PSI - Issue 13

F. Bülbül et al. / Procedia Structural Integrity 13 (2018) 590–595 Fatih Bülbül / Structural Integrity Procedia 00 (2018) 000 – 000

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experiments, micro-notches were milled in the middle of the mechanically and electrolytically polished ultrasonic specimens by means of the Focused-Ion-Beam (FIB) technique. Crack initiation in vacuum under VHCF loading was successfully performed due to the particular shape of the FIB micro-notch [Bülbül et al. (2018)] and its minimum length of about 250 µm corresponding to a length of about two to three grain diameters.

3. Results 3.1. VHCF long crack propagation in vacuum at Δơ /2 = 120 MPa

Fatigue experiments were carried out in vacuum at a constant stress amplitude of 120 MPa. From the fatigue experiments, it was ascertained that slip bands formed on the material surface and crack propagation takes place along an activated slip band. Moreover, crack path extension by slip bands on the material surface was found in particular areas, where the long fatigue crack changes its direction to an adjacent grain. All in all the crack propagation predominantly takes place very straight through many grains whereby the crack path strongly deviates from the direction perpendicular to the loading axis, even when the fatigue crack is very long. Due to the calculation of the glide traces of the individual grains, a clear correlation between the crack path, the formation of slip bands on the material surface and the glide traces for each grain has been found over the entire investigation area. The features of the long fatigue crack propagation behaviour in vacuum exemplarily can be seen in the fatigue experiment shown in Fig. 2.

Fig. 2. Crack propagation in vacuum at Δơ/2 = 120 MPa: (a) crack path with barrier effect at the right crack tip; (b) EBSD analysis with calculated glide traces; (c) crack propagation rates of both crack tips.

In Fig. 2a, it can be seen that the crack path of the long fatigue crack significantly deviates from the direction perpendicular to the loading axis whereby the long fatigue crack propagation takes place in a straightforward manner through most of the grains. Also in this experiment a clear correlation between the crack path, the slip band formation on the material surface and the calculated glide traces of the EBSD analysis was found (Fig. 2b). Figure 2c shows the crack propagation rates separately for both crack tips which are in the range of 10 -12 – 10 -9 m/cycle. Here, it is apparent that the left crack tip propagated faster than the right crack tip and the crack propagation velocity increased with increasing crack length. However, the right crack tip completely stopped at a crack length of about 150 µm. In Fig. 2a (blue rectangle), it can be noted that the slip band formation on the material surface was impeded by a grain boundary. As a result, new slip bands formed which are blocked again at the grain boundary leading to a parallel arrangement of slip bands on the material surface. The irregular long crack propagation behaviour also can be detected in the fracture