Project

DevTMF

Goal: Development of Experimental Techniques and Predictive Tools to Characterise Thermo-Mechanical Fatigue Behaviour and Damage Mechanisms in two novel superalloys for turbine disc applications

Methods: Predictive Modeling, Prediction, TMF

Date: 1 February 2016 - 31 January 2019

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Project log

Svjetlana Stekovic
added a research item
For critical component application, such as aerospace turbine rotors, it is imperative to be able to make accurate in-service material behaviour and component life predictions for both design and monitoring of component life. The development of such predictive capability is dependent on the quality of the experimental data from which the material parameters are derived. This paper shows the effect that scatter which may be present within experimental data, manifesting itself within the constitutive parameters derived from this data, has on the resulting fatigue crack initiation life of the nickel-based superalloy RR1000. Industrial relevance was added to this investigation by the use of flight representative thermomechanical fatigue loading cycles and state of the art material behaviour and fatigue crack initiation models used within the finite element simulations conducted. The effect of the ‘scatter in’ to the modelling approach on the outcoming predictions is made via a Monte-Carlo analysis. This analysis consisted of running the same simulation several times, but with the experimentally determined and validated ‘baseline’ constitutive parameters varied via correction factors built into the model, for each run via a singular value decomposition procedure. It was found that small ‘scatter in’ has only a very localised ‘scatter out’ effect on the crack initiation predictions under the flight representative loading.
Daniel Leidermark
added a research item
The current paper presents work on identification and evaluation of a range of factors influencing accuracy and comparability of data generated by three laboratories carrying out stress-controlled thermo-mechanical fatigue crack growth tests. It addresses crack length measurements, heating methods and temperature measurement techniques. It also provides guidance for pre-cracking and use of different specimen geometries as well as Digital Image Correlation imaging for crack monitoring. The majority of the tests have been carried out on a coarse grain polycrystalline nickel-base superalloy using two phase angles, Out-of-Phase and In-Phase cycles with a triangular waveform and a temperature range of 400-750 oC.
Benedikt Engel
added 2 research items
The polycrystalline nickel-base superalloy RR1000 is used as turbine rotor material in Rolls-Royce aero engines and has to withstand a wide variety of load and temperature changes during operation. In order to maximize the potential of the material and to improve component design, it is of great interest to understand, and subsequently be able to accurately model the crack propagation caused by thermo-mechanical fatigue conditions. In this work, experimental data is analysed and used to inform unified modelling approaches in order to predict the crack propagation behaviour of RR1000 under a variety of stress-controlled thermo-mechanical fatigue conditions.
An understanding of rate dependency over a wide range of time scales is vitally important in approximating the transient response of critical components operating in extreme environments. Many examples of viscoplastic model formulations can be found in the literature, wherein all rate dependency is assumed to occur after yielding. Such models neglect any viscous effects during elastic deformation. In the present work, a unified viscoelastic - viscoplastic material model is developed for the Nickel superalloy RR1000. Particular emphasis is placed on model parameter determination, which is accomplished using standard cyclic plasticity and stress relaxation experimental data.
Jonathan Jones
added 3 research items
Thermo-mechanical fatigue data is critical for the generation of appropriate lifing methodologies for a range of in-service applications where non-isothermal conditions are prevalent. Recently the development of more standardised testing approaches through appropriate code of practice documents and international standards has proved crucial. In the current paper, several methods of undertaking TMF testing are explored, with the benefits and pitfalls of each test type investigated. It is shown that bespoke test setups are often required, dependent on material, TMF cycle and specimen type. Further developments are suggested, along with a suggested methodology for TMF crack growth tests.
The current paper describes TMF crack growth behaviour in an advanced nickel-based superalloy. Changes in behaviour are examined which occur as a function of the phase angle between applied stress and temperature. The fractography of the failed specimens reveals changes from transgranular to intergranular growth between high and low phase angle tests as a result of the onset of high temperature damage mechanisms. More targeted testing has also been undertaken to isolate the contributions of these mechanisms, with specific transitions in behaviour becoming clear in 90° diamond cycles, where dynamic crack growth and oxidation strongly interact.
The crack driving mechanisms in a coarse grained nickel-base superalloy RR1000 when subjected to in- and out of phase thermo mechanical fatigue are investigated. It is found that the difference in fatigue crack growth rate between these two load conditions is accounted for by the different mechanical conditions at the crack tip region, rather than oxidation effects. This is based on digital image correlation and finite element analyses of the mechanical strain field at the crack tip, which demonstrate that in phase leads to larger crack tip deformation and crack opening. Notably, it is demonstrated that in- and out of phase crack growth rates coincide when correlated to the crack tip opening displacement.
Svjetlana Stekovic
added an update
The project organised a separate panel discussion focusing on development of Code of Practice for experimental TMF crack growth test method and further activities towards standardisation of TMF crack growth. Its agenda was to review any previous standardisation activities, sharing of good assessment practice and discuss further activities and processes. Before the panel discussion, Jonathan Jones gave a presentation of development of the experimental methodologies for TMF testing while Svjetlana presented work on the TMF crack growth round robin exercise between the labs involved in DevTMF (article pending publishing).
Questions raised during the panel discussion were study of other modes of fracture, sensitivity of DCPD to emf, influence of temperature on DCPD, best technique for temperature measurements, effect of different coil designs on TCs, use of ACPD and comparison to DCPD, use of compliance method, through thickness temperature gradients, effect of humidity, crack tip heating, effect of plastic deformation on crack growth measurements, small cracks and their measurements, etc.
The discussions contributed to further exchange of knowledge in experimental development of TMF CG testing and potential future collaborations outside the project.
 
Svjetlana Stekovic
added an update
A new paper from Jonathan Jones on The effect of phase angle on crack growth mechanisms under thermo-mechanical fatigue loading has been published in International Journal of Fatigue:
 
Svjetlana Stekovic
added an update
A paper "On the mechanistic difference between in-phase and out-of-phase thermo-mechanical fatigue crack growth" has been published by Viktor Norman in International Journal of Fatigue. The final version is available online now for free until 2nd of April:
 
Daniel Leidermark
added a research item
The computational efficiency in analysing cyclically loaded structures is a highly prioritised issue for the gas turbine industry, as a cycle-by-cycle simulation of e.g. a turbine disc is far too time consuming. Hence, in this paper, the efficiency of two different procedures to handle computational expansive load cases, a numerical extrapolation and a parameter modification procedure, are evaluated and compared to a cycle-by-cycle simulation. For this, a local implementation approach was adopted, where a user-defined material subroutine is used for the cycle jumping procedures with good results. This in contrast to a global approach where the finite element simulation is restarted and mapping of the solution is performed at each cycle jump. From the comparison, it can be observed that the discrete parameter modification procedure is by margin the fastest one, but the accuracy depends on the material parameter optimisation routine. The extrapolation procedure can incorporate stability and/or termination criteria.
Svjetlana Stekovic
added an update
Together with Malcolm S. Loveday from the High Temperature Mechanical Testing Committee (http://www.htmtc.net/w/Main_Page) and Hellmuth Klingelhöffer from Bundesanstalt für Materialforschung und -prüfung (BAM), DevTMF is organising TMF Workshop 2019 in Berlin between13-15 November 2019. More information can be found at https://netzwerke.bam.de/Netzwerke/Navigation/EN/Event-Series/TMF-Workshops/tmf-workshops.html. Welcome!
 
Svjetlana Stekovic
added an update
DevTMF is co-organising B8 symposium, High Temperature Material Characterisation, Testing and Mechanical Properties at EUROMAT 2019 conference in Stockholm: https://euromat2019.fems.eu/. Welcome!
 
Svjetlana Stekovic
added a research item
A fatigue crack initiation model based on damage accumulation via a fatigue memory surface in conjunction with a plastic strain energy parameter was evaluated for thermomechanical fatigue loading in a gas turbine disc alloy. The accumulated damage in each hysteresis loop was summed up, and it was assumed that the damage at the stable state is repeated until failure occurs. Crack initiation occurs when enough fatigue damage has been obtained, and the number of cycles can thus be directly determined. The fatigue damage is highly coupled to the constitutive behaviour of the material, where the constitutive behaviour was modelled using a non-linear hardening description. Based on this, a stable state was achieved and the obtained damage could be extracted. A user-defined material subroutine was implemented, incorporating both the constitutive description and the fatigue damage accumulation. The framework was adopted in a finite element context to evaluate the thermomechanical fatigue crack initiation life of the disc alloy RR1000. From the evaluation it could be seen that a good prediction of the thermomechanical fatigue life was achieved compared to performed experiments.
Svjetlana Stekovic
added an update
As part of the work in DevTMF, the teams at Swansea University have published a new paper in International Journal of Fatigue:
 
Svjetlana Stekovic
added an update
The TMF CG Workshop that was held at Swansea University at the end of November 2017 aimed towards gaining experience of past attempts to provide guidance towards the TMF CG approaches and identifying a pathway towards standardisation of TMF CG testing.
Svjetlana introduced the project, its main aim and objectives, work plan and dissemination activities that include exchange of knowledge and practice, and possible standardisation of the experimental method addressed in the workshop. Then Rolls-Royce proceeded with a presentation of background to DevTMF and future trends.
Afterwards, Mark T Whittaker, Jonathan Jones, Johan Johansson Moverare, Chris Hyde, Lee Waterhouse from Wood plc and Hellmuth Klingelhöffer from BAM/HTMTC talked about their experience with TMF CG testing and standardisation activities (strain controlled TMF). Peter Biley from Instron went through his project on temperature verification measurements for dynamic fatigue testing systems to support needs of industry for tighter specifications. This work is building on and supports the ISO 12111, the force-controlled TMF CoP and the NPL led EMPRESS project. And also, DevTMF.
Malcolm from the High Temperature Mechanical Testing Committee (HTMTC, http://www.htmtc.com/w/Main_Page) talked about the EMPRESS project which is funded by the EURAMET program European Metrology Program for Innovation and Research (https://www.euramet.org/research-innovation/search-research-projects/details/?eurametCtcp_project_show%5Bproject%5D=1315) and which could also be relevant for DevTMF. This 3-year project is intended to substantially increase the efficiency of high value manufacturing processes by improving temperature measurement techniques at the point of use.
Hellmuth mentioned that he is convener for a new ISO working group (number 6) that has been established on Elasto-plastic strain and crack growth conditions. Regarding TMF CG, it was expressed to develop a TMF CG standard to the TC 164 SC4 (fatigue testing of metals) to chairman Mike McGaw (USA) few years ago, which he appreciated very much. USA industry would gladly support it but input has been missing until now. Hellmuth suggested that the HTMTC has excellent external preconditions to support progress for implementation of TMF CG standard. Malcolm agreed with Hellmuth suggesting to publish a local code of practice by the DevTMF partners through them (the HTMTC), then let the members comment and then send it via Hellmuth to the ISO for further comments.
The HTMTC also organise workshops and seminars on TMF topics and could help to organise the final DevTMF workshop (planned for end of 2019 or beginning of 2020). Hellmuth, Malcolm and Svjetlana will see if the DevTMF final workshop could be organised under the HTMTC umbrella, as a part of TMF workshops organised by BAM in Berlin or as a technical session in the framework of, for example, the EUROMAT 2019 conference or International Conference on Low Cycle Fatigue (one of the main ESIS sponsored events). Conference proceedings from the HTMTC workshops are published in International Journal of Fatigue, Engineering Failure Analysis, Engineering Fracture Mechanics or Theoretical, Applied Fracture Mechanics and Materials at High Temperatures.
For the standard, validation of the techniques will be needed, but this is out of the scope of the project in the current form and budget.
The participants were happy to get involved in some way and support the CoP.
 
Svjetlana Stekovic
added an update
The project partners are organising a TMF Crack Growth workshop in the Institute of Structural Materials at Swansea University’s Bay Campus on 24th of November 2017. The main goal is to disseminate and communicate the project activities and its first results regarding development of TMF crack growth experimental method to wider audience outside the project. More information can be found in the attached poster. If you have any questions please contact myself or Mark T Whittaker.
 
Svjetlana Stekovic
added an update
Mark T Whittaker from Institute of Structural Materials, Swansea University, is organising a course called An Introduction into Thermo-Mechanical Fatigue in Swansea between 20-22 of November 2017. There are two places left. More information can be found in the attached pdf.
 
Svjetlana Stekovic
added an update
At the end of September, we hold our first project review meeting in Brussels and submitted the 1st periodic report to the Commission. As part of the reporting we wrote a short publishable summary of the project objectives, activities, achievements and impact, which can be found in the attached file. However, a short summary from the experimental and modelling work is given below.
During the first 18 months of the project, work has been performed on to characterise static and dynamic thermal gradients for the validation of both TMF crack initiation and propagation test methods. Thermal gradients were measured with thermocouples, pyrometry and thermography across and along the gauge length. It was found that infrared thermography (IT), which is a non-invasive temperature measurement technique, combined with Rolls-Royce HE23 black thermal paint to provide a stable emissivity value, gave consistent accuracies within ±2 oC to type N thermocouples (TCs). Pyrometry have proven accurate for temperature control and monitoring provided specimens are heat treated before testing but still there is a problem as emissivity changes during long duration tests. TCs have shown to be sensitive to cooling air displaying faster cooling rates and lower temperatures than those observed by the IT. They also absorb heat energy during tests and as result faster heating rates are observed with peak temperatures exceeding the target temperature by as much as 10 oC. Further, a comparison between isothermal CP tests run with different heating methods were studied to determine their effect on CP rates and crack monitoring techniques in order to establish a framework for the larger test programme (Figure 1 in the attached document). Finally, 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. However, the possibility of localised heating near the crack tip due to an increased surface area was questioned and required investigation to allow for confidence in induction coil testing and consequently in the testing methods.
At present sets of temperature dependent material parameters have been determined for the material investigated and corresponding interpolation functions fitted (with temperature as the argument in these functions) allowing both isothermal and an-isothermal cyclic behaviour to be estimated over the majority of the test range and the corresponding plastic strain increment quantified at the onset of damage (along with various other material/loading history dependent quantities). Various damage functions are explored in order to estimate tertiary material softening and crack initiation, Figure 2 (note that hysteresis loops presented in (a) are not from the actual material). Development of hardening parameter evolution equations (back and drag stress) is expected to yield notable developments due to the presence of heavily temperature dependent phenomenon (for example, isotropic hardening at low temperature and isotropic softening at high temperature). The Forman model is currently used for the estimation of CP due to fatigue loading, with extensions in the near future looking to add creep enhancement and oxidisation/acceleration/retardation terms. Initial results are promising. Particular emphasis is being placed on the development of fatigue/creep damage interaction laws (replacing the simple linear Miner’s rule variants that are often applied in the literature). Further, an-isothermal cyclic plasticity models have been developed and incorporated into MATLAB codes (for uniaxial loading validation) and UMAT subroutines (for Abaqus FEA implementation). Initial validation of these codes and the corresponding material coefficient functions is underway using standard TMF (cyclic plasticity) data.
Enjoy,
Svjetlana
 
Svjetlana Stekovic
added an update
Swansea University has undertaken assessment and subsequent development of multiple heating, cooling and crack growth measurements, and recording techniques have been investigated and compared towards international standardisation of dynamic temperature CG test method, which we would like to share with a wider audience. Therefore, we are organising a workshop on TMF CG to share our experience, especially with those in the field who have undertaken TMF CG experiments. For more information contact me or Mark T Whittaker. A flyer is being finalised and will be distributed shortly.
 
Svjetlana Stekovic
added an update
First project periodic reporting is on-going now.
 
Svjetlana Stekovic
added an update
Swansea University, Mark T Whittaker, Robert Lancaster, Jonathan Jones, plan to run a TMF course in Swansea between 6-8th of June 2017. The course objectives include:
i) A detailed understanding of the nature of TMF problems in service,
ii) Interpretation of the International TMF test standard ISO12111,
iii) Hands on experience of mechanical test methods for TMF,
iv) Approaches to data analysis of TMF tests, and
v) Understanding of the test data
For more information please contact Mark T Whittaker (m.t.whittaker@swansea.ac.uk).
 
Svjetlana Stekovic
added an update
We are happy to announce that three TMF crack growth tests have been successfully completed at Swansea University by Jonathan Jones, Mark T Whittaker and Robert Lancaster. Looking forward some discussions on damage mechanisms.
 
Svjetlana Stekovic
added an update
First commissioning TMF crack growth test is running at Swansea University. A slight modification has been done to the pre-cracking procedure to ensure appropriate formation of a pre-crack.
 
Svjetlana Stekovic
added an update
In accordance with the work plan, the 2nd project meeting was held on 18-19 October 2016 in Linköping, Sweden. It was hosted by the partners from Linköping University, Divisions of Engineering Materials and Solid Mechanics. The partners presented the main results of the last nine months and agreed on the forthcoming activities within the next six months. D.J. Child gave a presentation on oxidation damage. The key topics addressed were (i) test data, (ii) test conditions, (iii) literature review of TMF crack initiation and propagation (both experimental and modelling part) and (iv) selection of initial approaches for the modelling work.
 
Svjetlana Stekovic
added a research item
Presentation of the DevTMF project and activities towards standardisation of TMF crack growth method
Svjetlana Stekovic
added a project goal
Development of Experimental Techniques and Predictive Tools to Characterise Thermo-Mechanical Fatigue Behaviour and Damage Mechanisms in two novel superalloys for turbine disc applications