Influence of the dispersion on measurement of phase and group velocities of Lamb waves

Abstract

Influence of the dispersion on the measurement of phase and group velocities of Lamb wave in a plate is investigated in the work presented. The velocity of propagating guided waves is important parameter of their application in non-destructive testing. Accurate determination of the Lamb wave velocity is complicated by the fact that it is frequency dependent. In order to determine in what way the dispersion affects the uncertainties of the phase and group velocity measurements the investigation was carried out. The finite element model of the 2 mm thickness and 200 mm length aluminium plate was used in order to obtain the signals for analysis. The A0 mode was excited by adding a shear force to one of the ends of the plate. The excitation signal was 300 kHz burst with the Gaussian envelop. Determination of the velocities was based on measurement of the propagation time. The propagation time was measured using a zero-crossing technique. Investigations demonstrated that the obtained values of phase velocities depend essentially on the number of periods in the burst which was used for measurements by the zero-crossing technique. The obtained regularities enable to compensate part of uncertainties of the phases velocity measurement. Moreover it enables to reconstruct the segment of the phase velocity dispersion curve in narrow frequency ranges.