PSI - Issue 7

M. Wicke et al. / Procedia Structural Integrity 7 (2017) 235–241 M. Wicke et Al./ Structural Integrity Procedia 00 (2017) 000–000

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(1983)]). Specimens taken in transverse direction (-TS) further tend to exhibit a slightly higher threshold compared to samples machined in rolling direction (-LS) for both material conditions. 4.2. Crack Growth at ΔK = const. The influence of the microstructure and precipitates on crack growth in dependency of the heat treatment condition and specimen orientation was investigated in experiments at quasi-constant ΔK . On the basis of the determined threshold, SIF values between 1.2 and 1.6 MPa√m were defined. A comparison of the crack paths of the investigated materials 6PA and 6OA tested at a load of 1.3 MPa√m is shown in Fig. 3. Since the crack of the TS-oriented specimen of material 6OA repeatedly stopped at this load before reaching the starting crack length of 250 µm, no results are depicted for this material. The crack in the TS-oriented specimen of material 6PA already stopped after ca. 310 µm, while it grew to a length of 410 µm (6PA) and 435 µm (6OA), respectively, in the LS-oriented samples

Fig. 3. SEM-micrograph showing the crack path at 1.3 MPa√m : (a) 6PA-LS; (b) 6PA-TS; (c) 6OA-LS.

No significant differences can be detected when comparing the crack paths of material 6PA and 6OA. The predominant crack propagation direction is mode-I with a certain amount of crack kinking highlighted by the black arrows in Fig. 3 or even crack branching, which is commonly related to the occurrence of irregular clusters of primary precipitates formed in transverse direction (-TS). A closer inspection of the crack paths shows that there are occasionally sections in which the crack extension seems to be shear-dominated (white arrow in Fig. 3a). This effect is independent of the microstructure and rolling texture as it was observed in both the LS- and TS-oriented specimens. 5. Discussion The crack paths depicted in Fig. 3 indicate that shear-dominated crack extension of long cracks in the near-threshold regime is even possible at a stress ratio of R = 0.1 with primary precipitates acting as microstructural barriers. These barriers can deviate the crack from its original propagation direction and seem to have a stronger influence in the TS direction as the crack is deflected at several locations. [Stein et al. (2017)] revealed that this effect is correlated with the spatial distribution of the primary precipitates, see Fig. 4. In the LS-oriented specimen, the crack grows across the elongated grains and passes through well-defined precipitate lines decorating the grain boundaries. It either stops completely or overcomes this line, i.e. the complete line acts as a microstructural barrier. In the TS-specimen, on the