Metamaterials enable ultra-efficient mechanical energy storage

Berlin, Germany (SPX) Apr 03, 2025
Mechanical energy storage plays a crucial role across numerous technologies, from energy-absorbing springs to robotic components and energy-efficient machines. By converting kinetic energy into elastic energy, systems can later recover and reuse that energy on demand. Central to this conversion is enthalpy, the measure of how much recoverable energy a material element can store. Achieving high enthalpy, however, is a complex materials challenge. "The difficulty is to combine conflicting properties: high stiffness, high strength and large recoverable strain," explained Peter Gumbsch, Professor for mechanics of materials at KIT's Institute for Applied Materials (IAM).

Strategic Use of Twisted Rods in Engineered Structures

Metamaterials, synthetic structures engineered from unit components, can be designed to surpass the performance of conventional materials. Gumbsch, who also directs the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg, led an international collaboration involving researchers from China and the United States. Together, they developed a class of mechanical metamaterials capable of storing exceptional amounts of elastic energy.

The team first identified a technique for maximizing energy storage in a simple cylindrical rod, ensuring the rod neither fractured nor deformed irreversibly. Building on this, they devised an architectural configuration that incorporated the rods into a functional metamaterial. Unlike conventional bending springs, which suffer from localized stress concentrations that can lead to failure, their design relies on twisting the rods. This torsion places the entire rod surface under stress, while minimizing stress in the interior, allowing for a more efficient use of material volume.

This effect is driven by extreme torsion that leads to a complex form of deformation known as helical buckling. This structural behavior distributes stress more effectively throughout the material, boosting its energy storage capacity.

Performance Exceeds Conventional Metamaterials by Orders of Magnitude

The researchers embedded these helically buckled rods into metamaterials that perform under uniaxial loads on a macroscopic scale. Computer simulations projected that the design would exhibit high stiffness and absorb significant mechanical forces. Tests using compression experiments on mirrored chiral metamaterial structures confirmed these predictions. The measured enthalpy was between 2 and 160 times greater than that observed in comparable metamaterials.

"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," Gumbsch noted. Potential applications extend beyond energy storage to include damping, impact absorption, and elastic joints in robotics or adaptive machine components.

Research Report:Large recoverable elastic energy in chiral metamaterials via twist buckling