Mode-conversion between fundamental Lamb waves (A0-S0 mode) during scattering at a part-thickness notch

To ensure aircraft safety, non-destructive evaluation (NDE) and structural health monitoring (SHM) methods are applied for the early detection of defects such as part-thickness fatigue cracks. Guided ultrasonic waves can propagate significant distances along large plate structures, allowing the monitoring with a limited number of sensors. The scattering and mode conversion of the fundamental Lamb wave modes (A₀-S₀ mode) at a part-thickness notch was investigated to characterize the influence of size (length and depth) and incident wave direction on the detection sensitivity. A piezoelectric transducer was used to experimentally excite the A₀ mode and a laser vibrometer was employed to measure the scattered S₀ mode. Good agreement was obtained between scattering patterns and normalized amplitude predicted by three-dimensional FE simulations and experimental validation for a variation of the defect length. The amplitude increased mostly linearly as the damage length increased. It was found that the maximum scattered mode converted S₀ mode amplitude occurred for ¾ defect depth. The polar plot patterns changed, and the amplitude dropped significantly as the incident direction changed away from perpendicular to the notch orientation. For the future development of SHM based on the A₀-S₀ mode conversion, sensor placement to achieve as close as possible perpendicular wave incidence should be considered. This study contributed to the detection sensitivity and localization accuracy of part-thickness defects for baseline-free SHM algorithms.