Double-Negative Mechanical Metamaterials Displaying Simultaneous Negative Stiffness and Negative Poisson’s Ratio Properties

Intuitively, materials become both shorter and wider when compressed along their length. Here we show how a composite material or structure can display a simultaneous reversal in the direction of deformation for both the axial and transverse dimensions, corresponding to negative values of effective stiffness and effective Poisson’s ratio, respectively. A negative Poisson’s ratio[1] (NPR or auxetic[2]) host assembly stabilising (otherwise unstable) embedded negative stiffness[3] (NS) elements is presented and modelled analytically. Composite assemblies containing 3 alternative NS elements are demonstrated experimentally, confirming both NPR and NS responses under quasi-static loading over certain strain ranges and in good agreement with model predictions. We report systems demonstrating NS values over two orders of magnitude, ranging from -1.4 N mm-1 to -160 N mm-1. Such systems are scalable and are expected to lead to increased enhancements in other useful properties such as vibration damping, finding application across transport, healthcare, defence and space sectors, amongst others.