  Mechanical metamaterials are artificial composites with tunable advanced
mechanical properties. Particularly interesting types of mechanical
metamaterials are flexible metamaterials, which harness internal rotations and
instabilities to exhibit programmable deformations. However, to date such
materials have mostly been considered using nearly purely elastic constituents
such as neo-Hookean rubbers. Here we explore experimentally the mechanical
snap-through response of metamaterials that are made of constituents that
exhibit large viscoelastic relaxation effects, encountered in the vast majority
of rubbers, in particular in 3D printed rubbers. We show that they exhibit a
very strong sensitivity to the loading rate. In particular, the mechanical
instability is strongly affected beyond a certain loading rate. We rationalize
our findings with a compliant mechanism model augmented with viscoelastic
interactions, which captures qualitatively well the reported behavior,
suggesting that the sensitivity to loading rate stems from the nonlinear and
inhomogeneous deformation rate, provoked by internal rotations. Our findings
bring a novel understanding of metamaterials in the dynamical regime and opens
up avenues for the use of metamaterials for dynamical shape-changing as well as
vibration and impact damping applications.
