13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- locally with an adequate lateral and temporal resolution. After the experiments, SEM investigations were performed to analyze the fracture surface and to correlate the results with DIC. To distinguish between volumes with different chemical compositions, a BSE detector was chosen for the investigations. Therefore, volumes containing heavy elements as gold are shown in bright contrast whereas comparably light elements are shown in dark contrast. 3. Results Figure 1 shows the maximum applied loading amplitude of the defect-free assumed cross section as a function of the numbers of cycles to failure. Figure 3: S,N curve of defect-free and defect containing T-joint specimens The results of the experiments show that defects significantly influence the joint strength. As could be expected, defects lead to a decrease of the joint strength. The larger the defect, the more pronounced the decrease of the fatigue strength. Defect-free specimens can be loaded even up to the ultimate tensile strength of the substrate material for 50 loading cycles, as shown in Figure 3. This can be explained by the ductile deformation and hardening behavior of the substrate material. Loadings around the yield strength of the material lead to failure of the defect-free specimens at approx. 104 loading cycles. S,N-curves can be used to describe the influence of one defined defect, but they are not suitable to describe the influence of defects varying in form, shape and position reasonably. In a previous study, a fracture mechanical approach to estimate the influence of brazing defects on a universal scale is based on the stress intensity caused by a defect was successfully applied to make life-time predictions for joints of a less ductile steel [10]. However, this approach did not lead to reasonable results for the material used in this work. The local strain evolution was characterized experimentally by DIC. Figure 6 shows a defect-free specimen at several stages during cyclic loading at σmax = 650 MPa until Nf = 13518 cycles. The strain distribution in the first and in the loading cycle before failure occurred is shown in Figure 4 together with the speckle in the background.
RkJQdWJsaXNoZXIy MjM0NDE=