CMM-2011 – Computer Methods in Mechanics 9–12 May 2011, Warsaw, Poland Ductile fracture prediction in bulk metal forming using multiscale and continuum damage mechanics models Mohand OULD OUALI 1 , Madjid ALMANSBA 2 and Nacer Eddine HANNACHI 3 1 Laboratoire Elaboration et Caractérisation des Matériaux et Modélisation – LEC2M. Université Mouloud MAMMERI de Tizi-Ouzou, BP 17 RP, 15 000 Tizi-Ouzou. ALGERIA e-mail: m_ouldouali@mail.ummto.dz 2 Laboratoire LAMOMS. Université Mouloud MAMMERI de Tizi-Ouzou, BP 17 RP, 15 000 Tizi-Ouzou. ALGERIA e-mail: almansbm@mail.ummto.dz 3 Laboratoire LAMOMS. Université Mouloud MAMMERI de Tizi-Ouzou, BP 17 RP, 15 000 Tizi-Ouzou. ALGERIA e-mail: hannachina@yahoo.fr Abstract Two damage models are used in this study to predict ductile fracture of an aluminium alloy during metal forming process. The first one is developed in the framework of phenomenological approach of damage mechanics. The second model is the micromechanical Gurson, Tveergaard and Needleman (GTN) constitutive law describing the three physical mechanisms of ductile fracture: nucleation, growth and coalescence of cavities. In the second model, the voids coalescence onset is modelled using the critical porosity. The value of this material parameter is determined from calibration with experimental tensile test results. These two models have been implemented into the finite elements code Abaqus using the Vectorized User MATerial (VUMAT) subroutine and employed to simulate the forging process of cylindrical and flanged specimens. The confrontation between the predictions of these models and the experimental results shows the capability of these two constitutive laws to predict the evolution forging force. However, the GTN model fails to capture the failure of the two workpieces, which are essentially subjected to compressive loading. Keywords: forging process, damage mechanics, micromechanical modeling, experiment, porosity, numerical simulation. 1. Introduction Ductile fracture of metals is today recognized governed by three physical mechanisms: nucleation of initially inexistent voids, growth of theses voids under an appropriated loading and finally coalescence of the neighbouring voids. Several approaches are used to model this phenomenon, generally observed at large deformations over the last three decades. These efforts have been mainly concentrated on modelling this progressive material degradation, namely called material damage. The most used models in metal forming are either phenomenological or micromechanically based. The numerical simulation is generally used to study the workability of materials which can be defined as the degree of deformation that can be supported in a particular metal forming process without generating any undesirable condition, such as cracks, fracture, buckling… In this paper we introduce some numerical and experimental results obtained from studying axisymmetric forging process. The experimental results concern the response of cylindrical and flanged specimens during metal forming. The numerical aim of this work is to compare two constitutive laws. The first one is developed in the framework of the phenomenological approach, called in this paper the Saanouni and co-workers model [3,33-36,18,19,23]. The second law is the micromechanical Gurson, Tveergaard and Needleman (GTN) model [2,14,15,42,43]. These formulations have been implemented into Abaqus/Explicit finite element package. Both qualitative and quantitative comparison between simulations and experiments results will be done. 2. Experiments The material used is a commercial aluminium alloy. All the specimens used in this study are machined from one bar with 6m length and 35mm diameter. The mechanical properties of this alloy are measured from tensile tests. These tests are carried out on cylindrical samples standardized in accordance with ISO527-2 standard. The geometry of the tensile specimens is represented in the figure 1. This specimen bar is standardized with respect to the 527-2 standard. We have make choose of the following values: 0 50 L mm = , 115 L mm = , 1 80 L mm = , 1 10 b mm = , 4 h mm = , 3 150 L mm = and 2 20 b mm = . The main characteristics of the aluminium alloy obtained during the tensile tests are the Young’s modulus 70 000 E MPa = , the Poisson's ratio 0,34 ν = , the yield strength 378,34 e MPa σ = and the ultimate strength 527,39 m MPa σ = .