Testing composites for next-generation aircraft engines
Whether operators or manufacturers, the entire aerospace industry shares the same key goals: reduction of fuel consumption, mitigation of harmful emissions and noise decrease. Being part of Clean Sky's Sustainable and Green Engines programme, the Open Rotor demonstrator is a new engine architecture featuring two contra-rotating, unshrouded fans. Designed to power upcoming generations of single-aisle commercial jets, the Open Rotor aims to reduce fuel consumption and CO2 emissions by about 30 % compared with current CFM56 engines. The EU-funded HEGEL project was established to support the design process of this new aero-structure configuration, by delivering advanced assessment capabilities and procedures for composite materials. “Overall, our primary aim was to develop advanced testing tools, both virtual and experimental, to evaluate the ability of composite laminates to cope with dynamic and vibration loading, excesses of which can lead to fatigue damage in components,” notes Damaso De Bono, HEGEL coordinator.
Noise pollution – a major preoccupation
A major challenge associated with the open rotor aircraft engines is the noise levels generated by the propulsion system. This is mainly due to the rotating blades which are open and not ‘quieted’ by the fan case and the nacelle (as they are on a turbofan engine). “A solution to this challenge is to use open rotor engines in a pusher configuration, where the rotors with the engines are mounted at the rear of the fuselage. Such configuration requires major changes to the current aircraft architecture and thorough tests for structural integrity,” explains De Bono. Researchers at the Netherlands Aerospace Centre (NLR), one of the partners involved in HEGEL, developed a unique, sound source and amplification system that was put through its paces at Fraunhofer’s Institute for Building Physics test facilities. “The state-of-the-art system assesses the response of structural components and materials to different sound pressures and associated vibrations typically present in engine environments, such as the contra-rotating open rotor,” explains De Bono. “Laboratory tests could by-pass expensive large-scale tests at least at the early stages of the design process of structural components.” 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. One of the ways in which the composite laminate specimen can be tested is through the use of a 500x500 mm2 flat plate that covers the opening. The sound system is then activated, causing sound pressure to hit the plate. Sound pressure levels can vary according to test-specific requirements.
A fatigue life prediction framework
“Current numerical and computational prediction tools for high cycle fatigue are not fully validated for aerospace applications and only consider a limited number of parameters,” notes De Bono. HEGEL expands on existing fatigue prediction methodologies by incorporating additional environmental parameters (e.g. temperature, humidity) and dependent factors occurring at elevated frequencies (e.g. self-heating effects) during high-cycle fatigue. The newly developed prediction tools address the study of high-cycle fatigue via a semi-empirical method based on master curves and shift factors as well as advanced finite element models. “Our new testing and assessment tools can significantly improve the effectiveness of how composite materials in aircraft parts are assessed for structural integrity during the design process,” concludes De Bono. Overall, project results constitute an important step towards the development of future aircraft engines and their integration in new aircraft architectures.
Keywords
HEGEL, open rotor, composite materials, fatigue prediction, sound pressure, contra-rotating open rotor, high-cycle fatigue
