PSI - Issue 42

Hiroshi Nishiguchi et al. / Procedia Structural Integrity 42 (2022) 1442–1448 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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a force of 1.96 N. Figure 2 shows the magnified views of the crack extension in Fig. 1. The dotted lines in Fig. 2 show that the cracks propagated slightly after hydrogen exposure. Such crack extensions were observed even for lower hydrogen gas pressure. Figures 3 (a-c) show the images of the cracks after the hydrogen exposures with pressures of 70, 35, and 10 MPa, respectively. As observed from the figures, under the same loading and hydrogen gas exposure conditions, some cracks clearly propagated while others did not. In this study, 10 indentations were introduced in each specimen and each test load, and longitudinal and transverse crack lengths were measured. Figure 4 shows the relationship between the hydrogen gas pressure and the crack propagation ratio (the ratio of the propagation length to the initial crack length), which shows the crack growth ratio with a confidence interval of 95%, assuming a normal distribution of the crack growth length variation. For the hydrogen gas pressures in the range of 10 to 35 MPa, the amount of crack propagation increases with increasing gas pressure. By contrast, for hydrogen pressures above 35 MPa, the crack growth ratio no longer increases and is retained at the upper limit value of approximately 9%. According to Sieverts law, the amount of hydrogen introduced into material increases with increasing hydrogen gas pressure and temperature. However, the crack growth ratio does not change with increasing hydrogen gas pressure, indicating that either there is a limit to the crack growth ratio or that the hydrogen content is saturated. In addition, in Fig. 4, the crack propagation ratio at the hydrogen pressure of 0 MPa becomes zero. This result was obtained from an experiment where the specimen was exposed to Ar gas at 270 o C for 24 h. This experiment was conducted in the same conditions as those of the hydrogen exposure experiments except for the hydrogen gas pressure. Figure 5 shows the crack morphologies before and after the Ar gas exposure, and Fig. 6 shows the magnified views of the cracks in Fig. 5. As observed from Figs. 5 and 6, no crack growth was induced by Ar gas exposure. This indicates that the crack growth is not due to the effect of temperature, but rather is due to the introduction of hydrogen into silicon by exposure to the high-pressure and high-temperature hydrogen gas.

Fig. 3 Effect of hydrogen gas exposure on the crack length before and after H 2 gas exposure (the hydrogen gas pressures, p H2 , were 70, 35 and 10 MPa) .

Fig. 4 Relationship between the hydrogen gas pressure and the crack propagation ratio.

Fig. 5 Morphologies of the Vickers indentations and cracks before and after 270 o C Ar gas exposure.

Fig. 6 Magnified view of the crack in Fig. 5.