THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

17.2 Industrial Applications of Ultrasound 483mounted on a Plexiglas wedge and the longitudinal waves are directed to the surfacewith an angle of incidence greater than the first critical angle (this is the angle ofincidence sufficiently large that the refracted ray is directed along the boundaryrather than into the medium). The figure shows that the wedge is shaped in such amanner that the longitudinal waves that are reflected at the surface become totallyabsorbed by subsequent reflections. In Figure 17.3(b) a similar probe is positionedat a suitable location to receive the waves from the defect after a reflection atthe base of the specimen. In a given substance, the transverse wave velocity isgenerally about half that of the longitudinal wave velocity, so the sensitivity of thismethodology is twice that for longitudinal procedures.Surface defects can be discerned through the means of surface waves. Theseare produced by a probe similar to that shown in Figure 17.3(a), but the incidentlongitudinal waves are directed to the surface at the second critical anglewhere the transverse waves are refracted at an angle of 90 ◦ (i.e., along the boundarysurface). Laminar defects that exist just below the surface, which are hardto detect by normal longitudinal wave methods, can be located by Lamb waves(Worlton, 1957). According to Lamb, a solid plate can resonate at an infinitenumber of frequencies. The portion of specimen between the surface and a laminationclose to it forms such a plate. If surface waves are directed toward thisplate, it will resonate and generate a signal that can show up on an oscilloscopescreen.Determination of Propagation Velocity and Attenuationthrough an InterferometerThe interferometer is a continuous wave device that can accurately measure velocityand attenuation in liquids and gases that can sustained standing waves. Itconsists of a fluid column that contains a fixed, air-backed piezoelectric transducerat one end and a moveable rigid reflector at the other end. A fixed frequencyis selected. The reflector is moved with respect to the transducer by a micrometeradjustment mechanism. As the reflector moves, the reflected waves becomeperiodically in and out of phase with the transmitted waves, as a result of the correspondingconstructive and destructive interference. The effect of the interferenceon the crystals influences the load impedance detected by electronic system. Theload current in the electronic amplifier fluctuates accordingly. The wavelength ofthe sound is established by the distance the micrometer moves the reflector overone cycle of load current fluctuation, with the distance between two successivemaxima being equal to half-wavelength λ/2.Optical interference methods also have been used in this fashion to accuratelymeasure the wavelengths of standing waves at high frequencies (near 1.0 MHzor above). An accuracy of 0.05% is typical for the interferometer, which dependson the quality of micrometer readings, the parallelism between transducer andreflector surfaces, and the accuracy of the frequency determination. The velocityof sound is found by simply multiplying the frequency by the wavelength. The