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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2. Materials and methodology The chemical composition of the alloy investigated in this study is represented in Table 1. The as-received material was austenitized at 990°C during 5 min, cooled down at 5 K/s to 250° C, and subsequently bainitized at 250°C for 14 hours. This heat treatment was conducted in a dilatometer. The used dilatometer was a High-Resolution Dilatometer, Bahr 805D (TA Instruments, Germany), at CENIM-CSIC (Spain).

Table 1. Chemical composition of the steel used in this study, in wt. %.

Abbreviation C

Mn

Si

P

S

Cr

Ni

Mo

V

Cu

Al

1C2.5Si

0.99

0.74

2.47

0.008

0.003

0.97

0.12

0.028

0.003

0.17

0.024

Three fatigue flat specimens (Specimen 1, Specimen 2, and Specimen 3) were longitudinally extracted, by machining, from dilatometer samples. The geometry of the flat fatigue specimens is shown in Fig. 3a. One of the main faces of the specimens were metallographically prepared following standard procedures, finishing with a step of polishing with a suspension of colloidal silica. Subsequently, a manufactured FIB defect was induced at the edge of one of the fatigue specimens (corresponding to the polished face), Specimen 3, within the gauge section, by using a Gallium ion beam with a 47pA current using Dual beam Helios 650 Nano Lab microscope at TU-Kaiserslautern /Nano Structuring Centre (Fig. 3b). The selected position was at the edge and not in the center due to angular restrictions from the FIB technique with regards to obtaining a sharp tip. Fatigue specimens were subjected to tension-tension sinusoidal stress-controlled fatigue tests with a stress ratio (R) of 0.1, using Bose Electroforce 3230 set-up at the frequency of 50 Hz at TU-Kaiserslautern/AWP. After interrupted fatigue testing, the crack path of Specimen 3 was examined by means of scanning electron microscope (SEM), using a TESCAN FIB-SEM, GAIA3 model 2016.

Fig. 3. a) Geometry of the flat fatigue specimen, bold arrows show the direction of loading; b) SEM micrograph showing the manufactured FIB defect. 3. Results and discussion 3.1. Fatigue testing condition and design of pre-fabricated FIB defect Considering that the local microstructural features govern the short-crack fatigue behaviour, especially during the stage I of crack propagation, the FIB defect of Specimen 3 was designed to have a size ensuring that the initial crack