The Pulse-Echo Method; Design and Performance of a Pulse-Echo Flaw Detector

Abstract

Figure 10.1 shows the principle of the method in which an ultrasonic pulsed wave, usually in form of a damped oscillation, is generated by a probe and propagates into a specimen with the ultrasonic velocity corresponding to the material concerned. Part of the ultrasound will be reflected if it strikes an obstacle in the form of an inhomogeneity and, if this is not too large, the remainder will travel further to a boundary of the specimen and will be reflected back to a receiver, if the rear surface and the receiver are in favorable positions. The signal obtained from the receiver is displayed as a peak on a base line of a CR tube (Fig. 10.2). The horizontal sweep is proportional to the time, so that the transit times of the pulse to and from the reflector, and to and from the back wall, correspond respectively to the distances on the screen from the initial peak to the echo peaks corresponding to reflector and back wall. To obtain a standing image the pulses and the sweep of the CR tube are synchronised at the so-called pulse-repetition frequency.