Influence of transparent coating thickness on thermoelastic force source and laser-generated ultrasound waves

A numerical model is established to investigate the influence of transparent coating thickness on the laser-generated thermoelastic force source and ultrasound waves in the coating-substrate system by using the finite element method (FEM). Taking into account the effects of thermal diffusion, the finite width and duration of the laser source, as well as the temperature dependence of material properties, the transient temperature distributions are obtained firstly. Applying this temperature field to structure analyses as thermal loading, the thermoelastic stress field and laser-generated ultrasound wave in the specimen are obtained. The generation and propagation of the laser thermoelastically induced stress field and ultrasonic waves in coating-substrate system are presented in detail. The influence of transparent coating thickness on the transient temperature distribution, thermoelastic force source and the laser-generated ultrasound waveforms is investigated. The numerical results indicate that the thermoelastic force source and laser-generated ultrasound waveform are strongly affected by the coating thickness due to the constraint of coating. This method can provide insight into the generation and propagation of the laser-generated stress field in coating-substrate system consisting of a transparent coating and an opaque metallic substrate. It provides theoretical basics to optimize ultrasonic signal generation in particular applications and invert the physical and geometrical parameter of the coating-substrate system more accurately in the experiment.     

Numerical modeling of thermoelastic generation of ultrasound by laser irradiation in the coupled thermoelasticity

Laser-generation of ultrasound is investigated in the coupled dynamical thermoelasticity in the presented paper. The coupled heat conduction and wave equations are solved using finite differences. It is shown that the application of staggered grids in combination with explicit integration of the wave equation facilitates the decoupling of the solution and enables the application of a combination of implicit and explicit numerical integration techniques. The presented solution is applied to model the generation of ultrasound by a laser source in isotropic and transversely isotropic materials. The influence of the coupling of the generalized thermoelasticity is investigated and it will be shown, that for ultra high frequency waves (i.e. 100 GHz) generated by laser pulses with duration in the picosecond range, the thermal feedback becomes considerable leading to a strong attenuation of the longitudinal bulk wave. Moreover, the coupling leads to dispersion influencing the wave velocities at low frequencies. The numerical simulations are compared to theoretical results available in the literature. Wave fields generated by a line focused laser source are presented by the numerical model for isotropic and for transversely isotropic materials.     

Numerical simulation of evaluation of surface breaking cracks by array-lasers generated narrow-band SAW

Most of the factors limiting the extensive application of laser-based ultrasonic for nondestructive evaluation of surface breaking crack are its poor sensitivity, low efficiency relative to conventional contact ultrasonic methods and limit on the dimension of the cracks. For this reason, a new technique that multiplepulse narrow-band ultrasound generated by laser arrays has been proposed. It is found that crack detection dependent on spectrum of narrow-band ultrasound generated by laser arrays can be operated with low amplitude requirements. In this paper, the narrow-band ultrasound generated by pulse laser arrays interacting with surface breaking cracks has been simulated in detail by the finite element method (FEM) according to the thermoelastic theory. The pulsed array lasers were assumed to be transient heat source, and the surface acoustic wave (SAW) which propagating on the top of the plate was computed based on thermoelastic theory. Then the frequency spectrums of both reflected waves by crack and transmission ones through crack were compared with the direct waves. Results demonstrate that multiple-frequency components of the narrow-band ultrasound were varied with change of the depth of surface breaking cracks significantly, which provides the possibility for precise evaluation of surface breaking cracks.     

Interaction of a scanning laser-generated ultrasonic line source with a surface-breaking flaw

The scanning laser source (SLS) technique has been proposed recently as an effective way to investigate small surface-breaking cracks. By monitoring the amplitude and frequency changes of the ultrasound generated as the SLS scans over a defect, the SLS technique has provided enhanced signal-to-noise performance compared to the traditional pitch-catch or pulse-echo ultrasonic methods. In previous work, either a point source or a short line source was used for generation of ultrasound. The resulting Rayleigh wave was typically bipolar in nature. In this paper, a scanning laser line source (SLLS) technique using a true thermoelastic line source (which leads to generation of monopolar surface waves) is demonstrated experimentally and through numerical simulation. Experiments are performed using a line-focused Nd:YAG laser and interferometric detection. For the numerical simulation, a hybrid model combining a mass-spring lattice method (MSLM) and a finite difference method (FDM) is used. As the SLLS is scanned over a surface-breaking flaw, it is shown both experimentally and numerically that the monopolar Rayleigh wave becomes bipolar, dramatically indicating the presence of the flaw.     

Laser generation of narrow-band and directed ultrasound

An aluminium hemicylindrical sample has been irradiated with an array of laser lines, with each line acting as a source of acoustic waves. Detection of the generated ultrasonic waves was performed using both a wide-band stabilized Michelson interferometer and a 20 MHz piezoelectric transducer. Experimental and theoretical results are presented which reveal that the use of a spatially modulated laser source produces significant narrow-banding of the detected ultrasound, compared with a single point or single line source case. Additionally, for a given line spacing, ultrasound of a particular frequency can be directed. Owing to the nature of the acoustic signals generated by each individual array element, superposition of several signals does not result in any energy directivity similar to that encountered in phase electromagnetic array antennas. While time or frequency feature enhancement may be obtained in a desired direction, in most cases the far field energy directivity pattern is simply the incoherent sum of the energy directivity of each array element.     

Mass spring lattice modeling of the scanning laser source technique

The scanning laser source (SLS) technique is a promising new laser ultrasonic tool for the detection of small surface-breaking defects. The SLS approach is based on monitoring the changes in laser generated ultrasound as a laser source is scanned over a defect. Changes in amplitude and frequency content have been observed for ultrasound generated by the laser over uniform and defective areas. In this paper, the SLS technique is simulated numerically using the mass spring lattice model. Thermoelastic laser generation of ultrasound in an elastic material is modeled using a shear dipole distribution. The spatial and temporal energy distribution profiles of typical pulsed laser sources are used to model the laser source. The amplitude and spectral variations in the laser generated ultrasound as the SLS scans over a large aluminum block containing a small surface-breaking crack are observed. The experimentally observed SLS amplitude and spectral signatures are shown to be captured very well by the model. In addition, the possibility of utilizing the SLS technique to size surface-breaking cracks that are sub-wavelength in depth is explored.     

Numerical simulation of laser-generated ultrasound in non-metallic material by the finite element method

The use of a pulsed laser for the generation of the elastic waves in non-metallic materials in the thermoelastic regime is investigated by using finite element method (FEM), taking into account not only thermal diffusion and the finite spatial and temporal shape of the laser pulse, but also optical penetration and the temperature dependence of material properties. The optimum finite element model is established based on analysis of two important parameters, meshing size and time step, and the stability of solution. Temperature distributions and temperature gradient fields in non-metallic material for different time steps are obtained, this temperature field is equivalent to a bulk force source to generate ultrasonic wave. The laser-generated ultrasound waveforms at the epicenter and surface acoustic waveforms (SAWs) are obtained and the influence of optical penetration into the material on the temperature field and the ultrasound waveforms are analyzed. The numerical results indicate that the heat penetration into non-metallic material is caused mainly by the optical penetration, and the ultrasound waveforms, especially the shape of the precursor, are strongly dependent on the optical penetration depth into non-metallic material.     

Analysis of laser-generated ultrasonic force source at specimen surface and display of bulk wave in transversely isotropic plate by numerical method

Taking into account the effects of thermal diffusion and optical penetration, as well as the finite width and duration of the laser source, the laser-generated ultrasonic force source at surface vicinity is presented. The full acoustic fields of laser-generated ultrasonic bulk wave are obtained and displayed in transversely isotropic plate. The features of laser-generated ultrasound bulk waves are analyzed. The features of laser-generated ultrasonic bulk wave are in good agreement with the theoretical results (the phase velocity surfaces), demonstrating the validity of this simulation. The numerical results indicate that the features of laser-generated ultrasound waveforms in anisotropic specimen, different from the case in isotropic materials, have a close relation with the propagating plane and propagation direction. This method can provide insight to the generation and propagation of laser-generated ultrasonic bulk wave in transversely isotropic material.     

激光在管道中激发周向导波的有限元模拟

以超声的热弹激发机理为基础，建立了模拟激光在管道中激发周向导波的有限元模型.为了验证模型的正确性，根据Gao等人报道的实验条件［J. Appl. Phys. 91, 6114 (2002)］进行了模拟，理论模拟结果与Gao等人的实验结果符合很好，说明本数值模型的正确性.在此基础上，模拟激光在不同厚度、不同曲率半径的管道内激发的周向导波波形，同时分析了激光在管道中激发的周向导波波形与平板中激发Lamb波的差异，以及管道厚度和曲率半径对激光激发周向导波的影响.     

基于激光超声的微裂纹检测技术的研究

在固体中利用激光产生超声波 ,可作为超声测量和材料无损检测的一种新方法。介绍了激光超声表面波的产生机理、微裂纹的检测方法及其应用。采用Nd∶YAG脉冲激光器、扩束与聚焦透镜组、PZT探头、数据分析仪等器件设计并构建了一套基于接触式检测方法的激光超声微裂纹检测实验系统。通过对大量实验数据进行处理 ,得出了相应的各种关系曲线 ,说明了线光源产生的超声表面波非常适用于材料表面微裂纹的检测     

Non-contact optical fibre phased array generation of ultrasound for non-destructive evaluation of materials and processes

The use of non-contact laser techniques for the generation of ultrasound has extended the limits of the application of traditional ultrasonic techniques. This paper focusses on the use of one such non-contact laser technique, known as ‘optical fibre array’, to generate shear and surface waves. The shear wave experimental directivity pattern results are presented and compared with the theoretical results of a single source and an array source. The experimental directivity results for the surface wave are also presented, and compared with the theoretical results. The data show that the array enhances signal generation in the forward direction for both shear and surface waves. The array gain is also discussed. The receiver for the directivity measurements was a contact piezoelectric transducer.     

Laser-generated thermoelastic acoustic sources and Lamb waves in anisotropic plates

The effect of anisotropy and temperature on the dispersive Lamb wave generation and propagation in a transversely isotropic thin plate has been investigated. A quantitative numerical model for the laser-generated transient ultrasonic Lamb waves propagating along arbitrary directions is presented by using a finite-element method. All factors, such as spatial and time distributions of the incident laser beam, optical penetration, thermal diffusivity, thickness of the plate, and source–receiver distance, can be taken into account. The effects on the ultrasound waveform of the size of the optoacoustic source are investigated; in the limit of strong optical absorption, a subsurface thermal source gives rise to both vertical and lateral shear tensions. The lateral shear tension is equivalent to applying a shear dipole at the top face; the amplitude of the dipole is a function of material symmetry, contrary to the isotropic case, and the character and strength of the equivalent surface stress are a function of propagation direction. The specific results for the lower anti-symmetric and symmetric mode propagation in all planar directions are presented in the thermoelastic regime; the spatial dispersion (variation of the velocity with the direction of propagation) as well as the frequency dispersion is analyzed. PACS 43.35.+d; 02.70.Dh; 42.62.-b; 78.20.Nv; 81.70.Cv     

Experiments and molecular-dynamics simulation of elastic waves in a plasma crystal radiated from a small dipole source

The radiation of elastic waves from a localized source is observed experimentally in a two-dimensional plasma crystal. An initial shear stress applied by a laser forms a small dipole source. The emerging complex wave pattern is shown to consist of outgoing compressional and shear wave pulses. Subsequent structures are identified as inward-going waves due to the finite size of the source region, which reappear on the opposite side. The compressional wave forms a trailing wave train due to strong dispersion, while the nondispersive shear wave evolves into a vortex-antivortex pair on either side. The experiments are compared with a molecular-dynamics simulation.     

Scholte Wave at Air-Metal Interface Generated by a Pulsed Disc-Like Source

The normal displacement and pressure of Scholte and Leaky Rayleigh waves at air-metal interface generated by a pulsed disc-like source are simulated theoretically by the Cagniard-de Hoop method and studied by laser ultrasound technique experimentally. It is found that the Scholte wave detected by a photorefractive interferometer is mainly contributed by the surface pressure and the Leaky Rayleigh wave is dominated by the surface displacement. It is also proven that the pulse width of these interface waves is mainly determined by the acoustic time delay on the generating source size under our experimental conditions.     

Numerical simulation of laser-generated surface acoustic waves in the transparent coating on a substrate by the finite element method

The optimum finite element model in the system consisting of a transparent coating and an opaque substrate is established based on the analysis of two important parameters: meshing size and time step, and the stability of solution. Taking into account the temperature dependence of material properties, the transient temperature and temperature gradient field are obtained. According to the thermoelastic theory, this temperature gradient field can be taken as a buried bulk source to generate ultrasonic wave. The surface acoustic waves (SAWs) are obtained. The influence of the coating thickness on the SAWs is analyzed. The model provides a useful tool for the determination of modes which are generated by a laser source in transparent coating on opaque substrate. The surface skimming longitudinal wave exists for the multiple oscillations and it charges from unipolar waveforms to dipolar.     

激光超声检测带过渡圆角平板表面缺陷的数值研究

为了研究激光激发出的超声波在带过渡圆角的金属平板上的传播规律和检测表面缺陷的方法，采用有限元法模拟了该类平板中的激光超声现象，分析了表面波在圆角区域的传播规律和与表面缺陷的作用过程。数值结果表明:激光激发出纵波、横波和Rayleigh波等，其中Rayleigh波主要存在于表面mm量级，并且在过渡圆角处发生模式转换生成了直达波R′和模式转换波R_R等多种表面波；经过过渡区域后的声波在表面缺陷处发生了反射和透射现象，通过B扫图可以检测缺陷的位置。随着缺陷深度的增加，表面波的透射系数不断减小，且透射波R_t和R_st存在0.5 μs左右的到达时间差，该时间差与缺陷深度近似成线性正相关。数值结果为激光超声检测带过渡圆角的平板表面缺陷提供了有价值的参考。     

Finite element modeling of the interaction of laser-generated ultrasound with a surface-breaking notch in an elastic plate

The interaction of surface acoustic waves generated by laser line source in the thermoelastic regime with surface notches are investigated. The finite element method is used to establish the model of the transient displacement field for surface notches with various depths and orientation. The magnitude of the signal enhancement in the near field and the mechanism by which this occurs are explained. The positions of notches were evaluated by the reflected Rayleigh wave. The depths and orientations of the notches were also determined using a shear wave that was generated through mode conversion of a surface acoustic wave at the notch tip. The results agree with previously published experimental measurements.     

Numerical simulation of the ultrasonic waves generated by ring-shaped laser illumination patterns

The generation of ultrasound in aluminum plate subjected to ring-shaped laser beam illumination has been studied quantitatively by using the finite element method. The superposition effects of surface acoustic waves on the top surface and the bulk ultrasonic waves on the rear surface of specimen have been obtained in a single simulation. The typical displacement profiles of the bulk ultrasonic wave at various depths along the central axis of the ring are obtained for three different radii, and the effect of the ring radius on the focal depths of the compression and shear mode are determined. The numerical results confirm that the focal depth of a bulk acoustic mode is determined by the directivity patterns of the acoustic mode generated by point-like laser sources via a thermoelastic mechanism, which depends on the physical constants of elastic medium.     

Effect of optical penetration on laser generatedthermoelastic ultrasound in solids

I.IntroductionTbegenerationofu1trasoundbytheirradiationofpulsedlaseratso1idsurfacehasbeenwidelystlldied,boththeorctica1lyandexpcrimentallyI1-8].Intheprocessoflaserthermoelasticg6nerationofu1trasound,temperatureriseinduccdbytheabsorptionoflaserenergyproducestherma1expansion,andthcnanultrasonicsourceiscreated.Sofar,thestudyof1asergenera-honofultrasoundinso1idsconcentratesmain1yonmeta1s,anduntilrecent1y,1ittleattentionhasbenpaidtonon-mctals.Inametal,1aserenergyisabsorbedonlyatthesurfaceofthesamp…     

Non-destructive evaluation of composite materials with ultrasonic waves generated and detected by lasers

Measurement of the stiffness properties of composite materials with laser generated and detected ultrasound requires proper understanding of waves emanating from a line or point source in anisotropic and viscoelastic media. The paper briefly presents calculation results of waves radiated by such a source through or at the surface of a composite plate. Dispersion is represented as well as the multiple wave arrivals connected with the folded shape of the quasi-shear ray surface. Moreover, internal diffraction at the cusp edges is properly depicted. An identification method with specific signal processing have been used to measure the stiffness coefficients of composite materials. From group velocity data, the stiffness tensor of materials showing an orthorhombic symmetry can be identified. The stiffness tensor changes induced by elevated temperatures in a composite material were then measured. An alternative approach was developed which allows to measure the phase velocities of waves generated with laser line sources. The material characterisation reliability is then improved. Moreover, the method can be used in practical cases where the front side of the structure only is accessible with the experimental devices. Despite reflection at the rear interface of transient divergent waves which ray surfaces may contain caustics, this inverse problem can be solved in a simple and efficient manner.