Evaluation of instantaneous characteristics of guided ultrasonic waves for structural quality and health monitoring

The analysis and extraction of the appropriate signal's features in structural health monitoring applications is one of the major challenges on which the robustness of the designed systems relies. Many strategies have been developed in the past, which utilise the identification of amplitude-based parameters for the evaluation of structural integrity. However, these parameters usually require a baseline reference, which might be extensively affected by noise, environmental or mounting conditions. This paper illustrates the applicability of Hilbert transform and Hilbert–Huang transform on the postprocessing of guided ultrasonic waves for evaluating the condition of relatively complex structural health monitoring applications. Two case studies are presented to demonstrate the suitability of the techniques, namely, the damage monitoring of an aluminium repaired panel and the cure level monitoring of symmetric carbon fibre–epoxy composite laminates. In the first case study, the technique exhibits sensitivity in propagation paths within damaged areas and shows good agreement with the developed damage, enabling the identification of critical areas. In the second study, the technique demonstrates a significant advantage over the traditionally adopted approaches and predicts accurately the cure level of the polymeric composite system.