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DevTMF: raising the standards of European aviation

Improving the sustainability of European aviation correlates directly to the development of new materials – or improvement of existing ones – that are used in aeroengines. And to ensure safety, materials used within the hostile internal environment of an aircraft turbine engine – especially the rotating turbine discs – must withstand dramatic variations in temperature and mechanical stress. Clean Sky's DevTMF project is dedicated to developing methods and models so that the European aerospace industry can predict how materials in turbine discs perform in conditions where both stresses and temperatures vary with time.

Coordinated by Linköping University, with the participation of Swansea University and The University of Nottingham, Clean Sky's DevTMF (Development of Experimental Techniques and Predictive Tools to Characterise Thermo-Mechanical Fatigue Behaviour and Damage Mechanisms) project is on a mission to put European aeroengine manufacturing at the forefront by improving the industry's ability to predict the behaviour of materials under extreme and variable conditions. The project is a part of WP5 of Clean Sky's Engines ITD – Very High Bypass Ratio engine demonstrator – focused around technologies including engine core optimisation and integration, compressor efficiency, and structural design for low pressure turbines.

There are three strands to the project: Improvement and development of advanced standard and non-standard cutting-edge Thermo-Mechanical Fatigue (TMF) experimental methods and harmonisation of the test methods to enable standardisation by performing studies into the phenomena for a range of representative parts; advanced metallurgical assessment of structural disc alloys, looking at multiple variables to determine active damage mechanisms that control the life under TMF operating conditions; and physically based coupled models, with experimental validation, capable of predicting TMF initiation and propagation lives of components subjected to complex engine cycles and which are suited to implementation in the computer programmes used to predict component lives. The project will take these technologies to TRL5. 

"Low emissions, improved fuel efficiency and advanced temperature capability are pivotal to Rolls-Royce's AdvanceTM and UltraFanTM engine demonstrator programmes. To improve gas turbine efficiency necessitates weight reductions or increases in temperature. Either way, materials are key" says Svjetlana Stekovic, Senior Researcher and EU Senior Research Officer at Linköping University (LiU). "DevTMF will demonstrate the capability of increasing the operational window of the existing disc alloy and of using a new developed alloy in relevant engine components. In both cases, the capability of alloys and TMF technologies to cover existing and future requirements will be validated. Knowledge of TMF failure is critical in understanding component reliability because TMF cycles are caused by uneven temperature distribution across components. Therefore reliable TMF prediction depends on reproducing the right temperature cycles homogeneously across and along specimen gauge length, as well as establishing high quality material and mechanical TMF property data". 

Work has been performed at LiU to characterise static and dynamic thermal gradients as well as use of different heating methods for validating TMF crack initiation and propagation test methods. Also, the effect of static and dynamic crack tip heating in induction field was studied to provide more in-depth analysis as high heating rates and good accessibility for crack monitoring techniques make induction heating the preferred heating method for TMF tests in this programme. 

"DevTMF has made significant developments to the existing test methods to ensure robust TMF testing with repeatability and standardisation a top priority. With preferred test methods in place, the main programme of TMF testing is underway. A round robin test series (undertaking comparable tests but in different laboratories) has been conducted to generate a technical base towards a standard protocol" – and that's a key point, says Dr. Stekovic: "The EU has an active standardisation policy that promotes standards for better regulations to enhance European industry competitiveness and ensure interoperability of products and services, reducing costs, and improving safety. Standardisation bridges the gap between research and the market by enabling the fast and easy transfer of research results to the European market. One of the main activities in DevTMF is to produce a Code of Practice for the TMF experimental methods, which will be a base for future standardisation through the High Temperature Mechanical Testing Committee of the European Structural Integrity Society (ESIS) and International Organization for Standardization (ISO)".

In pursuit of this standardisation drive, several foundational tasks have been carried out under the modelling efforts, which will facilitate advanced material modelling of the candidate turbine disc alloy for TMF crack initiation and crack propagation mechanisms, and material parameter identification and optimisation tasks have been conducted using isothermal test data.

"Initial results are promising and a great deal of work has also been conducted on comparing computational 'cycle jumping methods' which reduce computational time when the developed material models are applied to production components" says Dr. Stekovic.

By the end of the project the materials understanding and lifing models will help optimise the performance of existing individual aero engine components, enabling reduction of fuel consumption by allowing them to run at higher temperatures and pressures, thereby increasing engine efficiency and reducing CO2 emissions without hardware modifications. Over a longer timescale the project will influence the development of new disc alloys and ultra-efficient future aero engine designs. 

"By the end of the project, DevTMF will achieve a more accurate prediction of product life of gas turbine components under TMF conditions, potentially prolonging engine service life which will translate into quantifiable environmental and economic benefits for Europe's aeroengine industry" concludes Dr. Stekovic. "A rough performance study indicates that if DevTMF delivers material and TMF lifing methodologies capable of allowing a turbine disc to operate at 750ºC instead at 700ºC under complex cycles whilst maintaining component integrity, it will result in fuel burn saving and CO2 reduction of 1.7%. A 1% specific fuel consumption in these products is equivalent to 350kg less fuel used per engine per transatlantic flight, which reduces the cost of ownership by approximately €100,000 per engine per year".