Papers by Author: Min Rae Lee

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: 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.

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.