Constitutive Model with Rotating Active Plane and True Stress

Abstract

A novel constitutive model for concrete, which approximately describes the basic known test data on nonlinear triaxial behavior including strain‐softening, 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.