Solid cellular bodies are materials made up of many cells that have been packed together, like a honeycomb. The shape of these cells largely determines the mechanical properties of the material, including its stiffness or strength. For example, bones are filled with a natural material that allows them to be lightweight but stiff and robust.
Inspired by bones and other solid cells found in nature, humans have used the same concept to develop architectural materials. By changing the geometry of the unit cells that make up these materials, scientists can adjust the mechanical, thermal, or acoustic properties of the material. Architectural materials are used in a variety of applications, from cushioning packaging foam to heat-regulating radiators.
Using kirigami, the historic Japanese art of paper folding and cutting, MIT researchers have now produced a type of high-performance architectural material known as a plate lattice, on a much larger scale than scientists were previously able to achieve using additive manufacturing. The technique allows them to create these structures from metal or other materials with custom shapes and tailored mechanical properties.
“This material is like a steel cork. It’s lighter than cork, but it has high strength and stiffness,” says Professor Neil Gershenfeld, who directs the Center for Bits and Atoms (CBA) at MIT and is senior author new paper in this approach.
Scientists have developed a modular construction process in which many smaller components are molded, assembled, and assembled into 3D shapes. Using this method, they have produced ultralight and ultrastrong structures and robots that can change shape and maintain it under a given load.
Because these structures are lightweight yet robust, stiff, and relatively straightforward to mass produce on a larger scale, they can be particularly useful in architectural, aerospace, automotive, or space components.
Gershenfeld is joined on the paper by co-authors Alfonso Parra Rubio, a research assistant at CBA, and Klara Mundilova, an MIT graduate student majoring in electrical engineering and computer science; along with David Preiss, a CBA graduate student; and Erik D. Demaine, a professor of computer science at MIT. The research will be presented at the ASME conference Computers and Information in Engineering Conference.
Production by folding
Plate gratings are cellular structures made from three-dimensional intersections of plates rather than beams. These high-performance structures are even stronger and stiffer than lattices, but their sophisticated shape makes them tough to manufacture using common techniques such as 3D printing, especially for large-scale engineering applications.
MIT researchers overcame these manufacturing difficulties by using kirigami, a technique for creating three-dimensional shapes by folding and cutting paper that dates back to 7th-century Japanese artists.
Kirigami has been used to produce plate trusses from partially folded zigzag folds. However, to make a sandwich structure, flat plates must be attached to the top and bottom of this corrugated core at the narrow points created by the zigzag folds. This often requires robust adhesives or welding techniques that can make assembly leisurely, high-priced, and tough to scale.
MIT researchers modified a common origami fold pattern, known as the Miura-ori pattern, so that the piercing points of the wavy structure were transformed into facets. The facets, like those on a diamond, provide flat surfaces to which the plates can be more easily attached with screws or rivets.
“The plate networks outperform the beam networks in terms of strength and stiffness, while maintaining the same weight and internal structure,” says Parra Rubio. “The achievement of the upper limit of HS for theoretical stiffness and strength was demonstrated by nanoscale fabrication using two-photon lithography. The construction of plate networks has been so difficult that few studies have been done at the macroscale. We believe that folding is the path to easier use of this type of plate structure made of metals.”
Customizable properties
What’s more, the way researchers design, fold, and cut the pattern allows them to tune certain mechanical properties, such as stiffness, strength, and flexural modulus (a material’s tendency to resist bending). They encode this information, as well as the 3D shape, in a crease map that’s used to create these kirigami folds.
For example, depending on how the folds are designed, some cells can be shaped to maintain their shape when compressed, while others can be engineered to bend. In this way, scientists can precisely control how different areas of the structure deform when compressed.
Because the flexibility of the structure can be controlled, these undulations can be used in robots or other energetic applications where parts move, twist and bend.
To make larger structures, such as robots, scientists have introduced a modular assembly process. They mass-produce smaller patterns of bends and assemble them into ultralight, ultrastrong 3D structures. The smaller structures have fewer bends, which simplifies the manufacturing process.
Using a customized Miura-ori pattern, the researchers create a fold pattern that will give the desired shape and structural properties. They then employ a unique machine—the Zund cutting table—to score a flat metal panel, which they then fold into a 3D shape.
“To make things like cars and airplanes, you have to invest a lot of money in tools. This manufacturing process is tool-free, like 3D printing. But unlike 3D printing, our process can set the limit for record-breaking material properties,” Gershenfeld says.
Using this method, they produced aluminum structures with a compressive strength exceeding 62 kilonewtons while weighing only 90 kilograms per square meter. (A cork weighs about 100 kilograms per square meter.) Their structures were so robust that they could withstand three times the force of typical corrugated aluminum sheeting.
This versatile technique can be applied to a wide range of materials, including steel and composites, making it ideal for producing lightweight, shock-absorbing components for aircraft, cars and spacecraft.
However, the researchers found that their method can be tough to model. Therefore, in the future, they plan to develop user-friendly CAD design tools for these kirigami plate lattice structures. In addition, they want to investigate methods that reduce the computational cost of simulating a design that yields the desired properties.
“Kirigami undulations have exciting potential for architectural construction,” says James Coleman MArch ’14, SM ’14, co-founder of design, fabrication, and installation firm SumPoint and former vice president of innovation and R&D at Zahner, who was not involved in this work. “In my experience working on complex architectural projects, current methods for constructing large curved and double-curved elements are material-intensive and wasteful, and therefore considered impractical for most projects. While the authors’ technology offers novel solutions for the aerospace and automotive industries, I believe their cell-based method could also have significant impact on the built environment. The ability to fabricate different plate truss geometries with specific properties could enable more efficient and expressive buildings with fewer materials. Goodbye heavy steel and concrete structures, hello lightweight trusses!”
Parra Rubio, Mundilova and other MIT students also used the technique to create three vast, foldable aluminum composite works of art that are on display at the MIT Media LabDespite the fact that each work of art is several metres long, the construction took only a few hours to create.
“At the end of the day, an artistic work is only possible because of the mathematical and engineering input that we put into our work. But we don’t want to ignore the aesthetic power of our work,” says Parra Rubio.
This work was supported in part by the Center for Bits and Atoms Research Consortia, an AAUW International Fellowship, and a GWI Fay Weber Grant.