Measuring the stress field around an evolving crack in tensile deformed Mg AZ31 using three-dimensional X-ray diffraction |
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Authors: | Jette Oddershede Bettina Camin Søren Schmidt Lars P Mikkelsen Henning Osholm Sørensen Ulrich Lienert Henning Friis Poulsen Walter Reimers |
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Affiliation: | 1. Center for Fundamental Research: Metal Structures in 4D, Materials Research Division, Risø Danmarks Tekniske Universitet, Frederiksborgvej 399, DK-4000 Roskilde, Denmark;2. Institut für Werkstoffwissenshaften und -technologien, Metallische Werkstoffe, Sekr. BH18, Technische Universität Berlin, Ernst-Reuter-Platz 1, DE-10587 Berlin, Germany;3. Danmarks Tekniske Universitet Wind Energy, Frederiksborgvej 399, DK-4000 Roskilde, Denmark;4. Advanced Photon Source, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA |
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Abstract: | The stress field around a notch in a coarse grained Mg AZ31 sample has been measured under tensile load using the individual grains as probes in an in situ high energy synchrotron diffraction experiment. The experimental set-up, a variant of three-dimensional X-ray diffraction microscopy, allows the position, orientation and full stress tensor of each illuminated grain to be determined and, hence, enables the study of evolving stress fields in coarse grained materials with a spatial resolution equal to the grain size. Grain resolved information like this is vital for understanding what happens when the traditional continuum mechanics approach breaks down and fracture is governed by local heterogeneities (e.g. phase or stress differences) between grains. As a first approximation the results obtained were averaged through the thickness of the sample and compared with an elastic–plastic continuum finite element simulation. It was found that a full three-dimensional simulation was required to account for the measured transition from the overall plane stress case away from the notch to the essentially plane strain case observed near the notch tip. The measured and simulated stress contours were shown to be in good agreement except at the highest applied load, at which stress relaxation at the notch tip was observed in the experimental data. This stress relaxation is attributed to the initiation and propagation of a crack. Finally, it was demonstrated that the measured lattice rotations could be used as a qualitative measure of the shape and extent of the plastic deformation zone. |
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