4D-printed reusable metamaterial via shape memory effect for energy dissipation

The present study aims at developing reusable metamaterials fabricated by 4D printing technology. Honeycomb metamaterials were manufactured via fused deposition modeling (FDM) with shape memory polymers (SMPs). The reusability of these metamaterials was determined through cyclic cold programming experiments, where each cycle involved a loading-unloading-heating (shape recovery)-cooling process. The novelty of this paper lies not only in experimentally demonstrating the recoverability of metamaterials by reversing plastic deformation based on the shape memory effect of SMPs, but also in studying their reusability of SMP metamaterials under cyclic programming and the effect of printing materials and unit-cell types on the mechanical degradation. The results reveal that, under one single compression cycle, the polylactic acid (PLA) hexagonal honeycomb dissipated 22% more energy than the polyethylene terephthalate glycol (PETG) counterpart because the higher elastic modulus of PLA leads to a larger critical buckling load for segments in honeycomb structures. Furthermore, the PETG re-entrant honeycomb dissipated 25% more energy than the hexagonal counterpart due to its negative Poisson's ratio and the overall uniform deformation pattern. More importantly, it is found that under multiple compression cycles, the PETG hexagonal honeycomb maintained an energy dissipation capacity of 78.3% at Cycle 6, nearly 3.5 times that of the PLA counterpart as a result of the better ductility of PETG. Moreover, the PETG re-entrant honeycomb could be reused for 17 cycles, while the hexagonal counterpart could only be reused for 12 cycles. This is because the re-entrant unit cells are failure-resistant and of less concentration in plastic deformation. The results demonstrate that the constituent materials with better ductility and the unit-cells with more failure resistance can reduce mechanical degradation, thereby exhibiting better reusability of metamaterials.