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A hierarchical programmable mechanical metamaterial unit cell showing metastable shape memory

: Berwind, M.F.; Kamas, A.; Eberl, C.


Advanced engineering materials 20 (2018), Nr.11, Art. 1800771, 6 S.
ISSN: 1438-1656
ISSN: 1615-7508
Fraunhofer IWM ()

Hierarchically structuring materials open the door to a wide range of unexpected and uniquely designed properties. This work presents a novel mechanical metamaterial unit cell with strain‐dependent solid–solid phase changes resultant from hierarchically structured “mechanisms” built into an auxetic unit cell, and further presents a realization of this kind. The interaction of auxetic structure and mechanism allows stable or metastable elastic energy states to be reached as a result of mechanical deformation. The result is a principally elastic analog to a shape memory material with a functional dependency on its negative Poisson's ratio. Prototypes are additively manufactured using direct laser writing, and are subsequently subjected to uniaxial compression with a customized micromechanical test set up. Experimental results depict reversible states initially triggered by deformation; the unit cell is a building block for a programmable material with a nonlinear "if… then…" relationship. Implementing interior mechanisms as a hierarchical level unlocks new directions for mechanical metamaterials research, and the authors see potential impacts or applications in multi‐scale modeling, medicine, micro‐actuation and ‐gripping, programmable matter/materials.