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Non-contact leaky Lamb wave imaging based on pulsed laser and ultrasound microphone

A local wavenumber estimation (LWE) method based on a scanning laser Doppler vibrometer (SLDV) is derived using a frequency-wavenumber domain filtering method. LWE methods based on a standing wave have recently been investigated. This technique utilizes SLDV data in the steady-state excitation of a fixed frequency. Despite its various advantages, the LWE method uses contact actuators to provide sufficient vibrating energy. In this study, we solved the cost and complexity problems of using SLDV by replacing it with an ultrasound microphone. We also measured the shallow defect of an aluminum plate using a noncontact-LWE method. A Nd:YAG laser with wavelength of 532 nm and an ultrasound microphone with a frequency range of 2200 kHz were both used to detect the shallow defect in the aluminum plate. The LWE image resulting from the pulsed laser has a low signal-to-noise ratio (SNR) and presents wave pattern artifacts because of a low excitation energy. To address this problem, we acquired several data from different laser point positions and constructed LWE images. Because the artifacts in these images are non-correlated, the averaging process removes the artifacts and significantly improves the SNR.