Explorations into the structural instabilities of mechanical metamaterials have led to the development of bistable cellular structures. Although structural instabilities have generally been regarded as entities that are best avoided in practice, rapid emergence of nonlinear mathematics, computational power and additive manufacturing techniques, have demonstrated that the exploitation of the geometrically nonlinear range is increasingly feasible. This three-pronged approach: analytical, numerical and experimental is utilized in the present work to generate a novel lattice configuration that is capable of enhancing energy absorption and structural isolation performance.
In conventional structural design, the principal concern is to maintain strength and stiffness of the component including after any instability. However, for energy absorbency or dynamic isolation applications, although the desired property still includes a reasonably high load-carrying capacity, a crucial property is a diminished post-buckling stiffness. The latter property ensures that any transfer of displacement to any connected structures is minimized, whereas the former ensures significant energy is absorbed in the deformation process. Therefore, with these criteria in mind, preliminary finite element analyses on sandwich structures explore the effects of variations to parameters such as the panel aspect ratio, to achieve the initial high load-carrying capacity, and disruption within the lattice geometry, to achieve the subsequent low post-buckling stiffness, which was found to be successful within the two-dimensional realm.
Additional complexities in the construction of three-dimensional lattices, predominant- ly owing to density and tessellation, warranted a different approach than incorporating disruptions within a repeating array of unit cells. Through a ‘bottom-up’ approach of examining the behaviour at the unit cell scale and mesoscale, a new lattice is devised that can snap from a conventional ‘flint’ to an auxetic arrowhead structure such that the post-buckling structural stiffness is maintained to be close to zero. This geometri- cal phase transition of the lattice, for a panel under axial compression and without face plates, enabled sequential shortening of the panel length prohibiting the unfavourable global-type buckling response. Tuning of the post-buckling response was also possible through geometrical grading. Furthermore, so long as the relative density is maintained, through alterations to strut dimensions, this desirable mechanical response was found to be scalable. Lastly, the results from experimental tests on specimens that were 3D printed compared well, thus verifying the numerical work. The responses were repeatable, with the full-scale panel able to achieve some level of recoverability when unloaded prior to material failure. The novelty of the present work lies in the predominant use of the elastic buckling range through geometrical variations of cellular structures to dissipate impact.