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

Abstract

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. Inspection with guided ultrasonic waves is a wave-propagation-based method, which has lately attracted the attention of the scientific community, offering various industrial applications in monitoring pipes, pipelines and rails [7]. Guided waves propagate in a bounded media, which interacts with the boundaries of the structure and propagate through its thickness in two possible modes; symmetric (S n ) and antisymmetric (A n ). The mathematical equations that describe their behaviour are the same with bulky waves. The only difference is that, in the case of guided waves, the equations should satisfy the boundary conditions. The most widely used technique for health monitoring is inspection with Lamb waves. Lamb waves are elastic perturbations that occur in solid plates with free boundaries. Those perturbations are a combination of displacements that occur both in the direction of wave propagation and perpendicularly to the plane of the plate [8]. Lamb waves have been extensively used for health monitoring of advanced, complex structures. The scope of the current investigation concentrates on aspects of Lamb wave's application for health monitoring of two case studies, the first on damage monitoring of bonded patch repairs and the second on monitoring the cure process of a composite laminate.Adhesively bonded repair patches have lately received a considerable amount of interest for their potential repair applications for temporary (emergency or field) solutions to structural damage. In the aerospace industry, the interest in bonded patches emerges from the need to increase the operational life of ageing aircrafts. Such aircrafts are estimated to be approximately 30% of the worldwide fleet [9]....

Novel method for true guided waves spectral characteristics estimation using a logistic function fit and nonlinear least square algorithm

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