Effect of steam conversion on the cellular structure, Young’s modulus and negative Poisson’s ratio of closed cell foam

Duncan, Oliver ORCID: https://orcid.org/0000-0001-9503-1464, Allen, Tom ORCID: https://orcid.org/0000-0003-4910-9149, Birch, Alana, Foster, Leon, Hart, John and Alderson, Andrew (2021) Effect of steam conversion on the cellular structure, Young’s modulus and negative Poisson’s ratio of closed cell foam. Smart Materials and Structures, 30 (1). 015031-015031. ISSN 0964-1726

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Abstract

Open cell auxetic foams have shown potential for use in sporting and other protective equipment. Previous conversion methods for auxetic closed cell foam used a bespoke pressure vessel. This work expands upon a recent method, using steam absorption followed by cooling and condensation, to change the cell shape of closed cell foam to impart a negative Poisson’s ratio. Aiming to present a simple, repeatable conversion method, foam samples (20 ⅹ 10 ⅹ 100 mm) floating in water filled (~20°C), covered ceramic containers (300 ⅹ 200 ⅹ 100 mm) were heated in an oven at 105°C for between 1 and 6.5 hours. Based on the results for the smaller samples, a larger sample of foam (30 ⅹ 100 ⅹ 100 mm) was also converted, with a steaming time of 8 hours. Final volume ratio (original/final volume) increased with heating time, up to a maximum of 4.6. The amount and angle of re-entrant (inward folding) cell walls increased with final volume ratio as cells contracted further, evidenced by micro computed tomography. Poisson’s ratios, measured using digital image correlation, were as low as -0.3 in tension and -1.1 in compression. Tensile Young’s modulus increased from ~2 MPa up to ~6 MPa with final volume ratio and compressive Young’s moduli reduced from ~1.5 MPa for low values of final volume ratio between one and three, then remained close to 1.5 MPa.