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

17.2 Industrial Applications of Ultrasound 485line, which can be a length of nickel wire or a column of liquid terminating in areflector. The latter pulse is indicated on the oscilloscope by a trace following thatrepresenting the pulse passing through the sample. The delay line is adjusted inlength by means of a micrometer device until the two traces on the screen coincidepositionwise. The thickness of the specimen is derived from the predeterminedcalibration of the delay line.A major advantage of using ultrasound for thickness measurement is that accessto only one surface is needed. This is especially useful in measuring the extentof corrosion in infrastructures such as viaducts, sewers, gas pipes, and chemicalconduits. The thicknesses of ship hulls can be monitored at sea without resorting totaking sample borings, an expensive and tedious process that is not 100% effective.In the livestock industry, ultrasonic thickness measurement is used to measure theamount of fat on the bodies of live animals.Mechanical Stress MeasurementsWhen a solid material undergoes a change in mechanical stress, changes alsooccur in its elastic moduli and hence in its acoustic velocities (Shahbender, 1971).This method can also determine the variations of stress in real time. In orderto render possible the use of this procedure, a calibration curve is necessary toprovide the reference data, namely, plots of velocity (in terms of percentage) ina direction as a function of stress applied normal to that direction. Figure 17.5displays typical calibration curves for longitudinal waves and also for shear wavespolarized perpendicular to the direction of the stress and also polarized parallelto the direction of the stress. It can be inferred that the variations of the twocomponents of shear velocity with time are different. Thus, at any given timethe corresponding wave vectors will be out of phase with each other. This phaseFigure 17.5. Calibration curves for longitudinal waves.