Research And Application On Elastoplastic Damage Constitutive Model For Concrete Based On Ideally Undamaged State

Damage Mechanics based on internal variable theory and irreversible thermodynamic theory provides a novel theoretic framework in developing the constitutive model of engineering materials such as concrete et al. The damage theory by means of hypothesis of strain equivalence and the concept of effective stress can be conveniently used to extend the damage mechanics to engineering application, during which it is the essential problem to determine the reference undamaged state. The existing concrete damage models are almost established on the subjectively presumed undamaged stress-strain relationship and so it is urgent to find the ideal stress-strain curve through experiments. In this paper the author tries to present a new type of concrete constitutive model combining damage theory and plastic theory, and as the result, a subsection curved damage model that can account for both compressive damage and tensile damage is proposed on the basis of the ideal stress-strain curve obtained in this paper, and an elastoplastic damage constitutive model is finally proposed based on the reference ideal stress-strain curve. The model is implemented by efficient and stable numerical method to simulate the nonlinear behaviors of concrete structures. The main tasks and conclusions of this paper are shown below.On the basis of a broad review of the existing references about plastic theory and damage theory, the author points out that it is the key point to determine the ideal state with no damage influence in order to set up the damage model on the basis of the hypothesis of strain equivalence, and proves the importance of the reference undamaged state for establishing the damage constitutive model.The paper carried out compression experiments of concrete specimen with surrounding pressure of different pressure ratio which can effectively prevent the onset and expansion of cracks. In this way the process of ideal state with no damage growth is simulated and some distinct conclusions have been drawn which is different with the similar experiments by others. And also the confining effect of surrounding pressure on uniaxially compressed concrete is systematically studied.The ideally undamaged tress-strain curve of compressed concrete is proposed based on the compression experiments with surrounding pressure, and the behavior of cyclically loaded concrete with proper confining pressure is studied. It is proved in different ways that the ideal stress- strain curve can be taken as the reference curve subjected to no damage. Theoretical and experimental basis has been provided to establish 3dimensional elastoplastic damage constitutive model.On the basis of comparing the ideal curves and the experimental points, the thermodynamically consistent damage growth equations for tension and compression in form of subsection curves are established , the parameters of which are determined by the condition of stress continuity. Simulations to the uniaxially compressed and tensioned concrete prove the validity and the accuracy of the damage model .A triaxial elastoplastic damage model for the failure of concrete is presented, which is thermodynamically consistent and requires only few conventional material properties. The plasticity part is based on the effective stress using a yield function with two hardening variables, one for tensile loading history and the other for compressive loading history. The hardening rule is gained from the ideal stress-strain curve in compression. The damage part is based on the damage growth equation of one dimensional tension and compression. In order to reflect the two distinctly different damage mechanisms in tension and compression, the stress tensor and damage variable are decomposed into positive part and negative part respectively.Constitutive integration of stress evaluation algorithm based on the standard split into an elastic predictor, a plastic correction and also a damage correction using backward Euler scheme is deduced. It is proved for the model proposed that the process of elastic prediction gives the information of the principal directions and the lode angle of the final stress for which it is evaluated. So the number of the unknowns can be further reduced and only the stress invariants need to be solved, and the algorithm efficiency can be largely improved.High efficiency computer program is compiled according to the algorithm consistent stiffness of the model, and simulations in a wide range of loading cases prove the validity of the model in nonlinear structural analysis...