High Crush Force Efficiency (CFE)
CFE is the ratio of the load (force per unit area) when crushing begins, Fcrush, to the load during the compression, Fcompression:
When CFE = 1, the impact decelerations are mitigated. When CFE becomes smaller and approaches 0, sudden changes in acceleration are transmitted through the material, causing trauma (e.g., head and neck injury) or damaging cargo.
High Specific Energy Absorption (SEA)
The SEA is the integrated area under the load-compression curve divided by the mass of the material that’s become crushed. A large SEA (> 20 kJ/kg) means lots of energy has been absorbed by the material on impact or that the material absorbing the impact is very lightweight. Many lightweight, high SEA materials are functional in one direction, meaning they have low SEA in the other two directions. Therefore, off-axis impacts are generally not effectively dissipated. MetaCORE is a pro-isotropic high-SEA metamaterial, which sidesteps the uncertainty of knowing the impacting direction.
Some materials have special properties in one or two distinct directions, making them anisotropic. Other materials have properties that are independent of direction, making them isotropic. Pro-isotropic materials are materials whose characteristics are enhanced to make them more isotropic.
Low Mass Density (Lightweight)
Mass per unit volume of material is a common metric used to determine the weight of a final product. Because many of our metamaterials are designed with open cellular geometries, their density often reaches 1/10th the density of the base material, making it an ideal choice for lightweight applications.
Our metamaterials are typically fabricated using widely available raw materials with a value-added design and manufacturing process. This method keeps costs low relative to other advanced materials available in the commercial market.
We have a variety of strategies to increase the effective strength of a material. Some are similar to plywood, which uses alternating plies bonded together to prevent fracture propagation. Other strategies use the metamaterial geometry to convert large bulk compression/tension to internal torsion, or conversely large bulk torsion to internal compression/tension. In both cases, we’re essentially compensating for the weakest failure mode (typically shear) by forcing the structure to activate a different type of deformation and therefore exhibit a higher effective strength. A third strategy we’ve introduced is the use of curved surfaces to guide failure-inducing stress away from critical regions and toward “sacrificial structures.” This Crack Denial Strategy is inspired by the goals of self-healing materials, but is realized in materials available today.
Delamination: Sandwich panels fail for a variety of reasons related to their design and the loading patterns experienced in applications. The most common problem is delamination. Delamination occurs when the panels separate from the core material, inducing core shear and reducing the panel’s effective thickness. As a result, delamination leads to out-of-plane buckling, collapse from in-plane loads, and complete failure. MetaCORE-LD is engineered to radically shift the bounds of a standard failure mode map, replacing the costly failure of core shear with a no-fuss face wrinkling.
Low Thermal Conductivity
Many of our products are cellular materials whose volume is ~90% or more air. The effective thermal conductivity is governed by the same physics of conventional disordered foams commonly used in applications where thermal insulation is required. What distinguishes our metamaterial products is that they are ordered cellular materials, which means they retain significant structural loading capacity. Most designs, including those for MetaCORE-LD, contain continuous open-air channels that run the full length of the panel for additional thermal engineering, heating, and cooling opportunities.
Controlled Poisson Effect
Most solid materials bulge outward when squeezed or compressed – the volume doesn’t want to change, so the material has to find somewhere to go. This effect is quantified by Poisson’s Ratio, which is typically positive. In contrast, many metamaterials have a negative Poisson’s Ratio, which introduces interesting opportunities for engineering materials with reduced frictional wearing. We can target a specific value for the Poisson’s Ratio in each direction, so that some directions have an extraordinary bulging effect when loaded, while others contract, or do not move at all.
Controlled Thermal Expansion
Heating solids typically causes them to expand. We’ve developed techniques to engineer this effect, and even induce negative thermal expansion. Precise engineering of thermal expansion is useful for multi-material interfaces like plastic caps on metal vessels. When heated, the differential swelling creates thermomechanical stress, fatigue, and eventually the plastic cracks. With metamaterial enhancements to create equal-but-opposite thermally-induced strain, we mitigate the expansion stresses so the two parts can coexist without failure.
We fabricate metamaterials with a variety of base materials including polymers, metals, and composites. Some of our base materials, like PETG and PEEK, are highly resistant to chemical and environmental corrosion.