Chapter 3 Early Strain Localization in Strong Work Hardening Aluminum Alloy (2198 T3): 3D Laminography and DVC Measurement Ante Buljac, Lukas Helfen, François Hild, and Thilo F. Morgeneyer Abstract The effect of strain hardening on localization in front of a notch is assessed by following the interactions between strain concentrations, damage, initial microstructure and grain orientations. A CT-like specimen made of strong work hardening 2198 T3 aluminum alloy is subjected to an in situ synchrotron laminography experiment. Kinematic fields are measured via digital volume correlation. The final results are bulk displacement and strain fields including their corresponding resolutions. The reported results refer to the portion of the specimen around 1 mm away from the notch root. With the selected spatial resolution, damage nucleation and growth is evaluated in strained bands until the very end of the loading process. Keywords Digital Volume Correlation (DVC) • Flat-to-slant transition • High work hardening material • Laminography • Plastic flow 3.1 Introduction One of the examples that summarize well the current challenges in ductile damage understanding and modeling is ductile tearing. It has been reported [1–6] that during mode I opening of Compact Tension (CT) specimens, the initial crack starts to propagate normal to the loading direction but then tilts and continues in a slant manner as a combination of modes I and III. The investigation of the origin for such behavior has its engineering relevance since it has been shown that mixed-mode I/III leads to reduced toughness when compared to pure mode I fracture [7]. The parallel use of laminography [8] and global Digital Volume Correlation (DVC) [9] enables in situdisplacement fields to be measured at the microscale. Laminography as a non-destructive imaging technique allows flat specimens to be analyzed (Fig. 3.1a) and wider stress states to be achieved when compared with tomography. Thanks to DVC, bulk displacement fields can be measured and strain fields calculated [10]. Up to now, each new portion of the results obtained by laminography and DVC reveals interesting phenomena at micrometer resolutions. This is per se a strong motivation for further analyses on other aluminium alloys. However, to be consistent it was decided to proceed in a systematic manner by selecting a specific family of alloys using different heat treatments. In particular, aluminum alloys AA2139 and AA2198 have been studied [10–14]. Representing the latest generation of aeronautical alloys, the understanding of the underlying failure mechanisms is of great interest. Extensive analyses on CT-like specimen made of AA2198 T8 [11, 13] have shown that in the zone close to the notch root, i.e. 800 m from the notch [11] and even in its immediate vicinity [13], strained bands appeared early in A. Buljac ( ) Laboratoire de Mécanique et Technologie (LMT), ENS Paris-Saclay / CNRS / Université Paris-Saclay, 61 avenue du Président Wilson, 94235, Cachan Cedex, France MINES ParisTech, PSL Research University, Centre des Matériaux, CNRS UMR 7633, BP 87, 91003, Evry, France e-mail: buljac@lmt.ens-cachan.fr L. Helfen ANKA/Institute for Photon Science and Synchrotron Radiation, Karlsruhe Institute of Technology (KIT), 76131, Karlsruhe, Germany European Synchrotron Radiation Facility (ESRF), 38043, Grenoble, France F. Hild Laboratoire de Mécanique et Technologie (LMT), ENS Paris-Saclay / CNRS / Université Paris-Saclay, 61 avenue du Président Wilson, 94235, Cachan Cedex, France T.F. Morgeneyer MINES ParisTech, PSL Research University, Centre des Matériaux, CNRS UMR 7633, BP 87, 91003, Evry, France © The Society for Experimental Mechanics, Inc. 2018 L. Lamberti et al. (eds.), Advancement of Optical Methods in Experimental Mechanics, Volume 3, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-63028-1_3 15
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