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Fatigue Test

Final Report Summary - FATIGUETEST (Fatigue Test)

Executive Summary:
The main objective of the Fatigue test project, was to increase the understanding of composite aerostructures fatigue behaviour. To achieve this goal, an instrumented test panel was devised and tested under static and fatigue conditions.
The instrumentation in the panel was based on acoustic and fibre optic sensors provided by the Topic Leader. These had to be integrated within the panel laminate, which in case of the optic fibres proved to be a significant challenge.
A comprehensive study had to be performed, towards developing a manufacturing sequence which could guarantee a reliable integration of the optical fibres. These are extremely fragile and are easily damageable when demoulding the components.
After a series trials, a relatively reliable process was developed and the final test panel produced.

Following on the manufacturing of an appropriate test panel, the test campaign was initiated, associated with a comprehensive NDT inspection campaign of the various testing phases.
This campaign collected a significant amount of data that will be useful for future analysis, and to improve the knowledge of composites fatigue behaviour.

Project Context and Objectives:
Aviation needs to envolve into higher efficiencies and a greener footprint. This evolution is driven by several parameters, but one of them, and also a fundamental one, is weight. Composites have for decades been contributing for a lighter aircraft.
Due to their complex nature, non-isotropic, to optimize a composite structure, it is necessary to understand intrinsically its failure behavior, and all its failure modes. This requires extensive test campaigns.
Even with quite comprehensive test campaigns scope exists for unforeseen degradation, which forces the designers to take conservative methodologies to cover them. If the structures could be monitored during their lifetime this conservativeness level could be reduced, and consequently the aircraft made lighter.
Structural Health Monitoring is therefore quite relevant for any future greener aviation.
Within the Fatigue Test project an aeronautical panel with integrated stiffeners was developed. This panel has integrated acoustic and fiber optics sensors. With these sensors integrated the panel is tested in an undamaged condition. After this initial test the panel is subjected to further loading and re-tested to evaluate its degradation.
This testing combined with intermediate NDT inspections, allows correlating the damage initiation and propagation in the panel with the data recordings from the sensors.

The information acquired will permit a further understanding of the fatigue behavior of panel and these sensors, which will contribute to revised and more accurate methodologies.

Fatigue Test project objective was to develop the understanding of the fatigue behavior of a composite test panel. For that understanding acoustic and fiber optics sensors are integrated in the panel. The integration of the sensors itself is a significant part of the project development.
Following on the design and manufacturing of the panel, these will be tested statically and in fatigue. The data recorded by the sensors embedded will be processed to evaluate the damage generation and growth.
The development of this project will contribute to the development of the lighter airframes of the future.
During the project development, a major hurdle was identified, which its degree was not foreseen initially. The fiber optic sensors integration is an extremely complex process, especially in the application selected.
Due to the sensitivity of the fiber optics cables these are quite prone to damages during the cure cycle, this lead to 5 different panels need to be manufactured till a stable solution was encountered. This solution was based on mold tool modifications, which protected the cable while being cured and prevented significant resin flows into the sensors cables.
Following on the panels manufacture these were NDT inspected and subjected to rig testing to validate their properties and degradation with load.

Project Results:
The main objective of the technology being developed in this project, is to further enhance the understanding of the fatigue behavior of composite aeronautical components. The better the material is understood, the further it is possible to optimize the future airframes and contribute to a more sustainable industry.
In this context and following the results obtained in the tests carried out it can be concluded that a very efficient inspection solution was developed and implemented with successful results. The overall sensitivity of the integrated sensors was very high and even small pin holes areas were clearly identified.
The D4.6 report highlights the main results obtained.

Potential Impact:
The successful implementation of the current project topic will support profoundly the achievement of the impact objectives listed in the ITD Green Regional Aircraft since it will contribute particularly to meet the ACARE goals for Reduction of fuel consumption (CO2) and NOx emissions by addressing the Technology Domains as Aircraft weight reduction, through new structural design concepts, load control and new materials.
The project will also enable the aerospace industry to achieve a higher level of reliability in aircraft structures by means of a fully confident validation.
The economic growth around the world has led to a continuous increase of air-traffic numbers during the past decades. This increase is expected to continue at an even stronger pace for the next two decades. As the operating fleet grows, the operating and maintenance costs also increase. Despite the recent difficulties faced by the industry, the market forecast over the next twenty years for commercial aircraft is expected to see strong growth. The Air Transport Market remains a highly competitive one and any aspect with commercial advantage must be sought. This topic is directly related with key elements of competitive advantage for the industry, those of aircraft reliability during operation and maintenance costs.
Furthermore, with the large-scale introduction of composite fibre reinforced plastic structures, representing a step change in airframe manufacturing, new technological challenges have emerged. One fundamental aspect is the damage behaviour of composites and bonded components on airframe elements. Aircraft predating the Boeing 787 and Airbus A350XWB are characterised by a metallic fuselage where impact damages are clearly visible and can easily be detected during the routine walk around of pilot and maintenance personnel. Composite laminates no longer fail in the same way. Due to their high stiffness, thin structures spring back following an out-of-plane impact, leaving no obvious indication of the impact event. Within the material however, the stiff fibre layers bonded together by resin may have experienced sufficient shear to cause a delamination of the layers or in the worst case even fibre fracture. As this state of affairs is obviously not acceptable in practical aircraft structures, laminate components tend to be oversized to ensure that impacts sufficiently severe to cause internal damage will always also cause clearly visible external damage. Obtaining a reliable, accurate and rapid inspection tool for this type of damage would allow significant structural weight reductions.
By improving the pre-flight detectability of delaminations and disbondings, significant further weight savings can be anticipated. In addition, reliable bond verification allows completely new design and repair opportunities such as permanent bonded repairs yielding further improved aircraft availability.
The air transport industry is a key sector for Europe as a whole. The implementation of sound technological innovation means related with Health monitoring technologies will enable the European air transport industry to maintain its competitive advantage over overseas competitors and hold its global leadership in the forthcoming years. By heading global aerospace technological innovation, Europe will be able to maintain its status as the world’s leader in high-technology industrial applications. The testing and validation of the monitoring technologies related to the current topic will support profoundly the strong technological progress of the European aerospace industry.

The applicants will make use of the results of the project and knowledge gathered in other future related projects and industry support activities. It is expected to present the results in potential conferences related with composite behavior and NDT applied technologies.

List of Websites:
Website: Not applicable

Contact details:

António Reis