Abstract
Methods for the exact determination of the surface waves velocity by the electromagnetic-acoustic method are proposed. The method is suitable for rectangular samples of ferromagnetic materials, in particular for structural steels. The velocities of ultrasonic waves in the material are shown to be related to the elastic moduli of the material. The determination of the elastic modulus values of the material is an important materials science problem. The best method for the determination of the speed of surface waves is that of electromagnetic-acoustic conversion, which combines both non-contact measurement and quick operation. Wire meander transducers are used as sensors, which can operate in two modes: a combined mode (one sensor generates a wave and receives its reflection) and a separate mode (one sensor generates a wave while the other one receives it). The main inaccuracies of the method have been analyzed. The main inaccuracy has been shown to be associated with the determination of the time interval of the wave propagation. This is due to the difficulty of obtaining the reference point since the probe pulse has a damped shape. A detuning from the probe pulse in determining the time interval of the wave propagation has been proposed. Another measurement inaccuracy is related to the accuracy of determining the base of the meander transducers (the distance between the centers) operating in separate mode. Simple formulas for calculating the surface wave velocity and the base of the meander transducers have been obtained. Technique testing was carried out using the sample of Armco iron. The accuracy of determining the surface Rayleigh wave was + 0.5 %. It has been shown that the use of modern digital detectors of time interval will allow one to obtain the accuracy of + 0.05 %. The velocity values obtained with the specified accuracy can be used to determine the elastic moduli as well as to assess the elastic properties of the material, for example, sheet metal anisotropy as an independent information parameter.
Keywords
Surface acoustic waves, Rayleigh waves, ultrasound, ultrasonoc velocity, elastic moduli, anisotropy, electromagnetic-acoustic method, ferromagnetic materials, accuracy of measurements.
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