Thickness gaging of thin plates by multi-peak frequency decomposition of Lamb wave signals

In the present work ultrasonic Lamb wave testing has been applied for thickness gaging of thin aluminum plates with 2, 3, and 4-mm thickness. Angle probes operated in pulse-echo arrangement with 0.98 MHz central frequency and 7°, 12°, and 20° incident angles were utilized for investigations. High order modes (A₁, S₁, A₂, and S ₂) were selected and evaluated for measurements and a fast Fourier transform was utilized for frequency decomposition. The variation with thickness and frequency of the phase velocity was the basis of measurement. Two methods were proposed for thickness gaging: (1) a probe with 6000-8000 m/s phase velocity and one or two other probes with higher than 10 000 m/s phase velocity were utilized simultaneously to inspect each test area and the peak frequency with the highest amplitude excited by each probe was evaluated, and (2) a single probe was utilized and all visible peak frequencies were evaluated. These methods overcome the limitations of the previous research, which applied a peak frequency shift method for corrosion monitoring of plates only for a known mode and known initial thickness and low thickness reductions. The second method is more practical and accurate and has potential for on-line automatic thickness gaging of rolled plates.