Rayleigh wave techniques

Figure 6.5 Shear modulus, Ga, of an epoxy adhesive, as determined by an ultrasonic Rayleigh wave technique, as a hmction of the thickness of the adhesive layer from the adhesive/aluminium alloy interface [20].

Anyone who has successfully used a microscope to image properties to which it is sensitive will sooner or later find himself wanting to be able to measure those properties with the spatial resolution which that microscope affords. Since an acoustic microscope images the elastic properties of a specimen, it must be possible to use it to measure elastic properties both as a measurement technique in its own right and also in order to interpret quantitatively the contrast in images. It emerged from contrast theory that the form of V(z) could be calculated from the reflectance function of a specimen, and also that the periodicity and decay of oscillations in V(z) can be directly related to the velocity and attenuation of Rayleigh waves. Both of these observations can be inverted in order to deduce elastic properties from measured V(z). 

The images shown in Figure 6 were obtained by surface wave (Rayleigh wave) inspection of 4 silicon nitride modulus of rupture test bars with laser-machined simulated voids. The technique utilized a focused 50 MHz transducer designed to produce a radial surface wave. Centrally positioned surface voids of 100,50, and 20 micrometers were readily detected. One bar (shown on bottom) had a 10 micrometer surface void, indicated by a marker pen, which would probably not have been detected. Longitudinal grinding marks were also apparent. 

In the forced Rayleigh scattering technique, the diffraction lattice of the excited state is formed by the interference of the two laser fluxes. A probe laser then follows the disappearing process of this lattice [21], The disappearance process of this lattice reflects the lifetime of the excited state T and molecular diffiision D. A plane wave laser light with a wavelength X is divided into two and they cross each other at an angle q to observe the interference pattern with a period A. In the constructive interference area, the probe will be excited and the striation of the excited state can be observed. Period A of the striation is expressed as... 

Using a ray-tracing technique, one may understand this mechanism understood as follows. The period of this variation results from interference between the two components. Figure 6 shows one component which is spectrally reflected at normal incidence, while the second one undergoes a lateral shift on incidence and reradiates at the critical phase-matching angle for the surface acoustic wave (also referred to as leaky Rayleigh waves ). 

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