Auxetic foam revisited: understanding the origin of negative Poisson's ratio using micro-CT and pore structure analysis

Porous foam with negative Poisson’s ratio, namely auxetic foam, is a typical mechanical metamaterial. One of the commonly used approach in producing auxetic foam is the thermomechanical compression of porous thermoplastic materials such as Polyurethane (PU) foams. While qualitatively it is well understood that the negative Poisson’s ratio arises from the reentrant cell shape, quantitative relation between the auxetic behavior and pore structure is still lacking. Using micro-CT, we systematically quantify the pore structures of a series of auxetic PU foams prepared with various degrees of thermomechanical compression, and correlate with their mechanical behaviors measured from tensile tests. We find that the fraction of bucked ribs of the pore structure is related to the initial value of Poisson’s ratio upon stretching while the extent of buckling is related to the maximum tensile strain for negative Poisson’s ratio. Our findings can shed light in designing mechanical metamaterials with targeted auxetic behaviors.