Thermoelastic Sound Source: Waveforms in a Sensing Application

Photoacoustically generated sound pulses are widely used in various NDT, NDE and sensing applications when a non-touching method is preferred. The generation mechanisms are relatively well known, including types of waves generated, directional patterns, sound pressures and damage thresholds for the laser intensity [1]. The so-called thermoelastic regime is attractive to many applications despite of its low efficiency (usually about sub 0.1%). It is because that the process is nondestructive to samples and the theory is well established [2,3,4]. The current study addresses the prediction of the temporal ultrasound pulse shape of an optimum sound generation scheme using a low power diode pumped high repetition rate Nd:YAG pulse laser [5]. A model is proposed in which the radiation from the thermoelastic sound source is treated as an instantaneous piston source at the solid-fluid interface

Optimization of Sound Pulse Generation for Photoacoustic Sensing Applications

Photoacoustically generated sound pulses are currently used in various NDT, NDE and sensing applications, often because this method generates ultrasound without touching the sample. The generation mechanisms are relatively well known, including directional patterns, sound pressures and damage thresholds for the laser intensity. Our study addresses the optimal conditions for sound generation for sensing purposes in a liquid using a low power diode pumped Nd:YAG pulse laser.</p

Laser-Induced surface acoustic waves for material testing

Surface acoustic waves are elastic vibrations which propagate along the surface of the material. They are very sensitive to films and surface treatments, since the wave energy is concentrated near the surface. Therefore, there is a growing interest in using this acoustic wave mode for nondestructive testing. Whereas the wave velocity is constant for homogenous materials, the velocity c depends on frequency f for coated and surface-modified materials. This phenomenon, termed dispersion, can be used to determine important film parameters such as Young’s modulus, density, or film thickness. Especially, Young’s modulus is an interesting parameter for nondestructive characterization of film materials, since it depends on the bonding conditions and the microstructure. In order to determine the parameters of the film material, the dispersion curve c(f) is measured and fitted by a theoretical curve. Many experimental setups use pulse lasers to create surface acoustic waves. Short laser pulses can create wideband acoustic impulses. The laser is a non-contact acoustic source that can precisely be positioned on the material surface, which enables an accurate measurement of the dispersion. Five examples of application are presented which demonstrate that surface acoustic waves can be used for very different problems of surface characterization: diamond-like carbon films (ta-C) with thickness down to few nanometers, porous metal films of titanium with a thickness in the micrometer range, thermal-sprayed ceramic coatings with a thickness of some hundreds of micrometers, laser-hardened steels up to the depth of one millimeter, and subsurface damage in semiconductor materials