Nieuwe methoden voor structurele gezondheidsmonitoring in de luchtvaart: Detectie van door vermoeiing veroorzaakte materiaalveranderingen door het volgen van modulatie van ultrageluid in dynamisch belaste structuren Detectie van binnendringend vocht

The research presented in this manuscript is situated in the field of Structural Health Monitoring (SHM) of airplanes, which could be defined as the continuous monitoring of the 'state' of the constitutive material of different parts, or of the full assembly. This monitoring allows to keep track of the state of parts of the structure, and employing this information for diagnosis, decision-making and maintenance, finally leading to predictive maintenance. Ideally, the parameters describing the state of the structure must remain near their nominal values throughout its lifetime. Performing SHM can aim at replacing, being complementary or optimizing non-destructive inspections. The latter approach is starting to emerge in existing aircrafts, where SHM systems are attached to existing designs without altering their properties. SHM can also involve the use or incorporation of smart materials and components, which make part of a structure or part of interest and sense its state of health. The current work presents the concept, implementation and validation of two approaches. The first one is respectively for metallic parts that are subjected to fatigue inducing loads, and the second approach is for composite structures that are subjected to moisture ingression. The first part of this work presents a SHM method based on the use of ultrasonic waves for early crack detection in an aluminium plate subjected to cyclic loads. The majority of airplane structures are frequently subjected to dynamic loads that cause fatigue damage at locations of high stresses, like in slat-tracks during take-off or landing, or in riveted areas. Therefore, continuous monitoring of these designated areas with a dedicated SHM system can provide an early warning of material degradation for triggering maintenance operations. In the performed experiments, a fatigue machine was used for the cyclic loading and to initiate a fatigue crack, while the modulation of sinusoidal ultrasonic wave packets traveling from a sending to receiving transducer around high and low loads in the fatigue cycle was monitored. Digital image correlation was used to determine the strain and to confirm crack formation. The ultrasonic wave parameters at different load conditions were found to be very sensitive not only to the initiation of the crack but also to plastic deformation, which, according to digital image correlation monitoring, preceded the crack formation and changed the effective elastic modulus probed by A0 and S0 Lamb wave packages. Single-value parameters aimed at tracking material changes were designed. The parameter values exhibited deviations from their baseline value at about 70% of the fatigue life, whereas the first crack visibility that could be detected by the naked eye appeared only around 90-95% of the lifetime. Experimental analysis and simulations showed that the high sensitivity to microstructural changes was resulting from their substantial effect on the propagation velocity of the A0 and S0 Lamb wave packages, causing strong interference effects. The second part of this work addressed the problem of water ingression in airplane honeycomb sandwich composites, by introducing a SHM system for moisture detection. In the past decade, the use of composites in airplanes has increased significantly due to the possibility to produce lightweight assemblies with high stiffness and strength. However, despite the substantial advantages, e.g., design flexibility, resistance to fatigue, etc., water can be trapped in the honeycomb core due to substantial fluctuations in humidity and temperature during operation. Freeze-thaw cycles can subsequently lead to material degradation and delamination. Thus, an early warning of the water presence can be used as a protective measure in order to prevent further damage and reduce downtime. Percolation sensors were successfully integrated into the honeycomb composite plates during their manufacturing process, which involved the application of high temperatures and pressures in an autoclave. The sensors are made of an organo-ceramic composite that consists of conductive Titanium Carbonitride (TiCN) powder and Polyvinyl Alcohol (PVA). The concept of this sensor relies on the collapse of the percolation conductivity of the sensor due to swelling from water absorption. This results in a sudden change in the electrical resistance of the sensor. The functionality of this smart material was tested during moisture cycling to study its ability to react to the ingress of water. A climate chamber was used to simulate ground-air-ground (GAG) cycles in an accelerated fashion. After exposure of the samples to GAG cycles for around half a year, the sensors showed a reaction to moisture ingress. Additional tests were carried out to investigate the sorption/desorption cycles and sample behaviour. To showcase a possible implementation of the percolation sensor, prototype samples with an embedded percolation sensor and RFID tag were manufactured.