PSI - Issue 51

2

Abdelhak Nehila et al. / Procedia Structural Integrity 51 (2023) 152–159 A.Nehila and W. Li / Structural Integrity Procedia 00 (2022) 000–000

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The stress concentration effect on fatigue life predictions of mechanical components is an important issue in fatigue reliability evaluation. The problem has been investigated in many studies but still has not been fully solved. Different approaches have been proposed to include the effects of notch and stress concentrations. These approaches can be grouped into several categories. One approach is the so-called theory of critical distance (TCD) stated by Turnbull et al. (1959). Another approach is based on the stress gradient formulation for the fatigue analysis of notched components was suggested by Suresh (1998). Hansson (2012) stated that the cohesive zone model (CZM) is another important methodology for fatigue analysis of notched specimens. However, the application seems to be mostly for composite and concrete materials, which show brittle and/or quasi-brittle behavior as explained by Schutz (1996). A comprehensive review of the fatigue analysis approaches for notched specimens is not the focus of this research. Only two of them, which are dominantly used in the research community and industry, are discussed below. One is based on the classical fatigue theory, i.e. S-N curve-based approach. The other is the fracture mechanics-based approach, i.e. da/dN ∼ Δ K curve-based crack growth analysis. In this paper, the HCF and the VHCF characteristics of the notched specimens of the carburized 17CrNi high strength steel were investigated under the axial loading with a stress ratio R = ‒1 and the S-N characteristics were revealed. Based on the microscopic observation of fracture surfaces using OM the failure mechanisms were studied. Further, the predicted fatigue crack growth life was estimated and compared with the experimental results. After that, based on small crack theory, calculation of fatigue notch factor ( K f ) denotes that K f is slightly less than the theoretical stress concentration factor ( K t ). Finally, K f was used to study notch effect and stress ratio effect on fatigue life, then fatigue life prediction model associated with R and K f was established. The notched specimens of the carburized 17Cr2Ni2Mo steel were used in this study. The elastic stress concentration factor at the notch is, K t = 1.89. The chemical composition is given in Table 1. The heat treatment process of the 17CrNi specimen is given in previous work (Nehila et al. (2018)). After, the axial polishing was performed on the middle of the notch using 80-2000 abrasive papers, each abrasive paper is used for 15 min polishing time. Finally, the grade 80 of the abrasive paper was used for about 1 h of time to ensure that the surface roughness of the notch is less than 3.2 μm. The specific shape of the notch specimen is shown in Fig. 1, the minimum cross-sectional diameter and the notch tip radius at the root of the middle notch is 6 mm and 1.5 mm, respectively. The cross-sectional diameter of the unnotched part was 12 mm. Based on the monotonic tension test, values of Young's modulus ( E ), Poisson’s ratio ( υ ) , and tensile strength ( σ b ) of 17Cr2Ni2Mo steel are 205 GPa, 0.3, and 1510 MPa, respectively. Table 1. Chemical composition of CrNi steel (wt %). Steel C Si Mn P S Cr Ni Mo V Nb Al Cu CrNi 0.16 0.05 0.50 0.004 0.001 1.62 1.50 0.29 0.10 0.036 0.032 0.08 2. Material and methods 2.1. Material and specimen

Fig. 1. Shape and dimensions of notched specimen (units: mm).