On Structural Health Monitoring of Aircraft Adhesively Bonded Repairs

The recent interest in life extension of ageing aircraft and the need to address the repair challenges in the new age composite ones, led to the investigation of new repair methodologies such as adhesively bonded repair patches. The present thesis focuses on structural health monitoring aspects of the repairs, evaluating their performance with guided ultrasonic waves aiming to develop a monitoring strategy which would eliminate unscheduled maintenance and unnecessary inspection costs.
To address the complex nature of the wave propagation phenomena, a finite element based model identified the existing challenges by exploring the interaction of
the excitation waves with different levels of damage. The damage sensitivity of the first antisymmetric mode was numerically investigated. An external bonded patch and a scarf repair, were further tested in static and dynamic loadings, and their performance was monitored with Lamb waves, excited by surface-bonded piezoelectric transducers. The response was processed by means of advanced pattern recognition and data dimension reduction techniques such as novelty detection and principal component analysis. An optimisation of these tools enabled an accurate damage detection under complex conditions. The phenomena of mode isolation and precise arrival time determination under a noisy environment and the problem of inadequate training data were investigated and solved through appropriate transducer arrangements and advanced signal processing respectively. The applicability of the established techniques was demonstrated on an aluminium repaired helicopter tail stabilizer. Each case study utilised alternative nondestructive techniques for validation such as 3D digital image correlation, X-ray radiography and thermography. Finally a feature selection strategy was developed through the analysis of the instantaneous properties of guided waves for damage detection purposes.