On the Structural Health Monitoring of Repaired Aerospace Structures

A major challenge of aircraft manufacturers is to ensure the operational life extension of ageing aircraft with an affordable maintenance cost. Composite adhesively bonded repair patches are a promising approach which could meet the increasing aerospace industry demands for aircraft maintenance in an efficient and relatively easy to apply manner. Structural health monitoring (SHM) techniques are an essential part of the design of a reliable and robust repair system. In-situ continuous damage assessment of the repaired region could provide information about the integrity of the repaired structure when it is subjected to mechanical and thermal loads, hence eliminating unscheduled maintenance and unnecessary inspection costs. This work aims to investigate the key challenges in different repaired configurations in terms of health monitoring through guided ultrasonic waves. Two representative composite repairs are tested, namely a scarf patch repair and an external bonded patch, while their structural integrity are assessed under static and cyclic loads, respectively. The panels are interrogated with Lamb waves that are actuated and captured by surface bonded piezoelectric transducers. The investigated techniques aim to characterise the integrity of the repairs by means of pattern recognition and data dimension reduction techniques such as outlier analysis and principal component analysis. In addition, the applicability of the proposed methods is demonstrated on an aluminium helicopter's stabilizer in which a pre-introduced crack is repaired with a composite patch and the demonstrator is subjected to bending fatigue. Each case study utilised alternative nondestructive techniques for validation such as 3D digital image correlation (DIC) and X-ray radiography. It will be shown that the obtained results successfully identify the presence of damage, hence providing a potential tool for built in on-line monitoring of adhesively bonded patch repairs.