PSI - Issue 71

Haru Fujishima et al. / Procedia Structural Integrity 71 (2025) 18–25

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Fig. 1 Comparison of experimental results of tension-compression test with prediction of S-N curve for annealed 0.37% C steel (Murakami et al., 2023)

Fig. 2 Comparison of experimental results of rotating bending test with prediction of S-N curve for annealed 0.37% C steel (Murakami et al., 2023)

2. Material and experimental procedure The material was 0.37% carbon steel (JIS S35C) annealed at 860°C. The stress gradient is one of the essential parameters used in this study. Siebel and Stieler define the stress gradient χ at the root of a notch by the following equation (Siebel and Stieler, 1955): = 1 | ( ) | =0 (3) where σ ( x ) represents the stress distribution ahead of a notch root as a function of the distance x from the notch root, and σ max is the maximum stress at x = 0. They also show the approximate expression of χ for a cylindrical specimen with a circumferential notch in bending as = 2 + 2 (4) where ρ is the notch radius, and d is the notch-root diameter of the specimen. Eq. (4) is reduced to χ = 2/ d for a smooth specimen in bending since ρ = ∞, while for a smooth specimen in tension, χ = 0. Fig. 3 shows the shapes and dimensions of specimens. The stress gradient was chosen as χ = 0 mm -1 , 0.2 mm -1 , or 0.4 mm -1 , which was set by changing the specimen geometry and loading type combination. The specimens were polished with an emery paper of a grade up to #2000 to remove the surface machined layer. Artificial defects with a dimension of √ = 93 μm, as displayed in Fig. 4, were drilled onto the surface of the smallest cross-section of the specimens. After introducing the defects, the hole depths were fine-tuned by emery paper polishing and buffing, and then specimens were slightly electropolished.