Auxetic PVC foam as a new core material for sandwich structures

An auxetic material has a negative Poisson's ratio in that when loaded in uniaxial tension, it expands in all directions. An auxetic foam has many advantages over conventional ones. We convert an ordinary PVC foam (H45 blue) into an auxetic foam by three-dimensional compression resulting in a global volumetric change of more than 50%. The material is then baked in an oven above its critical temperature for a certain period which locks in the deformation permanently. The material is then left to cool down to room temperature. Small specimens are cut from an auxetic foam block for testings. We perform tension tests to determine the degree of auxeticity. A typical stress-strain curve is such that both the longitudinal and transverse strains are positive indicating a negative Poisson's ratio. We find that the value of the resulting Poisson's ratio is a function of the volumetric change. The larger the volume reduction, the larger the absolute magnitude of the negative Poisson's ratio. In a shear test, we also find that the shear modulus is substantially increased as a result of the conversion from ordinary to auxetic foam. We do a series of tests to show that the auxetic foam also resists indentation and low velocity impact damage, respectively. However, the conversion of ordinary foam to auxetic foam reduces the materials stiffness substantially. We need to know if a sandwich plate using the auxetic foam as the core material will resist bending better. Two sandwich beams with the same face sheets and ordinary foam and auxetic foam as the core materials, respectively, are made and tested under three-point bending load. The results show that indeed the sandwich beam with the auxetic foam resists deflection much better.