###### Abstract  
Liquefied hydrogen storage vessels (LHSVs) are vulnerable to surface-crack initiation, propagation, and fracture on their surfaces because they are under high-pressure, low-temperature conditions. Defects can also occur in the coatings of the storage containers used to prevent hydrogen permeation, and these lead to surface defects such as pitting corroosions. Together, these increase the probability of liquid hydrogen leaks and can cause serious accidents. Therefore, it is important to detect surface defects during periodic surface inspections of LHSVs. Among the candidate non-destructive evaluation (NDE) techniques, testing using guided waves (GWs) is effective for detecting surface defects. Because of the ability of GWs to travel long distances without significant acoustic attenuation, GW testing has attracted much attention as a promising structural monitoring technique for LHSVs. In this study, an ultrasonic NDE method was designed for detecting surface defects of 304 SS plate, which is the main material used for fabricating LHSVs. It involves the use of linear discriminant analysis (LDA) based on short-time Fourier transform (STFT) pixel information produced from GW data. To accomplish this, the differences in the number of STFT pixels between sound and defective specimens were used as a major factor in distinguishing the two groups. Consequently, surface defects could be detected and classified with 97% accuracy by the newly developed pixel-based mapping method. This indicates that the newly developed NDE method with LDA can be used to detect defects and classify LHSVs as either sound or defective. on-destructive evaluation (NDE); guided wave (GW); liquefied hydrogen storage vessel (LHSV); short-time Fourier transform (STFT); 304 SS; linear discriminant analysis (LDA) +
Footnote †: journal: Applied Sciences  
friendly energy source, has been actively conducted [3,4]. Now, hydrogen is being used in almost all fields, from the basic materials of the steel industry and semiconductors, to fuel cells and energy systems in such as hydrogen vehicles [4]. In terms of the amount of storage per weight or volume, the storage of hydrogen in a liquefied state is more effective than in a metal hydride or pressurized gas state [5,6]. Pressurized storage vessels have been used to store liquefied hydrogen at temperatures <20 K [7,8] because of its boiling point (about 20 K), so liquefied hydrogen should be kept in a freezer. Hence, storage vessels for liquefied hydrogen should have highly effective insulation properties to store or transfer liquefied hydrogen [9]. In particular, because hydrogen leakage from a storage vessel could cause a fire or explosion, containment safety is a critical factor. Hydrogen affects the mechanical properties of materials, so that the quasi cleavage caused by fractures due to the brittleness of the material surface in a hydrogen environment can appear on fractured surfaces. This can cause brittleness due to a loss of elongation due to hydrogen filling [10, 11, 12]. Moreover, in cases where steel equipment is exposed to wet hydrogen sulfide (H\({}_{2}\)S), internal cracking occurs due to hydrogen-induced cracking (HIC) [12, 13, 14, 15]. Accordingly, it is necessary to understand the effect of the container materials on mechanical properties according to the amount of hydrogen to solve this problem. In addition, the defects (such as pitting corrosion) that occur in the film coating of storage vessels, which are used to prevent hydrogen permeation, are one of the causes of explosion [16, 17, 18]. Early detection of defects and the ability to monitor their growth using a non-destructive evaluation (NDE) method is therefore required. So far, because related research on the safety of liquefied hydrogen storage vessels (LHSVs) has been focused on improving storage efficiency, NDE methods suitable for LHSVs have not yet been sufficiently developed. [PERSON] et al.