ISSN: 0090-3973
Published Online: 12 February 2008
Page Count: 9

Thickness Gaging of Thin Plates by Multi-Peak Frequency Decomposition of Lamb Wave Signals
Edalati, K.
Department of NDT and Inspection, Radiation Protection Technological Center of Iran,

Edalati, A.
Kankash Parto Technical Inspection Company,

Kermani, A.
Department of NDT and Inspection, Radiation Protection Technological Center of Iran,

(Received 24 May 2006; accepted 1 January 2008)

Abstract

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 S²) 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.

Keywords:
Lamb wave, thin plates, thickness gaging, frequency, fast Fourier transform

Paper ID: JTE100662
DOI: 10.1520/JTE100662
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