Foam-filled auxetic mechanical metamaterials: effects of foam properties on strength and energy absorption

Foam filling of hyperelastic lattice structures has emerged as a promising strategy for enhancing mechanical strength and energy absorption capacity. In auxetic lattices, foam filling can also mitigate undesirable deformation mechanisms, such as premature buckling. This work investigates the mechanics of foam-filled auxetic metamaterials through a combined experimental-modeling approach, emphasizing key processing-performance relationships. Auxetic structures are fabricated from 3D printed hyperelastic lattices filled with polyurethane foams of varying densities and mechanical properties. Unfilled and foam-filled lattices are subjected to uniaxial compression, and their load-bearing response, structural Poisson’s ratio, and energy absorption characteristics are evaluated. Incorporating low-density foams into hyperelastic auxetics yields considerable improvements in structural performance, including increased stiffness, elevated plateau stress, and enhanced energy absorption, with only a modest reduction in auxeticity. The mechanisms governing these trends are examined using full-field measurements in conjunction with a model established to capture the effects of the foam-solid interface and the volumetric compaction of the filler foam confined within the lattice voids. The benefits of foam filling are shown to depend strongly on the plateau stress of the filler foam and on the fabrication method used for the lattice-foam assembly.