Dynamics of Polymeric Re-Entrant Auxetic Structures: Cyclic Compression Studies

The present study investigated the dynamic behavior of structures made of re-entrant unit cells subjected to cyclic compressive loading limited to the elastic range. The structures were assembled from printed polymer re-entrant cells in six combinations. Through the given compression cycles for three different amplitude values, strain-force relationships, which had the shape of a hysteresis loop, were obtained. Under compression, all unit cells of the structures deformed uniformly, though only for a certain amount of strain, whereas with larger changes, they underwent uncontrolled deformation. Experiments showed that structures composed of more than one unit cell exhibit different mechanical characteristics. It was observed that the width of the hysteresis loop depended on the degree of closing the structure and on the compression amplitude. The obtained hysteresis curves for different amplitudes also testify to the occurrence of the Mullins effect for these polymeric auxetic structures. Taking into account the maximum values of changes in dimensions for a given compression cycle, Poisson's ratio values were determined, which were negative and below unity. The effect of strut thickness on the NPR was confirmed, decreasing its negative value along with the increasing thickness.