We systematically investigate the incremental response of various equilibrium
states of dense 2D model granular materials, along the biaxial compression path
(\sigma 11 < \sigma 22, \sigma 12 = 0). Stress increments are applied in
arbitrary directions in 3- dimensional stress space (\sigma 11, \sigma 22,
\sigma 12). In states with stable contact networks we compute the stiffness
matrix and the elastic moduli, and separate elastic and irreversible strains in
the range in which the latter are homogeneous functions of degree one of stress
increments. Without principal stress axis rotation, the response abides by
elastoplasticity with a Mohr-Coulomb criterion and a non-associated flow rule.
However a nonelastic shear strain is also observed for increments of \sigma 12,
and shear and in-plane responses couple. This behavior correlates to the
distribution of friction mobilization and sliding at contacts.Comment: 4 page
