Hyunjo Jeong

Wavelet analysis of plate wave propagation in composite laminates

Abstract A new approach is presented for the analysis of transient waves propagating in composite laminates. The wavelet transform (WT) using the Gabor wavelet is applied to the time–frequency analysis of dispersive plate waves. It is shown that the peaks of the magnitude of WT in the time–frequency domain are related to the arrival times of group velocity. Experiments are performed using a lead break as the simulated acoustic emission source on the surface of quasi-isotropic and unidirectional graphite/epoxy laminates. For predictions of the dispersion of the flexural mode, Mindlin plate theory is shown to give good agreement with the experimental results. The planar source location based on the flexural wave is performed using a triangulation method. The use of frequency-dependent arrival time of output signal and angular dependence of group velocity provides accurate results of source location for anisotropic laminates.

Fracture source location in thin plates using the wavelet transform of dispersive waves

A new signal processing approach was presented for acoustic emission source location using the dispersive waves in a thin plate. For wave propagation in dispersive media, the accuracy of source location can be improved by using the arrival times of a single frequency component in the output signals at an array of sensors. The wavelet transform (WT) was used to resolve this problem. By utilizing the time-frequency data of the WT, the frequency-dependent arrival time traveling with the group velocity was shown to be easily determined. Experiments were performed using a lead break as the simulated fracture source on the surface of an aluminum plate. Two plate modes corresponding to the S/sub 0/ and A/sub 0/ Lamb waves were identified, and their group velocities were accurately measured. The source location results based on the WT method agreed well with the true locations. The WT method was also compared with the cross correlation technique, and both methods provide similar results.A new signal processing approach was presented for acoustic emission source location using the dispersive waves in a thin plate. For wave propagation in dispersive media, the accuracy of source location can be improved by using the arrival times of a single frequency component in the output signals at an array of sensors. The wavelet transform (WT) was used to resolve this problem. By utilizing the time-frequency data of the WT, the frequency-dependent arrival time traveling with the group velocity was shown to be easily determined. Experiments were performed using a lead break as the simulated fracture source on the surface of an aluminum plate. Two plate modes corresponding to the S(0) and A(0) Lamb waves were identified, and their group velocities were accurately measured. The source location results based on the WT method agreed well with the true locations. The WT method was also compared with the cross correlation technique, and both methods provide similar results.

Analysis of plate wave propagation in anisotropic laminates using a wavelet transform

A new approach is presented for the analysis of transient waves propagating in anisotropic composite laminates. The wavelet transform (WT) using the Gabor wavelet is applied to the time-frequency analysis of dispersive flexural waves in these plates. It can be shown that the peaks of the magnitude of WT in a time-frequency domain is related to the arrival times of the group velocity. Experiments were performed using a lead break as the simulated acoustic emission source on the surface of unidirectional and quasi-isotropic laminates. A method was developed to obtain the group velocity of the flexural mode as a function of frequency. Theoretical predictions were made using the Mindlin plate theory, which includes the effects of shear deformation and rotatory inertia. Our predictions on the dispersion of the flexural mode showed good agreement with the experimental results.

High-density mesh flow computations by building-cube method

With the conventional computational fluid dynamics (CFD) approaches like unstructured high-density mesh method, the problem about the solution dependencies on the grid and on the physical models isn’t completely resolved. In this study, thus, a new approach named Building-Cube Method based on a Cartesian mesh is proposed using a high-density mesh in order to solve the problem of the current CFD approaches. In the present method, a flow field is divided into a number of cubes (squares in 2D) of various sizes. Each cube is a computational sub-domain with Cartesian mesh of equal spacing and equal number of nodes. The geometrical size of each cube is determined by adapting to geometry and flow features so as to cope with broadband characteristic lengths of the flow. Equal spacing and equal number of Cartesian mesh in each cube make it easy to parallelize the flow solver and to handle huge data output. The method is applied to several airfoils including NACA0012 and two-element airfoils at relatively low Reynolds number, RAE2822 airfoil with transition trip at high Reynolds number and a four-element airfoil at high Reynolds number. These results validate the capability of the present approach.

A nondestructive method for estimation of the fracture toughness of CrMoV rotor steels based on ultrasonic nonlinearity

The objective of this paper is to develop a nondestructive method for estimating the fracture toughness (K(IC)) of CrMoV steels used as the rotor material of steam turbines in power plants. To achieve this objective, a number of CrMoV steel samples were heat-treated, and the fracture appearance transition temperature (FATT) was determined as a function of aging time. Nonlinear ultrasonics was employed as the theoretical basis to explain the harmonic generation in a damaged material, and the nonlinearity parameter of the second harmonic wave was the experimental measure used to be correlated to the fracture toughness of the rotor steel. The nondestructive procedure for estimating the K(IC) consists of two steps. First, the correlations between the nonlinearity parameter and the FATT are sought. The FATT values are then used to estimate K(IC) using the K(IC) versus excess temperature (i.e., T-FATT) correlation that is available in the literature for CrMoV rotor steel.

Anisotropic conductivities of multiphase particulate metal-matrix composites

Abstract A theoretical model has been developed to investigate the effective conductivity of a general multiphase composite with arbitrarily oriented constituent phases. The theory utilizes the concept of orientation-dependent average fields and concentration factors to provide a closed-form expression for the anisotropic conductivity. The concentration factors were evaluated by the equivalent-inclusion method for steady-state heat conduction and the Mori–Tanaka mean-field theory. Aluminum-matrix composites reinforced with SiC particles (SiC p ) were fabricated and their electrical conductivities measured by eddy-current techniques. On the basis of microstructural information available from metallographic analysis of these composites, a two-phase model was first considered to account for the effects of the matrix and reinforcement phases. The effects of the intermetallic phase formed during the processing stage were then accounted for in a three-phase model. Comparisons of theoretical predictions with experimental results showed good agreement. The three-phase model was found to be more effective in estimating the effective conductivities particularly when a large amount of intermetallic is present in the composite.

Significance of accurate diffraction corrections for the second harmonic wave in determining the acoustic nonlinearity parameter

The accurate measurement of acoustic nonlinearity parameter β for fluids or solids generally requires making corrections for diffraction effects due to finite size geometry of transmitter and receiver. These effects are well known in linear acoustics, while those for second harmonic waves have not been well addressed and therefore not properly considered in previous studies. In this work, we explicitly define the attenuation and diffraction corrections using the multi-Gaussian beam (MGB) equations which were developed from the quasilinear solutions of the KZK equation. The effects of making these corrections are examined through the simulation of β determination in water. Diffraction corrections are found to have more significant effects than attenuation corrections, and the β values of water can be estimated experimentally with less than 5% errors when the exact second harmonic diffraction corrections are used together with the negligible attenuation correction effects on the basis of linear frequency de...

Finite-Element Analysis of Laser-Generated Ultrasounds for Wave Propagation and Interaction with Surface-Breaking Cracks

ABSTRACT A finite-element method was used to simulate the wave propagation of laser-generated ultrasound and its interaction with surface-breaking cracks in an elastic material. The thermoelastic laser line source on the material surface was approximated as a shear dipole and loaded as nodal forces in the plane-strain finite element (FE) model. The shear dipole FE model was tested for the generation of ultrasound on the surface with no defect. The model was found to generate the Rayleigh surface wave and to give the correct directivity patterns for longitudinal and shear bulk waves. The model was then extended to examine the interaction of laser-generated ultrasound with surface-breaking cracks of various depths. Both fixed and scanning laser sources (SLS) were considered. For the case of the fixed source, the crack-reflected Rayleigh waves were monitored to detect a crack. In the SLS technique, the laser-generated signal arriving at the Rayleigh wave speed was monitored as the laser source was scanned. The proposed model clearly reproduced the experimentally observed features that can be used to characterize the presence of surface-breaking cracks.

A Baseline-Free Defect Imaging Technique in Plates Using Time Reversal of Lamb Waves

We present an analytical investigation for a baseline-free imaging of a defect in plate-like structures using the time-reversal of Lamb waves. We first consider the flexural wave (A0 mode) propagation in a plate containing a defect, and reception and time reversal process of the output signal at the receiver. The received output signal is then composed of two parts: a directly propagated wave and a scattered wave from the defect. The time reversal of these waves recovers the original input signal, and produces two additional sidebands that contain the time-of-flight information on the defect location. One of the side-band signals is then extracted as a pure defect signal. A defect localization image is then constructed from a beamforming technique based on the time-frequency analysis of the side band signal for each transducer pair in a network of sensors. The simulation results show that the proposed scheme enables the accurate, baseline-free imaging of a defect.

Simultaneous evaluation of acoustic nonlinearity parameter and attenuation coefficients using the finite amplitude method

A novel method to determine acoustic parameters involved in measuring the nonlinearity parameter of fluids or solids is proposed. The approach is based on the measurement of fundamental and second harmonic pressures with a calibrated receiver, and on a nonlinear least squares data-fitting to multi-Gaussian beam (MGB) equations which explicitly define the attenuation and diffraction effects in the quasilinear regime. Results obtained in water validate the proposed method. The choice of suitable source pressure is discussed with regard to the quasilinear approximation involved. The attenuation coefficients are also acquired in nonlinear regime and their relations are discussed.