PSI - Issue 39

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

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Fig. 1. Schematic illustration showing Kitagawa-like diagrams, including the transitional section modified by El-Haddad (red curve, corresponding to Eq. 1 or Eq. 3 with α = 1 ), or by Atzori (solid black curves, co rresponding to Eq. 3 with different α values). The dashed red line 2 corresponds to the Eq. 4 for α = 1. Regarding long cracks, different solutions for the stress intensity factor have been proposed in the literature, which correspond to a variety of defect/notch geometries and positions, as widely covered by Anderson(2017). In this work fatigue tests were conducted on flat specimens of a nanostructured bainitic steel. One of the specimens, containing an artificial defect manufactured by the focused ion beam (FIB) technique, was subjected to interrupted fatigue testing and the crack path at the surface was examined. The manuscript is divided into two parts. The first part deals with the design of the FIB defect, with the purpose to ensure that the initial crack does not enter the long-crack regime. In this sense, the equivalent initial crack size corresponding to the FIB defect will be enclosed within the range between 0.1 0 2 and 0 2 (green dashed region in Fig. 1). For the reader´s convenience, the flowchart in Fig. 2 summarises the steps by which the FIB notch was designed, which will be explained in the section Results and Discussion in detail. The second part is devoted to the morphological investigation of the crack path by means of the scanning electron microscopy (SEM) technique.

Fig. 2. Flowchart illustrating the steps to design the FIB defect.