BENDING LIMIT PREDICTION OF AN ALUMINUM THIN SHEET S. Thuillier * , N. Le Maoût, P.Y. Manach LIMATB, Université de Bretagne-Sud, Université européenne de Bretagne ABSTRACT: The aim of this work is to predict the occurrence of rupture during bending of an aluminum alloy thin sheet by considering ductile damage. The first step consists in characterizing the mechanical behaviour of the material under different strain paths, such as tension of straight and notched samples and equibiaxial tension, up to necking and final rupture. The parameters of Gurson-Tvergaard-Needleman model are then identified for different triaxiality ratios ranging from 0.3 up to 0.67. Inverse identification is performed, by coupling the optimization software with a finite element code. A value of 0.15 for the critical void volume fraction corresponding to the void coalescence is obtained. In order to determine experimentally the onset of rupture in bending for this material, square specimen of length 60 mm are bent over a small radius of 0.2 mm, with a designed-on-purpose device. The area just beneath the bending tool is observed with a scanning electron microscope, for different tool displacements. The bending test is then simulated and a good correlation between the numerical onset of rupture, defined when the void volume fraction equals its critical value, and the occurrence of cracks on the sample surface is found. KEYWORDS: Mechanical behavior, Ductile damage, Aluminum alloy, Forming process 1 INTRODUCTION Numerical validation of the parameters of a drawing or assembly process, such as hemming, necessitates the taking into account of the material limits, such as necking and rupture. This is usually performed by using forming limit curves. However, in the case of flanging and hemming processes, using these curves is no longer possible due to the bending over a small radius [1]. Figure 1: Aspect of the bent area during hemming. Figure 1 shows the bent area, which can exhibit either a weak peel orange aspect (acceptable in the automotive industry), or fine and localized cracks hardly to be seen with bare eyes (just acceptable) or a complete failure (non-acceptable) [2]. To predict these different states, it was assumed to relate them with the void volume fraction of a ductile damage model of Gurson's type [3]. This method, whatever the damage model, has already been described in the literature [4-6]. Tensile tests on both straight and notched samples and equibiaxial tests have been performed, to identify material parameters [1]. In this paper, a dedicated bending test for square samples have been designed, to reproduce a bending similar to the one occuring during hemming. SEM observations of the bent area as well as numerical simulation of the test are performed, in order to correlate the aspect of the bent area with the critical void volume fraction. 2 MATERIAL AA 6016 aluminium alloy thin sheet of thickness 1 mm is used in this study. Its mechanical behavior has been investigated with tensile tests on both straight and notched samples and with equibiaxial tests, up to rupture (see Figure 2). Gurson-Tveergard-Needleman (GTN) [3] model is used and material parameters have been optimized by inverse identification. As the tests are non- homogeneous, either since the very begining or after necking, the finite element code Abaqus is linked to the optimization software SiDoLo [1]. Hill's 1948 yield criterion has also been used but, as in this work only tests in the rolling direction are taken into account, an isotropic yield criterion is considered. Aluminum alloys are very sensitive to ageing and tests were performed at different maturation times (30 days and more than 6 months). This phenomenon is accounted for by modifying the hardening curve, as shown in Figure 3. Hardening is fitted with Voce-type equation, see ____________________ * Corresponding author: Université de Bretagne-Sud, centre de recherche C. Huyghens, LIMATB, rue de Saint Maudé, BP 92116, F- 56321 Lorient Cedex, sandrine.thuillier@univ-ubs.fr DOI 10.1007/s12289-010-0747- © Springer-Verlag France 2010 7 Int J Mater Form (2010) Vol. 3 Suppl 1:223 226 –