Papers by Author: Joon Hyun Lee

Abstract: The long-term corrosion resistances for the carbon steels have been investigated under high temperature pressurized water atmosphere, in the conjunction with the analysis of nondestructive properties by the ultrasonic wave. The corrosion test for carbon steels was carried out at the temperature of 200 °C under a water pressure of 10 MPa. The corrosion test cycles for carbon steels were changed up to 65 weeks. The mechanical properties of carbon steel suffered from the corrosion cycle were investigated by a tensile test, attaching an acoustic emission sensor on the test sample. The tensile strength of carbon steels greatly decreased beyond the corrosion cycle of 35 weeks, accompanying the increase of weight loss by the creation of corrosion damages. The attenuation coefficient of carbon steels by the ultrasonic wave increased with the increase of corrosion cycles.

Abstract: A study on corrosion evaluation by using ultrasonic waves and acoustic emission technique is presented. The experimental equipment was established to improve the corrosion process of carbon steel pipe. The carbon steel pipe was under 473K temperatures and 10Mpa pressure conditions, and ultrasonic wave and acoustic emission techniques were used to inspect the degree of corrosion after a certain period of time. Ultrasonic bulk waves are limited by the poor time resolution when used in the measurement of corrosion depth in thin wall structures because the corroded surfaces cause unclear echo signal edges. Therefore, in this study, the ultrasonic guided waves were generated on the pipe because the thickness of pipe was thin. Various wave modes were subsequently generated on the pipe to evaluate the implications of corrosion thinning on group velocity, transmission and reflection amplitudes. The amplitudes of the transmitted and the reflected waves are influenced by couplent material. In order to reduce the effect of coupling acoustic emission sensor was used. Acoustic emission technique has lots of parameters to evaluate the corrosion besides amplitude parameter. Among parameters energy, count, and frequency were useful parameters to measure the degree of corrosion inside the carbon steel pipe under 473K temperatures.

Abstract: The nuclear power plant has lots of pipes that the fluid of high temperature and high pressure flows. Among the pipe materials used at secondary circuit of the power plant the carbon steels are sensitive to corrosion due to their material properties. In this study, both ultrasonic test and acoustic emission test were used to study the corrosion effect for the carbon steel pipe nondestructively. The carbon steel specimens were in the pipe under 473K temperatures and 10MPa pressure conditions for corrosion processing. According to the degree of corrosion the strength of the specimen was evaluated, and the thickness of the corrosion specimens was also measured by using the ultrasonic wave. The experimental results showed that the attenuation factor was also increased as a depth of corrosion increased. The measured depth of the real corrosion by ultrasonic test shows the good agreement with that by an optical microscope. In order to understand the corrosion effect for the failure mechanism of carbon steel, a failure test on the specimen with various corrosion conditions was performed. An acoustic emission technique was also used to evaluate the degree of damage of corrosion specimen in real time. Acoustic emission technique is proved a useful method for on-line monitoring the microscopic failure mechanism and the damage location for the structures.

Abstract: This study dealt with the corrosion resistance for the carbon steels under a pressurized water atmosphere at the elevated temperature. The nondestructive test was also used to evaluate the damage degree of corrosion test specimen. The corrosion test for carbon steels was carried out at the temperature of 200 °C under a water pressure of 10 MPa. The corrosion time for carbon steel was changed up to 20 weeks. The strength of carbon steel by the degree of corrosion was investigated by a tensile test. The carbon steel showed an average tensile strength of about 500 MPa after the corrosion period of 20 weeks, accompanying the weight loss of about 2.5 %. The attenuation coefficient of ultrasonic wave can be utilized as useful parameters to inspect the corrosion damages of carbon steels.

Abstract: Creep testing of Alloy 718 has been carried out at various loads in the temperature range near 650°C in constant load control mode in order to understand how to predict the creep behavior including tertiary creep. The test results have been used for evaluating the existed models, such as Theta projection and Omega method that have been widely used for predicting long term creep strain and rupture time. After determining variables and material parameters of each method with the test results, estimated creep data from each model have been compared with the each measured creep data from the creep tests. The root cause of the discrepancy between estimated and measured data has been analyzed in order to improve the existed methods. The reliability of the improved model has been evaluated in relation to creep data.

Abstract: In this study, elastic waves of ultrasonic and acoustic emission were used to evaluate the propagation characteristic of the wave in pipe, and study on mode conversion of the elastic wave due to the cracks in the pipe was also performed. An acoustic emission (AE) sensor was used to receive the propagated ultrasonic wave. AE technique has a merit that it can identify the received ultrasonic wave by the analysis of the AE parameters such as count, energy, frequency, duration time and amplitude. For transmitting and receiving of the wave, a wedge for universal angle was manufactured. The optimum angles for transmitting of ultrasonic wave and signal receiving at the attached AE sensor on the pipe were determined. Theoretical dispersion curve was compared with the results of the time-frequency analysis based on the wavelet transformation. The received modes showed a good agreement with theoretical one. The used ultrasonic sensor was 1MHz, and AE sensor was broadband (100kHz – 1200kHz). The artificial cracks were induced in the pipe to measure the propagation characteristics of the elastic wave for the cracks. AE parameters for the received signals were also varied with the crack types in the pipe. AE parameters of amplitude and duration time were more effective factors than the analysis of mode conversion for evaluation of the cracks in the pipe.

Abstract: This paper is focused on the capability of the Acousto-Ultrasonic (AU) technique and the non-contact technique to provide diagnostic information useful to detect defect in composite. An acousto-ultrasonics (AU) is to simulate stress wave that resemble acoustic emission waves but without disrupting the material. One launched inside the material sample, the wave are modified by stochastic processes like those that affect spontaneous acoustic emissions from internal sources during stressing, deformation, etc. Moreover, acousto-uloasonic waves are launched periodically at predetermined times and with predetermined reparation rates. A fiber reinforced composite materials should be inspected in fabrication process in order to enhance quality by prevent defects such as delamination and void. In conventional ultrasonic technique for the evaluation of FRP, the transducer should be contacted on FRP. Therefore, in this study, advanced conventional contacting method (AU) and non-contact technique using air-coupled transducer can make contact and noncontacting ultrasonic technique available in evaluation of FRP. This paper demonstrates first results using an acousto-ultrasonic technique.

Abstract: Local wall thinning is one of the major causes for the structural fracture of pipes of nuclear power plants. Therefore, assessment of local wall thinning due to corrosion is an important issue in nondestructive evaluation for the integrity of nuclear power plants. In this study, lasergenerated guided waves were used for pipe inspection, where a laser beam illuminated through linear slit array was used as the transmitter and the air-coupled transducer was used as the receiver. Slits was used in order to enhance the mode-selectivity of guided waves, since the space of slits is equal to the wavelength of the generated wave. The air-coupled transducer detected the selected single mode by turning its detection angle that was calculated from the relations between the wave propagation velocity in air and the phase velocity in dispersion curves. Experimental results for a 4- mm thick carbon steel pipe showed that the detection of the specific mode was useful in the distinction of the wall-thinning thickness in the carbon steel pipe.

Abstract: The objective of this paper is to investigate the applicability of ultrasonic wave technique to monitor the progress of the thermal shock damage on alumina ceramic. For this purpose, alumina ceramic specimen was heated in the furnace and then was quenched into the water tank. The initiation, growth behaviors of surface micro-cracks as a function of the number of thermal shock cycle have been discussed by taking into account the change of ultrasonic wave velocity and attenuation. The change of both velocity and attenuation of ultrasonic wave showed good relation with the surface crack density changing due to the number of thermal shock cycle. Measuring the change of attenuation gives more effective information to evaluate thermal shock damage than that of velocity nondestructively. The flexural strength was also measured for the thermal shocked specimen. The flexural strength was decreased rapidly at the point of time of observation of microcracks on the surface of specimen, and the flexural strength decline by the crack growth caused thermal shock cycles was slight.

Abstract: This paper dealt with the fabricating process of liquid phase sintered (LPS) SiC ceramics containing the oxide additives of Al2O3 and Y2O3, in conjunction with the evaluation of their mechanical properties. LPS-SiC ceramics was sintered at the temperature of 1820 oC under an applied pressure of 20 MPa and a pressure holding time of 2 hour. A commercial SiC powder with an average size of about 0.3 μm was used as a starting powder. LPS-SiC ceramics with additive composition ratios of 1.5 and 2.3 (Al2O3/Y2O3) represented an excellent density of about 3.2 Mg/m3. LPS-SiC ceramics had a flexural strength of about 800 MPa and a fracture toughness of about 8.0 MPa⋅m0.5 at an additive composition ratio (Al2O3/Y2O3) of 1.5.