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A noncontact laser–ultrasonic technique is described that allows determination of material properties of anisotropic platelike samples. The method requires access to only one side of the sample. Plane waves are synthesized by scanning the surface with a laser line source at judiciously chosen intervals. By doing so, the intricacies associated with the reflection of an acoustic ray off an interface of an anisotropic medium are avoided. The plane-wave reflections off the opposite free surface of the plate are simply governed by Snell's law. The identification of the stiffness-tensor components is based on the inversion of the phase velocities of these reflected plane waves. The procedure developed is presented and applied to both simulated and experimental signals. The method is easy to implement, and provides a means for noncontact measurement of elastic properties where there is limited access to test specimens. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, acute focusing of the laser pump beam (≈0.5μm) on the sample surface allows picosecond acoustic diffraction in thin metallic films. The resulting wavefronts propagate at a group velocity which differs from phase velocities in anisotropic films. Waveforms have been experimentally recorded in a gold layer (2.1μm thick) for several distances between pump and probe on the sample surface. A specified signal processing based on a Synthetic Focalization Technique allows analyzing the space repartition of the acoustic wave vectors for both longitudinal and shear waves. Stiffness coefficients of the gold layer are then identified from wave arrival times. [Copyright &y& Elsevier]

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 noncontact laser-ultrasonic technique is described that allows determination of materials anisotropic properties. Plane waves are synthesized by scanning the surface with a laser line source at judiciously chosen intervals. The identification of the stiffness-tensor components is based on the inversion of the phase velocities of these transmitted plane waves. The procedure developed is presented and applied to both simulated and experimental signals. The influence of the spatial extent of the laser source in the identification of the stiffness-tensor components is also discussed; the effect of the spot size is analyzed in the ablation regime for which the acoustic source is localized at the surface, whereas the influence of the optical penetration depth is quantified in the thermoelastic regime. [ABSTRACT FROM AUTHOR]

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. [Copyright &y& Elsevier]

Abstract: A point source-point receiver technique, based on laser generation and laser detection of acoustic waves, allows determination of mechanical properties of an anisotropic cylinder. The nature of the material and the geometry of the sample give a dispersive behaviour to the diffracted waves and make the acoustic signature difficult to interpret. To overpass the intricacies, wave fronts (conical waves in the volume and helical waves on the surface) are synthesized from signals provided by scanning the primitive line of the cylinder with a laser point source. In order to distinguish between direct bulk conical waves and other contributions in the acoustic response, some considerations on line surface waves and on reflected bulk conical waves are supplied. The identification of the stiffness tensor components, based on the inversion of the bulk waves phase velocities, is applied to signals simulated for a composite material. [Copyright &y& Elsevier]

The acoustic signature provided by laser-generated ultrasound in polymer matrix composites is difficult to interpret on account of the dispersive nature of these materials. In order to simulate such wave forms, a double numerical inverse transform is used to calculate the surface displacement generated by a line impact. This calculation method is preferred due to the frequency dependence connected with the viscoelastic rheology. The focusing effects that anisotropy gives rise to are represented. In addition, the spreading of the signals issued from absorption is correlated with the anisotropic changes of the slowness versus frequency. The predicted displacements are in good agreement with the result of experiments performed with a composite plate. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: The transient response of a transversely isotropic cylinder under a laser point source impact is solved theoretically. The radial displacement generated by the laser under the ablation regime is numerically calculated by introducing Fourier series expansion and two-dimensional Fourier transform. The validity of this theoretical solution is demonstrated on a fiber reinforced composite cylinder with a strong anisotropy. Experimental displacements are detected at the cylinder surface by the laser ultrasonic technique, and are analyzed by the ray trajectories. Corresponding theoretical displacements are calculated numerically and compared to the experimental signals. Good agreement is found. The diffraction effect caused by the cusp is observed in both theory and experiment. [Copyright &y& Elsevier]