PSI - Issue 39

524 10

Lucia Morales-Rivas et al. / Procedia Structural Integrity 39 (2022) 515–527 Author name / Structural Integrity Procedia 00 (2019) 000–000

Fig. 6. (a) BSE-SEM micrograph showing the overview of the crack path after the interrupted fatigue test (100,000 cycles) and detail of the section II of the crack path; and (b) hypothesis regarding the sequence of the fatigue crack initiation and propagation event and correlation with the schematic illustration of the crack path. In this sense, a hypothesis regarding the sequence of the crack propagation in this work is that crack initiation takes place at the section II of the crack profile by the activation of the single primary slip system, probably also leading to surface relief in the form of intrusions and extrusions. Plastic slip ahead of the growing microcracks in this section would be blocked by the grain boundaries and phase boundaries that may act as barriers against the slip transmission into the adjacent grains until the stress on a dislocation source in the neighboring grain exceeds a critical value. Then slip would be extended into the next grain. With increasing the crack length, a transition from stage Ia to stage Ib, i.e., crack growth on multiple slip planes, would take place, corresponding to the crack propagation in section III. Crack growth according to stage Ib can also explain the crack morphology of section I. This might be justified due to the presence of the FIB defect, changing the stress state at areas near the defect tip and creating a localized condition of biaxial or triaxial stress [Dieter(1986)]. In addition, in section III the crack length might be long enough to enter the stage II of the crack propagation. This hypothesis (shown in Fig. 6b) requires validation taking advantage of microstructural and, specifically, crystallographic analyses. The crack morphology observed in section II may have been also caused by the interaction of parallel cracks, which, in that case, are called en-passant cracks (EPCs), as the tips of the parallel cracks tend to grow towards each other. It has been stated that such interactions are more typical for opening cracks (Mode I) under tension. In that sense, the formation of the EPC is justified as a tendency of a Mode I crack to grow perpendicular to the maximum principal stress and to keep the energy release rate at maximum. Further details can be found in [Schwaab, et al.(2017)]. This behavior has been also recently reported in martensitic microstructures [Seleznev, et al.(2020)]. A complete understanding of the mechanisms involved in stage I is challenging for complex microstructures. The study of crystallographic boundaries and effective grain sizes relevant for the first stages of crack growth is intense.