Baiqiang Xu

Numerical simulation of laser-generated ultrasound by the finite element method

The results obtained from the finite element model of laser-generated ultrasound are presented in terms of temperature and displacement. According to thermoelastic theory, considering the temperature dependence of the thermophysical parameters of the material, the transient temperature field can be precisely calculated by using the finite element method; then, laser-generated surface acoustic wave forms are calculated in Al plates of various thicknesses. The elastic waves excited by a pulsed laser in a thin plate are typical Lamb waves, and the numerical results demonstrate that the surface vibration is mainly determined by the lower frequency components of the symmetric mode s0 and antisymmetric mode a0 of the lowest order in very thin plate materials. It is also indicated that, when the sample thickness increases, both the higher frequency components of the lower Lamb wave modes and the higher order Lamb wave modes should be considered. In a relatively thicker plate, the numerical model can still captur...

Finite element model of laser-generated surface acoustic waves in coating-substrate system

The generation of ultrasound in coating-substrate systems subjected to laser beam illumination has been studied quantitatively by using the finite element method. Taking into account the temperature dependence of material properties, the transient temperature and temperature gradient field can be obtained in different coating-substrate systems. According to the thermoelastic theory, these temperature gradient fields are taken as bulk sources to generate ultrasound in coating-substrate systems. The typical surface acoustic waves (SAWs) in systems, a slow coating on a fast substrate and a fast coating on a slow substrate, are obtained. The influence of the propagation distance and the coating thickness on the SAWs is analyzed.

Numerical simulation of laser-induced transient temperature field in film-substrate system by finite element method

The transient temperature fields generated by a pulsed laser in film-substrate system are obtained by using the finite element method. Time integrations of the semi-discrete finite element equations are achieved by using approximate one order derivative of temperature. The temperature dependences of material properties are taken into account, which has a great influence on the temperature fields indicated by the numerical results. The pulsed laser-induced transient temperature fields in aluminum/methyl-methacrylate system and aluminum/copper system are obtained, which will be useful in the research on thermoelastic excitation of laser ultrasonic waves in film-substrate system.

Thermal stability and electrical properties of titanium-aluminum oxide ultrathin films as high-k gate dielectric materials

As potential high-k gate dielectric material for metal-oxide-semiconductor field effect transistor, the TiO2 incorporated Al2O3 [(TiO2)x(Al2O3)1−x (TAO)] films have been prepared by pulsed laser deposition. The thermal stability and electrical properties of the TAO films annealed at different temperatures have been systematically investigated by x-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and capacitance-voltage measurements. These results show that the TAO films keep amorphous state after annealed at 900°C, and the interfacial reaction between the TAO film and silicon substrate is closely related to the deposition and postannealing temperatures. The dielectric constant of the film is about 30. For a 5nm TAO film after annealed at 400°C, the thickness between interfacial layer and the silicon substrate is only two atomic layers (about 0.6nm) and a small equivalent oxide thickness (1.2nm) is achieved. A possible explanation for interfacial interaction has been propos...

Thermoelastic finite element modeling of laser generation ultrasound

Thermoelastic finite element modeling of laser-generated ultrasound in aluminum plates is presented based on a numerical formulation for the transient response in terms of the characteristics of the source of the thermoelastic waves. The model accounts for the effects of thermal diffusion, as well as the finite width and duration of the laser source. The stress can be related to the laser energy and material properties. The numerical results indicate that the temperature dependence of the thermophysical parameters has a significant influence on the laser-generated surface-acoustic waves with high frequencies.

Numerical simulation of laser-generated ultrasonic waves in layered plates

Thermo-elastic laser-generated ultrasound in layered plates has been studied using the finite element method, after considering the temperature dependence of the thermo-physical parameters of the materials. It is confirmed that the temperature dependence of the thermo-physical parameters has a significant influence on thermo-elastically generated ultrasound with high frequencies. Numerical results calculated in two kinds of two-layer system indicate that the features of the surface acoustic wave (SAW) generated are dependent on the thickness of the surface layer. Numerical results from three-layer systems indicate that the skimming surface longitudinal wave is sensitive to the thickness of the surface layer, while the SAW is much more sensitive to the thickness of the bonding layer, and it can be used to extract the thickness parameter of the bonding layer in an effective way.

Numerical simulation of the reflected acoustic wave components in the near field of surface defects

The interaction of a laser generated surface acoustic wave with a surface crack has been simulated in detail by the finite element method, where a surface notch of rectangular shape has been introduced to represent the fatigue crack for the convenience of modelling. It is shown that four distinct reflected components are present in the captured waveforms; the first component is the direct reflection of a compressive pulse from the left side of the notch, while the second one is assumed to be the Rayleigh wave converted from the laser generated compressive wave when it impacts the left face of the notch. The arrival time of the third peak has demonstrated clearly that it comes from the direct reflection of an initial Rayleigh wave on the near side of the notch. The origin of the last component has been assumed to be the mode conversion occurring at the base of the notch, which is based on the fact that the deeper the notch the longer the arrival time of the fourth peak.

Influence of viscoelastic property on laser-generated surface acoustic waves in coating–substrate systems

Taking account of the viscoelasticity of materials, the pulsed laser generation of surface acoustic waves in coating–substrate systems has been investigated quantitatively by using the finite element method. The displacement spectra of the surface acoustic waves have been calculated in frequency domain for different coating–substrate systems, in which the viscoelastic properties of the coatings and substrates are considered separately. Meanwhile, the temporal displacement waveforms have been obtained by applying inverse fast Fourier transforms. The numerical results of the normal surface displacements are presented for different configurations: a single plate, a slow coating on a fast substrate, and a fast coating on a slow substrate. The influences of the viscoelastic properties of the coating and the substrate on the attenuation of the surface acoustic waves have been studied. In addition, the influence of the coating thickness on the attenuation of the surface acoustic waves has been also investigated ...

Numerical simulation of laser-generated Lamb waves in viscoelastic materials by finite element method

The effect of material viscoelasticity on the propagation of Lamb wave in a viscoelastic thin plate has been investigated quantitatively. Based on the plane strain theory, a numerical model of the laser-generated Lamb wave is developed in the frequency domain by using the finite element (FE) method. To verify the correctness of the FE model in the frequency domain, a classical FE model in the time domain is established for an elastic plate. The transient waveforms calculated in the frequency domain are in very good agreement with those obtained in the time domain. And then, the laser-generated Lamb waves in the elastic and viscoelastic thin plates are calculated, respectively. By comparing the waveforms of Lamb wave in the two plates, the influence of the material viscoelasticity on the propagation of Lamb waves is studied. Moreover, the effect of the material viscoelastic rate on Lamb waves is analyzed in detail.

Finite Element Analysis of the Scanning Laser Line Source Technique

A scanning laser line source (SLLS) technique is simulated numerically using a finite element method in this study, and changes in amplitude and frequency content have been observed for ultrasound signals generated by laser scanning over a large aluminum block containing a small surface notch. The laser generation of surface acoustic waves with the frequency range from 4 to 15 MHz in an elastic material is modeled using a thermoelastic model for finite element analysis, where a transient heat source is employed to represent a pulsed laser source. The experimentally observed SLLS amplitude and spectral signatures are shown to be captured very well by this model. In addition, the possibility of utilizing the SLLS technique to size surface notches that are sub-wavelength in depth is explored.