13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- In the brazing zone, the filler metal consists of a gold-rich phase and an iron-nickel-rich phase whereas in the region between substrate material and braze layer, the penetration of gold into the substrate material was observed, as is also reported in the literature [11]. Adjacent to the diffusion zone, a layer of the precipitate-free gold-rich phase is formed, that provides the lowest hardness values of 235 HV 0.05. For the actual experiments T-joints with and without artificial brazing defects were investigated. T-joints are characterized by a sudden change of the cross section area at the brazing zone, as shown in Figure 2. They were designed with respect to the original geometry of a turbo compressor impeller. Figure 2: a) Geometry of the T-joint specimen and b) defects introduced into the braze layer To simulate typical brazing defects like partly unfilled brazing gaps or large pores, straight defects with sizes of a = 0.5 mm, a = 1 mm and a = 2 mm (cf. Fig. 2b) were introduced into the braze layer by electrical discharge machining. The stress-controlled cyclic loading experiments were performed on a servo-hydraulic testing machine at a frequency of 5 Hz. To simulate the influence of start/stop cycles, the experiments were performed at a load ratio of R = 0.1 until Nmax=2·10 4 cycles. Besides the lifetime oriented investigation of the fatigue behavior, event-oriented experiments were performed with DIC. The DIC analysis was performed with a black speckle pattern on a white coating on the side surfaces of the T-specimens. To guarantee a sufficient resolution of the object displacements, the pattern was prepared with an average speckle size of 35 µm. The spray painting system used was an airbrush device with a nozzle diameter of 0.2 mm and gravity feed. The image acquisition was performed with a high speed camera (Redlake MotionExtra HG100K) that was focused on the brazing zone. Series of pictures were taken at a recording speed of 500 fps and an aperture time of 500 µs. The DIC was performed with the software Moiree Analysis V0.950© by correlating images of the maximum and the minimum force at defined loading cycles. The use of a high frequency camera allows investigating the development of strains during cyclic loading. Especially for brazed specimen, as a typical representative for heterogeneous systems, this technique allows to study the crack initiation, the crack propagation and the influence of stress concentrations as e.g. defects
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