Toward Utilizing Full-Field Laser-Ultrasound for Practical Nondestructive Inspection with Acoustic Wavenumber Spectroscopy

摘要

This study concerns the use of steady, harmonic excitation in place of repeated transient excitation for full-field laser ultrasound inspection. With harmonic excitation, we realized several orders of magnitude improvement in signal level in ultrasonic laser Doppler vibrometer measurements, enabling scans with eye safe lasers on unmodified inspection surfaces at speeds of up to five square meters per minute. We've found that two classes of full-field analysis techniques to be especially effective when properly modified for harmonic response measurements: wavenumber spectroscopy and local gradient estimation. This paper focuses on the former. Wavenumber spectroscopy, which is effective at detecting in-plane defects, involves local analysis of the wavelengths of the ultrasonic waves in order to quantify changes in effective thickness using the Rayleigh-Lamb equations. Using the techniques briefly described in this paper, we achieved effective nondestructive evaluation with scan rates of up to 320 square centimeters per second on metallic samples and 80 square centimeters per second on carbon-fiber-reinforced polymer composites.