Instability-Triggered Triply Negative Mechanical Metamaterial

We report a three-dimensional mechanical metamaterial that simultaneously possesses negative stiffness, negative bulk modulus, and negative Poisson's ratio. This metamaterial is a periodic arrangement of binder-shell elements. Under compression, the spherical shells dent inwards which cause the material to contract in the lateral directions. At larger deformations, snap through instability occurs and the material exhibits negative incremental stiffness. Interestingly, both incremental stiffness and incremental Poisson's ratio approach negative infinity (under displacement control) when snap-back is observed. We further showed that the multi-negative index metamaterial satisfies the strong ellipticity condition, therefore, a block of the metamaterial with many unit cells is stable under displacement constraint.