PSI - Issue 7

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

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M. Wicke et Al./ Structural Integrity Procedia 00 (2017) 000–000

contrary, local precipitate clusters, visible as black dots in Fig. 4, are formed with crack deflection or even crack branching being related to the occurrence of these clusters. Local crack propagation is thus possible in precipitate free areas leading to kinked crack paths as seen in Fig. 3.

Fig. 4. CT scan and micrographs illustrating the position of the crack front according to the primary precipitates

Overaging further seems to increase the barrier effect, which is seen in terms of pronounced crack path kinking even in the LS-oriented specimen (see Fig. 3c). It can be conjectured that the coarsening of the secondary precipitates by the heat treatment is accompanied by a migration towards the existing primary precipitates which would then, in turn, explain the increased crack deflection by precipitates. The coarsening of the secondary precipitates also explain the increased pinning potential of primary precipitates in the overaged case observed in the experiments at R = -1, where ridges of ductile fracture and pronounced crack front kinking was detected even in the TS-oriented specimens of material 6OA (see [Stein et al. (2017)]). From the results presented above it can be concluded that the behavior of long-cracks in the near-threshold regime at R = 0.1 is apparently affected by similar mechanism as in the case of a stress ratio of R = -1. However, further investigations are required to explain the crack growth behavior at R = 0.1 in detail. Analysis of the fracture surfaces can give evidence for local pinning of the crack front by primary precipitates, while the evaluation of the crack tip fields derived from EBSD analyses will lead to further insights into the mechanism of shear-dominated crack growth. Moreover, high-resolution X-ray computed tomography (µ-CT) enabling the reconstruction of the three-dimensional morphology of the crack front can give additional information on the mechanisms keeping the crack from continuous extension. 6. Conclusion Results of fatigue crack growth experiments in the near-threshold regime on peak-aged and overaged specimens of an aluminum alloy pre-cracked by cyclic in compression are presented. The long crack threshold was determined with flat dog-bone specimens taken in rolling and transverse direction using the method described in [Pippan et al. (1994)]. On this basis, crack growth experiments with a nearly constant Δ K-value propagation rate were performed. The results point out that primary precipitates contribute in many ways to the morphology of the crack path and that shear-