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Longitudinal critically refracted (LCR) waves are considered bulk longitudinal waves and penetrate into an effective depth beneath the surface parallel to the material surface. Such LCR waves can be employed to measure residual stresses because the acoustoelastic effect is the basis for ultrasonic residual stress measurements. This effect is described by the relationship between change of wave travel time and stress applied when such waves propagate in a stressed medium. In this paper, stresses applied in a rail were evaluated by using a developed LCR probe. With this transducer, it was verified how the difference in the arrival times of the LCR waves showed a trend as the tensile stresses increased. The acoustoelastic coefficients were calculated using the relationship between the stresses and the travel times, and the residual stresses of the used rails were measured using these coefficients. In addition, the difference in residual stress distribution according to the characteristics of the wheel-rail contact surface was analyzed from the obtained residual stress value. It was concluded that this non-destructive evaluation technique using LCR waves could be employed for accurate stress measurement of rails because differences in stress applied to the rail can be detected.

Background Recently, some adverse effects of moxibustion has been reported such as burns, smoke, allergies, and so on. To overcome the adverse effects of traditional moxibustion, an ultrasonic moxibustion device (UMD) was designed, simulated, fabricated, and tested. The objective of this study is to provide detailed information about the main design parameters, simulation outcome, and performance-test results. Methods The main components of the UMD are a 1-MHz ultrasonic transducer (UT) with concave lens, and its applicator. The acoustic pressure and temperature distribution of the UT was simulated and described graphically using COMSOL software, which is based on the finite element method (FEM). Experimental verification of the temperature distribution was performed on the skin of pork. The temperature-change profiles of pork in relation to increase of therapy time were obtained at an unfocused point (2 mm) and at a focal distance of 13 mm. For the performance test, moxibustion therapy was conducted on the abdominal skin of mice for 120 min using the new UMD and its histological images were acquired to analyze the skin-tissue damage. Results The FEM simulation of temperature distribution and acoustic pressure agreed with the experimental outcome. Histological images showed that there was no skin-tissue damage to the mouse abdomens after therapy. The results clearly show that the newly developed UMD can overcome the disadvantages of traditional moxibustion therapy and achieve the proposed design parameters. Conclusion The FEM simulation and performance tests provided valuable information about developing future UMDs. In addition, its performance can be compared with traditional moxibustion therapy for future study.

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 corrosions. 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 304SS 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.

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