Mechanical properties of foam-filled hexagonal and re-entrant honeycombs under uniaxial compression

Auxetic materials have excellent mechanical properties, e.g., indentation resistance, shear resistance, fracture toughness and energy absorption. However, the stiffness of auxetics is normally lower than that of solid structures due to the existence of voids. In this study, to improve the mechanical properties of re-entrant honeycombs, a buffer material called slow recovery foam is filled into re-entrant honeycombs. The mechanical properties and deformation patterns of slow recovery foam-filled re-entrant honeycombs are investigated numerically and experimentally. Parametric studies are conducted to investigate the effects of geometrical parameters on the Poisson's ratio and energy absorption capacity. The results show that filling foam into re-entrant honeycombs will prevent lateral buckling of the structure. Compared with foam-filled hexagonal honeycombs, foam-filled re-entrant honeycombs have higher stiffness. With the increase of the strain rates, the stiffness and energy absorption capacity of slow recovery foam-filled re-entrant honeycombs will increase. With the increase of cell wall thickness and the decrease of cell angle, the energy absorption capacity of slow recovery foam-filled re-entrant honeycombs will also increase. The results indicate that slow recovery foam-filled re-entrant honeycombs are promising in the field of protective engineering.