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Aircraft Integrated Structural Health Assessment (AISHA)

Final Report Summary - AISHA (Aircraft Integrated Structural Health Assessment)

The AISHA project aimed at realising an aircraft monitoring technology by using ultrasonic Lamb waves as the basic sensing principle. The special potential of Lamb waves for damage detection arises from their propagation capabilities, i.e. Lamb waves are guided waves which propagate in-plane structures. There are a number of Lamb modes being a function of frequency and plate geometry. These different Lamb modes show selective sensitivity to different kinds of defects, such as cracks, delaminations and uniform thickness degradation. Both active and passive wave inspection were explored. The information from these guided waves, combined with signal analysis routines and models for remaining lifetime prediction, was used in a full scale testing action during which the possibilities for large-scale application were explored.

The final goal of the project was to apply the selected automated NDT techniques to full-scale parts which can represent a broad spectrum of possible applications in operating aircraft. A special focus was the use of realistic environmental conditions which is in contrast to 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.

A digital and searchable database has been constructed, containing an inventory of common structural aircraft materials together with their relevant properties and degradation mechanisms. In this context, a summary of 22 types of representative aircraft material groups was established (metallic: aluminium, steel, titanium, long fibre composites), it contained information on their degradation under different loading conditions (failure by distributed and interacting damage modes, fatigue, environmental degradation, interlaminar fracture) and NDT inspection techniques. As a result, complete information about 56 different materials is available by a software-based database.

A special attention was given to the development or selection of the dedicated hardware (electronics and sensors). In this context, a Lamb wave driver and receiver system was designed. It enables the generation of Lamb wave signals over a wide range of frequencies. It was used by the partners in diverse experiments and it has the potential to be a prototype for a series of tailored electronics for Lamb wave generation and detection. The specific demands of Lamb wave drivers concern the specific frequency range, moderate amplitude requirements and dynamic ranges. A further important point is size and cost of the proposed systems.

An essential part of the project was devoted to establishing of quantitative relations between growing damage phenomena and detected signals. This step was 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 was 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, conclusions could be drawn about the fitness for service of the structure and the need for repair.

There is a strong correlation between signal and damage (fatigue crack) growing for all three kinds of samples:
- RMS is a stable parameter representing the signal intensity, and it decreases as a function of the length of a crack.
- The coupling of the sensors with the surface of the sample remained stable during the testing.
- If a sensor is installed in direction of the load and it is coupled directly with deformable surface, then the fatigue effect to coupling appears for used piezoceramics sensor at middle level of the stress.

The first model applied was the remaining lifetime model at the fatigue damage of aluminium structure. Basic regularities of the fatigue failure of thin-walled structure were analysed as a base for the remaining lifetime model. Both the remaining lifetime and remaining strength estimation were estimated by the methods of linear fracture mechanics. The problem of the damage tolerance in aircraft structures was discussed as a very important element of the model. Finally, the programme code for remaining lifetime prediction was elaborated and tested. After simple adaptation this code can be included in a SHM system. The second model was the model of environmental degradation of an aluminium structure.

Fundamentals of the predicting of remaining lifetime are the same as at pure fatigue stress. Special features of the simulation of the corrosive damage should be:
- The corrosion crack grows at dynamic and static stresses.
- The special constants of crack-resistance for small crack at the modelling of corrosive damage.

Some versions of composite degradation and remaining lifetime predicting were analysed. Three kinds of the model of remaining lifetime predicting:
- environmental model;
- mechanical model;
- combined environmental-mechanical model.
Remaining stiffness prediction was selected as the main phenomena for the environmental models for damaged composite. The most effective is the damage growth models.

Successful efforts were made to develop novel sensors / actuators for selectively generating and detecting Lamb wave modes. Methodologies for the integration of sensors and actuators into the structure were 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 150 K have to be tolerated by the measurement system.

This required an adequate modelling of damage states, calculating residual properties and predicting the remaining lifetime. A final research action was devoted to a full-scale testing of the obtained laboratory results. Results were obtained for the detection of impact damage at a helicopter beam made of composite material, a helicopter beam made of aluminium alloy and a slat track made of maraging steel.

The main requirements in structural health measurement technology were the following:
- high integration of the system into the structure (wireless sensors, long battery life, relevant location);
- validation on full scale tests for qualification;
- integration into the operational scenario.
In AISHA, promising results were obtained with ultrasonic and optical fibre techniques, especially for the online impact detection.

There were full-scale tests performed on EC 135 with an optical fibre sensor network, the Mil 8 helicopter and the Airbus A 320 slat track.

EC 135 with an optical fibre sensor network
Signal processing methods based on comparing the time-frequency images of signals obtained before and after the impact were used for detecting damage in composite materials. The method was tested on data obtained in a large-scale experiment made on a helicopter tail, using of real impacts and a small-scale experiment of using pseudo-defects. The results showed that in spite of big fluctuations a global view of all seven parameters calculated and averaged can show, with a very good approximation, the position of the damage.

Mil 8 helicopter
The used configuration of the non-destructive inspection based on Lamb wave technology was able to reliably detect the fatigue cracks in the riveted aluminium structure. All cracks located in an inspecting zone cause the response reaction that can be detected by the sensors. Piezoceramic sensors were able to keep their functional ability under intensive mechanical vibrations during the whole fatigue test. But investigation on the reliability of the system should be continued.

Airbus A 320 slat track.
The test was performed using an uni-axial test rig and here, and the sensor connection was ensured during the whole process, such as proven by measurements of the electric impedance of the sensors. Also in this case, the growth of the crack was visible by a strong response of the ultrasonic signal. A particular feature of this measurement was that the cracks are only open if the slat track is under load. This corresponds to operational conditions, where potential cracks would only be open during the start and landing of the aircraft. Therefore, ultrasonic measurements were performed under load and without load to get information on both cases.

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