PSI - Issue 66

Yamato Abiru et al. / Procedia Structural Integrity 66 (2024) 525–534 Author name / Structural Integrity Procedia 00 (2025) 000–000

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450,000), and hydrogen-precharged specimens (at cycles N = 4,000, 5,000, 50,000, and 78,000). The experimental procedure involved striking points A to C (as shown in Fig. 4a) 20 times with a striking rod, while the PDC-200A system recorded the impact strength (see Fig. 3b). Each specimen was subjected to repeated compressive loads up to a maximum of 20 kN. After the cyclic loading, each specimen was heated to approximately 400 °C for 3 h to promote oxidation on the crack propagation surfaces, followed by rapid cooling in liquid nitrogen to induce fracture. This method facilitated the identification of regions within the pipe where cracks had propagated during fatigue testing.

Fig. 3. Impact inspection system.

Fig. 4. Macroscopic crack growth behavior for each number of cycles (Pmax = 20 kN).

3.

Experimental results

3.1. Crack growth property Fig. 4 shows the macroscopic crack growth behavior at various cycle counts. The results indicate that hydrogen significantly accelerated crack growth. At P max = 20 kN, the fracture surface of the crack exhibited linear patterns radiating from the inner to the outer surface of the pipe, suggesting that the roughness originated from the formation and merging of multiple cracks in high-stress areas on the inner surface. This roughness was less pronounced in hydrogen-precharged materials compared to uncharged materials. In hydrogen-precharged specimens, once a crack was initiated, hydrogen promoted its propagation, preventing the formation of multiple cracks and thus resulting in a smoother fracture surface. Fig. 5 shows the relationship between crack growth in the thickness direction and the number of cycles. The crack growth rate in hydrogen-precharged specimens was faster than in uncharged specimens. Crack initiation also occurred sooner in hydrogen-precharged specimens. Overall, the number of cycles for crack initiation and propagation to the