Fabrication, characterization and analytical modeling of gradient auxetic closed cell foams

Abstract

Auxetic (negative Poisson’s ratio) and gradient open cell foams have shown promise for their conformability and high impact energy absorption—useful in applications like protective equipment, footwear and prosthetics. Recent methods fabricated auxetic closed cell foam by steaming conventional closed cell foam. Methods developed herein control the cell structure of auxetic closed cell foam, to produce novel intentionally anisotropic and gradient auxetic closed cell foam. Pins passed through the foam constrained or stretched regions during steaming to further modify cell structure, and impart gradients in cell structure, Young’s modulus and Poisson’s ratios. Fabricated foams had Poisson’s ratios between 1 and −1. Imparted Young’s moduli of 1–12 MPa were similar to foams used in footwear, prosthesis, helmets and other impact protection. The effect of changes to cell structure on Young’s moduli and Poisson’s ratio are explained by combining analytical models of auxetic open cell and conventional closed cell foam.

Effect of Compressive Strain Rate on Auxetic Foam

“…The anisotropic open cell foam had elongated cells in the stretched directions (y-axis, Figure 3b), and slightly inward angled, or re-entrant ribs, in the compressed x and z directions. The triaxially compressed open cell foam had re-entrant like cells (Figure 3c), as expected [47,58,71]. Also as expected [62,71], the unconverted closed cell foam had hexagonal cells with no visible cell rise direction (Figure 3d), and the triaxially converted closed cell foam had re-entrant like, kinked cell walls (Figure 3e) [62,71].…”

“…A summary of all data described below is in Supplementary Table S1. The unconverted (UC) open cell foam had hexagonal shaped cells, elongated in the cell rise direction (z-axis, Figure 3a), as expected [47,58,71]. The anisotropic open cell foam had elongated cells in the stretched directions (y-axis, Figure 3b), and slightly inward angled, or re-entrant ribs, in the compressed x and z directions.…”

“…Here, we test auxetic open and closed cell foam at a range of compressive strain rates (0.05 to 200 s −1 ). We measure Young's modulus and Poisson's ratio at higher compression rates, and in a more controlled way, than previous work (e.g., [36,52,[61][62][63]71]). A dynamic mechanical analyser applied constant rate-controlled compression to foam samples, with concurrent high-speed optical full-field strain measurement.…”

Cancellation of Auxetic Properties in F.C.C. Hard Sphere Crystals by Hybrid Layer-Channel Nanoinclusions Filled by Hard Spheres of Another Diameter

“…Auxetic properties were found not only in metamaterials [14][15][16] or model structures [17,18], but also in crystals, e.g., cubic systems [19], both theoretically [20][21][22][23] and experimentally [24,25]. Novel structures [26][27][28][29][30][31][32][33][34], nanostructures [35,36], and models [37][38][39][40][41][42][43][44], real materials (e.g., polymers [45,46], composites [47], or foams [48,49]), and metamaterials [50,51] with auxetic properties have been designed and reported. These advances in real metamaterials would not have been possible without more basic research [52][53][54], analysis [55][56][57][58][59], and optimization [60,…”

Removing Auxetic Properties in f.c.c. Hard Sphere Crystals by Orthogonal Nanochannels with Hard Spheres of Another Diameter

“…Since their discovery, auxetics have been extensively studied both theoretically [ 14 , 15 , 16 , 17 , 18 ] by computer simulations [ 19 , 20 , 21 ] and experimentally [ 22 , 23 , 24 ]. Auxetic properties, i.e., the existence of negative PR in at least some crystallographic directions (such materials are called partial auxetics [ 25 ]), were reported not only in model structures [ 13 , 26 , 27 , 28 , 29 ], but also in real cubic materials [ 30 ], polymers [ 31 , 32 ], composites [ 33 ], and foams [ 34 , 35 ]. Today, novel structures [ 36 , 37 ], nanostructures [ 38 ], and metamaterials [ 39 , 40 , 41 ] with auxetic properties are being developed.…”