This paper describes work towards the development of a Lamb wave scanning method for the detection of defects in thin plates. The approach requires the generation of an ultrasonic S o -Mode Lamb wave using an incident transmitter excited with a tone burst centered at a near non-dispersive frequency. A pair of receiving transducers, with a fixed relative separation, remotely scans line sections of the thin plate. The global position of the receiver pair is moved to cover a large plate area. The arrival time information coming from incident and reflected waves contain information associated with the location of reflection surfaces or potential flaws. The cross-correlation between the excitation signal and the receivers' waveforms is obtained and subsequently demodulated using a quadrature amplitude method in order to facilitate the determination of arrival times. Distances from the source, to the reflection surface and to the receivers are found from the arrival times of the reflected waves and the Lamb wave phase velocity. The distances and the source and receiver locations are incorporated in an elliptical solution to find coordinates of the reflection points. In a line scanning the set of predicted reflection points define the extent of the defect. The Lamb wave scanning approach is tested using 1.6 mm-thick Aluminum plates with notches of various lengths and orientations from 0, 22.5 and 45 degrees with respect to the far edge of the plates. The results are summarized with defect maps that compare favorably to the actual notch locations.
This paper describes a Lamb-wave scanning method for the detection of notches simulating cracks at rivet holes in thin plates. The approach requires the generation of an ultrasonic S o -Mode Lamb wave using an incident transmitter excited with a tone burst centered at a near non-dispersive frequency. Area scans are performed on a plate with a hole with a notch to generate times series information which is used to create animations illustrating the wave propagation characteristics. The time series are subject to a sifting process to obtain intrinsic mode functions which contain narrow frequency banded information of the signals. The Hilbert-Huang transform is applied to the intrinsic mode functions which permit the computation of the signal energy as a function of time, proportional to the square of the amplitude of the analytical signal. Animations of the propagation of the Lamb-wave energy illustrate that a potential scanning approach is to acquire time series along a line between the transmitter and the hole, capturing wave scattering from the hole and reflections from the notches. The times of flight and amplitudes of the notch-reflected energy are used to calculate coordinates of the source of the reflections by a geometric approach. The identified coordinates of the reflections outline the extent of the notch at the rivet hole. Results of experiments conducted on thin square plates with a single hole with notches of various sizes compare favorably with the actual notches.
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