  The purpose of this study is to propose a modified micromorphic continuum
model for granular materials based on a micromechanics approach. In this model,
Cauchy stress and the couple stress are symmetric conjugated with the symmetric
strain and the symmetric curvature respectively, and the relative stress
measures are asymmetric conjugated with the asymmetric relative strain
measures. This modified micromorphic model considers a continuum material point
as a granular volume element whose deformation behavior is influenced by the
translation and the rotation of particles. And this study proposes that the
microscopic actual motion is decomposed into a macroscopic motion and a
fluctuation between the macro-micro motion. Based on this decomposition, the
micromorphic constitutive relationships are derived for granular materials. In
the constitutive relationships, the macroscopic constitutive relationships are
first-order because of the introduce of the independent rotation of particle
instead of the second-order micro-deformation gradient. Furthermore, the
complex constitutive moduli in the micromorphic model are obtained in the
expressions of the microstructural information such as the contact stiffness
and the internal length.
