Materials with negative Poisson's ratio: A qualitative microstructural model

Materials with negative Poisson's ratio are peculiar in that they expand laterally when stretched. Examples of such type of behavior have been discovered with foams by Lakes (1987); however, it is only recently that isotropic materials with negative Poisson's ratio have been shown by Milton (1992) to exist within the framework of classical theory of elasticity. In this Note, we demonstrate qualitatively that a composite material with a reentrant microstructure can also have a negative Poisson's ratio, even though the composite may be isotropic owing to a completely random distribution of the microstructure.