Figure 3: 3D view of the volume meshing with quadratic tetrahedrons of the fatigue sample 3. Results and analysis The fatigue lives (Wöhler curves) and the damage mechanisms (crack initiation sites and propagation) have been characterized. 3.1 Fatigue mechanisms of brazed material The sample analysed by X-ray tomography and shown in section 2.1 (Figure 2(b)) has been submitted to 933744 fatigue cycles. The test was stopped before fracture when the sample contained a 1mm long fatigue crack. Figures 4(a) and 4(b) present the zone of interest (ZOI) used for DIC at reference (750 cycles) and final states. The strain (Exx) fields for 5 specific steps, respectively 47%, 75%, 97% and 100% of the fatigue lifetime, are presented on figures 5(a-c). Large localized plastic deformation leading to large displacements (figure 5(c)) appears during the last part of the cycling, which probably corresponds to the stable crack propagation. Before the few last cycles (97% of the fatigue life, i.e. 28 000 cycles), no strain heterogeneity can be visualized by DIC. The presence of a crack can be evidence by using a discrepancy map (Figure 6(c)) as described in [10]. Figure 4: Zone Of Interest (ZOI) for the digital image correlation (DIC) at: (a) the initial state (750 cycles) and (b): the final state (933744 cycles). The painting black points (speckles) are used to calculate displacement field by DIC. On fatigue crack can be easily visualized on the final state image (b). The shadow clearly visible near the crack on figure 4(b) highlights the presence of a Clad Solidification Drop (CSD). Systematic fractographic analysis of the fatigue samples confirms the presence of a CSD close to the crack initiation zone in most cases. The stress concentration induced by the CSD roughness of the sample surface is likely to promote local plasticity and induce eventually crack initiation. The influence of CSD on local (elastic) stress distribution during cycling can be studied in detail from the 3D FE meshes generated from tomography; this is described in the next section. 65
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