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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3.2. Effect of hydrogen gas exposure on residual stress field Figure 7 shows the residual stress field induced around the indents for which the indentation force was 1.96 N as observed via Raman spectroscopy. This result was obtained under the same conditions as those used for the observation of the cracks in Figs. 1 – 3, namely for four different hydrogen gas pressures ( p = 10, 35, 70, and 100 MPa) and at 270 °C. In Fig. 7, images (I) and (II) show the stress fields before and after the hydrogen gas exposure, respectively. In images (I) and (II), compressive residual stress fields (blue images) were clearly observed around all indentations before and after the hydrogen exposure. In addition, a small tensile residual stress field was certainly formed at the crack tip, although it is difficult to discern it in the figures. Figure 7 shows that there is a clear difference between the images before (I) and after the exposure (II), that is, the blue areas of the compressive residual stress fields decrease due to the effect of the hydrogen gas exposure. To demonstrate this difference, the stress values of image (I) were subtracted from those of image (II), and the difference values are plotted in image (III) in Fig. 7. In image (III), red areas indicate the positive change due to the weakening of the compressive stress field or the strengthening of the tensile stress field. Conversely, blue areas indicate a negative change due to a weakening of the tensile stress field or an increase in the compressive stress field. Therefore, the red areas observed around the indents in image (III) indicate a general decrease in the compressive residual stress. In addition to this change, in image (III), a small blue region appears at each crack tip, suggesting the decrease in the tensile stress at the crack tip during the hydrogen exposure. Higashida et al. (28) introduced Vickers indentation into silicon wafers and observed the compressive residual stress fields by the photoelastic method. According to their observations, the area of the compressive residual stress field reduced after annealing at 500 o C for 5 min. However, the temperature used in this study was 270 o C, which is sufficiently low to suppress such a recovery of the internal stress field, and thus the temperature effect for the residual stress is regarded as negligible. In fact, we confirmed that little change was observed in the residual stress around the indent in the specimen exposed to Ar gas at 270 o C for 24 h. In the next section, the mechanism of crack growth and the residual stress change due to the hydrogen gas exposure are discussed.

Fig. 7 Raman spectral images of the Vickers indentations for various hydrogen pressures (Indentation force: 1.96 kN).