Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - HEGEL (High Cycle Fatigue Prediction Methodology for Fibre Reinforced Laminates for Aircraft Structures in CROR Environment – Development and Validation)

Teaser

The achievement of more eco-efficient aircraft able to reduce CO2 and NOx strain has led the way to exploring innovative advanced engine solutions. One of the most valuable technologies is seen in the Contra Rotating Open Rotor (CROR) propulsion system. The integration of this...

Summary

The achievement of more eco-efficient aircraft able to reduce CO2 and NOx strain has led the way to exploring innovative advanced engine solutions. One of the most valuable technologies is seen in the Contra Rotating Open Rotor (CROR) propulsion system. The integration of this engine technology requires changes in the aircraft architecture as well as giving rise to several challenges in the field of structural integrity. In particular, high dynamic loads are transferred to the aircraft primary structures and the fuselage can have highly stressed interfaces, with very high demands for vibration loading and potential high cycle fatigue (HCF) issues. In this context, in order to enable a successful integration of new engine technology solutions, tools capable to predict the long-term fatigue life of CFRP materials becomes an important aspect of the design of composite aero-structures.

The overall objective of the HEGEL project is to develop high cycle fatigue (HCF) testing capabilities and a methodological framework to study the long-term fatigue life of composite laminates used in new structural architectures, subjected to high sound pressure loading in contra rotating open rotor (CROR) environment. The achievement of the overall project aim will be tackled through the accomplishment of two main technical objectives:

• Development of a sound source and amplification system representative of the high sound pressure generated by the CROR;
• Development and validation of an enhanced accelerated fatigue prediction methodology framework for HCF life prediction of CFRP laminates.

Work performed

The development of new testing capabilities within the HEGEL project is achieved through two main areas of investigation. The first area aims to the development and manufacturing of a sound source and amplification system (WP2) able to replicate the acoustic pressure and loading typically generated in CROR environment. A second area of investigation is the development of an HCF prediction methodology framework, through undertaking a comprehensive technical programme involving both experimental (WP3, WP4, WP5, WP7) and numerical modelling activities (WP6).

Activities in WP2 were undertaken by NLR and aimed to the development and manufacturing of a sound source and amplification system. As a result of preliminary experimental trials and theoretical calculations, the final design was identified and successively manufactured, delivered and installed at Fraunhofer IBP in Stuttgart.

A parametric experimental investigation with respect to temperature (-50°C to 250°C) and humidity (‘dry’ and ‘fully saturated’) was undertaken in WP3 (led by NLR and supported by TWI) to determine fatigue master curves. The material used during the investigation was T700-M21 from Hexcel. The methodology framework involved the following tests: dynamic mechanical analysis (DMA), four-point bending constant strain rate (CSR) tests and four-point bending fatigue test at zero stress ratio. Low frequencies loading (<10 Hz) were used during fatigue testing. A second part of experimental activities in WP3 was related to the generation of material properties (Task 3.3) to be used as input into the FE models activities in WP6.

In WP4 physical analysis of the specimens via destructive and non-destructive techniques were undertaken by NLR and TWI, to support the experimental and numerical activities respectively in WP3 and WP6. Microscopy and NDT analyses included: optical microscopy, scanning electron microscopy and fluorescent penetrant inspection.

The technical work being undertaken in WP5 is led by UoT and aims to extend the work carried out in WP3 by studying the high cycle fatigue phenomenon at higher frequencies (>100 Hz). In presence of the higher frequency range, self-heating effect will be included in the study, in addition to the effects due to temperature and humidity. An experimental programme was developed to correlate a series of mechanical properties. The test sequence was designed to extract material, modal and geometrical parameters. The material used for the preparation of samples for the test programme was T700-M21 from Hexcel, as in WP3.

WP6 in the HEGEL project involves all those activities undertaken by TWI and aimed to the development of the FE-based predictive model for high cycle. As part of Task 6.1, TWI developed a model to predict the onset and accumulation of intra-laminar damage based on continuum damage mechanics (CDM). Damage accumulation was monitored through degradation of the material stiffness. The development, calibration and verification of the CDM model in Task 6.1 were supported by the experimental activities in WP3 and WP4. The material properties implemented in the models were those experimentally obtained in WP3 for carbon fibre reinforced epoxy T700/M21. Damage modelling approach adopting Puck’s criteria appeared to offer the most stable, accurate and less computational expensive way to investigate damage initiation and accumulation during fatigue life. As part of Task 6.2, the development activities of a damage propagation FE model that can be successively combined with the damage initiation model capabilities developed in Task 6.1 have been initiated by TWI.

Final results

Expected results until the end of the project can be listed as follows:

• Design, development and delivery of a sound source and amplification system able to replicate sound pressure present in engine environment.
• Development and validation of a semi-empirical predictive model for high cycle fatigue based on master curves and shift factors, and able to take into account the effects of temperature, humidity and frequency (frequency dependant factors).
• Development and validation of a numerical HCF predictive model that can be used as an extension of the semi-empirical fatigue model based on master curves and shift factors.

Progress beyond the state of the art associated with the project results mentioned above are as follows:

• The availability of a laboratory-based sound system able to replicate sound pressures typical of CROR environment is limited or not existent. The development of the sound source and amplification system within the HEGEL project will allow to go beyond the state of the art.
• Enhanced models will be developed and validated in the HEGEL project, accounting for temperature, humidity and frequency-dependent phenomena (e.g. self-heating). Such expanded models will be able to reduce number of experimental tests during the design process and will open the potential to reduce the overall time of testing.

Ultimately, the availability of such testing capabilities will facilitate the implementation of new disruptive engine solutions, therefore contributing to the achievement of the ACARE SRIA agenda by 2050 (75% reduction in CO2 emissions per passenger kilometre to support the ATAG target and a 90% reduction in NOx emissions. Reduction of perceived noise emission of flying aircraft by 65%). In addition, the successful achievement of the technical deliverables within the HEGEL project will help contributing towards the ACARE Flightpath 2050 goals in terms of maintaining a European leading edge in design, manufacturing and system integration and decreasing development costs (including a 50% reduction in the cost of certification) by streamlining systems design and certification during upgrade processes.