Effective thickness mapping in composite materials based on local changes of the dispersion curves

Local wavenumber estimation (LWE) has been extensively employed as a powerful tool for damage detection, localization, and thickness estimation in composite structures. However, its practical application is constrained by challenges in determining an appropriate excitation frequency for local wavenumber estimation and the inherent narrowband constraints of traditional LWE techniques. To address these challenges, a novel approach is proposed utilizing broadband excitation for more accurate thickness mapping, leveraging local variations in the dispersion curves. In this method, broadband guided waves are generated by piezoelectric actuators and captured using a scanning laser Doppler vibrometer at a dense grid of points covering the inspected area. The S0 mode is suppressed with a mode filter in the wavenumber-frequency domain. Subsequently, a circular tube-like filter is applied to eliminate most of the A0 mode, isolating a dataset containing only a narrowband of wavenumbers within the A0 mode. After transforming the data from the frequency-wavenumber domain to the space-frequency domain, a frequency shift index is introduced for each point in the wavefield spectrum, which correlates with local effective thickness. To validate the effectiveness of the proposed technique experimental measurements were performed on a glass fiber-reinforced polymer (GFRP) specimen with various Teflon tape inserts to simulate delamination.