An active-passive acoustics approach for bond-line condition monitoring in aerospace skin stiffener panels

This article demonstrates the effectiveness of acoustic nondestructive inspection methods to monitor the condition of the bond-line in carbon fiber reinforced polymer stiffened panels. In particular, acoustic emission and guided ultrasonic waves were utilized experimentally to identify the onset of debonding in the stiffened panels and track its evolution under constant amplitude fatigue loading. Before testing, numerical simulations of wave propagation were carried out to evaluate the wave characteristics and optimize the placement of sensors along the spar. A damage index based on numerically produced guided waves was developed to estimate the extent of debonding which was tested and proved effective using actual experimental data. The panels were subjected to both quasi-static and fatigue loading conditions, while continuously recording acoustic emission and triggering guided ultrasonic waves at predetermined load levels. The acoustic emission activity associated with debonding was found to have a dual dominant frequency content and a low frequency centroid, which were also confirmed with wavelet analysis. In view of potential aerospace applications of the investigation presented herein, pattern recognition algorithms were also implemented and showed great potential for real-time detection of such damage.