Time–Frequency Energy Analysis for Active Guided Wave Monitoring in Thick Aluminum Structures Using a Flexible Transducer

Structural health monitoring of thick-walled industrial components remains challenging due to modal superposition and dispersion effects that limit conventional time-of-flight-based guided-wave analysis. This study proposes an active excitation-based monitoring framework using a self-developed flexible Macro Fiber Composite (MFC) transducer for defect characterization in 25 mm thick aluminum plates. Controlled three-cycle tone-burst excitation at 350 kHz was introduced, and the resulting elastic wave responses were analyzed using Short-Time Fourier Transform (STFT)-based time–frequency energy and spectral bandwidth metrics. Artificial V-shaped notches with depths of 10% and 30% of the plate thickness were introduced to evaluate defect severity. Compared to the intact specimen, the 10% notched plate exhibited a 22.5% reduction in STFT-based energy and a 3.37% decrease in spectral bandwidth, while the 30% notched specimen showed reductions of 37.5% and 7.78%, respectively. The results demonstrate that defect-induced structural discontinuities in thick plates not only attenuate overall guided-wave energy but also alter frequency distribution characteristics. The proposed approach enables quantitative defect evaluation without explicit modal separation and validates the dual actuation and sensing capability of the flexible MFC transducer, supporting the feasibility of transitioning from passive acoustic emission monitoring to an active, self-diagnostic structural health monitoring framework for thick industrial structures.