Experimental and numerical investigation of conventional and stiffened re-entrant cell structures under compression

A classical re-entrant cell is a type of metamaterial known as auxetic. While the most unusual and advantageous feature of auxetic materials is that they have negative Poisson's ratios, having low stiffness-as seen in the classical re-entrant cell-may be a drawback. A study was conducted to increase the stiffness of the classical re-entrant cell while maintaining the negative Poisson's ratio. This paper reports the nonlinear experimental and numerical works of three re-entrant cells one of which is a well-known classical re-entrant cell, and the latter two were modified based on classical re-entrant cell. In the work, the cellular structure specimens were fabricated with a 3D printer using polylactic acid (PLA) material and crushing tests were conducted until the full crush phase. The specimens were also modelled using solid finite elements considering wall-to-wall frictional contacts and analysed. The linear mechanical properties of the cells were also determined by employing analytical expressions that were developed for modified cells. Thus, both the theoretical and the nonlinear numerical results were validated using experiments. In conclusion, the modified cells exhibited an increase in stiffness, energy absorption capacity, and plasticity, compared to the classical re-entrant cell. All benefits and drawbacks of the modifications to achieve stiff cells are reported in this paper.