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Development of Experimental Techniques and Predictive Tools to Characterise Thermo-Mechanical Fatigue Behaviour and Damage Mechanisms

Periodic Reporting for period 3 - DevTMF (Development of Experimental Techniques and Predictive Tools to Characterise Thermo-Mechanical Fatigue Behaviour and Damage Mechanisms)

Reporting period: 2019-02-01 to 2021-01-31

The overall aim of DevTMF is to enable further increase of operation temperature and service life in aero engine while providing environmental and economic benefits. DevTMF has been a success as it has accomplished the aim through:

• Improvement and development of advanced standard and non-standard TMF crack initiation and TMF crack growth (CG) experimental methods including harmonisation of TMF CG testing by a round robin to enable its standardisation.
• Generation of accurate high quality TMF test data under different loading conditions such as temperature range, phase angle, phase direction, R-ratio, waveform, dwell, etc., including component relevant complex flight cycles.
• Mechanical and metallurgical assessments of two structural disc alloys considering effect of multiple critical loading and material variables to determine active damage and deformation mechanisms controlling TMF crack initiation and crack propagation lives.
• Development of unified materials models and methods, with experimental validation, capable of predicting TMF crack initiation and crack propagation lives of components subjected to complex engine cycles and suitable for implementation in industrial computer programmes. In addition, procedures to handle computationally heavy cycles were developed and evaluated.

The DevTMF results show that the operational window capability of the investigated materials can be increased resulting in improved fuel efficiency and lower emissions. During the time, the consortium could not quantify the benefits due to difficulties in obtaining turbine inlet temperature data. However, DevTMF support structural and fatigue life analysis in realistic thermal and mechanical loadings for better optimisation of current and future aero engine components towards development of more environmentally friendly engines.
WP1 Project management & Dissemination and exploitation set up necessary processes for the project management, dissemination, communication and exploitation. The website and social media channels were launched, and templates produced. The project has resulted in 9 peer-review journal papers, 2 conference papers, 22 conference presentations, 3 posters and 20 other events including TMF courses. One of the main achievements was co-organisation of an international conference, TMF Workshop 2019, Berlin, Germany. The consortium also facilitated a panel discussion for sharing know-how, and further engagement and collaboration towards standardisation of TMF CG testing. To promote a wider uptake of the results, DevTMF organised a symposium under the EUROMAT 2019 conference in Stockholm.

WP2 Experimental activities including characterisation work performed extensive TMF testing and characterisation of two alloys to determine effect of different loading variables and component flight-like complex cycles. One of the main achievements under WP2 is the TMF CG round robin testing forming the basis for a local Code of Practice and standardisation. Precise and stable non-invasive temperature measurements were developed using infrared thermography (IRT). This approach was also used to measure and understand crack tip heating in induction coil as well as innovative measurements of TMF CG rates, Figure 1. The method enables measurement of crack length and removes complications with PD probes and coil interferences. Detailed investigations of other temperature measurement methods were also performed.
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Various characterisation techniques were used, e.g. SEM, EBSD, TEM, FIB-SEM, DIC, nanoindentation, to understand material behaviour, Figure 2.

WP3 Modelling activities including validation work developed a viscoelastic-viscoplastic constitutive material model capturing cyclic plasticity and viscous flow, Figure 3. A subroutine for the material model was written for integration in commercial FEA codes. Crack initiation life was estimated using a damage accumulation model based on the memory surface concept of Jiang. To account for elastic damage, a simple critical plane search algorithm, calculating stress tensors experienced by a particular plane in the material affecting fatigue life, was considered. To reduce CPU time and increase efficiency, additional subroutines for cycle jumping procedures were developed.

For simulation and prediction of TMF CG, a model accounting for fatigue, creep crack growth and a participation size effects was developed, Figure 4.

In addition, generalised crack tip modelling scripts to correlate plastic strain with degradation in Young’s modulus (a function of accumulated plastic strain) and SIF were formulated, Figure 5.

The models and methods were validated and evaluated for various loading conditions showing good agreement with the experimental data in most cases.
Main innovations of DevTMF and their potential impact are:

• The IRT based non-invasive temperature measurement and control technique with improved accuracy over traditional methods providing a major development for TMF testing.
• Early stage development of a non-invasive IRT system for TMF CG control.
• Investigation of effect of static and dynamic crack tip heating on CG in induction field. Better knowledge of effect of different heating methods and induction coil set ups on TMF behaviour.
• The TMF CG round robin test aimed at harmonisation and standardisation is the first inter-laboratory comparison TMF CG testing.
• The TMF data is improved and more reliable providing realistic validation data for numerical analysis and reducing need for full scale engine testing and costs.
• Better understanding of TMF material behaviour will enable higher operating temperature leading to improved engine efficiency and reduction in fuel consumption and CO2 emissions.
• Several approaches for crack initiation prediction delivered: critical plane approach searching for the most “damaging” orientation of computed stress/strain tensors, fatigue damage model based on memory surface, and unified constitutive models accounting for multiple hardening mechanisms in both isothermal and an-isothermal loading conditions.
• A unified TMF CG model accounting for microstructure properties, creep and fatigue effects validated under various loading conditions.
• FEA simulations of Young’s modulus effect enabling modelling of planar and non-planar crack growth towards more precise prediction of TMF CG behaviour.
• Additional feature addressing computational efficiency in FE analysing and simulation of cyclic loaded structures reducing development time and costs and saving energy.

Linköping, Swansea and Nottingham universities have benefited from DevTMF by advancing knowledge and know-how in TMF experimental techniques, material behaviour, simulation and prediction. Through improved understanding of TMF, DevTMF will sustain growth potential of Rolls-Royce and its market share in manufacturing of current and future aero engines. DevTMF might also improve competitiveness of other Clean Sky 2 ITD leaders and associates focusing on manufacturing of aero engines, and on research and development activities in similar fields. The knowledge, technologies and tools are relevant to other sectors such as energy and automotive, which is reflected in a new proposal targeting MSCA funding comprising wider partnerships. DevTMF has a positive societal impact by contributing to increase of jobs and career prospects of the researchers.
A schematic (rheological) representation of the viscoelastic-viscoplastic model and example cyclic d
Secondary cracking under TMF CG out-of-phase loading. Courtesy of Dr Svjetlana Stekovic, Linköping U
Non-invasive TMF crack growth measurements with IRT. Courtesy of Dr Jon Jones, Swansea University, U
A schematic representation of the TMF crack growth model. Courtesy of Dr Benedikt Engel, the Univers
A schematic representation of SIF simulation based on plasticity-induced changes to Young’s modulus.