Materials that are both light and strong have many engineering applications, and UC Santa Barbara mechanical engineer and materials scientist Jonathan Berger has come up with a high-performance solid foam material whose secret lies within its structure.
Berger conceived of a solid foam material called Isomax, which is light, strong and versatile, allowing it to be used in a variety of applications, from buildings to vehicles to packaging and transport.
The reason this material performs so well is due to the three-dimensional pyramid-and-cross cell geometry within. While normal foam materials have a structure resembling bubbles or honeycombs, the ordered cells within Isomax are set apart by walls forming the shapes of pyramids with three sides and a base. These pyramids join together into octahedra, which are reinforced inside with the 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. In fact, due to the unique geometry inside, the material is able to achieve the theoretical limit of isotropic elastic stiffness, where it is able to handle stresses in all directions equally well.
While conventional geometries like honeycomb may be able to resist forces from one direction, the cell is vulnerable to collapse if the force comes from a different direction. Isomax's cell structure allows it to resist crushing and shearing forces, without any penalties in weight or density.
“The Isomax geometry is maximally stiff in all directions,” explained Berger.
While Berger started with theory and computer modelling, he enlisted the help of Robert McMeeking, a UCSB material and mechanical engineering professor, to help provide additional scientific and mathematical support.
"I carried out some simplified paper and pencil 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.
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.
The development of this material may have widespread implications. 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.
Due to the simple geometry, the material is versatile enough to fabricate for a variety of situations, and can also be used to create objects with varying levels of stiffness from one end to another, such as prosthetics and replacement joints. The design is compatible with various advanced manufacturing methods from origami-like folding to bonding and 3D printing.
Berger and his team are now following up with experimental analysis and are looking into manufacturing methods that may allow for efficient fabrication.