PSI - Issue 68

Ritsuki Morohoshi et al. / Procedia Structural Integrity 68 (2025) 701–707 R. Morohoshi et al. / Structural Integrity Procedia 00 (2024) 000–000 this issue, small-scale hydrogen storage tanks (Tomoya (2023)) currently employ primarily metastable (austenitic) stainless steels, as they are less susceptible to hydrogen embrittlement and are among the most affordable face-centered cubic (FCC) materials. However, lower-cost stainless steels with reduced nickel content are more prone to ′ (martensitic, BCC) transformation. Therefore, it is essential to re-examine the search for materials that balance both safety and cost-effectiveness. Moreover, due to significant temperature variations during liquid exchange, safety must be considered across a range of temperatures, not only at the liquid hydrogen temperature of 20K. The primary issue is that the fracture assessment requires finite element analysis (FEA), but the transformation-induced plasticity (TRIP) phenomenon in metastable stainless steels cannot be adequately captured using FEA. For example, several experiments (Lebedev and Kosarchuk (2000); Beese and Mohr (2011); Polatidis et al. (2021); Uranaka et al. (under review)) have demonstrated that the ease of ′ trans formation varies significantly under different stress triaxialities. According to the assumption of Elselby’s effective medium approximations(Eshelby (1957)), the same stress-strain relationship should be obtained under conditions of equal ′ transformation rates, and vice varsa. Therefore, stress triaxiality is expected to have a substantial influence on the stress-strain relationship. In other words, the results of uniaxial tensile tests, which are typically used as input data for finite element analysis (FEA), are insufficient for accurately simulating the stress and strain fields near a crack tip under high stress triaxiality conditions. Moreover, the literature on the dependence of ′ transformation on stress triaxiality presents conflict ing results, particularly under conditions of high triaxiality. The reasons for these discrepancies remain unresolved. Therefore, the goal of this experiment is to quantitatively evaluate the effect of different stress triaxialities ( ) on the ′ transformation by using specimens that represent these three values of . In previous studies, XRD (Lebedev and Kosarchuk (2000)) and ferrite scopes (Beese and Mohr (2011)) were employed to observe the transformation amount for similar evaluations. However, this study utilizes SEM EBSD for this purpose because it has the high spatial resolution, previous experimental methods have struggled to capture. 2. Method The main experimental equipment used in this study includes a JEOL JIB-4700F SEM and an Oxford Instruments Aztec 3.4 for EBSD. EBSD post-processing was performed using MTEX. The test material, SUS301L, was provided by Nippon Steel Stainless. The tensile testing machine was a custom-made model from Sanwa Kiki Seisakusho. table 1 is the composition table of the plate material used for preparing the test specimens. Table 1: Chemical composition of the used material Mo Cu N 30 d [ C ] 1.0mm 0.022 0.51 1.08 0.028 0.0018 7.1 17.1 0.1 0.2 0.12 23.7 Next, the results of the tensile test performed on the flat specimens using the above-mentioned plate material are presented. The final calculated mechanical properties are as follows: Young’s modulus is 198 GPa, is 1.93 GPa, is 0.09, and is 0.68. It follows swift equation T = T , plastic ) . The specimen shapes are illustrated in fig. 1. The shapes of the shear and notched specimens were based on Zhang et al. (2019). The experimental procedure is as follows: (1) Apply a sufficiently small load to fix the alignment of the specimen. In this case, 1 N was applied. (2) Capture the initial EBSD image. (3) Capture SEM images at intervals of a few newtons until plastic deformation occurs. (4) Once plastic deformation occurs, capture another EBSD image. (5) Capture EBSD images at intervals of 5-10 N. (6) The test concludes when the upper limit of 200 N on the testing machine is reached. 2 Thickness C Si Mn P S Ni Cr

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