Ultrasonic Health Monitoring of Thermoplastic Composite Aircraft Primary Structures

The adoption of composites as aircraft primary structural material has created a real need for a new aircraft maintenance philosophy to enable almost real-time structural condition assessment. It is thus crucial to develop structural health monitoring (SHM) systems capable of performing damage diagnostic and remaining useful life prognostic. At the same time, the urge to reduce production costs has led to consistent developments in thermoplastic composite (TpC) technology. In particular, new possibilities have been unlocked for automated assembly processes based on welding. This context constitutes a unique opportunity for integrating research on SHM into the advances of TpC technology, in order to contribute to a combined reduction of production and maintenance costs, and thus to the development of a truly cost-effective composite airframe. Over the last three decades, ultrasonic guided waves (GWs) have been recognised as having a great potential for detailed quantitative diagnostic of damage in composite structures. However, there are still no certified GW based SHM (GW-SHM) applications for civil aircraft. The reason for that is a limited understanding about the interaction between measurement variability factors associated with real operational environments, damage types, materials and geometric complexity. Therefore, the aim of the research presented in this thesis was to accelerate the bridging of those knowledge gaps and thereby to improve the reliability of GW-SHM for composite aircraft.
The research presented in this thesis has put forward three possible paths for improving the reliability of GW-SHM systems for composite aircraft. First, to the early detection of manufacturing defects by investigating the relationship between GW propagation and assembly process parameters at structural element scale. Second, to reduce the uncertainty in the damage diagnostic by increasing the systematisation of the GW-SHM system design. Third, to increase the robustness of damage diagnostic capabilities by studying the effects of real operational-environmental variability factors on GW propagation at real scale.