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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Moreover, slip bands formed on the material surface which are pronounced mostly parallel to the crack path on both sides. At this stage, a clear correlation between the calculated glide traces for each grain, the crack path and the slip band formation on the material surface exists. With increasing stress amplitude (green ellipse in Fig. 4a), the crack does not propagate continuously along an activated slip band. Rather, for the most part on the left side of the crack, the crack bends towards the normal direction and predominantly propagates very straight through many grains. The comparison of the crack growth direction with the calculated glide traces reveals that in some sections no clear correlation between the crack path and the glide traces exists (Fig. 4b). Figure 5c documents that the crack propagation rates are increasing with increasing crack length and increasing stress amplitude, while the right crack tip is blocked at a grain boundary, even when Δơ/2 = 145 MPa is applied. A further fatigue experiment in vacuum was carried out at a constant stress amplitude of 150 MPa. The results are presented in Fig. 5. One can see at a first glance from Fig. 6a that the crack path is aligned perpendicular to the stress axis and differs significantly from the fatigue crack growth experiments shown before (e.g., the crack propagation path at Δơ/2 = 120 MPa ). For the most part, there is no correlation between the crack path orientation and the calculated glide traces. Only in a particular section (red arrow in Fig. 5a), the crack propagated along an activated slip band and shows a clear correspondence to the glide trace with the highest Schmid factor (Fig. 5b). Furthermore, crack branching (green ellipse Fig. 5a) was detected on the right side of the crack path. The crack growth rates shown in Fig. 5c are in the range of 10 -11 – 10 -9 m/cycle. The fracture surfaces of the fatigue experiments shown in Fig. 4 and Fig. 5 are illustrated in Fig. 6. The fracture surface morphology of the fatigue experiment with increasing load amplitudes shows a non-elliptical shape (Fig. 6a). Especially the left part of the fracture surface exhibits strong changes in the direction. However, in the regime where Δơ/2 = 150 MPa was applied, a very smooth surface prevails (green arrows in Fig. 6a). Similarly to the observations of Stein et al. [Stein et al. 2017], a pinning effect most probable caused by a primary precipitate cluster was found in the fracture surface morphology (red ellipse in Fig. 6a).

Fig 6. (a) Fracture surface of the fatigue experiment with increasing load amplitudes; (b) fatigue fracture surface of the fatigue crack propagation experiment at Δơ/2 = 150 MPa ; (c) detail view of Fig. 7b showing a pinning effect in the fracture surface morphology.

The fracture surface of the fatigue experiment carried out at a constant stress amplitude of 150 MPa appears to be elliptical (Fig. 6b). All in all the fracture surface morphology is homogeneous. Just two particular areas show specific features. Behind the micro-notch marked as a green ellipse, facetted structures were found. Moreover, on the right side of the micro-notch, again a pinning effect can be detected on the fracture surface (red mark in Fig. 6c).

4. Discussion

In a previous study [Bülbül et al. (2018)] the main crack propagation mechanism in the aluminium alloy EN-AW 6082 (pa) in vacuum was found to be shear-stress-controlled. This was attributed to (i) the absence of air humidity and (ii) the presence of (semi-) coherent shearable secondary precipitates in the alloy. During VHCF-loading, very localized plastic deformation within individual slip bands takes place leading to stage I crack propagation as it can be seen from the fatigue experiment represented in Fig. 2. The VHCF long crack propagation of this material