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Laser-Induced surface acoustic waves for material testing

 
: Schneider, Dieter

:

Ida, Nathan (Hrsg.):
Handbook of Advanced Nondestructive Evaluation
Cham: Springer Nature, 2019
ISBN: 978-3-319-26552-0 (Print)
ISBN: 978-3-319-26553-7 (Online)
pp.171-234
English
Book Article
Fraunhofer IWS ()

Abstract
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.

: http://publica.fraunhofer.de/documents/N-569688.html