  We investigate the constitutive response of two-dimensional packed samples of
polygons using molecular dynamics simulation. The incremental elasto-plastic
response is examined in the pre-failure regime. Besides the Young modulus and
the Poisson ratio, an additional parameter must be included, which takes into
account the anisotropy of the elastic response. The plastic deformations are
described by the introduction of the yield and the flow directions. These
directions do not agree, which reproduces the non-associated feature of
realistic soils. In order to detect the yield surface, different
loading-unloading-reloading tests were performed. During the reload path, it is
found that the yielding develops continuously with the amplitude of loading,
which does not allow to identify a purely elastic regime.

Condensed Matter