13th International Conference on Fracture June 16–21, 2013, Beijing, China -7- A further experiment was performed with a defect containing specimen with a = 1 mm that was subjected to σmax = 350 MPa. Failure occurred after Nf = 7711 loading cycles. Figure 6: DIC of a defect containing specimen at a) N = 100 b) N = 0.8·Nf and b) N = Nf – 20 The results of the DIC show the inhomogeneous distribution of local strains due to increased stresses at the tip of the defect already in an early state of the experiment. After 100 loading cycles, a slight increase of the strains can be measured at the tip of the defect (Figure 6a) At N ≈ 6200 loading cycles (≈ 0.8·Nf), severe deformations indicate increasing strains as a result of accumulated fatigue damage at the approximate position of the braze layer at the tip of the defect (Figure 6b). Further cycling leads to crack initiation and crack growth. Figure 6c shows the local deformations 20 loading cycles before failure occurred. Due to the lacking possibility of differentiating between crack opening and a black speckle, the results in the direct proximity of the crack should be considered only qualitatively. Especially the volume ahead of the crack exhibits high strains that are distributed over a relatively large volume compared to the layer thickness of d = 100 µm. The wing shaped strain field that expands over 1 mm in front of the crack tip corresponds also to the strain field that has been determined for the defect free specimen (Figure 4b). To investigate the mechanisms of crack growth, SEM investigations were performed on the fracture surfaces of the specimens shown in Figure 4 and Figure 6.
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