Papers by Keyword: Laser-Generated Ultrasound

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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: Eric B. Flynn, Gregory S. Jarmer, See Yenn Chong, Jung Ryul Le
Abstract: We describe a method to effectively image and characterize structural features and defects using local estimates of wavenumber for propagating guided Lamb waves at a fine grid of spatial sampling points. The guided waves are rapidly excited at each grid point using a scanning Q-switched laser system and sensed by a single fixed ultrasonic transducer. Through reciprocity, this produces a full-wave-field time history of a virtual wave being excited from the transducer.We first demonstrate the unique capability of the measurement system by showing frequency-wavenumber intensity diagrams that resolve up to 5th order guided Lamb wave modes. Well then show how, using frequency-wavenumber processing, localized wavelength estimates can be obtained by isolating each guided Lamb wave mode, extracting a specific frequency bin, and identifying the center-wavelength of a sliding wavenumber band-pass filter that maximizes the energy at each grid point. This procedure is repeated across all frequency bins in order generate estimates of the local dispersion curves at each spatial sample point on the structure. The approach was capable of producing detailed images of hidden wall-thinning in an aluminum plate and local impact delamination in a complicated composite component.
Authors: Won Su Park, Sang Woo Choi, Joon Hyun Lee, Kyeong Cheol Seo, Joon Hyung Byun
Abstract: For improving quality of a carbon fiber reinforced composite material (CFRP) by preventing defects such as delamination and void, it should be inspected in fabrication process. Novel non-contacting evaluation technique is required because the transducer should be contacted on the CFRP in conventional ultrasonic technique during the non-destructive evaluation and these conventional contact techniques can not be applied in a novel fiber placement system. For the non-destructive evaluation of delamination in CFRP, various methods for the generation and reception of laser-generated ultrasound are applied using piezoelectric transducer, air-coupled transducer, wavelet transform technique etc. The high frequency component of laser-generated guided wave received with piezoelectric sensor disappeared after propagating through delamination region. Air-coupled transducer was tried to be adopted in reception of laser-generated guided wave generated by using linear slit array in order to generate high frequency guided wave with a frequency of 1.1 MHz. Nevertheless, it was failed to receive high frequency guided wave in using air-coupled transducer and linear slit array. Transmitted laser-generated ultrasonic wave was received on back-wall and its frequency was analyzed to establish inspecting technique to detect delamination by non-contact ultrasonic method. In a frequency spectrum analysis, intensity ratio of low frequency and center frequency was approvable parameter to detect delamination.
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