In the present paper the constitutive equation for polycrystalline materials, such as ice at wide temperature and strain rate range, is studied. The following deformation mechanisms of the material are examined: instantaneous linear elasticity, viscoelasticity, and viscoplasticity. The uniaxial material model is based on the theory proposed by Sinha. The delayed elastic strain is predicted by the modification introduced by Ashby and Duval to the equation of Sinha. A multiaxial theory to represent the reponse of isotropic polycrystalline material is developed. The multiaxial material model, called the SADS-model, coincides in case of uniaxial tension with the Ashby and Duval modification of the equations proposed by Sinha.