Energy absorption is a vital quality for materials used in protective systems, like helmets or crumple zones of cars. Materials that absorb energy can do it in a recoverable fashion through elastic deformation, e.g. viscoelastic materials, or in a non-recoverable fashion through plastic deformation or fracture/comminution of the material, e.g. cellular metals and polymer-matrix composites arranged in arrays of tubes. Whether a recoverable or non-recoverable material is preferred depends on the application. Architected materials (or metamaterials), e.g. cellular solids with carefully designed unit cell topologies, can provide unique solutions for both kinds of deformation, while also offering multifunctionality. In this thesis, we investigate two novel concepts for the optimal design of metamaterials for energy absorption: (i) the incorporation of negative stiffness elements (originating from mechanical and magnetic forces) in an architected material, leading to two recoverable designs and (ii) layer-by-layer fracture and comminution of a ceramic nano-architected material, resulting in a non-recoverable system.
Viscoelastic materials are commonly used to dissipate kinetic energy in case of impact and vibrations. Unfortunately, dissipating large amounts of energy in a monolithic material requires high combinations of two intrinsic properties – Young’s modulus and loss factor, which are generally in conflict. This limitation can be overcome by designing cellular materials incorporating negative stiffness elements. We investigate and present a configuration comprising two positive stiffness elements and one negative stiffness element. This unit cell possesses an internal degree of freedom, which introduces hysteresis under a loading-unloading cycle, resulting in substantial energy dissipation, while maintaining stiffness. We perform a detailed study of an implementations of this concept designed for large strokes. Subsequently, we investigate the possibility of exploiting magnetic forces in order to embed steep negative stiffness elements in lattice materials, further improving performance.
Finally, we study the energy absorption in nano-architected materials through progressive fracture and comminution of the constituent ceramic, yielding a non-recoverable protective system. We develop a fabrication approach for nanoscale shell-metamaterials made of pyrolytic carbon. By exploiting the unique features of the architecture as well as materials size effects at the nanoscale, we demonstrate non-catastrophic irreversible deformation up to 80% strain, and energy absorption up to one order of magnitude higher than for any existing nano-, micro- and macro-architectures and solids, as well as state-of-the-art impact protection structures. At the same time, strength and stiffness are on par with the most advanced, yet brittle nanolattices, demonstrating true multifunctionality.
By combining analytical modeling, computational approaches for mechanical modeling and optimal design, and sophisticated additive manufacturing approaches and mechanical characterization techniques over multiple length scales, this thesis unveils three new classes of mechanical metamaterials with unique energy absorption capabilities.