Wavefield dislocations, so named because of their morphological similarity to dislocations in crystals, are singular lines in the phase of a wave. These features exist in wavefields formed by the scattering of pulsed ultrasound from rough surfaces and from the internal microstructure of metals. The aim of this paper is to show that changes in the statistics of wavefield dislocations can be correlated with changes in the scattering object-a fact that has potential application in ultrasonic characterization. Dislocations were observed using pulses of ultrasound of central frequency 20 MHz, reflected from the specimen and detected in water immersion using a coincident source-receiver. The phase of the wave is defined in terms of the position of the wave crests and troughs in the signal detected as a function of time at a fixed receiver position. Various grades of sandpaper served as a set of surfaces of varying roughness. The internally scattered ultrasound from samples of both brass and steel was observed and changes due to annealing operations were detected by the method of dislocations. The position of a wavefield dislocation can be determined with a precision two orders of magnitude less than the wavelength. Thus the method of dislocations is very well suited to the study of slowly evolving systems. [References: 6]
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