Jin-Chul Hong

The matching pursuit approach based on the modulated Gaussian pulse for efficient guided-wave damage inspection

The success of the guided-wave damage inspection technology depends not only on the generation and measurement of desired waveforms but also on the signal processing of the measured waves, but less attention has been paid to the latter. This research aims to develop an efficient signal processing technique especially suitable for the current guided-wave technology. To achieve this objective, the use of a two-stage matching pursuit approach based on the Gabor dictionary is proposed. Instead of truncated sine pulses commonly used in waveguide inspection, Gabor pulses, the modulated Gaussian pulses, are chosen as the elastic energy carrier to facilitate the matching pursuit algorithm. To extract meaningful waves out of noisy signals, a two-stage matching pursuit strategy is developed, which consists of the following: rough approximations with a set of predetermined parameters characterizing the Gabor pulse, and fine adjustments of the parameters by optimization. The parameters estimated from measured longitudinal elastic waves can be then directly used to assess not only the location but also the size of a crack in a rod. For the estimation of the crack size, in particular, Loves theory is incorporated in the matching pursuit analysis. Several experiments were conducted to verify the validity of the proposed approach in damage assessment.

Dispersion-based short-time Fourier transform applied to dispersive wave analysis

Although time-frequency analysis is effective for characterizing dispersive wave signals, the time-frequency tilings of most conventional analysis methods do not take into account dispersion phenomena. An adaptive time-frequency analysis method is introduced whose time-frequency tiling is determined with respect to the wave dispersion characteristics. In the dispersion-based time-frequency tiling, each time-frequency atom is adaptively rotated in the time-frequency plane, depending on the local wave dispersion. Although this idea can be useful in various problems, its application to the analysis of dispersive wave signals has not been made. In this work, the adaptive time-frequency method was applied to the analysis of dispersive elastic waves measured in waveguide experiments and a theoretical investigation on its time-frequency resolution was presented. The time-frequency resolution of the proposed transform was then compared with that of the standard short-time Fourier transform to show its effectiveness in dealing with dispersive wave signals. In addition, to facilitate the adaptive time-frequency analysis of experimentally measured signals whose dispersion relations are not known, an iterative scheme for determining the relationships was developed. The validity of the present approach in dealing with dispersive waves was verified experimentally.

Waveguide damage detection by the matching pursuit approach employing the dispersion-based chirp functions

If the wave mode used in guided wave non-destructive inspection is dispersive, reflected pulses from damaged parts may be significantly distorted due to wave dispersion. The main concern, in this case, is how to detect the reflected pulses in noisy signals, and to extract meaningful damage information from the detected pulses. However, current signal processing techniques used for guided wave inspection do not account for pulse dispersion, so the extracted information is often not so accurate. The objective of this study is to develop an efficient technique to deal with dispersed pulses for guided-wave nondestructive evaluation. Our idea is to model dispersed pulses by chirp functions of special form that can simulate up to quadratically varying group delay. To determine the parameters of the chirp functions approximating dispersed, reflected pulses, an adaptive matching pursuit algorithm is employed. Once the characterizing parameters are found, the damage location and extent can be estimated. The proposed method is tested with experimentally measured signals of longitudinal waves in a circular cylinder.

The Mathematical Model on Crosstalk Effect of Airborne Noise Sources and Verification based on Comparison between Transfer Path Analysis Methods

In this paper, the mathematical model on crosstalk effect between acoustic noise sources is presented. Based on this model, a theoretical value to quantitate crosstalk effect of sources can be defined on the authority of reverberation property of surrounding which sources are located, relative magnitudes of sources and phase deference between acoustic wave transferred from all of sources. In order to verify crosstalk effect factor, experiments are made for two cases, weak and strong crosstalk effect condition, by using toro representative transfer path analysis(TPA) methods.

The Use of Vibro-acoustical Reciprocity to Estimate Source Strength and Airborne Noise Synthesis

In this paper, an alternative method was introduced to conduct a transfer path analysis for airborne noise. The method used the transfer function matrix composed of acoustic transfer functions that are referenced by the input voltage of a calibration source. A calibration factor which is converting a virtual voltage to source strength was deduced by vibro-acoustical reciprocity theorem. The calibration factor is then multiplied to the virtual input voltage to estimate the operational source strength. Three loudspeakers were used to noise sources of acrylic half car model. The method was applied to airborne noise transfer path analysis of the half car. The estimated source strength by transfer path analysis was compared the deduced source strength by vibro-acoustical reciprocity to verify the method.

Matching Pursuit Approach for Guided Wave-based Damage Inspection

For successful guided-wave damage inspection, the appropriate signal processing of measured wave signals is very important. The objective of this paper is to introduce an efficient signal processing technique especially suitable for the guided-waves used for damage detection. The key idea of this technique is to model guided-waves by chirp functions of special form considering the dispersion phenomenon. To determine the parameter of the chirp functions simulating guided-waves, the matching pursuit algorithm is employed. The damage information in waveguides can be extracted by pulse-characterizing parameters. The effectiveness of present method is checked with the guided wave-based damage inspection.

Waveguide damage detection by the matching pursuit using the Gaussian chirp pulses

Guided-waves have been widely used for the long-range nondestructive health inspection of various waveguides. Though nondispersive waves are best for damage detection, transmitted waves are often dispersed as they travel a long distance. The objective of this work is to develop an advanced technique to deal with dispersed pulses for guided-wave nondestructive evaluation. To this end, chirp functions are proposed to simulate dispersed pulses. The parameters of the chirp functions simulating dispersed pulses are determined by the matching pursuit method. Then, the damage location can be directly estimated from the parameter characterizing the pulse arrival time. The performance of the present approach is checked by the actual damage detection in a cracked cylinder.

The optimal selection of mother wavelet shape for the best time-frequency localization of the continuous wavelet transform

The continuous wavelet transform (CWT) has been utilized as an effective and powerful time-frequency analysis tool for identifying the rapidly-varying characteristics of some dispersive wave signals. Particularly, in the applications of continuous Gabor wavelet transform, its effectiveness is strongly influenced by the shape of the applied Gabor wavelet so the determination of an optimal shape tracing well the time-frequency evolution of a given signal. Since the characteristics of signals are rarely known in advance, the determination of the optimal shape is usually difficult. The main objective of this work is to propose a method to determine the signal-dependent optimal shape of the Gabor wavelet for the best time-frequency localization. To find the optimal Gabor wavelet shape, the notion of the Shannon entropy which measures the extent of signal energy concentration in the time-frequency plane, is employed. To verify the validity of the present approach, a set of elastic bending wave signals generated by an impact in a solid cylinder are analyzed.