Scanning Laser Vibrometry Imaging of Fatigue Cracks via Nonlinear Ultrasonic Guided Wave Scattering and Mode Conversion

摘要

This paper presents the Scanning Laser Vibrometry (SLV) imaging of fatigue cracks by taking advantage of the nonlinearultrasonic guided wave scattering and mode conversion phenomena. The investigation starts with the numerical modelingusing the Local Interaction Simulation Approach (LISA) to demonstrate the distinctive scattering and mode conversionfeatures at rough fatigue cracks. During the wave crack interactions, nonlinear higher harmonics are generated fromContact Acoustic Nonlinearity (CAN). In addition, the microscale rough crack surface condition may introduce modeconversion between the symmetric and antisymmetric Lamb modes. After the theoretical analysis, SLV experiments areconducted on an aluminum plate, where fatigue cracks are nucleated from a rivet hole. The damage imaging schemeutilizes the post-processing techniques via Fast Fourier Transform (FFT), frequency domain filtering, and Inverse FastFourier Transform (IFFT) to eliminate the linear wave field, leaving only the scattered higher harmonics in the images. Inthis way, the fatigue cracks can be distinguished from structural features such as rivet holes and stiffeners. This paperfinishes with summary, concluding remarks, and suggestions for future work.