Papers by Author: Joon Hyun Lee

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Authors: Min Rae Lee, Joon Hyun Lee
Abstract: This study presents an approach to leak detection of pipeline review in terms of theoretical analysis such as acoustics and hydromechanics that should be accompanied by explanation of leakage. The acoustic emission signals during leak from circular hole of different geometries were studied both analytically and experimentally. The relationships between acoustic parameters and fluid mechanical parameters also were derived analytically. A quadrupole aerodynamic model was applied for the analysis of leak from the circular hole. Computer simulation results demonstrate the effectiveness of the proposed approach. In addition, it was confirmed that the wavelet transform (WT) was an effective tool to determine source location. That is, arrival times of each frequency component needed in the velocity calculation could be determined from the peak of the magnitude of wavelet transform data on the time-frequency plane.
Authors: Jin Kyung Lee, Young Chul Park, Sang Ll Lee, Joon Hyun Lee, Jong Baek Lee
Abstract: Tensile residual stress happen by difference of coefficients of thermal expansion between fiber and matrix is one of the serious problems in metal matrix composite (MMC). TiNi alloy fiber was used to solve the problem of the tensile residual stress as the reinforced material. TiNi alloy fiber improves the tensile strength of composite by occurring compressive residual stress using shape memory effect in the matrix. A hot press method was used to create the optimal condition for the fabrication of shape memory alloy (SMA) composite. The bonding effect between the matrix and the reinforcement within the SMA composite was strengthened by the cold rolling. The fabricated composite by these processes can be applied as a part of the aircraft, and this part is operated under severe flying condition such as low temperature and high pressure. In this study, an acoustic emission technique was used to quantify the microscopic damage behavior of cold rolled TiNi/Al6061 SMA composite at low temperature condition. The results showed that the tensile strength of the TiNi/Al6061 SMA composite increased with the TiNi reinforcement at low temperature condition, but the strength for the specimen subjected to the cold rolling decreased. AE parameters of AE counts, amplitude and energy were useful to evaluate the microscopic damage behavior of the composite.
Authors: Won Geun Yi, Min Rae Lee, Joon Hyun Lee, Sang Woo Choi, Bo Young Lee
Abstract: Pipelines of nuclear power plants undergo high pressure and temperature. Thermal stratification typically occurs in the surge line and the main feed water lines by flow and this stratification will initiate and propagate thermal fatigue cracks. This may cause rupture and leakage and it is a serious problem to nuclear power plants operation. Therefore it is very important to detect and measure thermal fatigue cracks. In this study, thermal fatigue cracks were generated in austenitic stainless steel specimens by a thermal cycle in notched pipes and weld jointed pipes. Ultrasonic techniques were used to evaluate the thermal fatigue crack depth. When ultrasonic waves propagate from an angle beam probe to thermal fatigue cracks, waves are reflected and diffracted. Crack depth was evaluated by the reflected signals from back wall and diffracted signals from the crack tip, but diffracted signals were too weak to detect so the reflected signals were more useful. The TOFD and dB drop methods were used in this study. The TOFD method is uses a time delay of diffracted signal from the crack tip. The dB drop method is an application of an amplitude decreasing rate by a probe moving distance.
Authors: Youn Ho Cho, Won Deok Oh, Joon Hyun Lee
Abstract: This study presents a feasibility of using guided waves for a long-range inspection of pipe through investigation of mode conversion and scattering pattern from edge and wall-thinning in a steel pipe. Phase and group velocity dispersion curves for reference modes of pipes are illustrated for theoretical analyses. Predicted modes could be successfully generated by controlling frequency, receiver angle and wavelength. The dispersive characteristics of the modes from and edge wall-thinning are compared and analyzed respectively. The mode conversion characteristics are distinct depending on dispersive pattern of modes. Experimental feasibility study on the guided waves was carried out to explore wall thinning part in pipe for data calibration of a long range pipe monitoring by comb transducer and laser.
Authors: Min Rae Lee, Joon Hyun Lee
Authors: Sang Ll Lee, Jin Kyung Lee, Joon Hyun Lee
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.
Authors: Joon Hyun Lee, Min Rae Lee, Jung Taek Kim, Jung Soo Kim
Authors: Jong Ho Park, Joon Hyun Lee, Gyeong Chul Seo, Sang Woo Choi
Abstract: In carbon steel pipes of nuclear power plants, local wall thinning may result from erosion-corrosion or flow-accelerated corrosion(FAC) damage. Local wall thinning is one of the major causes for the structural fracture of these pipes. 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, laser-generated ultrasound technique was employed to evaluate local wall thinning due to corrosion. Guided waves were generated in the thermoelastic regime using a Q-switched pulsed Nd:YAG laser with a linear slit array. . In this paper, time-frequency analysis of ultrasonic waveforms using wavelet transform allowed the identification of generated guided wave modes by comparison with the theoretical dispersion curves. Modes conversion and group velocity were employed to detect thickness reduction.
Authors: Sang Woo Choi, Joon Hyun Lee, An Jin Nam, Sridhar Krishnaswamy
Authors: Sang Woo Choi, Joon Hyun Lee
Abstract: The reactor vessel body and closure head are fastened with the stud bolt that is one of crucial parts for safety of the reactor vessels in nuclear power plants. It is reported that the stud bolt is often experienced by fatigue cracks initiated at threads. Stud bolts are inspected by the ultrasonic technique during the overhaul periodically for the prevention of failure which leads to radioactive leakage from the nuclear reactor. The conventional ultrasonic inspection for stud bolts was mainly conducted by reflected echo method based on shadow effect. However, in this technique, there were numerous spurious signals reflected from every oblique surfaces of the thread. In this study, ultrasonic phased array technique was applied to investigate detectability of flaws in stud bolts and characteristics of ultrasonic images corresponding to different scanning methods, that is, sector and linear scan. For this purpose, simplified stud bolt specimens with artificial defects of various depths were prepared.
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