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The generation of acoustic waves by a line-focused laser pulse in an optically absorptive cylinder is studied experimentally and theoretically. Experiments are performed on a 5 mm diameter NG5 colored glass rod using Nd:yttrium aluminum garnet laser, which delivers 5 ns pulses. The numerical simulation is based on the semi-analytical model of a radially distributed thermoelastic source, which takes into account penetration of laser energy into the bulk of the sample. Good agreement between the experimental and calculated wave forms is observed. Comparison of these wave forms with an auxiliary simulation, which assumes the model of a dipole source located at the cylinder surface, reveals the effect of optical penetration on the shape of the wave form and also on the relative amplitude of bulk and surface waves. [ABSTRACT FROM AUTHOR]

A point-source-point-receiver technique, based on laser generation and laser detection of acoustic waves, allows determination of mechanical properties of anisotropic cylinders. The anisotropic nature of the material and the geometry of the samples make the acoustic signature difficult to interpret. In addition to multiple surface waves, quasi-longitudinal and quasi-shear bulk waves are diffracted and acoustic rays are reflected with or without mode conversion at the cylinder surface. Moreover both bulk and surface diffracted waves have a dispersive behavior. To bypass the intricacies, wave fronts are synthesized with signals provided by scanning a straight line on the cylinder with the laser point source. Conical waves propagating in the volume and helical waves propagating along the surface are then numerically produced. The recovery of the stiffness-tensor components is based on the inversion of the bulk waves, phase velocities. The method is presented and applied to signals simulated or experimentally recorded for a composite material. The five independent stiffness coefficients of the hexagonal symmetry are thus measured with waveforms provided by a single scan along the cylinder surface. The method provides a unique mean for the noncontact measurement of elastic properties of cylindrical parts. [ABSTRACT FROM AUTHOR]

A three-dimensional (3D) model is presented to predict the acoustic waves generated by a laser point pulse in a transversely isotropic cylinder. The Fourier series expansion and the two-dimensional Fourier transform are introduced to calculate the 3D transient response under either the ablation or the thermoelastic generation. The presented physical model and the numerical inverse scheme are applied to a fiber reinforced composite cylinder with a strong anisotropy. Experimental radial displacements of the cylinder surface are detected by the laser ultrasonic technique and analyzed by the ray trajectories for both generation regimes. Corresponding theoretical displacements are obtained numerically and compared to the experimental signals. Good agreement is found between theoretical and experimental results. The focusing effects that anisotropy gives rise to are observed in both theory and experiment under either regime. [ABSTRACT FROM AUTHOR]

The published model [Appl. Phys. Lett. 82, 4379–4381 (2003)] for the two-dimensional transient wave propagation in a cylinder is modified to avoid the inherited integration of the numerical inverse scheme. The Fourier series expansion is introduced for one spatial coordinate to resolve the transient response problem: theoretical radial displacements in either the ablation or the thermoelastic regime are obtained with little numerical noise and short computation time. The normal mode expansion method fails to deliver results with the same accuracy. Acoustic waves are fully identified by the ray trajectory analysis. These identified waves are further verified on the experimental results observed with the laser ultrasonic technique. © 2005 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

A model is proposed to predict acoustic waves generated in a transversely isotropic cylinder by a laser line pulse extended in beamwidth and time duration, and an application to elastic constants measurement is presented. Documented good agreements are observed in the comparison of experimental and theoretical normal displacements for aluminum cylinders under either ablation or thermoelastic generation. Bulk waves are identified and processed for the elastic constants measurement. The effects of source beamwidth and time duration on wave forms and on the elastic constants measurement are predicted by numerical simulations. For nondestructive evaluation applications using bulk waves, a radius of 0.3 mm appears as a minimum limit for the sample size using a laser source of 0.1 mm beamwidth and 20 ns time duration. Elastic constants of aluminum rods are experimentally measured with very good accuracy. [ABSTRACT FROM AUTHOR]

An inverse method is proposed in order to determine the viscoelastic properties of composite-material plates from the plane-wave transmitted acoustic field. Analytical formulations of both the plate transmission coefficient and its first and second derivatives are established, and included in a two-step inversion scheme. Two objective functions to be minimized are then designed by considering the well-known maximum-likelihood principle and by using an analytic signal formulation. Through these innovative objective functions, the robustness of the inversion process against high level of noise in waveforms is improved and the method can be applied to a very thin specimen. The suitability of the inversion process for viscoelastic property identification is demonstrated using simulated data for composite materials with different anisotropy and damping degrees. A study of the effect of the rheologic model choice on the elastic property identification emphasizes the relevance of using a phenomenological description considering viscosity. Experimental characterizations show then the good reliability of the proposed approach. Difficulties arise experimentally for particular anisotropic media. © 2002 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

This paper describes a method which allows the synthesis of acoustic plane waves by means of laser ultrasonic techniques. When a laser line source radiates at the surface of an anisotropic material, intricate effects are observed on signals obtained by transmission experiments. The material characterization from such a set of signals is not a trivial task since it requires correctly selecting the data necessary for the elastic constants determination, by distinguishing them from all the parasitic phenomena. In the present paper, it is shown that a judicious sum of signals obtained for various positions of the line source at the surface of a sample leads to the synthesis of an acoustic plane wave for which the odd phenomena disappear. Moreover, by applying a constant delay between the various signals, acoustic plane waves can be synthesized with varied refracted angles. One of the advantages of this technique is to offer convenient access to the phase velocity, providing an easier determination of the elastic constants. This method is successively applied to recover the stiffness coefficients of a silicon crystal and of a composite material. The good agreement between the so-recovered elastic constants and values given by other processes shows the contribution of such a method in the field of material characterization. © 2001 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

A point source-point receiver technique, based on laser generation and laser detection of acoustic waves, is used to measure the stiffness tensor of a composite material. Because of the dispersive nature of these materials, the acoustic signature provided in polymer matrix composites is difficult to interpret. A model for acoustic propagation in composites is developed to analyze the accuracy of the stiffness coefficients determined from the measured acoustic signals. The frequency dependence associated with viscoelastic rheology leads to the numerical implementation of a double inverse transform to calculate the surface displacement generated by a line impact. The model accurately predicts the focusing effects caused by anisotropy and the spreading of the signals caused by dispersion and attenuation. The predicted displacements are in good agreement with the result of experiments performed with a composite plate. The model is used to test a signal processing method based on wavelet transform to measure the group velocities of bulk waves propagating through the plate. Despite dispersion and echoes overlapping, the stiffness coefficients are identified from group velocities with good reliability. The process is then successfully applied to an actual viscoelastic composite material. © 1999 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

Zero-group velocity (ZGV) Lamb modes are associated with sharp local acoustic resonances and allow, among other features, local measurement of Poisson's ratio. While the thermoelastic generation of Lamb waves in metal plates has been widely studied, the case of materials of low-optical absorption remains unexplored. In materials such as glasses, the generation of bulk elastic waves has been demonstrated to be sensitive to the refracted light distribution. In this paper, a detailed analysis of the effect of light refraction on the laser-based generation of ZGV Lamb modes is presented. Experiments are performed on a bare glass plate without the need for an additional layer for light absorption or reflection. Using an appropriate tilted volume source, it is shown that the laser-ultrasonic technique allows non-contact measurement of the Poisson's ratio.

The symmetry of a thermoelastic source resulting from laser absorption can be broken when the direction of light propagation in an elastic half-space is inclined relatively to the surface. This leads to an asymmetry of the directivity patterns of both compressional and shear acoustic waves. In contrast to classical surface acoustic sources, the tunable volume source allows one to take advantage of the mode conversion at the surface to control the directivity of specific modes. Physical interpretations of the evolution of the directivity patterns with the increasing light angle of incidence and of the relations between the preferential directions of compressional- and shear-wave emission are proposed. In order to compare calculated directivity patterns with measurements of normal displacement amplitudes performed on plates, a procedure is proposed to transform the directivity patterns into pseudo-directivity patterns representative of the experimental conditions. The comparison of the theoretical with measured pseudo-directivity patterns demonstrates the ability to enhance bulk-wave amplitudes and to steer specific bulk acoustic modes by adequately tuning light refraction.

When a laser beam is absorbed in a semi-transparent material, a volume acoustic source is created owing to penetration of the laser beam inside the material and to thermoelastic transduction. Many experimental and theoretical studies have been conducted to better understand this ultrasound generation process with normal laser light incidence on the material surface. The purpose of this paper is to analyze the effects of the asymmetry caused by oblique incidence of a laser line source on the generation of acoustic waves in semi-transparent isotropic materials. Experiments on a glass plate demonstrate that such an obliquely incident laser light strongly affects bulk acoustic waves generation. Compressional and shear waves are enhanced and the loss of symmetry of the acoustic source causes asymmetrical behavior of the acoustic waves. Surprisingly, compressional-wave amplitude decreases whereas shear-wave amplitude increases in the region where the electromagnetic energy is refracted. This feature is explained by semi-analytical calculations., (© 2011 Acoustical Society of America)