13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Figure 7: SEM investigations of the fracture surface for a) defect-free and b) defect containing T-joint The SEM analysis correlates with the DIC calculations considering crack initiation and crack growth. Figure 7a shows the crack initiation side in the fillet and its growth at an angle of 45°, as was also shown in Figure 4b. The crack path through the substrate material is characterized by a discontinuous course. This could be an indicator for inter-crystalline crack growth that could result from the diffusion of liquid gold into the steel substrate between the grain boundaries, as described in [11]. Next to the main crack, multiple secondary cracks were found in the substrate material and in the braze layer. The crack growth follows the same direction until it has passed the braze layer and reached the opposite interface. There, it changes direction and follows the interfacial zone until it has reached a critical length and fast fracture occurs. The fracture surface of the defect containing specimen is shown in Figure 7b. Also for this specimen, the crack passes through the substrate material. Conclusions The results show that defects in the brazing zone strongly influence the fatigue lifetime of brazed components. It could be shown that DIC allows measuring local displacements during cyclic loading with a sufficiently high resolution. Continuing fatigue damage is indicated before final failure occurs by increasing strains around structural hot spots. Wing shaped strain fields in front of the crack tip extend over a comparable large volume of the substrate material. Furthermore, the results allow to draw conclusions on the mechanisms of crack initiation and to monitor crack growth during cyclic loading, which is important for braze joints. The results of the DIC show that for defect-free and for the defect containing T-joint specimen, the crack does not necessarily follow the softer filler metal but propagates through both the substrate material and the braze layer. Only when the crack reaches the opposite interface of the braze layer it changes its direction and follows the interfacial regions on its path. Further experiments will be performed to characterize the fatigue behavior of brazed joints with special interest on the influence of defects. In the future, DIC will be performed to investigate the fatigue crack growth behavior more precisely. In correlation with SEM investigations, the presented methods can be used to achieve a better understanding of the mechanisms that lead to failure of brazed steel joints. Acknowledgement The authors gratefully thank MAN Diesel & Turbo AG Zürich for financing this project
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