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Abstract

Negative Poisson's ratio foams have improved indentation resistance over conventional materials. The mechanism for this is thought to be strain dependent densification but experimental verification of this has been lacking. Novel Surface Displacement Analysis (SDA) software, which uses a speckle analysis technique, allows visualisation of displacements and strains over a surface. Cross sections of foam specimens with various Poisson's ratios, bulk densities and moduli undergoing indentation were examined using the SDA software. We define a Poisson's function analogous to a tangential Poisson's ratio. The specimens with negative Poisson's function demonstrated enhanced indentation resistance independent of bulk density and modulus. The results of the tests with the SDA software showed that auxetic foams do densify under indentation, and the strain field under the indenter is much larger, probably due to enhanced shear stiffness.

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Strain Dependent Densification during Indentation in Auxetic Foams

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