Analysis of short fatigue crack propagation by 3D synchrotron imaging technique
Résumé
Short fatigue crack propagation in polycrystalline materials is strongly influenced by microstructural features, in particular grain orientation and grain (and phase) boundaries. This is the main source of dispersion in fatigue experiments. A better understanding of the local mechanisms responsible of the very first stages of short fatigue crack growth is needed to provide better estimations of fatigue life. In order to get the evolution of the crack during the fatigue testing and to investigate this three-dimensional problem, nondestructive observations are required, so synchrotron 3D imaging techniques are combined. We use Diffraction Contrast Tomography in order to reconstruct the polycrystalline microstructure of a notched Titanium alloy sample. The crack propagation history is monitored in-situ by repeated Phase Contrast Tomography acquisitions. After image post-treatment, such as registration and crack segmentation, the evolution of the crack is correlated to the polycrystalline structure.
The results showed dominant crystallographic propagation along slip planes changing from grain to grain. Analyzing the evolution of the crack front correlated with the orientation of the crack lips, we highlight that crack growth under double slip mode, as described by Neumann, does not occur inside polycrystals, except in rare cases. Indeed the local direction of propagation differs from the observed striation.
Crystal plasticity finite element computations are carried on the real microstructure in an attempt to compare numerically simulated against experimentally observed crack propagation behavior.