Vol.8 No.2 2015
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Research paper : Development of material testing equipment in high pressure gaseous hydrogen and international collaborative work of a testing method for a hydrogen society (T. IIJIMA et al.)−65−Synthesiology - English edition Vol.8 No.2 (2015) strength and high toughness with tensile strength of 950 MPa or less that is expected to be used in high-pressure gaseous hydrogen equipment. As a result of comparing the fracture toughness value calculated by the constant displacement method (KTHa, crack-arrest threshold) and the fracture toughness value calculated by the rising displacement method (KJH, crack-initiation threshold) in 103 MPa high-pressure gaseous hydrogen atmosphere, the KJH value was lower than the KTHa value, and as a fracture resistance value, KJH was shown to be a conservative value.[18][19] The constant displacement method is a testing method in accordance with ASTM E1681, where the bolt-load compact specimen (Fig. 5(a)) ,which is pre-cracked in advance, is used, the crack opening displacement is held constant by tightening the bolt, the load is applied to the tip of the crack, and the load is maintained until the crack grows and stops under certain conditions.[15] This is also called the delayed fracture test. At the Sandia National Laboratories, the fracture toughness value was calculated from the length of the crack that finally stopped after tightening the bolt in inert gas conditions and then maintaining the specimen to a maximum of 3,800 hours in high-pressure gaseous hydrogen. Since the fracture toughness value of the crack arrest is calculated, it can be considered as a crack growth stop test. The rising displacement method is a material test where the load is applied continuously to the pre-cracked compact specimen (Fig. 5(b)) in the high-pressure gaseous hydrogen atmosphere, so the crack opening displacement will increase, and this method is in accordance with ASTM E1820.[13] At the Sandia National Laboratories, the crack opening displacement was measured with the linear variable differential transformer (LVDT) and the crack length was measured by the direct-current potential difference (DCPD) method, and the fracture toughness value of the crack initiation under the continuously rising displacement is calculated from the load, opening displacement, and crack length. Therefore, this can be considered the crack growth starting test.4.2 Fracture toughness evaluation using the unloading elastic compliance methodIn our research group, the rising displacement test was conducted using the unloading elastic compliance method that is another crack length measurement in accordance with ASTM E1820, and we attempted direct comparison with the measurement data obtained at the Sandia National Laboratories.[20] The rising displacement test using the unloading elastic compliance method is a method of calculating the fracture toughness value of the crack initiation, as the crack opening displacement of the pre-cracked compact specimen (Fig. 5(b)) is increased at a certain rate, part of the load is removed at arbitral crack opening displacement, and then the crack length from the relationship of the crack opening displacement and load at that moment is calculated. For the experiment, SCM435 (Japan standard) and SA-372 Grade J (American standard; supplied by Sandia National Laboratories) were used. These are standard materials of the Cr-Mo alloy steel and are expected to reduce the cost of high-pressure gaseous hydrogen equipment in the future. Table 1 shows the material properties and composition of SCM435 and SA372 Grade J. The outline of the testing conditions by the unloading elastic compliance method is presented in Reference [20].4.3 Direct comparison of Japanese and American data for fracture toughness evaluationFigure 6 shows the load vs. crack opening displacement (P-COD) curve calculated using the unloading compliance method in 115 MPa gaseous hydrogen for SCM435. The relationship between the J integral value and crack growth length (R curve) was calculated, and the fracture toughness value (JIC) of crack-initiation was determined. Using the relation equation between J and K described in ASTM E1820 shown below, the stress intensity factor (KJIC,H) of the minimum limit of crack-initiation was derived. Here, Young’s modulus was E = 206 GPa and Poisson ratio was = 0.3.KF =EJF1-ν2The fracture toughness value of SCM435 obtained by this Fig. 5 (a) Bolt-loaded compact specimen and (b) compact specimenBal0.0040.0080.280.930.180.990.49889762SA-372 Grade JBal0.0040.0060.220.790.231.10.38828700SCM 435(mass%)(MPa)FeSPSiMnMoCrCTensile strengthYield stressTable 1. Material properties and compositions of SCM 435 and SA-372 Grade J(a)(b)

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