PSI - Issue 68

Birhan Sefer et al. / Procedia Structural Integrity 68 (2025) 1129–1139 Sefer et al. / Structural Integrity Procedia 00 (2025) 000–000

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3. Results and Discussion

3.1 Slow strain rate testing Figure 2 shows stress-strain curves for testing in Ar and H 2 for all three studied materials at room and high temperature using hollow specimen method.

Figure 2. Representative stress-strain curves obtained using hollow specimens in argon and hydrogen at room and high temperature for the studied materials a) SiMo51; b) CGI and c) NCF3015.

Table 3 summarises the yield strength (Rp0.2 h ), ultimate tensile strength (Rm h ) and elongation to leakage (A h ) evaluated from all SSRT tests performed in this work. Note the h stands for denomination of the properties using hollow specimens according to ISO 7039:2024. The values for the properties shown in Table 3 are average from duplicate tests in Ar and triplicate tests in H 2 . Their relative ratio calculated from average values between H 2 and Ar are also given in Table 3. Table 3. Tensile properties (Rp0.2 h , Rm h and A h ) obtained in 200 bar argon and hydrogen at room and high temperature for all studied materials. Material Temp Rp0.2 h Rp0.2 h Rel. H 2 /Ar Rp0.2 h Rm h Rm h Rel. H 2 /Ar Rm h (%) A h A h Rel. A h H 2 (MPa) Ar (MPa) (%) H 2 (MPa) Ar (MPa) H 2 (%) Ar (%) (%) SiMo51 RT 507±4 518±5 98 562±7 619±4 91 1±0.2 3±0.2 41 SiMo51 800 ° C 18±2* 29±4* 63* 29±4* 31±2* 95* 10±0.5* 26±1* 39* CGI RT 321±2 326±0 98 390±4 446±5 87 1±0.02 2±0.08 52 CGI 300 ° C 290 ±3 299±1 97 394±8 406±2 97 2±0.1 2±0.14 91 NCF3015 RT 644±2 642±0.5 100 1122±5 1126±7 100 32±0.6 34±0.5 94 NCF3015 600 ° C 638±4 636±0 100 785±11 838±3 94 6±0.5 8±0.7 82 In general, the tensile properties of all three materials tested at high temperatures as compared to room temperature were deteriorated, but to different extent. The immediate observation was that there is tremendous loss of the tensile properties for the SiMo51 material when tested at 800 °C compared to room temperature. In fact, at 800 °C the yield strength of SiMo51 is very close to the estimated hoop stress caused by the internal pressure of 200 bar. According to ISO 7039:2024 the hoop stress shall not exceed two thirds of the yield strength, and this is not the case for SiMo51 tested at 800 °C. Hence, for this set of tested specimens the failure was because of combination of tensile stress applied by the test rig and the hoop stress caused by the internal pressure of the gas in the hollow specimen regardless of if it was argon or hydrogen. This means that the mechanical properties evaluated for this set of tests are not plausible (marked with * in Table 3). Although the values are not reliable for SiMo51 when tested at 800 °C, the relative A h