Experimental validation of guided wave mode-conversion at part-thickness defects in metal plates

Low frequency guided waves have been used to develop Structural Health Monitoring (SHM) for the early detection of fatigue cracks in metallic aircraft structures. The scattering and mode conversion of guided waves at part-thickness defects was investigated to quantify the sensitivity for defect detection and the potential for the development of a baseline-free SHM methodology employing mode-converted guided waves. Finite Element Analysis (FEA) and experimental validation were conducted to investigate the mode converted scattering from the S₀ to the A₀ Lamb wave mode at part-thickness crack-like defects in an aluminum plate. A piezoelectric (PZT) transducer was experimentally used as a plate edge excitation for the S₀ mode and the out-of-plane displacement was measured using a laser vibrometer. Good agreement between the FEA and experimental results was obtained and the influence of defect depth and length was investigated and quantified.