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High Cycle Fatigue Prediction Methodology for Fibre Reinforced Laminates for Aircraft Structures in CROR Environment – Development and Validation

Periodic Reporting for period 2 - HEGEL (High Cycle Fatigue Prediction Methodology for Fibre Reinforced Laminates for Aircraft Structures in CROR Environment – Development and Validation)

Reporting period: 2018-08-01 to 2020-01-31

The achievement of more eco-efficient aircraft able to reduce CO2 and NOx strain has led the way to exploring innovative advanced engine solutions, such as the contra-rotating open rotor (CROR). The integration of new engine technologies 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, 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 demanding vibration loads in engine 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.
During the second reporting period of the HEGEL project, a framework of activities, including both experimental testing and numerical modelling, was undertaken.

An extensive experimental fatigue testing programme was performed by University of Twente, involving frequency, temperature (up to 50°C), humidity (dry and fully saturated) and strain levels as main parameters. Frequencies higher than 100Hz were considered during testing as this allowed to determine and assess how self-heating phenomena affected the fatigue life of the specimens. The experimental setup consisted of an electromagnetic shaker, instrumented with a single point Laser Doppler Vibrometer (LDV) to measure the vibration response (velocity), an accelerometer to record the base excitation and a thermal camera to capture self-heating caused by the vibration. The material considered during the experimental testing was T700-M21 from Hexcel. The specimen fatigue life was monitored by tracing the response phase of the specimen, instead of the resonance frequency. A major benefit in using this methodology is the higher sensitivity of the ‘phase’ parameters to small changes in the structural integrity, enabling identification of ‘critical events’ at early stages of the fatigue life. In addition, the low level of constraints associated with the test setup, compared to that of conventional tests, enables to replicate more closely real case scenarios of HCF loading conditions (e.g. free vibration caused in engine environment). Furthermore, the implemented methodology allows to significantly reduce the testing time compared to HCF conventional test procedures. The experimental activities were supported by NLR, through physical analysis and non-destructive inspections and fatigue data analysis.

Modelling activities were also performed by TWI and concerned the definition and implementation of two different modelling frameworks. One modelling framework is suitable for simulating conventional/standardised fatigue tests (often static or quasi-static), where a combination of relatively high level of constrains imposed by the test setup (e.g. tensile, three point bending, four point bending) and load conditions (e.g. low frequencies) allows neglecting dynamics effects. The mentioned modelling framework could cover the study of both damage initiation (based on continuum damage mechanics approach) and crack growth (based on fracture mechanics principles – e.g. VCCT technique). A second modelling framework was also investigated and aimed to replicate fatigue problems where the dynamic nature of the tests can no longer be neglected (the fatigue response is affected by the individual modes). This second modelling framework involved an iterative process of a sequence of different type of analyses, such as modal analysis, steady state dynamic and VCCT static analyses. The framework can be particularly suitable for fatigue assessments where the load envelope triggers dynamic responses of the test specimen, which are far more challenging to be replicated in a conventional/standardised mechanical testing environment. The modelling frameworks were verified against the experimental tests.

As a result of the activities described above, HEGEL provided the aerospace community with ‘know-how’ and a framework of fatigue prediction methodologies, which can facilitate the design process of composite structures subject to HCF and fatigue in general. The topic and findings from the project were disseminated via different routes, including conferences (AeroMat 2018, Chemnitzer seminar at Fraunohfer 2018, 7th ECCOMAS Thematic Conference on the Mechanical Response of Composites, Thermal Analysis Conference (TAC) 2019, International Modal Analysis Conference 2020), proceedings, newsletter and press releases.
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 and framwork of methodologies will be developed and validated in the HEGEL project, accounting for temperature, humidity and frequency-dependent phenomena (e.g. self-heating). Such expanded models and optimised experimental procedures 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.
Example of damage accumulation in FE models
Schematic sound source and amplification system