Online structural state assessment for aerospace composite structures using an acousto-ultrasonics-based multi-damage index identification approach

The development of aerospace manufacturing has promoted the application of lightweight composite materials into aerospace structures. Although the aerospace composite structures possess numerous advantages, invisible internal structural damage such as delaminations induced by various external factors can significantly reduce the mechanical affordability, safety, and life-cycle of the structure. Therefore, it is of great significance to monitor and assess the health state and predict accurately the lifetime of aerospace composite structures. An acousto-ultrasonics-based multi-damage index identification approach is thus proposed in this study to identify and quantify possible multiple damage in thin-walled aerospace composite structures. In this approach, two indices for damage quantification were proposed: the energy and phase divergence indices. The energy index defines the reflected energy resulting from damage, and the phase divergence index defines the instantaneous phase variation of propagating waves due to damage. The two damage indices are obtained through the developed mode decomposition and spectral element analysis using sensor response signals collected by a transducer array placed onto the examined structure. Through a series of relevant experimental tests on the fabricated laminated composite panels with/without damage, the proposed acousto-ultrasonics-based multi-damage index identification approach was validated. The developed damage indices are competent to evaluate a structural health state in terms of damage quantification, and all of the validation results fell well in the prospected ranges. Moreover, it shows a linear and consistent trend between the variation of two damage indices and damage extents. Based on the particular relation, the linear regressive prediction functions were established separately regarding the two damage indices. They can be used to assess a structural health state due to the damage growth in real time. The proposed multi-damage index identification approach demonstrates its potential to serve as an online assessment tool to be aware of the reliability condition of a composite structure.