Jeong-Ki Lee - Academia.edu (original) (raw)

Papers by Jeong-Ki Lee

The elastic waves in a plate are dispersive waves due to the characteristics of Lamb waves. Howev... more The elastic waves in a plate are dispersive waves due to the characteristics of Lamb waves. However, S0 symmetric mode is less dispersive in the frequency region below the first cut-off frequency. The wave Propagation velocities vary with the direction in anisotropic plates such as Carbon Fiber Reinforced Plastic (CFRP) Plates. The wave vector direction and energy flow vector direction are same in isotropic plates. However, the wave vector direction same as the phase velocity direction is not in accordance with the energy flow direction same as the group velocity direction in anisotropic plates. In this study. the dispersion curves or the phase velocity from anti-symmetric and symmetric Lamb wave dispersion equation are calculated for unidirectional laminated composite plate. Slowness surface is sketched using phase velocity under the first cut-off frequency. The direction and magnitude of group velocity are corrected with this slowness surface. The measured group velocities are in ...

Measurement Science & Technology - MEAS SCI TECHNOL, 2003

A new method is proposed for the simultaneous measurement of the ultrasonic wave velocity and the... more A new method is proposed for the simultaneous measurement of the ultrasonic wave velocity and the thickness of a solid plate with access from only one side. A line-contact transmitter and a line-contact receiver were placed on the same side of the specimen, and the times of flight were measured for varying distance between the transducers. The ultrasonic wave velocity and the thickness of the specimen can be simultaneously determined from the relationship between the time of flight and the ultrasonic beam path. A least-squares fitting routine was employed to increase the accuracy of the measurements. Experimental results for several specimens of different materials as well as different thicknesses show good agreement with those measured by conventional methods.

Welding in the World

Offshore chain may have the flaws such as the lack of fusion in the weld. The ultrasonic testing ... more Offshore chain may have the flaws such as the lack of fusion in the weld. The ultrasonic testing (UT) has performed to detect the lack of fusion in the weld. The conventional UT has been performed by the pulse-echo and/or tandem method; the probe is positioned on the straight part with cylindrical surface but has not completely secured the reliability of the flaw detection in the offshore chain. Phased array ultrasonic testing (PAUT) can generate the ultrasonic focused beam and sweep the ultrasonic beam with multiple angles, so it is possible to inspect the wide area on a fixed probe position. Therefore, PAUT can inspect the full area of the weld in the offshore chain as moved with single array probe in a finite area. The phased array probe is positioned off the straight part with the spherical surface. And, the procedure of PAUT and the criteria of the detection and sizing of the flaw are not established. In this study, ultrasonic beam generated with array probe in offshore chain is simulated with CIVA simulation software, and sound pressure distribution is analyzed for establishing the criteria of the flaw sizing.

The elastic waves in the isotropic plate are dispersive waves with the characteristics of Lamb wa... more The elastic waves in the isotropic plate are dispersive waves with the characteristics of Lamb wave, however, S0 symmetric mode is less dispersive in the frequency region less than the first cut-off frequency. In the anisotropic plates such as CFRP plates, the propagation velocities vary with the directions as well as the dispersion of the Lamb wave, and the phase velocity direction does not accord with the group velocity direction. The phase velocity direction is equivalent the wave vector direction, while the group velocity direction is equivalent the energy flow direction. In this work, the group velocity dispersion curves were obtained by the dispersion relation of the Lamb wave in unidirectional CFRP plate with an orthotropic structure. The group velocities of the S0 symmetric mode in the frequency region less than the first cut-off frequency were corrected by applying the slowness surface. The propagation velocities of Lamb wave were decided by measuring the arrival time of the Lamb wave signals received with the two pinducers varying the propagating direction in the laminated unidirectional CFRP plates of 8, 16 and 24 plies having a volume fraction of 67%. The measured velocities are better agreement with corrected group velocity curve, except near the fiber direction at the cusp region. When the propagating direction is not accorded with the principal axis, the direction of the group velocities inclines toward the fiber direction in the unidirectional CFRP plates, suggesting that the energy propagates preferentially toward fiber direction.

The conventional eddy current testing uses a sinusoidal signal with very narrow frequency bandwid... more The conventional eddy current testing uses a sinusoidal signal with very narrow frequency bandwidth. Whereas, the pulsed eddy current method uses a pulse signal with a broad frequency bandwidth. This allows multi-frequency eddy current testing, and the penetration depth is greater than that of the conventional eddy current testing. In this work, the pulsed eddy current instrument was developed for evaluating the metal loss. The developed instrument was consist of the pulse generator generating the square pulse of maximum 40 V, the amplifier controlled to 52dB, the A/D converter of 16bit 20MHz, and the industrial personal computer for operating with Windows program. And, the probe for the pulsed eddy current was designed as the pancake type in which the sensing coil was located in the driving coil. The peak voltage did not linearly increase with the voltage of the step pulse. For the driving coil with inductance of 670µH, the peak voltage linearly increased with the step pulse voltage to 30V. But, for the other driving coils with the inductance of 1.7mH, 2.7mH, 3.6mH, 22mH, the peak voltage linearly increased with the step pulse voltage to 20V. The output signals of the sensing coil rapidly increased when the step pulse driving voltage was off, and the latter part of the sensing coil output voltage exponentially decreased with a time. The decrement value of the output signals of sensing coil increased with the thickness of the aluminum test piece.

Experimentally measured Lamb wave group velocities in composite materials with anisotropic charac... more Experimentally measured Lamb wave group velocities in composite materials with anisotropic characteristics are not the same as the theoretical group velocities which is calculated with the Lamb wave dispersion equation. This discrepancy arises from the fact that the angle between the group velocity direction and the phase velocity direction in anisotropic materials exists. Wave propagation in a composite material with anisotropic characteristics should be considered with respect to magnitude correction in addition to direction correction. In this study, S0 mode phase velocity dispersion curves are depicted with the variation of degree with respect to the fiber direction using a Lamb wave dispersion relation in the unidirectional, bidirectional, and quasi-isotropic composite plates. Slowness surface is sketched by the reciprocal value of the phase velocity curves. The magnitude and direction of the group velocity could be calculated from the slowness surface. The recalculated group velocities with consideration of the magnitude and direction from the slowness surface are compared with experimentally measured group velocities. The proposed method shows good agreements with theoretical and experimental results.

Key Engineering Materials, 2007

The group velocity of the S0 mode of a Lamb wave under the first cut-off frequency is measured du... more The group velocity of the S0 mode of a Lamb wave under the first cut-off frequency is measured during the tensile loading for the unidirectional composite materials. The group velocity of the S0 mode has the characteristic of increasing with the strain. The propagation velocity of a wave is generally proportional to the square root of the ratio of the modulus to the density. The elastic modulus is considered as a constant as the stress linearly increases with the strain. The increase of the group velocity with the strain is caused by a decrease in the density. It is proposed that the measurement of the group velocity of the S0 mode is useful for making an evaluation of the tensile strain and of any degradation in the unidirectional composite materials.

Key Engineering Materials - KEY ENG MAT, 2005

Measurement Science and Technology, 2003

Journal of Materials Science, 1995

The effects of probe and pumping beam size and modulation frequency on photoreflectance were inve... more The effects of probe and pumping beam size and modulation frequency on photoreflectance were investigated for a silicon wafer by considering one- and three-dimensional generation and propagation of thermal and plasma waves,PR1D andPR3D. The magnitude ofPR1D decreased as the inverse square of the effective beam radius and that ofPR3D was 100 times smaller thanPR1D at 0.1 µm effective beam radius

Japanese Journal of Applied Physics, 1999

The scattered ultrasonic energy from a liquid/solid interface at the Rayleighangle, Es, was theor... more The scattered ultrasonic energy from a liquid/solid interface at the Rayleighangle, Es, was theoretically derived by considering the interactionbetween the energy of the penetrated Rayleigh wave, E0, and themicrostructure in the subsurface within the Schoch displacement,Deltas, as Es = 0.25 alphas Deltas E0, where alphas isthe attenuation coefficient due to scattering.The backscattered ultrasonic energy, EBs, at the Rayleigh angle was alsoderived as EBs ∝D3 f3 in the Rayleigh scattering region andEBs ∝D f in the stochastic scattering region, where D is theaverage grain size and f is the frequency.The grain size and the frequency dependence of the backscattered ultrasonicenergy at the Rayleigh angle were obtained by both integration and powerspectrum analysis of the backscattered signals from the interface between thewater/stainless steel plates with the average grain size varying between 5.6and 40.5 µm.The exponents of the grain size and the frequency dependence obtained by bothmethods were about 3.5 and 2 in the region of lambdaR >=6 piD andlambdaR <=6 piD, respectively.The deviation of the exponents is attributed to the direct scattering from thesurface irregularities in the region of lambdaR >=6 piD and thestochastic scattering due to grain size distribution in the region oflambdaR <=6 piD.This implies that the exponents depend on the ratio of the average size of thescatterer to the wavelength of the Rayleigh wave.The results suggested that the backscattered ultrasonic wave at the Rayleighangle was built up predominantly by the scattering from the grain boundaries inthe subsurface within the Schoch displacement and the direct scattering fromthe irregularities on the surface.

Japanese Journal of Applied Physics, 2005

A novel method is proposed for the simultaneous measurement of longitudinal and transverse wave v... more A novel method is proposed for the simultaneous measurement of longitudinal and transverse wave velocities and the thickness of isotropic solid plates from access on only one side. Line contact transducers were placed on the same side of the specimen, and waveforms were acquired at various distances between the transducers. Longitudinal wave velocity and the thickness of the specimen can

NDT & E International, 2011

In this work, we propose a crossed-coil magnetostrictive sensor consisting of a solenoid coil, a ... more In this work, we propose a crossed-coil magnetostrictive sensor consisting of a solenoid coil, a toroidal coil, and a ferromagnetic patch for generating and detecting torsional waves for pipe inspection. We show that the direction of the magnetic field of the ferromagnetic patch can be controlled by adjusting the input current to the toroidal coil. In addition, it is found that the amplitudes of the signals reflected from the pipe end using a crossed-coil sensor are much larger than those using pre-magnetization techniques, and excitation energies are used to generate the torsional waves only. From results on pipes with multiple circumferential notches, any defects larger in diameter than three times the pipe thickness were detected using the crossed-coil magnetostrictive sensor.