Unit Cell with Sign-switching Poisson's Ratio for Adaptive Mechanical Metamaterials

Abstract

Mechanical metamaterials are architected materials with unique properties derived from their internal structure, rather than the material they consist of. Introducing distinct stable states into the material architecture allows the creation of mechanical metamaterials with multiple effective properties that can be altered post-fabrication. So far these so-called reprogrammable mechanical metamaterials have only demonstrated responsive behaviour, where the state of the metamaterial is dictated by the external input, leading to complex actuation and limited functionality. This research introduces a transformative approach to overcome these limitations through state-dependent switching, enabling metamaterials to autonomously determine their state based on internal information. Leveraging internal instabilities in the form of bistable and slender buckling elements, a contactless and tessellatable 3D unit cell design that can switch between positive and negative Poisson's ratios upon a specified displacement threshold is introduced. State transition occurs based on internal state information, rather than the external input, enabling adaptive behaviour. Both, Finite Element Analysis (FEA) simulations and experimental validation demonstrate the ability of the unit cell to switch between positive and negative Poisson's ratio under the same repeated input. Preliminary FEA simulations further suggest that through the tessellation of this innovative unit cell, the adaptive behaviour can be exploited to create mechanical metamaterials capable of adaptive shape morphing and repeatable counting abilities.