In reply to the call released by the CleanSky team, named “Fatigue test of sensor integrated CFRP aircraft panels with stiffeners – JTI-CS-2009-2-GRA-01-025”, a consortium has been assembled to perform this project.
This consortium encompasses Optimal Structural Solutions Lda (www.optimal.pt) and ISQ – Instituto de Soldadura e Qualidade (www.isq.pt). Acting as as sub-contractor, and to guarantee the specimen will be manufactured to aeronautical standards, OGMA – Industria Aeronautica de Portugal S. A. (www.ogma.pt) will also participate. This team has a wide experience in composite aeronautical structures within their fields of expertise.
This project requires 3 core sets of competences, which are covered by each of the involved companies, 1) Engineering of aeronautical composite structures and test campaigns definition and coordination - Optimal Structural Solutions; 2) Testing of composite structures and NDT inspections - ISQ; and 3) Manufacturing composite structures to aeronautical standards - OGMA.
These competences have been acquired working in projects for leading aerospace companies. Details on the relevant projects are presented in this document.
Together, this consortium is confident it has all the required knowledge and experience to successfully perform the task in hand.
Aerospace industry moves to carbon fibres
The latest trend in aircraft manufacture is integral structures made of carbon fibre-reinforced polymers (CFRPs). Since carbon fibre airframes have been or are in the process of being designed, tests to verify required durability and damage tolerance are needed for their certification.
With financial support from the EU's Clean Sky Joint Technology Initiative (JTI), a consortium was assembled to assess if aircraft panels made of CFRPs can withstand the ultimate loads expected during service. The aim of the 'Fatigue test' (FATIGUETEST) project was to develop the experimental set-up to record and analyse their long-term behaviour.
Multi-step tests were carried out on five prototype aircraft panels with integrated stiffeners, designed and manufactured by FATIGUETEST partners. In addition, small piezoelectric sensors were embedded in the panel laminate to detect elastic waves generated by the rapid release of energy from sources within the CFRP material. Sources of acoustic emission include plastic deformations and fracture.
Complementing other non-destructive testing methods, acoustic emission and acousto-ultrasonics technology is sensitive enough to detect newly formed cracks, long before structural integrity is compromised. Notably, FATIGUETEST researchers opted not to test the carbon fibre panel to failure.
The custom-made test rig for static and dynamic loading of the panel consisted of two clamping devices and one anti-buckling device. The strains induced with a hydraulic servo-cylinder monitored and measured in real time. The huge amount of data recorded was then analysed and correlated against finite element models and simulation analysis of the panel.
Successful completion of the fatigue test by the fifth and final aircraft panel marks a major step in determining the strength of aircraft structures made of composite materials. The test rig and methodology proposed by the FATIGUETEST project is expected to go on to form part of future certification test programmes.