A Proposal for Parameters Identification of the GTN Model – Application to CT Specimen J. Chambert and Ph. Bressolette LERMES (Laboratoire d’Etudes et de Recherches en Mécanique des Structures), Université Blaise Pascal – Clermont-Ferrand II, 63177 Aubière, France – e-mail: chambert@lermes.univ-bpclermont.fr ABSTRACT: This paper deals with a new ductile fracture model based on incorporating coalescence criteria into the Gurson-Tvergaard-Needleman (GTN) model in order to determine the critical porosity of coalescence fC and the final void volume fraction fF (GTN2P model). Because this model is only valid for axisymmetric specimens, we propose a practical approach based on a two-step procedure. In the first step, using the GTN2P model on an axisymmetric notched tensile specimen, the parameters are identified. In the second step, the GTN model, with parameters values previously defined, is applied to structural components or fracture specimens to predict the ductile crack growth. We have performed a numerical analysis of a CT25 specimen made of 16MND5 in order to validate the proposed methodology. INTRODUCTION Ductile fracture of metals is classically described by three idealized stages: nucleation, growth and coalescence of micro-voids. Since classical fracture mechanics has not accurately predicted the geometric and loading effects, local approaches have been increasingly used to simulate ductile fracture. In this context, Gurson [1] has proposed a micro-mechanical model for porous plastic solids containing spherical voids. In order to better account for the void coalescence process, Tvergaard and Needleman [2] have introduced into Gurson’s original model two coalescence parameters: the critical void volume fraction Cf and the final void volume fraction Ff (GTN model). CONSTITUTIVE EQUATIONS In this context, the overall ductile porous material is modelled by a periodic array of axisymmetric unit cells (see Figure 1a). Each cylindrical cell of current 2H-height and L-radius contains one spherical microvoid of current R-radius, which is embedded in Von Mises matrix materials. Subsequently,
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