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Tunable stiffness using negative Poisson's ratio 331

this geometry was found to be 0.43 (Kolken and Zadpoor, 2017). The
detailed kinematic model of the missing rib configuration was presented
by Zhai (Zhai et al., 2018). The auxetic effect of such a design is obtained
by the concurrent out-folding of re-entrant cells and the missing ribs’ rota-
tion mechanism.

5.3.5 Double arrowhead structure
The double arrowhead design is another variant of the re-entrant honey-
comb, which was first founded through the numerical topology optimiza-
tion method. Based on the actual configuration of the arrowhead, any
extension will cause the triangles to expand in the transverse direction while
compressions will cause them to collapse. The auxetic behavior of such
geometries is shown in Fig. 12. The CAD drawing of the double arrowhead
structure is shown in Fig. 13. These structures are designed to exhibit a

Fig. 12 Auxetic behavior of double arrowhead structure.

Fig. 13 CAD of the double arrowhead structure.