  This paper presents an auxetic medium, consisting of a two-dimensional
perforated sheet where the holes are arranged in a repetitive pattern. The
hexagonal disposition of the perforations makes the medium isotropic in the
plane. It is shown that negative values of the Poisson's ratio can be achieved
for specific values of the dimensions and orientations of the holes. The
results of the numerical simulations are confirmed by experimental tests, in
which the Poisson's ratio of each specimen examined is evaluated from the
displacement field obtained from the Digital Image Correlation (DIC) technique.
The distribution of stresses in the medium is determined directly from
photoelastic images. The auxetic structure proposed in this paper is easy to
fabricate and can be very useful in several engineering applications.
