Mode conversion of fundamental guided ultrasonic wave modes at part-thickness crack-like defects

Guided ultrasonic waves can be employed for efficient structural health monitoring (SHM) and non-destructive evaluation (NDE), as they can propagate long distances along thin structures. The scattering (S₀ mode) and mode conversion of low frequency guided waves (S₀ to A₀ and SH₀ wave modes) at part-thickness crack-like defects was studied to quantify the defect detection sensitivity. Three-dimensional (3D) Finite Element (FE) modelling was used to predict the mode conversion and scattering of the fundamental guided wave modes. Experimentally, the S₀ mode was excited by a piezoelectric (PZT) transducer in an aluminum plate. A laser vibrometer was used to measure the out-of-plane displacement to characterize the mode-converted A₀ mode, employing baseline subtraction to achieve mode and pulse separation. Good agreement between FE model predictions and experimental results was obtained for perpendicular incidence of the S₀ mode. The influence of defect depth and length on the scattering and mode conversion was studied and the sensitivity for part-thickness defects was quantified. The maximum mode conversion (S₀-A₀ mode) occurred for ¾ defect depth and the amplitude of the mode-converted A₀ and scattered S₀ modes mostly increased linearly as the defect length increased with an almost constant A₀/S₀ mode scattered amplitude ratio. Similar forward and backward scattering amplitude was found for the mode converted A₀ mode. The mode conversion of the S₀ to SH₀ mode has the highest sensitivity for short defects, but the SH₀ mode amplitude only increased slightly for longer defects. Employing the information contained in the mode-converted, scattered guided ultrasonic wave modes could improve the detection sensitivity and localization accuracy of SHM algorithms.