Influence of the laser source position on the generation of Rayleigh modes in a layer-substrate structure with varying degrees of adhesion.

Non-Destructive Testing of adhesion using Surface Acoustic Waves (SAW) is an important issue in industrial and academic domains. Indeed, these waves are sensitive to the quality of adhesion at the interface between the substrate and the layer with a thickness comparable to the acoustic wavelength. Furthermore, their propagation distance allows a large majority of the sample to be tested quickly. Numerous studies have used SAW for the Non-Destructive Testing of adhesion. However, some recurrent experimental difficulties may lead to an incorrect interpretation of the results. This is the case when the layer thickness is non-uniform, for example. To provide a quasi-constant thickness, a PolyEthylene Terephthalate (PET) film was placed directly on the substrate surface without any glue and Laser-Ultrasonics was used to investigate this type of structure. As the film was transparent at the optical wavelength used, it was possible to focus the laser source on the substrate surface through the film. To the best of our knowledge, no paper has been published on the influence of the source position on adhesion testing. In this work, two source positions were investigated.