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‘Sandwich’ materials may lead to shape-shifting cars

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By optimizing the core elements of sandwich structures, researchers have created materials that are extremely light, robust, and adaptable at once—ideal for aerospace applications.

“It is our philosophy to develop modern composite materials for adaptive systems and, while doing so, to optimize their structural efficiency—that is, obtaining the same performance with fewer resources or better functionality with the same amount of material,” says Paolo Ermanni, professor for composite materials and adaptive structures at ETH Zurich.

At the same time, he and his collaborators investigate appropriate production processes that make the new materials interesting for practical applications.

Spare the rods, spoil the structures?

Ermanni’s PhD student Christoph Karl takes care of the “structural efficiency” aspect. “As they feature a large stiffness and stability whilst also being very light, sandwich structures are often used for lightweight construction,” he explains.

Sandwich structures typically consist of two thin and stiff cover layers and a low-density core material. “In our research we develop high-performance sandwich composites made of carbon fiber-reinforced polymers, also known as CRP’s or simply carbon fiber. In this approach, the core consists of a truss structure of carbon fiber rods,” says Karl. The good mechanical properties of carbon fiber mean that such core structures can have more stiffness and stability than conventional foam or honeycomb cores.

Another significant advantage of the truss cores, according to Karl, is the possibility of a load-optimized design. “The mechanical properties of the sandwich composite depend strongly on the core topology—in other words, on the arrangement and orientation of the rods inside the core,” he says. “With the help of numerical optimizations, we can tailor the orientation of the rods to specific external loads and thus maximize the structural efficiency for a particular application.”

The core of a sandwich material researchers construct and optimize in this way weighs less than 30 kilograms per cubic meter (a cubic meter of steel, by comparison, weighs in at almost 8,000 kilograms).

“This makes our materials particularly interesting for aerospace applications, where structural efficiency is of crucial importance,” says Karl. “Moreover, it is possible to integrate additional features, such as vibration damping, directly into the core structure.”

As part of the European Union project ALTAIR, which the French aerospace lab Onera leads, researchers are investigating real-life applications of these new sandwich structures. Ermanni’s research group, for instance, is involved in developing the load-bearing structures of new deployment systems for small satellites.

Shape-shifting cars

PhD student Oleg Testoni, on the other hand, specializes in flexible and adaptive structures. He develops techniques that allow the sandwich structures to adapt flexibly and dynamically. Those techniques could be used, for instance, to build futuristic spoilers or wheelhouses for sports cars that can deform while the vehicle is in motion in order to accurately optimize its aerodynamics for a particular velocity or wheel position when cornering.

To achieve such a degree of flexibility whilst maintaining the robustness of the material, semi-active elements—so-called mechanical switches—are embedded in the material.

“With such switches, the rods inside the core can be temporarily loosened in order to adapt the shape. After that, they are locked in place again so that the material regains its original stiffness,” Testoni explains.

Mechanical switches can be built using “intelligent materials” such as shape memory alloys. A component made of such an alloy can take on two different shapes depending on temperature. Above a certain critical temperature, its shape changes, but when it cools down it goes back to its exact original shape. By fitting many of those mechanical switches inside the rods of a sandwich structure, one can change the shape of the entire material.

Next up: 4D printing

Ermanni and his coworkers do not just carry out basic research on new materials, however. The spin-off company 9T Labs, cofounded by Ermanni’s PhD student Martin Eichenhofer, develops a 3D-printing technology that can produce high-quality carbon fiber components such as the rods for sandwich structure cores in a robust and flexible manner.

“First and foremost, this is about expanding the range of application of such materials through novel production techniques, which will enable smaller companies to use them as well. This democratizes lightweight construction technologies, as it were,” says Eichenhofer.

The first products for 3D-printing should hit the market as early as 2019. “This procedure also opens up the possibility of integrating active elements directly into the printing process in the future, thus realizing 4D-printing,” Ermanni adds.

Source: ETH Zurich