DUCTILE DAMAGE ACCUMULATION UNDER CYCLIC DEFORMATION AND MULTIAXIAL STATE OF STRESS CONDITIONS N. Bonora1 and A. Pirondi2 1DiMSAT – Dept. of Mechanics, Structures and Environment, University of Cassino 03043 Cassino, Italy 2Industrial Engineering Department, University of Parma 43100 Parma, Italy ABSTRACT The possibility to develop reliable predictive tools for the design of components undergoing plastic deformation is connected to the capability to incorporate damage mechanics into the constitutive model of the material. Even though many damage models for ductile failure are available in the literature, none of them, as far as the authors are aware of, is extended to reversal plastic flow occurring under compressive stress states. In this paper the damage model proposed by Bonora (1997) has been reformulated in order to account for compressive loading introducing a new internal variable associated to damage. The model has been implemented in a commercial finite element code and used to predict single element performance under cycling loading and damage accumulation in a round notch tensile bar. Some preliminary experimental results are also presented. KEYWORDS Ductile damage, cyclic loading, fatigue, CDM INTRODUCTION In the last decades it has been shown that local approaches have a great potential in predicting the occurrence of failure in specimens, components and structures. Today it is well assessed that ductile failure occurs as a result of microvoids nucleation and growth at inclusions. The local approach is based on the assumption that, if the microscopic mechanism of failure is known, the modification of the material constitutive response can be predicted from micromechanical considerations. Consequently, direct transferability from material to structure, without any geometry effect, would be one of the key features. Many theoretical models have been proposed in the literature that can be grouped in two main sets: continuum damage mechanics (CDM) based models and porosity models. Porosity models, derived from the Gurson type model, are based on the modification of the yield function as a result of the increasing porosity with plastic strain. Here, porosity plays the role of a softening variable that progressively implodes the yield surface in order to account for damage effects. CDM models are developed in the framework of continuum mechanics. Here, damage effects are accounted by a thermodynamic variable, D, that reduces material stiffness. Thus, the complete set of constitutive equations for the material undergoing damage is derived. Both approaches suffer major limitations. Porosity models usually require a large number of material parameters, none of which has a physical
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