Aluminum alloy damage evolution for different strain paths – Application to hemming process N. Le Maoût a,b , S. Thuillier a, * , P.Y. Manach a a Université Européenne de Bretagne, Université de Bretagne-Sud, LIMATB, BP 92116, F-56321 Lorient, France b PSA Peugeot Citroën, PCI Le Rheu, 89 Rue Nationale, ZAC Atlante Apigné, BP 95217, F-35652 Le Rheu, France article info Article history: Received 30 July 2008 Received in revised form 5 January 2009 Accepted 23 January 2009 Available online 6 February 2009 Keywords: Hemming Damage mechanics Aluminium alloy Plasticity Automotive components abstract This work deals with ductile damage characterization of a 6000 series aluminum alloy. Tensile tests on both straight and notched samples at different orientations to the rolling direction, and equibiaxial expansion tests are performed up to fracture. The Gurson–Tverg- aard–Needleman model, extended to the case of plastic anisotropy described by Hill’s 1948 yield criterion, is used to represent the material behavior. The parameters are identified by inverse analysis and by using finite element simulations for inhomogeneous tests. The coa- lescence criterion proposed by Tvergaard and Needleman is considered and a critical void volume fraction is then determined. The numerical simulation of a three-step hemming process of flat surface-straight edge sample is then performed, to investigate the influence of some process parameters on the damage development in the folded zone and thus to predict hemming limits. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction For several years now, to make vehicles lighter, to decrease fuel consumption, and to reduce CO 2 emissions, the automo- tive industry has been using aluminum, and particularly, for the production of vehicle outer panels. Alloys of 5000 and 6000 series, often used for vehicles, offer good mechanical properties with initial yield stress in the range 90–160 MPa, depending on the chemical composition and on thermomechanical treatment, and yield strength in-between 180 and 310 MPa. How- ever, compared to steels, aluminum alloys have a high strength over Young’s modulus ratio, which leads to enhanced spring- back, and a weak formability, at room temperature. Moreover, highly localized severe plastic deformation can be associated with damage development during forming operations. This is particularly the case during bending [1], with failure strain as high as 0.6, and hemming, which is an assembly process that consists in folding the edge of an outer piece over an inner one [2–6]. Fig. 1 illustrates the three-step hemming process involving flanging, pre-hemming and hemming, and shows damage development in the folded area, the magnitude of which depends on process parameters. Formability is usually assessed with forming limit diagrams (FLD), which are determined through the onset of striction or fracture during different experimental tests for linear strain paths [7] and numerous works have demonstrated its relevance during the stamping process [8]. Thus, using it during the hemming process may seem relevant to predict failure; however this tool have some limitations. Indeed, the methods used to determine it do not allow fracture to be predicted in the areas near the edges of the pieces, in the non-planar deformation zones (bending on small radii, for example [9]), or for non-linear strain paths [10]. However, the very principle of the hemming process cannot fulfill these use restrictions: deformation is never planar because the sheet must be folded in two, and the strain paths are rarely linear because the sheet underwent, 0013-7944/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfracmech.2009.01.018 * Corresponding author. Tel.: +33 2 97 87 45 70; fax: +33 2 97 87 45 72. E-mail address: sandrine.thuillier@univ-ubs.fr (S. Thuillier). Engineering Fracture Mechanics 76 (2009) 1202–1214 Contents lists available at ScienceDirect Engineering Fracture Mechanics journal homepage: www.elsevier.com/locate/engfracmech