New metamaterial is proved to be the world's first to achieve the performance predicted by theoretical bounds

February 21, 2017 by Sonia Fernandez, University of California - Santa Barbara
The Isomax cell configuration. Credit: UCSB Engineering

In 2015 UC Santa Barbara mechanical engineer and materials scientist Jonathan Berger developed an idea that could change the way people think about high-performance structural materials. Two years later, his concept is paying research dividends.

In a letter published in the journal Nature, Berger, with UCSB materials and mechanical engineering professor Robert McMeeking and materials scientist Haydn N. G. Wadley from the University of Virginia, prove that the three-dimensional pyramid-and-cross cell geometry Berger conceived is the first of its kind to achieve the performance predicted by theoretical bounds. Its lightness, strength and versatility, according to Berger, lends itself well to a variety of applications, from buildings to vehicles to packaging and transport.

Called Isomax, the beauty of this solid foam—in this case loosely defined as a combination of a stiff substance and air pockets—lay in the geometry within. Instead of the typical assemblage of bubbles or a honeycomb arrangement, the ordered cells were set apart by walls forming the shapes of pyramids with three sides and a base, and octahedra, reinforced inside with a "cross" of intersecting diagonal walls.

The combination of the pyramid and cross-shaped cells, said Berger, resulted in a structure that had low density—mostly air, in fact—yet was uncommonly strong for its mass.

"The Isomax geometry is maximally stiff in all directions," explained Berger. Other geometries—a honeycomb, for instance—may be able to resist forces from one direction, but approach it from a different direction and the cell will collapse easily. Isomax's cell structure makes it possible for the material to resist crushing and shearing forces without the need to make it heavier or denser.

However, for all the early interest that his proposed metamaterial generated and the computer modeling that supported his claims, Berger knew he couldn't rest until science backed him up.

"There was obviously a lot of positive feedback, but for me as a scientist, it's a bit too much hand waving until you have something in a peer-reviewed journal," Berger said.

And now the work has borne fruit.

"I carried out some simplified calculations of the stiffnesses of some of the foams and was able to see that the pencil-and-paper results agreed with the computer calculations," explained McMeeking, whose research focuses on computational science and engineering as well as the mechanics of materials, including their fracture and durability. "This gave us confidence that the computer calculations were both correct and being formulated accurately."

McMeeking's calculations also proved that, in the case of the lightest weight foams, they were identifying the optimal geometries of the foams that enabled them to achieve the maximum possible stiffness. "That finding also meant that we could be sure that the were also successfully obtaining the optimal geometries for the heavier weight foams, where pencil-and-paper calculations are almost impossible to carry out because they are much more complicated," he said.

The closed-form solutions and equations developed to create the mechanical model of the metamaterial's behavior "matched up beautifully" with the earlier computer models, said Berger.

Given its properties, Isomax "is going to be a very interesting metamaterial," said Wadley, whose research spans the synthesis, structure and performance of novel materials. "It will also be an excellent thermal insulating and sound absorbing material. Potential applications for this ultralight material are likely to emerge in aerospace structures, for lightweighting automobiles and in many robotic machines, especially mobile types that carry their own power and must maneuver."

The development could not have better timing. As resources become more limited and concern for energy efficiency grows, a material with this mass relative to its strength would require fewer resources to produce and less fuel to transport. The simple geometry makes it versatile enough to fabricate for a variety of situations, and, functionally graded, it can be used to create objects with varying levels of stiffness from one end to another, such as prosthetics and replacement joints, and the design is compatible with manufacturing methods from origami-like folding to bonding and 3-D printing.

The study is one of a series of steps investigating the potential of this metamaterial. Berger and team are currently following up with experimental analysis and are looking into manufacturing methods that may allow for efficient fabrication.

Explore further: New metamaterial can switch from hard to soft—and back again

More information: J. B. Berger et al. Mechanical metamaterials at the theoretical limit of isotropic elastic stiffness, Nature (2017). DOI: 10.1038/nature21075

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1 / 5 (2) Feb 21, 2017
" As resources become more limited"

What resources? That 'peak oil' thing is so ten years ago.
not rated yet Feb 22, 2017
Potential applications for this ultralight material are likely to emerge in aerospace structures

I wonder how thin/light such a structure could be and still be stiff enough to support the stress of a vacuum structure. Vacuum balloons, anyone?
not rated yet Feb 22, 2017
Are there any details as to what those theoretical bounds are?
not rated yet Feb 22, 2017
The theoretical upper bound is defined by the Hashin-Shtrikman metric
5 / 5 (3) Feb 22, 2017
I wonder how thin/light such a structure could be and still be stiff enough to support the stress of a vacuum structure. Vacuum balloons, anyone?

A vacuum balloon must weigh less than 1.28 grams per liter. A sphere of 1 m in diameter has a volume of 0.52 m^3 which means it lifts only 670 grams.

So suppose you have a millimeter thick sheet of material to form into the sphere. The material would have a density of ~50 kg/m^3 which is about 0.7% of the density of steel.

Of course a 1 mm thick sphere 1 meter wide cannot resist buckling no matter what it's made of. If the shell is 10 cm thick the density would be ~2 kg/m^3 which is on par with aerogels. If the metamaterial was made of steel, it would need to have a fill factor of 0.027% or less in order to float.

If each cell is 1 cm cubed, 9 cm^2 for all the surfaces, the material thickness in steel would have to be less than 0.3 micrometers. It would be too fragile to even touch with your fingers.
not rated yet Feb 22, 2017
Or to make the same calculation more explicitly:

If each pyramidal cell is 1 cm cubed, and the shell thickness is 10 cm for a 1 m diameter sphere, then the shell has a volume of 255,516 cells.

Each cell in separate has 6 faces and the two internal crosswise walls, which sum up to approximately 10 cm^2 in area together so that makes 25,551,162 cm^2 for all the cell wall surfaces.

This structure needs to weigh no more than 670 grams to float which is therefore 0.00026 g/cm^2 and knowing that steel weighs 7.85 g/cm^3 the wall thickness of each cube must be no more than 0.33 µm assuming the whole structure is under vacuum.

0.33 µm or 330 nm is the wavelength of ultraviolet light, so the structure would be fine enough to be transparent to the naked eye.
5 / 5 (1) Feb 22, 2017
It looks like an ideal geometry for 3D printing structural elements. Maximising strength while saving weight and material. Its resistance to stress and fatigue would have to be tested before it's used in critical applications. (It has a lot of tight corners focusing stress in its basic form.)

Though for pressure vessels such as cryo-oxygen tanks traditional hexagonal bracing is still stronger as it's under tension and not a structural component.

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