PSI - Issue 58

M.R.A. Rahim et al. / Procedia Structural Integrity 58 (2024) 9–16 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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2. Methodology 2.1. Material

The material used in this study is P91, a 9Cr-1Mo martensitic steel variation. Table 1 summarizes its mechanical parameters based on ASME Section II requirements and findings from Udoh and Speicher (2013). Rahim et al. (2023) specified this steel's chemical properties.

Table 1. Mechanical properties of 9Cr-1Mo (P91) at room temperature Mechanical properties (Udoh & Speicher, 2013) 24°C Yield stress (MPa) 550 Ultimate tensile stress (MPa) 690 Young’s modulus, E (MPa) 218,000 Poisson ratio, v 0.38 Shear modulus, G (MPa) 78,986

2.2. Four different microstructure morphologies Two-scale models were developed in Abaqus: a Global Model, as shown in Fig. 1(a), replicates the geometry of the actual fatigue test specimen. The Global Model and a 3D SubModel, functioning as a macromodel, facilitate the transmission of stress amplitude to the four microstructure morphologies under investigation. Electron backscatter diffraction (EBSD) is performed in order to examine the crystallographic orientation and particle morphology of the P91 microstructure in Fig. 1(b). Four artificial microstructures as shown in Fig. 1(c), which were generated based on the experimental microstructure with average grain size, d as 20 μm , are used in the simulations to achieve improved fatigue crack initiation results. Similar results were obtained experimentally for 9Cr-1Mo at various temperatures, with 20 μm grain sizes by Li et al. (2014) and Yang et al. (2023). Microstructure morphologies M1 to M3 are considered as irregular, while M4 exhibits in addition a rather homogeneous grain size. The models were assumed to behave elastic-plastically, which ensures they matched the actual material properties (Rahim et al., 2023).

Fig. 1. Two-scale models illustrating (a) Macrostructure as Global Model and 3D SubModel; (b) Scanning Electron Microscope (SEM) and EBSD Image; (c) Four microstructures as SubModel derived from EBSD Image.

2.3. Tanaka-Mura Model Tanaka and Mura (1981, 1982) elucidated the early stages of crack initiation with a dislocation model that included a double-pile up of dislocations in a single slip band. Estimating fatigue crack initiation in alloyed steels, such as 9Cr-