Optical Measurements of Interface Acoustic Waves Guided by the Boundary Between Two Elastic Substrates
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Surface acoustic waves propagating at the interface between two solids are very important for Non Destructive Evaluation of layered media . Dispersion features and transmission losses of such waves strongly depend upon the elastic constants of the solids as well as the mechanical boundary conditions at the interface between the solids. One particular kind of interface waves is the Stoneley wave, which can exist at the interface between certain specifically combined elastic half-spaces in rigid contact . The acoustic field of this wave, travelling with no loss along the interface, decays away from the interface in each medium (Fig. 1a). Murty studied interface waves in another extreme case, i.e. slip contact . For such type of contact, only the normal stress and displacement components are continuous but shear stresses are cancelled at the interface. In the slip contact case, Murty’s study revealed also that the conditions of existence of the Stoneley-like wave are much less rigorous. When material combinations and/or boundary conditions are not satisfied to support the Stoneley-like wave, interface waves may become lossy  and radiate acoustic energy through mode conversion into bulk waves (Fig. 1b), like the case of the leaky Rayleigh wave propagating at a solid-liquid interface. Besides, if the presence of a coupling layer between the two half-spaces is taken into account as in most practical conditions, the features of interface waves depends also on the viscoelastic parameters and thickness of the coupling layer[4,5]. Up till now, most of the investigations involving interface waves are indirect ones using ultrasonic reflection and transmission measurements , and some others are related to dispersion measurements using interdigital or wedge transducers [4,5]. Few experiments were reported illustrating the field distribution of interface waves. Claus and Palmer observed the Stoneley wave  using an optical interferometer. The wave displacements were only detected by the probe beam focused at the Nickel-Pyrex interface .