Three-dimensional quantitative in situ study of crack initiation and propagation in AA6061 aluminum alloy sheets via synchrotron laminography and finite-element simulations Yang Shen a,b,⇑ , Thilo F. Morgeneyer b , Je ´ro ˆ me Garnier a , Lucien Allais a , Lukas Helfen c,d , Je ´ro ˆme Cre ´pin b a CEA, DEN, DMN, SRMA, F-91191 Gif-sur-yvette Cedex, France b Mines ParisTech, Centre des Mate ´riaux, CNRS UMR 7633, BP87 91003 Evry Cedex, France c ANKA/Institute for Photon Science and Synchrotron Radiation, Karlsruhe Institute of Technology (KIT), D-76131 Karlsruhe, Germany d European Synchrotron Radiation Facility (ESRF), BP 220, F-38043 Grenoble Cedex, France Received 2 November 2012; received in revised form 20 January 2013; accepted 20 January 2013 Available online 14 February 2013 Abstract Ductile crack initiation and propagation in AA6061 aluminum alloy for a fatigue precrack have been studied in situ via synchrotron radiation computed laminography, a technique specifically developed for three-dimensional imaging of laterally extended sheet speci- mens with micrometer resolution. The influence of the microstructure, i.e. due to the presence of coarse Mg 2 Si precipitates and iron-rich intermetallics, on the void nucleation process is investigated. Coarse Mg 2 Si precipitates are found to play a preponderant role in the void nucleation and ductile fracture process. Void growth and void coalescence are then observed and quantified by three-dimensional image analysis during crack initiation and propagation. Parameters for a Gurson–Tvergaard–Needleman micromechanical damage model are identified experimentally and validated by finite-element simulations. Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Crack propagation; AA6061 aluminum alloy; In situ laminography; Micromechanical modeling; Ductile fracture 1. Introduction The improvement of the damage resistance is a critical criterion for the selection and use of light alloys. The toler- ance for the presence of cracks is one of the crucial material properties for engineering components. The assessment of the complete crack initiation and propagation process may help to improve the material microstructure and to predict the lifetime of components and to delay their final fracture [1,2]. AA6061 aluminum alloy is often used for its light weight, good mechanical properties and good resistance to intergranular corrosion [3]. However, its relatively low resistance to crack propagation limits its utility, for instance in pressurized structures. Coarse intergranular precipitates in the alloy are responsible for the damage since they progressively become cavities during the loading processes and define a preferred path of crack propagation [4]. Two types of precipitates at the micrometer scale are present in this material: coarse Mg 2 Si and iron-rich inter- metallics [5,6]. Several authors have revealed the role played by the iron-rich intermetallics in the damage sequence of AA6061 alloys. For example, Blind et al. [7] have shown that the more intermetallics are present in the microstructure, the more reduced is the toughness of the alloy. Few authors have differentiated the role played by the two types of coarse precipitates in the damage mech- anism. In fact, most authors showed that damage from coarse Mg 2 Si precipitates is often negligible compared to the iron-rich intermetallics for two reasons [6,8–10]. First 1359-6454/$36.00 Ó 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.actamat.2013.01.035 ⇑ Corresponding author at: CEA, DEN, DMN, SRMA, F-91191 Gif-sur-yvette Cedex, France. Tel.: +33 169082279; fax: +33 169087167. E-mail address: yang.shen@mines-nancy.org (Y. Shen). www.elsevier.com/locate/actamat Available online at www.sciencedirect.com Acta Materialia 61 (2013) 2571–2582