Thermodynamical Modeling of Elastic-Plastic Damage and Experimental Validation of Damage Potential

Formulation of constitutive and damage evolution equations of elastic-plastic damage materials are first discussed. In order to facilitate the description of the experimental damage surface in axialtorsional stress space, Gibbs thermodynamic potential is employed. The damage surface is expressed as a function of the damage conjugate force which is easily related to stress tensor. Then, the resulting equations are applied to the experimental results of the spheroidized graphite cast iron. The change in elastic moduli due to damage development and the initial and the subsequent damage surface expressed in stress space are described well by the proposed theory. Finally, experimental verification of the existence of a damage potential and the corresponding normality law is performed. By noting that the normality law must be discussed in the space of damage conjugate, the direction of the damage rate vector on each loading path is identified from the change in the elastic modulus due to damage. The resulting direction of the damage rate at each loading path coincides sufficiently well with the normal of the damage surface, and hence, the proposed damage surface is certified to be identical to the damage potential.