A novel 3D evaluation method for surface defects using broadband laser-generated Rayleigh waves with wavenumber analysis

To address the issues of large imaging errors for small defects and the difficulty in depth evaluation using local wavenumber estimation for surface defect imaging, a novel three-dimensional (3D) evaluation method for surface defects using broadband laser-generated Rayleigh waves with wavenumber analysis is proposed. A finite element model is established to investigate the interaction between the Rayleigh wave and the surface defect and reveal the wavenumber change mechanism of the non-dispersive Rayleigh wave in the case of defects. It is discovered that when the Rayleigh wave encounters the surface defect, various mode-converted scattered waves are generated, resulting in the appearance of new components with wavenumbers lower than that of the incident Rayleigh wave in the wavenumber domain. Additionally, the maximum amplitude of the Rayleigh wave in the B-scan image increases as the defect depth increases. Based on the simulation analysis, a 3D evaluation method for surface defects is proposed. Firstly, the scattered Rayleigh wave caused by the defect is extracted using frequency-wavenumber analysis. Secondly, a space-frequency-wavenumber analysis is used to determine the local wavenumber of the scattered Rayleigh wave for defect imaging. Finally, the defect depth is estimated by analyzing the maximum amplitude of the Rayleigh wave. A surface defect detection experiment is conducted to verify the effectiveness of the proposed method, and the experimental results demonstrate that the proposed method can suppress noise interference and accomplish high-precision imaging of small surface defects compared to the traditional method. Moreover, the method can establish a linear mapping relationship between the defect depth and the maximum amplitude of the Rayleigh wave for depth evaluation. The research results can provide a potential application for the 3D evaluation of surface defects.