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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investigated in vacuum is strongly affected by the microstructure showing strong interaction with microstructural features. The resulting morphological irregularities manifest themselves in the crack path as well as in the non elliptical fracture surface shown in Fig. 2 and 3. Figure 2a documents that specific grain boundaries can impede the slip band formation on the material surface leading to a complete crack stop in the vicinity of a grain boundary. Fatigue experiments with stepwise increasing stress amplitude reveal that the crack propagation mechanism changes to normal-stress-controlled crack growth at a stress amplitude of about 145 – 150 MPa. This behaviour will be confirmed by the crack path in Fig. 4a (green ellipse) as well as the smooth fracture surfaces in Fig. 6a (green arrows). Moreover, the effect of local pinning of the crack front (red ellipse Fig. 6a), which normally is observed for crack propagation in air [Stein et al. (2017)] [Wicke et al. (2017)], confirms the change of the crack propagation mechanism. At Δơ/2 = 150 MPa, stage II crack propagation predominantly takes place due to the activation of several glide systems leading to multiple slip at the front of the crack tip. Stage II crack growth manifests itself in a crack growth direction perpendicular to the stress axis (Fig. 5a and b) and the elliptical fracture surface in Fig. 6b. However, despite of the increased stress amplitude, stage I crack growth still can occur in few segments (red arrow in Fig 5a and green ellipse Fig 6b). At increased stress amplitudes, the effect of pinning the crack front by primary precipitate clusters gains in importance as the main crack propagation hindrance effect. However, at very low stress amplitudes, specific grain boundaries (depending on the misorientation between adjacent grains considering the combination of the tilt and twist angle of the activated slip planes) significantly can impede the long fatigue crack propagation in vacuum. The main crack propagation mechanism in EN-AW 6082 (pa) in vacuum in the VHCF regime is shear-stress controlled due to very pronounced single slip. The VHCF long crack propagation in vacuum of this material condition differs significantly from the conventional long crack growth in the Low and High Cycle Fatigue regime and seems to be rather insensitive to the anisotropy of the microstructure resulting from the existing rolling texture. The VHCF long crack growth is highly influenced by the microstructure leading to uneven crack propagation. Furthermore, the VHCF long crack propagation can be impeded by specific grain boundaries disturbing the slip band formation on the material surface. This can lead to a complete crack stop in the vicinity of a grain boundary. Stage II crack propagation prevails at increased stress amplitude ( Δơ/2 = 150 MPa). In this case, primary precipitate clusters can locally pin the crack front leading to a retardation of the long crack propagation in vacuum. 5. Conclusion

Acknowledgement

The financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged.

References

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