Progressive compaction of soft cellular structures with layer-wisely designed gradient porosity

Soft cellular structures possess excellent recoverable compacting characteristics, which have great potentials for developing a new branch of flexible metamaterials with emerging applications. Inspired by the microstructures of natural materials, a novel type of soft cellular structures with gradient porosity, which are different from the periodic cellular structures, is proposed and designed in this study. In this new design of soft cellular structures, the overall porosity is kept as a constant while the porosity is varying along each layer gradually. In doing so, a new kind of soft cellular structures possess a layer-wisely designed gradient porosity is achieved. This design enables the cellular structures to exhibit a distinct progressive compaction behavior which is completely different from the uniform densification of the soft periodic structures. The progressive compaction of the structure is controllable through designing the gradient of the porosity to regulate stiffness of each layer. The mechanical responses and properties of the gradient soft cellular structures with different porosities and different layer-wise patterns under uniaxial compression are investigated both experimentally and numerically. This study reveals that the maximum absolute values of negative Poisson's ratio of current gradient soft cellular structures is 2.113 time than that of periodic ones with a same overall porosity. The additional design freedom introduced by porosity gradient is beneficial for adjusting the mechanical properties of soft cellular metamaterials to meet the design demands of various potential applications.