Metamaterials: Highly twisted rods store large amounts of energy

Clever Arrangement of Helically Deformed Rods in Metamaterials

Metamaterials are artificially designed materials that do not occur in nature. They are assembled from individually defined units so that their effective material properties can be enhanced. Peter Gumbsch, who also heads the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg, and his international research team with members from China and the USA now succeeded in developing mechanical metamaterials with a high recoverable elastic energy density. "At first, we detected a mechanism for storing a high amount of energy in a simple round rod without breaking it or deforming it permanently," says Gumbsch. "By defining a clever arrangement of the rods, we then integrated this mechanism into a metamaterial."

The scientists compare this mechanism to a classic bending spring, whose maximum deformation is limited by high tensile and compressive stresses that occur at its top and bottom surfaces and lead to breaking or permanent plastic deformation. In such a bending spring, the stresses on the entire inner volume are very low. However, if a rod is twisted instead its entire surface is also exposed to high stresses but the interior volume at low stresses is considerably smaller. To leverage this mechanism, the torsion must be so high that it results in a complex helical buckling deformation.

Enthalpy Is 2 to 160 Times Higher Than in Other Metamaterials

The researchers managed to integrate such torsionally loaded and helically deformed rods into a metamaterial that can be used macroscopically under uniaxial loads. Simulations helped them predict that the metamaterial would have a high stiffness and thus could absorb large forces. In addition, its enthalpy is 2 to 160 times higher than that of other metamaterials. To confirm this, they conducted simple compression experiments on various metamaterials with mirrored chiral structures.

"Our new metamaterials with their high elastic energy storage capacity have the potential to be used in various areas in the future where both efficient energy storage and exceptional mechanical properties are required," says Gumbsch. Conceivable applications beside spring-based energy storage include shock absorption or damping as well as flexible structures in robotics or in energy-efficient machines. Alternatively, the twists occurring inside the metamaterials might be used for purely elastic joints.