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

17.2 Industrial Applications of Ultrasound 481Flaw Detection and Thickness MeasurementsA method of nondestructive testing, the pulse technique, is used extensively todetermine the propagation constants of solids, particularly in the MHz frequencyrange. This method consists of sending a short train of sound waves through amedium to a receiver. In the transmission mode of the pulse technique, the receiveris placed at a measured distance from the source. In the echo mode, a reversibletransducer acts as both source and receiver, with a reflector used to reflect thepulses. The speed of sound in a medium can be determined from the time of travelof the pulse over a given length of acoustic path. Longitudinal waves are generallyused. In gauging the thickness of a specimen, advantage is taken of the fact that abeam of pulses will reflect from the specimen surface opposite to the side of thereversible transducer.The use of the single-pulse method for flaw detection is fairly straightforwardwhen the specimen has two parallel surfaces and the defect is linear and roughlyparallel to these surfaces but not too close to a surface or another defect. If thepulse is followed on an oscilloscope and there is no defect present, two peaks,say, A and B, will appear on the screen. Peak A represents the instant of thetransmission of the pulse, and peak B that of its return after a simple echo. PeakB is referred to as the bottom echo. When a defect is present, a discontinuityof the characteristic impedance and some, or possibly all, of the sound energy isprematurely reflected back to the transducer. Another peak will then occur betweenA and B. The distance AC indicates the depth where the flaw exists and the heightof the peak C determines the extent of the defect.Figure 17.2 illustrates a schematic of a longitudinal wave probe used to detectflaws. A crystal transducer is normally used, and it is encased in a suitable housing.The crystal is mounted for heavy damping, which results in the propagation ofFigure 17.2. Longitudinal wave probe for detecting flaws.