-based curing methods, showing that when voltage was applied, the film generated heat, preventing ice from forming.

So how hot can you go? 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.

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The researchers manufactured a CNT film about the size of a Post-It note, and placed the film 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.

They then 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.

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.

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To do this, they tested the film’s ability to generate higher and higher temperatures, and found it topped out at over 1,000 F. In comparison, some of the highest-temperature aerospace polymers require temperatures up to 750 F in order to solidify.

The team is working with industrial partners to find ways to scale up the technology to manufacture composites large enough to make airplane fuselages and wings. The group’s carbon nanotube film may go toward improving the quality and efficiency of fabrication processes for large composites, such as wings on commercial aircraft. The new technique may also open the door to smaller firms that lack access to large industrial ovens.

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Gershenfeld likens the structure, which is made from tiny, identical, interlocking parts to chainmail. The parts, based on a novel geometry that Cheung developed with Gershenfeld, form a structure that is 10 times stiffer for a given weight than existing ultralight materials. But this new structure can also be disassembled and reassembled easily — such as to repair damage, or to recycle the parts into a different configuration.

The individual parts can be mass-produced; Gershenfeld and Cheung are developing a robotic system to assemble them into wings, airplane fuselages, bridges or rockets — among many other possibilities.

The new design combines three fields of research, Gershenfeld says: fibre composites, cellular materials (those made with porous cells) and additive manufacturing (such as 3-D printing, where structures are built by depositing rather than removing material).

With conventional composites used in everything from golf clubs and tennis rackets to the components of Boeing and Airbus’s new aircraft each piece is manufactured as a continuous unit. Therefore, manufacturing large structures, such as airplane wings, requires large factories where fibres and resins can be wound and parts heat-cured as a whole, minimising the number of separate pieces that must be joined in final assembly. That requirement meant, for example, Boeing’s suppliers have had to build enormous facilities to make parts for the 787.

The new technique allows much less material to carry a given load. This could not only reduce the weight of vehicles, lowering fuel and operating costs, but also reduce the costs of construction and assembly, while allowing greater design flexibility. The system is useful for anything you need to move, or put in the air or in space says Cheung, who will begin work this fall as an engineer at NASA’s Ames Research Centre.

In traditional composite manufacturing, the joints between large components tend to be where cracks and structural failures start. While these new structures are made by linking many small composite fibre loops, Cheung and Gershenfeld show that they behave like an elastic solid, with a stiffness, or modulus, equal to that of much heavier traditional structures — because forces are conveyed through the structures inside the pieces and distributed across the lattice structure.

What’s more, when conventional composite materials are stressed to the breaking point, they tend to fail abruptly and at large scale. But the new modular system tends to fail only incrementally, meaning it is more reliable and can more easily be repaired.

The researchers produced flat, cross-shaped composite pieces that were clipped into a cubic lattice of octahedral cells, a structure called a “cuboct” which is similar to the crystal structure of the mineral perovskite, a major component of Earth’s crust. While the individual components can be disassembled for repairs or recycling, there’s no risk of them falling apart on their own, the researchers explain. Like the buckle on a seat belt, they are designed to be strong in the directions of forces that might be applied in normal use, and require pressure in an entirely different direction in order to be released.

The possibility of linking multiple types of parts introduces a new degree of design freedom into composite manufacturing. The researchers show that by combining different part types, they can make morphing structures with identical geometry but that bend in different ways in response to loads: Instead of moving only at fixed joints, the entire arm of a robot or wing of an airplane could change shape.

Alain Fontaine, who directs the innovation program for aircraft manufacturer Airbus, says;

this new approach to building structures is really disruptive. It opens interesting opportunities in the way to design and manufacture aerostructures. These technologies can open the door to other opportunities” and have significant potential to lower manufacturing costs.

In addition to Gershenfeld and Cheung, the project included MIT undergraduate Joseph Kim and alumna Sarah Hovsepian (now at NASA’s Ames Research Centre). The work was supported by the Defence Advanced Research Projects Agency and the sponsors of the Centre for Bits and Atoms, with Spirit Aerosystems collaborating on the composite development.

Images by: Kenneth Cheung

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