13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- Figure 4: DIC with a defect free specimen During the first loading cycles, the strains are equally distributed over the whole cross section (Figure 4 a). During further cycling, the fatigue damage accumulation leads to increased local strains and subsequent crack initiation and propagation. Figure 4b shows the deformations in the last cycle before fracture. Furthermore, Figure 4b shows that the crack initiates in the fillet, not directly in the braze layer, as could be expected. Then, it grows at an angle of approximately 45° to the loading direction until it changes its direction and propagates in the vicinity of the brazing zone, perpendicular to the loading direction. Figure 5: DIC – Strain field in the vicinity of the fatigue crack Figure 5 shows a close-up image correlation of the crack tip region, the approximate position of the filler metal layer is indicated by dashed lines. Whereas the highest strains are found at the position of the crack due to the crack opening, the volume ahead of the crack tip exhibits also increased strains. With increasing distance these high strains are located in wing-shaped strain fields in the parent material. The filler metal is only subjected to a strain level comparable to the wing-shaped strain fields in a region of 300 µm ahead of the crack tip, whereas in further distance the enhanced stress levels are only observed in the parent material. Some of the data points shown in blue represent regions where no correspondence was found. The strain interpolation with the neighboring values leads to the vertical stripes in Figure 5.
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