Materials scientists have a few options for making lighter, yet stronger materials: improve the strength, lower the density, or both. But at a certain point, it’s tough to get much much stronger without added bulk.
Jens Bauer and his colleagues at the Karlsruhe Institute of Technology used a type of 3D printing to create a series of low density, high strength materials. By controlling the architecture, the authors created cellular materials that could take advantage of mechanical size effects, a principle by which strength increases as size decreases. The results are reported in the Proceedings of the National Academy of Sciences.
The strength of human bones, for example, comes from a structure made of nanometer-sized building blocks. “It’s a thing that nature takes advantage of quite often,” Bauer says.
Using 3D laser lithography, the authors fabricated artificial cellular materials comprised of a polymer core with an alumina coating. The simplest design is an open cubic cage-like structure with only horizontal and vertical support. From there, the design was complicated by adding diagonal braces first to the global cube structure, and then further by the addition of diagonal braces to the local cube structures.
Drawing on what is known about the strength and structure of bones, a second design approach moved toward optimization by converting the open cubic cage-like structure to an open hexagonal structure. A similar, closed hexagonal honeycomb-like structure exhibited the highest strength of all the designs.
To test the strength, the authors applied uniaxial mechanical pressure, and the honeycomb material performed best with a compressive strength up to 280 MPa, exceeding all natural and engineered materials with densities less than water, the authors report.
“In the bone, the material is aligned with the flow of forces,” he says. “If you look at (the honeycomb structure), that’s what we’ve done there.”
But there’s still room for improvement, Bauer says. The size could be reduced further, or the details could be further optimized such that each structural component receives pressure homogeneously, so that there are no weak links or places where stress is exceptionally high.