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

482 17. Commercial and Medical Ultrasound Applicationssmall pulses to allow for greater accuracy in locating defects and better resolutionof neighboring defects from one another. The transducer is protected by a plasticcover that is coupled to the crystal with oil so as to prevent wear by friction betweenthe surfaces of the crystal and the material being tested. It would be ideal for thecharacteristic impedances of the transducer material, the material of the protectivecover, and the oil should be similar. Sometimes it is more desirable to apply theimmersion method, whereby both the specimen and the probe are immersed inwater, with the probe at a fixed distance from the upper surface of the sample. Anadditional peak that represents the echo from the interface between the immersionliquid and the sample will appear on the oscilloscope screen.When a defect is not parallel with a surface, it is preferable to use an angledprobe, which consists of a transducer mounted on a wedge. This enables the soundwaves to be incident normally to the defect, and a greater degree of sensitivity isthus achieved. Using variable-angled probes makes it easier to better gauge thedirection of defects that might otherwise remained undetected. For samples havingirregular shapes, two probes are used, one acting as a sender and the other servingas a receiver in order to locate the defects. Care should be taken that couplingbetween the transducers should involve the medium and nothing else.Flaws, such as those that occur in butt welds oriented at right angles to thesurface, are commonly detected through the procedure of forward scanning. Herea beam of transverse waves is propagated, after refraction at the boundary, in themedium at a shallow angle to the surface. In Figure 17.3(a) the transducer is shownFigure 17.3. Transducer mounted on plexiglass wedge and setup of transverse waveprobes for locating a defect in a specimen.