Surface defect identification based on broadband laser-generated Rayleigh wave with wavenumber filtering

Abstract As a non-contact, couplant-free and nondestructive technique, the laser ultrasonic technology has great potential for detecting surface defects. In this paper, we propose a surface defect identification method based on broadband laser-generated Rayleigh wave with wavenumber filtering in an area scanning mode. Firstly, a particular lowpass filter is constructed in frequency-wavenumber domain to extract the scattered wave generated by the defects, and then a surface defect image can be reconstructed based on the broadband scattered wave wavenumbers. Secondly, a threshold denoising method is employed to enhance the signal-to-noise ratio and improve the image sharpness. Thirdly, the experimental validation is carried out, in which a laser ultrasonic detection system is used to detect the surface defects of aluminum alloy specimens and identify their different parameters. The experimental results verify that the proposed method can identify the location, size and orientation of surface defects effectively, meanwhile, its imaging effect shows significantly superior to that of the conventional reflected wave energy and standing wave energy methods. Furthermore, the correlation between maximum local scattered wave energy and the depth of defects is found, which can characterize the severity of the surface defects.