European HEGEL project develops fatigue prediction method for aerospace industry composites

19/02/2019

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The three-year long HEGEL project commenced in early 2017 following a successful bid for Clean Sky funding by a consortium comprising Netherlands Aerospace Centre (NLR), the University of Twente and TWI Ltd.

Clean Sky contributes to strengthening European aero-industry collaboration, global leadership and competitiveness, and is the largest European research programme developing innovative, cutting-edge technology aimed at reducing CO2, gas emissions and noise levels produced by aircraft.

The European aerospace industry is collectively working on solutions that will contribute to achieving reductions of: 75% in CO2 emissions per passenger kilometre; 90% in NOx emissions; and 65% in perceived noise by 2050, as specified in the updated Strategic Research and Innovation Agenda (SRIA).  This provides the roadmap for aviation research, development and innovation that accounts for both evolutionary and revolutionary technology, and was developed by the Advisory Council for Aviation Research and Innovation in Europe (ACARE).

These targets have led to the exploration of advanced options in aircraft engine design, the successful integration of which would require changes in aircraft architecture, with high dynamic loads being transferred to the primary structure and potentially resulting in: stressed interfaces on the plane’s fuselage; excessive vibration loading; and fatigue-related issues.  Such challenges in structural integrity have led to composite materials, for example carbon fibre reinforced polymer (CFRP), being investigated for their use in aerospace structures.

The HEGEL project was established to support the design process of new aero-structure configurations, expected in the demonstrators of the Clean Sky 2 Large Passenger Aircraft (LPA) programme, by delivering advanced assessment capabilities and procedures for composite materials.  Overall, the primary aim of HEGEL is to develop advanced testing tools with which to evaluate the ability of composite laminates to cope with dynamic and vibration loading, excesses of which can lead to fatigue damage in components.

The first 18 months of extensive research and development by the project partners, in collaboration with the Clean Sky Topic Manager, Fraunhofer, has resulted in the creation of a unique, lab-based, sound source and amplification system.  This was developed by project partner NLR and was recently put through its paces in the first set of laboratory trials, held at Fraunhofer IBP’s test facilities in Stuttgart, Germany.  The state-of-the-art system generates sound pressure levels typically present in engine environments such as contra-rotating open rotor (CROR). 

The hardware consists of eight speakers connected to an equivalent number of tubes guiding the sound waves, all symmetrically configured, and secured within an acoustically and thermally insulated housing box, with the sound tubes converging on an external opening.  One of the ways in which the composite laminate specimen can be tested is in the form of a 500ml by 500ml flat plate, clamped to cover the opening using appropriate fixtures.  The sound system is then activated, causing sound pressure to hit the plate. 

Sound pressure levels can be varied according to test-specific requirements, enabling the specimen’s likely response to the different sound pressures and associated vibrations generated in an engine-like environment to be investigated.  The system can operate in a frequency range of 0.1-1kHz, with a sound power level (PWL) of 150dB for 1/3-octave bandwidth filtered white noise and pure tones, and overall 140dB for white noise.

The next major deliverable for the project will be the development of a prediction methodology framework for high-cycle fatigue (HCF) of composite materials.  The prediction tools will approach the study of HCF via a semi-empirical method, based on master curves and shift factors, as well as through advanced finite element (FE) models.  The methodology framework will also be able to account for the effects of temperature and humidity as well as the influence of high frequency factors such as self-heating.

Damaso De Bono, Project Leader at TWI explained “Current accelerated fatigue prediction methodologies are not fully validated for aerospace applications and only consider a limited number of parameters.  Therefore, the technologies applied in HEGEL have the capacity to significantly improve the effectiveness of how composite materials as used in aircraft parts are assessed for structural integrity during the design process.”  “This project will also expand on existing fatigue prediction methodology by incorporating both additional environmental parameters and dependent factors occurring at elevated frequencies during high cycle fatigue.” he added.

Evidenced by the positive results from the project consortium’s research and development activities thus far, HEGEL is on target to achieve its ambition of tangibly contributing to an increase the capabilities and competitiveness of the European aircraft industry by the time it completes in 2020.

Image: 3D virtual visualisation - external perspective of the HEGEL

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