PSI - Issue 66

Yamato Abiru et al. / Procedia Structural Integrity 66 (2024) 525–534 Author name / Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction Amid growing concerns over environmental issues such as global warming and the depletion of fossil fuels, many countries are advocating for a decarbonized society. Hydrogen, a clean and renewable energy source, has emerged as a key component in achieving carbon neutrality, aiming to reduce carbon dioxide and other greenhouse gas emissions to virtually zero. In this context, nations are advancing research and development of fuel-cell vehicles (FCVs) and hydrogen stations. However, a major challenge lies in "hydrogen embrittlement," where metals exposed to hydrogen suffer mechanical degradation due to hydrogen ingress. Furthermore, the high cost of hydrogen-resistant materials, such as austenitic stainless steel SUS316L, significantly raises the cost of hydrogen-energy products, hindering broader adoption. According to the "Current Status of FCV and Hydrogen Station Projects" report (2021) by the Agency for Natural Resources and Energy, Ministry of Economy, Trade, and Industry, the operational costs of hydrogen stations in 2019 included labor expenses of 10 million yen and repair costs of 16 million yen. By ensuring the strength of materials with enhanced hydrogen resistance and improving inspection methods, operational costs could be reduced, thereby lowering total construction expenses. In facilities such as hydrogen stations, nondestructive testing (NDT) is periodically and manually conducted to detect cracks originating from internal components such as pipelines, tanks, and valves, ensuring safety. However, such inspections remain infrequent. This study seeks to develop a nondestructive testing technique using external sources to detect internal pipe cracks. Commonly used techniques, specifically eddy-current testing (ECT) and hammering test (HT), have been employed. Current research focuses on analyzing the effect of crack length on NDT results for both uncharged and hydrogen-precharged materials. Experimental procedure At hydrogen stations, pipelines are subjected to repeated pressurization to fill high-pressure hydrogen tanks for FCVs. Ideally, pressure fatigue tests using high-pressure hydrogen gas would replicate these conditions; however, such tests demand costly equipment. As reported by Yamabe et al. (2020), methods such as electrical discharge machining (EDM) can introduce pre-cracks to promote crack propagation. Nonetheless, introducing internal cracks in longer pipelines proves increasingly challenging. In this study, we used an ECT method. To avoid edge effects during crack detection, sufficient pipeline length is required, complicating the use of EDM for pre-crack introduction. Therefore, a quasi-cyclic internal pressure test was conducted to obtain a crack pattern similar to that in cyclic internal pressure tests. Here, pipes with internal cracks were fabricated for nondestructive testing experiments. Fig. 1 shows the pressure-simulated fatigue test. By cyclically applying compressive loads to the pipe specimen from the top and bottom, cracks propagated radially due to circumferential stresses induced along the radial direction of the pipe. The test material was STKN, and the pipe dimensions were selected to match those typically used in hydrogen stations. The outer and inner diameters were 14 mm, and 6 mm, respectively, the wall thickness was 4 mm, and total lengths was 300 mm. For this study, a Shimadzu UN 500 kN universal testing machine and Shimadzu servo pulser fatigue testing machine were used. 2.

Fig. 1. Illustration of quasi-cyclic internal pressure test.