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

Duncan, Oliver and Allen, Tom (2020) Effect of steam conversion on the cellular structure, Young’s modulus and negative Poisson’s ratio of closed cell foam. Smart Materials and Structures. ISSN 0964-1726

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Restricted to Repository staff only until 20 October 2021.
Available under License Creative Commons Attribution Non-commercial No Derivatives.

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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.

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