The safe use of complex engineering structures such as aircrafts can only be guaranteed when efficient means of damage assessment are in place. Whereas aircraft design is nowadays based on a damage tolerance approach and time based inspection cycles, it is envisaged that the large cost associated with this approach can be drastically reduced by switching to a condition based maintenance schedule. This does require continuous health monitoring capabilities using integrated sensing technology and autonomous damage assessment.
The present project aims at the development of advanced monitoring systems for the structural state of aircrafts using extended sensor networks. The project started with the establishment of detailed specification sheets where aircraft operators and producers summarised their technical and economical requirements for health monitoring systems. Finally, five solutions for damage detection are investigated in the consortium: fatigue cracks in slat tracks of an Airbus A 380, impact damage in the tail boom of the helicopter EC 153, fatigue cracks in the helicopter tail boom of a Mil8, as well as corrosion in floor beams and fatigue damage in doubler repairs of an Airbus A340.
Structural Health Monitoring can essentially be considered as a kind of automated sensor network. Therefore, the development and selection of appropriate sensors plays a key role. For the ultrasonic excitation and sensing, array systems are used in different configurations that depend on the size of the full-scale parts applied. These arrays allow a tailoring of waveforms due to their geometric shape and an electronically controlled signal delay. Despite of the sophisticated detection concepts, these sensors has to be low-profile, i.e. they are not made for versatile used and the price must be relatively low when a high amount of sensors has to be implemented. Beside piezoceramic sensors, electrochemical, optical fibre and EMAT sensors are successfully applied in the project.
The sensor systems selected require dedicated electronic steering and sensing, this especially holds for tailored electronics for array transducers working at specific values under operational conditions. Also in this case, not only piezoceramic transducers were considered, but also optical fibre equipment and systems for electrochemical sensing.
An essential challenge is durable integration of transducers. Adhesives are under development that are able to withstand typical temperature and stress variations. A special focus in this context is the ability of sensor self-testing e.g. by impedance analysis. Furthermore, we could prove that pre-stressing of sensors can essentially contribute to the mechanical stability of piezoceramic transducers.
Before the selected technologies were applied to full-scale parts, experimental feasibility was checked for simplified samples. Here, basic results were obtained e.g. for the detection of impact damage by acoustic emission using optical fibres, the use of EMAT’s for crack detection, the detection o
On the way towards the main project goals, a number of results were obtained. A digital and searchable database has been constructed, containing common structural aircraft materials together with their relevant properties and degradation mechanisms. Furthermore, it was suggested to use a limited number of Lamb wave modes in the detection process. As a first step in the project, optimum Lamb wave mode sets were selected, taking into account the materials under investigation, loading condition and damage type. An important result of the project is the improvement of the understanding of the interaction of propagating Lamb waves with material defects in metals and long fibre composite materials. The use of this knowledge is not confined to the study of aircraft materials, but can also be used for the inspection of other structural parts that exhibit defect formation, such as chemical process installations with corrosion cracks or structural composites that are subject to impact or fatigue damage evolution. Efforts were done to develop novel sensors/actuators for selectively generating and detecting Lamb wave modes. Methodologies for the integration of sensors and actuators into the structure are explored. This especially regards the trade-off between the needs for a sensitive detection of ultrasonic waves and the severe operational conditions in aircraft where for instance temperature differences of more than 150K has to be tolerated by the measurement system.
A major part of the project is devoted to establishing of quantitative relations between growing damage phenomena and detected signals. This step will be aided by the development of automated signal analysis strategies, which aim at providing either a visualisation of the data or a multidimensional analysis. A separate action will be devoted to providing the link between the monitoring results and the actual structural condition. Based on a sound knowledge of the amount of damage present, a conclusion will have to be drawn about the fitness for service of the structure and the need for repair. This will require an adequate modelling of damage states, calculating residual properties and predicting the remaining lifetime. A final research action will be devoted to a full scale testing of the obtained laboratory results. First results were obtained for the detection of impact damage at a helicopter beam made of composite material.
Full-scale parts investigated within the AISHA II project:
- Slat tracks of Airbus A320 and A380
- Tail Boom Helicopter Mil 8
- Tail Boom Helicopter EC 135
- Floor beam
- Doubler Repair
An important part of the project is the improvement of the understanding of the interaction of propagating Lamb waves with material defects in metals and long fibre composite materials. The use of this knowledge is not confined to the study of aircraft materials, but can also be used for the inspection of other structural parts that exhibit defect formation, such as chemical process installations with corrosion cracks or structural composites that are subject to impact or fatigue damage evolution. Efforts were done to develop novel sensors/actuators for selectively generating and detecting Lamb wave modes.
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