Multilayered recoverable sandwich composite structures with architected core

In this paper, we propose a novel design and fabrication strategy to produce architected core structures for use as the core in composite sandwich structures. A traditional foam core or honeycomb structure is lightweight and stiff, but susceptible to permanent deformation when subjected to excessive loading. Here we propose the use of an architected structure composed of arrays of hollow truncated cone unit cells that dissipate energy and exhibit structural recovery. These structures printed with a viscoelastic material rely on buckling of their sidewalls to dissipate energy and snap-back to prevent permanent deformation. We explore the mechanical response of these conical unit cells in terms of their buckling strength and post-buckling stability condition, and develop design maps for the same, by relating them to non-dimensional geometric parameters $\alpha$, $\beta$, $\gamma$, where $\alpha$ represents the slenderness of the curved sidewalls, $\beta$ is the angle of the sidewall to the base, and $\gamma$ represents the curvature of the sidewall. A validated finite element model is developed and used to investigate the effect of these parameters. We see that the peak buckling load is directly proportional to both $\alpha$ & $\beta$ and is not dependent on $\gamma$ when the load is normalized by the volume of material in the curved sidewall. Interestingly, the post-buckling stability is influenced by $\gamma$, or the initial curvature of the sidewall, where a larger radius of curvature makes the structure less susceptible to exhibit structural bistability. The structures presented here are printed using a viscoelastic material, that causes them to exhibit pseudo-bistability, or a time-delayed recovery. This allows the structures to buckle and dissipate energy, and then recover to their original configurations without the need for external stimuli or energy.