13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- rameters for the homogenizing and grain coarsening (required for 3D synchrotron tomography) is given in Table 1. Table 1: Chemical composition of the duplex steel (wt. %) and heat treatment parameters. C Cr Ni Mo Mn N P S Fe 0.02 21.9 5.6 3.1 1.8 0.19 0.023 0.002 bal. grain coarsening 1250°C (4h); cooling down to 1050°C (1K/min); water quench Figure 1: Microstructure of the investigated stainless duplex steel, austenite (red), ferrite (blue). The grain coarsening heat treatment resulted in a microstructure consisting of 50 % austenite and 50 % ferrite, with a mean grain size of 33 µm for austenite and a mean grain size of 41 µm for ferrite. The geometries of the specimens are given in Figure 2; (a) shows the cylindrical bulk specimen, (b) shows the miniature specimen which has been developed for the synchrotron experiments at ESRF Grenoble. Both kinds of specimens are electro-polished in the gauge length; therefore, the surface of the specimens can be observed by means of light and electron microscopy in combination with automated electron back-scatter diffraction (EBSD). a b Figure 2: (a) shallow notched specimen for the VHCF tests, (b) flat bone shaped specimen for the VHCF tests. For the cyclic loading of the samples an ultrasonic fatigue testing equipment designed and produced by BOKU Vienna was used, with an average frequency of f = 20 kHz and an load ratio of R = -1. To prevent heating of the specimens during cycling compressed air cooling is used. The observation of the fatigue dam-
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