An investigation of smart, inclusion-based mechanical metamaterials

Abstract

Mechanical metamaterials have captured the attention of researchers worldwide owing to their exceptional mechanical properties. They are poised to emerge as a pivotal category of materials that will shape the next generation of innovative materials. Research on auxetic materials, a class of mechanical metamaterials, has been ongoing for more than four decades, with recent years witnessing a surge in interest in the design of 3D auxetic metamaterials. Another class of metamaterials that has shown great potential is active metamaterials, which are materials that can have their geometry and/or mechanical properties tuned post-fabrication in response to external stimuli. This thesis delved into the realm of mechanical metamaterials, focusing on these two distinct categories. It introduced a novel design approach for creating 3D auxetic structures and conducted an in-depth exploration of magneto-mechanical active metamaterials. The 3D auxetic systems were intentionally designed to lay the foundation for potential future research, envisioning their transformation into active magneto-mechanical metamaterials. The mechanical properties of the 3D auxetic structures produced in this thesis were investigated through numerical simulations validated by experimental tests. It was demonstrated that a system, created through equally sized voids with a constant cross-sectional area into a solid material at specific locations in various planes, could exhibit a negative Poisson's ratio of approximately -0.5 in multiple directions. This behaviour was observed over a significant range of aperture angles for the cross-sectional areas, especially when the voids were positioned close to each other. A scalable inclusion-based active magneto-mechanical metamaterial consisting of magnetic inclusions embedded within a non-magnetic matrix was also produced. The proposed structure, based on an accordion-like structure, was shown to respond to the magnitude and direction of an external magnetic field by tuning its geometry. The basic unit was then used to create a number of active systems including the auxetic re-entrant honeycomb and egg-rack structures. Finally, iron nanoparticles inclusions were used instead of permanent magnets and successfully produced a magneto elastomer. Through numerical simulations validated by experimental prototypes, the response to an external magnetic field was investigated. The numerical model showed good agreement with the experimental tests and following this the effect of nanoparticle concentration and other geometric parameters were investigated.