Constitutive model with rotating active plane and true stress

A novel constitutive model for concrete, which approximately describes the basic known test data on nonlinear triaxial behavior including strainsoftening, is presented. The model rests on two basic ideas: (1) The stress-strain relation is defined as the relation between the normal and shear components of stress and strain on a certain special plane, called the active plane, the orientation of which varies as a function of accumulated inelastic strains; and (2) the stress-strain relation is written in terms of microstresses or true stresses that are obtained as the macrostresses divided by the resisting area fraction of the material. Strain-softening is obtained principally due to decrease of this area fraction. Thus, an incremental plasticity law satisfying the normality rule may be introduced on the microlevel, and a symmetric stiffness matrix is obtained. The loading surface for the active plane on the microlevel is an ellipse in the normal-shear stress space, similar to the critical state theory for soils. The model involves only six empirical inelastic material parameters, for which a simple sequential identification procedure is developed.