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J. Lachambre et alii, Frattura ed Integrità Strutturale, 25 (2013) 50-53 ; DOI: 10.3221/IGF-ESIS.25.08 52 Figure 2 : Mode I component of the displacement field of the corner crack after 180kc (left) and the surface crack after 200kc (right): X-DVC analysis. The scale is given in voxel (1voxel= 3.5µm). In this work, the studied cracks having a three-dimensional shape, a new method has been used for extracting K values (see Fig. 3 for a summary). An approximate position of the crack front is first derived from the DVC residuals (Fig. 2a). A series of regularly spaced planes is built along this first crack front and used to fit a first set of Williams series. By a minimization of the super singular term n=-1 of the series, a new position of the front is found ( red line in Fig. 3b) a new set of planes is build and the crack front detection is carried out again. The « correct » crack front position is found when the crack front position is stable between two iterations. The different mode I, II and III stress intensity factors can then be computed. Figure 3 : Schematic illustration of the iterative crack front extraction process : an initial crack front position is assumed (a) to build a series of planes which will be used for fitting the Williams series. Once the new position of the front is found (b) (see the text for details) a new series of planes is build (c) ; for non planar cracks, the process can be carried out in 3D (d). Examples of stress intensity profiles obtained along the crack fronts of the propagating cracks can be obtained with this method and used to plot local da/dN=f(DK) laws. S UMMARY he 3D propagation of small fatigue cracks initiating from an artificial defect in a cast iron sample has been characterized in situ and analyzed using laboratory X-ray computed tomography and DVC. The process of creating small artificial defects with laser beam gives an appropriate way to initiate a crack in this material. The DVC analysis with a relatively coarse mesh provides during cycling a valuable information on the crack propagation. The X-DVC analysis allows to estimate directly the crack opening displacement over the entire crack surface. Displacement fields extracted by DVC can be used to compute local values of the stress intensity factor along the curved crack fronts. A CKNOWLEDGEMENTS his work was funded by the French ANR program (grant ANR-09-BLAN-0009-01 - RUPXCUBE Project). T T