Overview
The structural health of engineering structures is increasingly threatened by material degradation. Reliable means of health monitoring are therefore required for safe operation. Based on this monitoring, maintenance actions can be undertaken. Whereas a time-based inspection scheme has resulted in excellent reliability records for aircraft, there is an economic drive for more innovative health monitoring procedures. A proposal was maade to switch from time-based, towards condition-based procedures, where maintenance is only performed when a component is known to be degraded. This required a means of continuously assessing the structural integrity of the aircraft by a continuous damage monitoring system.
The European project 'Aircraft Integrated Structural Health Assessment' was dedicated to the establishment of the basic elements of a health monitoring system based on ultrasonic Lamb waves. The project AISHA aimed to contribute to realising an aircraft monitoring technology, by using ultrasonic Lambwaves as the basic sensing principle. The special potential of Lambwaves for damage detection arises from their propagation capabilities. Lamb waves are guided acoustic waves propagating in plate-like structures. If there is damage, the propagation of an ultrasonic Lamb wave will be disturbed, resulting in a characteristic reflection and attenuation pattern.
During this interaction, there is also the possibility that the acoustic mode is partially changed. The particular reflection or attenuation characteristics can be detected, and if the correct functional relationship between signal-damage is known, the amount of damage can be assessed. The functional relationships between signal and damage are obtained theoretically or experimentally, but a combined approach including advanced data analysis provides the best results.
A consortium with broad and multidisciplinary expertise was formed, with contributions from high tech SMEs, university research groups, end-users and a certification laboratory.
The final goal of the project was to apply the selected automated non-destructive testing (NDT) techniques to full-scale parts which represented a broad spectrum of possible applications in operating aircraft. A special focus was the use of realistic environmental conditions which contrasted with many other results from different projects where structural health monitoring systems are only applied under idealised laboratory conditions.
For the investigations of widely used aluminium alloy structures, such as frame-stringer constructions, we chose a helicopter tail boom from a MI-8 helicopter to install a sensor network to follow stress induced crack propagation by ultrasonic waves. A dedicated test rig was installed to simulate real flight conditions to enable a realistic validation of the technique applied.
Another full-scale part was the slat-track of the AIRBUS A320. This structural part is a moving beam that connects the wing with the leading-edge slats to vary the surface of the wings available for the aerodynamic lift. The main problem in installing a sensor network is the complex structure of the beam and the tiny space available for the installation of sensors and cables. The final goal is to follow cracks occurring at points with high stress intensity.
Funding
Results
The AISHA project started with an inventory of materials, structures and damages typically occurring in aircraft. Based on this information, the required Lamb wave propagation properties were identified on a theoretical and experimental basis. This provided important clues for the selection of appropriate Lamb modes which could be applied in the further course of the project.
A further step was the development and selection of appropriate sensor systems and dedicated electronics. For the excitation of Lamb waves, piezoelectric patches were selected and piezoelectric patches as well as single mode optical fibres were used as sensors. The use of piezoelectric patches and optical fibre sensors for detection is complementary, in many cases and both methods can be implemented in real aircraft.
Experiments performed within AISHA on lab-scale and on selected full-scale parts showed the ability of Lamb waves or other guided waves to give information on correlations between acoustic parameters and damage in structural parts. The validation of the results was performed by established state-of-the-art NDT techniques, such as ultrasonic C-scan methods.
Technical Implications
In May 2008 started the project AISHA II. It was based on the consortium of AISHA, but the 'chain of expertise' was extended with a couple of new partners, InSensor (Denmark), Fraunhofer Institut IFAM (Germany), University of Leipzig (Germany), University of Basque Country (Spain), Free University of Brussels (Belgium) and Lufthansa Technik (Germany).
The goal was not only to continue the work which was started in the AISHA I project, but the work plan also contained further innovative aspects.