“Mechanical Metamaterials” – Versatile building blocks structure structures with amazing mechanical properties

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CBA researchers have created four new types of subunits, called voxels (a 3D variation of the pixels in a 2D image). From left to right: rigid (gray), compatible (purple), auxetic (orange), chiral (blue). “Credit:
Image: Benjamin Jenett, CBA

Subunits can be robotically assembled to create complex and large objects such as cars, robots, or wind turbine blades.

Researchers WITH ONEThe Center for Bits and Atoms has created tiny building blocks that exhibit a variety of unique mechanical properties, such as the ability to create bending motion when tightened. These subunits could be assembled using tiny robots with almost unlimited functionality with integrated functionality, such as vehicles, large parts of the industry, or specialized robots that can be reassembled in a variety of ways.

The researchers created four different types of these subunits, called voxels (a 3D variation of the pixels in a 2D image). Each type of voxel exhibits special properties not found in typical natural materials and, when combined, can be used to make devices that respond to environmental stimuli in predictable ways. As an example, the wings or turbine blades of an aircraft respond to changes in air pressure or wind speed by changing their overall shape.

The findings, which determine the creation of a family of discrete “mechanical metamaterials,” are described in a paper published in the journal on November 18, 2020. Advances in science, By Benjamin Jenett ’20 MIT graduate doctor, Professor Neil Gershenfeld and four others.

“This remarkable, fundamental, and beautiful synthesis promises to revolutionize the cost, suitability, and functional efficiency of ultralight structure and materials shed,” says Amory Lovins, assistant professor of civil and environmental engineering at Stanford University and founder of the Rocky Mountain Institute. , who was not associated with this work.

Metamaterials get the name because the large-scale properties are different from the micro-level properties of the material components. They are used in electromagnetic and as “architectural” materials, designed at the level of their microstructure. “But not much has been done to create macroscopic mechanical properties as a metamaterial,” says Gershenfeld.

Gershenfeld says that with this approach, engineers will be able to build structures that capture many of the properties of materials and produce them using the same shared production and assembly processes.

Voxels are assembled from four-piece injection-molded polymer structures, which are then combined into three-dimensional shapes that can be joined into larger functional structures. They are mostly open spaces and therefore provide a very light but rigid framework when assembled. In addition to the basic rigid unit, which offers an excellent combination of strength and light weight, there are three other variants of these vowels, each with a different unusual property.

“Auxetic” voxels have a unique property, that when a cube of material is compressed, instead of coming out of the sides, it comes out inside. This is the first demonstration of this material, which is produced using conventional and inexpensive manufacturing methods.

There are also “compatible” voxels that have a zero Poisson ratio, which are similar in auxetic property, but in this case, when the material is compressed, the sides do not change shape at all. Few known materials show this property, which can now be created through this new approach.

Eventually, “chiral” voxels respond to axial compression or elongation with a twisting motion. Again, this is not a common property; Research that produced such a material through complex manufacturing techniques was considered a significant discovery last year. This work allows this property to be easily incorporated into macroscopic scales.

“Each type of property we are showing before has been its own area,” says Gershenfeld. “People used to write papers on that property. It’s the first thing that shows them all in one system. “

To show the true potential of large objects built as LEGO from these mass-produced voxels, the team, in collaboration with Toyota engineers, produced a functional super-mileage race car. recep track this year at an international robotics conference.

Jenett says they were able to assemble a lightweight, high-performance structural structure in a month, which previously took a year to build a comparable structure using conventional fiberglass construction methods.

During the race, the track was affected by rain and the race car crashed into a barrier. To the surprise of all involved, the car’s grid-like internal structure was deformed and then bounced off, absorbing the shock without much damage. A custom-built car, Jenette says, is likely to have been thickly injured if it had been metal or broken if it had been compounded.

The car proved that these small pieces can be used to make functional devices on human-sized scales. And, Gershenfeld points out, in the structure of the car, “they are not parts related to something else. Everything is only made of these parts, ”except for the engines and power supplies.

Because the voxels are uniform in size and composition, they can be combined as needed to give the device different functions. According to Gershenfeld, “we can now expand the many properties of materials that were highly specialized.” “The fact is that you don’t have to choose a property. For example, you can make robots that bend in one direction and are rigid in another direction and only move in certain ways. So the big change in the work done before is the ability to extend to multiple mechanical properties. which were considered in isolation “.

Jenett, who completed much of this work as the basis of his doctoral dissertation, says, “These parts are low cost, easy to produce, and very fast to assemble, and you get that range of properties in a single system. They’re all compatible with each other, so there are all these kinds of exotic properties, but they all play well with each other in a scalable and affordable system. ”

“Think of all the rigid and moving parts of cars and robots and ships and planes,” says Gershenfeld. “And we can expand all of that with this system.”

Jenett argues that a key factor is that a structure composed of one type of these vowels will act as the subunit itself. “When we assembled the pieces together we were able to demonstrate that the joints disappear effectively. It acts as a continuous monolithic material.”

While robotics research is often divided between hard and soft robots, “that’s also not very much,” says Gershenfeld, because of its ability to mix and match these properties in a single device.

Jenette says one of the possible early applications of this technology could be the construction of wind turbine blades. As these structures grow, transporting shovels to their site of operation becomes a serious logistical problem, whereas if they are assembled from thousands of small subunits, this work can be done on site, eliminating the problem of transportation. Also, disposing of used turbine blades is a serious problem due to their large size and lack of recyclability. But shovels made up of tiny voxels could be disassembled there, and later reused for something else.

In addition, the blades themselves could be more efficient because they can have a mixture of mechanical properties designed in a structure that will allow them to respond passively to changes in wind force in a dynamic way.

Overall, Jenette says, “We now have this low-cost, scalable system so we can design whatever we want. We can make quartets, we can make swimming robots, we can make flying robots. That flexibility is one of the key advantages of the system.”

According to Stanford’s Lovins, this technology can “make cheap, durable and ordinary light bird surfaces similar to those that passively and continuously optimize the wing of a bird. It is also possible that the crushing force can allow a spherical shell (a helium-free) balloon to float in the atmosphere, lifting it a couple of times higher than the net jet charge of the jet. “

He adds, “Like biomimicry and integrative design, this new art of cellular metamaterials is a powerful new tool to help us do more with less.”

Reference: Benjamin Jenett, Christopher Cameron, Filippos Tourlomousis, Alfonso Parra Rubio, Megan Ochalek and Neil Gershenfeld, “Discretely Mounted Mechanical Metamaterials,” November 18, 2020, Advances in science.
DOI: 10.1126 / sciadv.abc9943

The research team included Filippos Tourlomousis, Alfonso Parra Rubio and Megan Ochal from MIT and Christopher Cameron from the U.S. Army Research Laboratory. He had the support of the work NASA, U.S. Army Research Laboratory and Center for Bits and Atoms Consortium.

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