13th International Conference on Fracture June 16–21, 2013, Beijing, China 3D synchrotron laminography assessment of damage evolution in blanked dual phase steels Mouhcine Kahziz1,2,*, Thilo Morgeneyer2, Matthieu Mazière2, Lukas Helfen3, Eric Maire4, Olivier Bouaziz1,2 1 ArcelorMittal Research S.A., voie Romaine, F-57239 Maizières-lès-Metz, France 2 Mines ParisTech, Centre des Matériaux, UMR CNRS 7633, BP 87, 91003 Evry Cedex, France 3 Institute for Synchrotron Radiation – ANKA, Forschungszentum Karlsruhe, D-76021 Karlsruhe, Germany 4 Université de Lyon, INSA-Lyon, MATEIS CNRS UMR 5510, 20 avenue Albert Einstein, 69621 Villeurbanne, France * Corresponding author: mouhcine.kahziz@mines-paristech.fr Abstract The mechanical performance of automotive structures made of advanced high strength steels (AHSS) is often seen reduced by the presence of cut-edges. Here an attempt is made to gain insight into the initial damage state and the damage evolution during loading of a cut-edge. This is assessed in 3D and in-situ by synchrotron laminography observation during simultaneous tensile and bending loading of a cut-edge produced by stamping. Laminography is a technique that allows to observe regions of interest in thin sheetlike objects. It is found for the DP600 laboratory steel grade that the fracture zone is very rough and that needle voids from the surface and in the material bulk follow ferrite-martensite flow lines. During loading the needle voids grow from the fracture zone surface and coalesce with voids in the bulk. The needle cracks coalesce with the burnish zone though narrow zones, called void sheets. The formed cracks are inclined by 45° compared to the load direction. Keywords: Dual phase steels, cutting edges, X-ray laminography, damage 1. Introduction Advanced High Strength Steels (AHSS) grades remain the most widely used and developed materials in the automotive industry in order to reduce the “weight” of structural parts. Among these AHSS grades, dual phase (DP) steels with their ferrite-martensite composite microstructure present a good compromise between strength and formability. DP steels consist of a ferritic matrix containing a hard martensitic second phase in the form of islands. They are produced by controlled cooling from the austenite phase (in hot-rolled products) or from the two-phase ferrite plus austenite phase (for continuously annealed cold-rolled and hot-dip coated products) to transform some austenite to ferrite before a rapid cooling transforms the remaining austenite to martensite. However, the forming processes could affect the mechanical behavior of these grades. Some observations have shown that the cutting step tended to alter the good mechanical properties of this grade [1,2]. These studies have shown that the cutting process of DP sheets affects the adjacent material that extends into the bulk region of the sheet. This affected zone is characterized by a hardening and microstructural deformation which leads to local decohesion of ferritic and martensitic phases [1,11]. This drop in mechanical performance can significantly reduce the properties and then the use of AHSS. While ductile fracture mechanisms of this steel and its base materials (i.e. ferrite and austenite separately) have been discussed in the past [5,6,7,8], the damage mechanisms of DP cut-edges are not well known. This study aims at offering knowledge of the microstructural initial state of a cutedge and the evolution of damage from using three dimensions in-situ X-ray synchrotron laminography adapted to 3D observation of regions of interest sheet-like specimen and to identify the damage mechanisms leading to the crack formation initiating from the cut-edge. X-ray synchrotron laminography has been used in the present study to visualize for the first time -1-
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