Multistep and Elastically Stable Mechanical Metamaterials

Materials and structures with tunable mechanical properties are essential for numerous applications. However, constructing such structures poses a great challenge since it is normally very complicated to change the properties of a material after its fabrication, particularly in pure force fields. Herein, we propose a multistep and elastically stable 3D mechanical metamaterial having simultaneously tunable effective Young's modulus and auxeticity controlled by the applied compressive strain. Metamaterial samples are fabricated by 3D printing at the centimetric scale, with selective laser sintering, and at the micrometric scale, with two-photon lithography. Experimental results indicate an elementary auxeticity for small compressive strains but superior auxeticity for large strains. Significantly, the effective Young's modulus follows a parallel trend, becoming larger with increasing compressive strain. A theoretical model explains the variations of the elastic constants of the proposed metamaterials as a function of geometry parameters and provides a basic explanation for the appearance of the multistep behavior. Furthermore, simulation results demonstrate that the proposed metamaterial has the potential for designing metamaterials exhibiting tunable phononic band gaps. The design of reusable elastically stable multistep metamaterials, with tunable mechanical performances supporting large compression, is made possible thanks to their delocalized deformation mode.