Elastic constants determination of anisotropic materials from phase velocities of acoustic waves generated and detected by lasers.
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.
Identification of laser generated acoustic waves in the two-dimensional transient response of cylinders.
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.
Refinements in Supersonic Reflectoscopy. Polarized Sound
One form of simple circuit for the generation of the short duration high frequency voltage trains and in the supersonic reflectoscope is shown. The required band width of the system, including the crystal, is discussed. Techniques for the radiation of longitudinal, shear, and Rayleigh waves are set forth. Either longitudinal or shear waves can be used to establish various modes of thickness resonance through a plate for measuring velocity of propagation or thickness. Shear waves have some of the properties of polarized light, for instance, double refraction has been measured by three methods in elastically aeolotropic solids and a technique for the direct indication of the amount of elastic aeolotropy developed. By means of a quarter‐wave plate, circularly polarized sound can be produced.