Long-term stability of guided wave structural health monitoring using distributed adhesively bonded piezoelectric transducers

Industrial interest in structural health monitoring of safety-critical assets is increasing. Guided wave structural health monitoring shows potential for practical use as it allows detection of small defects (1.5% reflection changes) in the presence of environmental changes. Guided wave structural health monitoring systems use bonded piezoelectric sensors to monitor wave propagation patterns and subtraction between signals and baselines from the “healthy” structure for robust discrimination of damage. Long-term stability of structural health monitoring using these techniques has been investigated in this article as this is essential for industrial deployment of a system. Plates that were exposed to thermal cycling in an environmental chamber were monitored using bonded S0 mode sensors operating in pitch catch. Analysis of structural health monitoring data revealed drifts in baseline subtraction results corresponding to progressive reduction in defect sensitivity levels. These drifts were correlated with increases in coherent variability and suspected to be due to variations in the adhesive properties of the bonds between the sensors and the structure. Finite element analysis and further experimental results confirmed that changes in adhesive properties were a likely cause of the degradation in monitoring capability with time.