PSI - Issue 39

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

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Fig. 5. Kitagawa-like diagram based on Atzori´s Eq. 3 (black, red, and orange lines). At defect depth = 30 μm : solution to Murakami´s Eq. 7 for a defect of √ = 50 µm; and selected initial conditions, S a , for fatigue testing of Specimen 3. 3.2. Morphological analysis of the crack path The fatigue test of Specimen 3 with the FIB-defect was carried out with R=0.1 and Sa=250 MPa and was stopped after 100,000 cycles. This number of cycles was selected based on the crack growth experiments carried out in a former project, MECBAIN [Sourmail, et al.(2016)]. The region of the FIB-defect of the specimen 3 after the interrupted fatigue test was investigated by backscattered electron (BSE)-SEM. The micrograph is shown in Fig . 6 a, illustrating an overview of the crack path. The crack grew, to a length of about 100 µm from the FIB defect root, macroscopically perpendicular to the loading direction. To interpret the crack path, it was grouped into three sections, namely section I, II, and III. In section I and section III, an approximately straight crack growth can be observed, where the crack propagated nearly normal to the direction of the applied load, which is supposed to be a characteristic of the stage II of crack growth. In contrast, in section II an oblique growth can be seen relative to the applied load direction, where the crack followed a zigzag-like path that is presumed to be typical of the stage I crack propagation. The traces of the microcracks in section II are shown in the detail of Fig . 6 a, where a crack branching backwards to the FIB defect position can be seen. This morphology implies that the material in that region experienced, at least, some damage before the crack grows through it. It is well-known that the fatigue crack growth comprises stage I, short-crack propagation; stage II, short- and stable long-crack propagation; and stage III, unstable crack propagation [Maierhofer, et al.(2015)]. As a result, from the observation of the crack path, short-crack growth corresponding in part to the stage I of the fatigue life can be suggested. There is no consensus, within the scientific community, on the mechanisms involved in stage I [Navarro and de los Rios(1988)], since this stage is mainly relative to the microstructure and therefore characteristic of each one. Künkler, et al.(2008) explained that the stage I crack growth is completed through stage Ia and stage Ib. In stage Ia, the microstructurally short cracks propagate through the single primary slip systems in which the resolved shear stress acts along the activated slip systems. According to this theory, as the crack length increases, the stage Ib starts, implying that the crack enters the physically short-crack regime where more slip systems are activated. Subsequently, the stage II of the propagation applies, covering both physically short-crack growth (described by the elastic plastic fracture mechanics theory) and/or long-crack propagation (described by LEFM).