INTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING Int. J. Numer. Meth. Engng 2004; 61:303–315 (DOI: 10.1002/nme.1074) Fold zone analysis in wiping die bending process by using experimental and numerical approaches Ali Mkaddem ∗,† and Alain Potiron ‡ LPMI-ERTGI, ENSAM, 2 Boulevard du Ronceray, 49035 Angers, France SUMMARY This paper deals with an analysis of the fold zone of bent parts within the sheet thickness by means of micro hardness measurement and numerical modelling. The failure risk has to be accurately predicted by using a continuum damage mechanics in order to avoid some expensive experiments. In this reason, a 3D numerical model, based on Lemaitre damage formulations, has been developed. The scalar damage variable D as well as the hardening law are taken to be isotropic within the sheet. The numerical approach is based on an incremental integration scheme of the constitutive equations coupled with ductile damage and von Mises criteria. The corresponding numerical algorithm is implemented into ABAQUS/Standard code. During bending operation, the damage variable evolution is computed and the hydrostatic stress effect is investigated within the sheet thickness. The hardening and plastic strain evolutions have been compared with the micro hardness one deduced from experimental tests. The comparison between experimental and numerical results illustrates the capability of the model in 3D process simulation, particularly in bending processes. Copyright  2004 John Wiley & Sons, Ltd. KEY WORDS: sheet metal; experiment; simulation; damage; bending process 1. INTRODUCTION The localization of deformations in different zones of forming parts is the main cause of fracture initiation. As defined by Lemaître [1], the damage depends directly on the density of micro defects and micro voids in the material. During part loading, the coalescence of micro defects and their propagation into the material, lead to the initiation of cracks and then to failure. Lemaître et al. [2] prove that micro hardness gives an idea of the material deterioration. Duyi [3] having shown that, during loading and unloading tests, micro hardness decreases when damage increases. In the case of bending operation, the bent zone has been strengthened as hardly as it is near the sheet’s surfaces. The cracks initiation, governed by the micro defects density, depends on the hydrostatic stress which prevent the damage evolution in compressive state. Then, the influence of the hydrostatic stress [4] has to be taken into account in analysis ∗ Correspondence to: Ali Mkaddem, LPMI-ERTGI, ENSAM, 2 Boulevard du Ronceray, 49035 Angers, France. † E-mail: ali.mkaddem@angers.ensam.fr ‡ E-mail: alain.potiron@angers.ensam.fr Received 13 November 2002 Revised 29 April 2003 Copyright  2004 John Wiley & Sons, Ltd. Accepted 15 December 2003