In situ observation of strained bands and ductile damage in thin Al alloy sheets

Ante BULJAC 1,2 , Thibault TAILLANDIER THOMAS 1,2 , Thilo F. MORGENEYER 2 , Lukas HELFEN 3,4 , Stephane ROUX 1 , Francois HILD 1 1 LMT, ENS Cachan/CNRS/Univ. Paris-Saclay, F‑94235 Cachan, France E-mails: { buljac,ttaillan,hild,stephane.roux}@lmt.ens-cachan.fr 2 MINES ParisTech, PSL Research University, MAT - Centre des materiaux, CNRS UMR 7633, F‑91003 Evry, France E-mail: thilo.morgeneyer@mines-paristech.fr 3 ANKA/Institute for Photon Science and Synchrotron Radiation, Karlsruhe Institute of Technology (KIT), D‑76131 Karlsruhe, Germany E-mail: lukas.helfen@kit.edu 4 European Synchrotron Radiation Facility (ESRF), F-38043 Grenoble, France While ductile fracture of metallic engineering materials at high stress triaxiality is fairly well established, failure at lower levels of stress triaxiality (i.e. below 1) is less understood.  An example of poorly understood localized failure phenomena is the flat-to-slant crack transition.  During this process, fracture typically initiates at crack-like features, with an initial flat crack forming normal to the loading direction, which evolves into a slant crack by so-called localized “fast-shear” decohesion (i.e. flat-to-slant transition).  Up to now, the interactions between plasticity and damage remain obscure when the flat-to-slant transition fracture of ductile sheet materials is studied.  The fundamental question becomes: Which mechanism is responsible for localized phenomena leading to the final failure? In this work, high-resolution in situ synchrotron X-ray laminography combined with digital volume correlation (DVC) is used to measure the damage and plastic strain fields ahead of a notch introduced within a 2198 Al–Cu–Li alloy sheet.  The alloy is recrystallized and tested in T8 artificial ageing condition involving relatively low work hardening.  Synchrotron laminography is a technique specifically developed for three-dimensional (3D) imaging of laterally extended sheet specimens with micrometer resolution.  Ductile damage (i.e., cavitation) is analyzed via the reconstructed volumes and the correlation residuals associated with DVC.  The latter allows displacement fields to be measured and strain fields to be estimated in the bulk of the material during the whole test benefitting from the 3D image contrast provided by the microstructure (i.e., iron-rich intermetallic particles). The first part aims to evaluate the resolution of the measurement technique under such very difficult conditions [1].  The second part shows that plastic strained bands occur very early on in what will be the slant region of the crack path as opposed to damage that develops at later loading stages.  Strained bands are shown to develop 800 µm from the notch prior to the onset of damage [2]. Damage in this region results mainly from the nucleation of voids on micrometer intermetallic particles and occurs at very late stages of deformation, followed by very limited void growth.  It is also shown that the activity of the different strained bands closer to the notch root is alternating between different locations over the loading history. However, the band leading to final rupture is always active. The region where slant fracture occurs is identified to be in plane strain condition with respect to the crack propagation direction. References [1]    Morgeneyer, T. F., Helfen, L., Mubarak, H. et al. (2013). 3D Digital Volume Correlation of Synchrotron Radiation Laminography Images of Ductile Crack Initiation: An Initial Feasibility Study. Experimental Mechanics 53(4), 543-556 [2]    Morgeneyer, T. F, Taillandier-Thomas T., Helfen L. et al. (2014). In situ 3D observation of early strain localisation during failure of thin Al alloy (2198) sheet. Acta Materialia, 69 78-91