Oxidation Damage at a Crack Tip and Its Significance in Crack Growth under Fatigue-Oxidation Conditions

Lead Research Organisation: Loughborough University
Department Name: Wolfson Sch of Mech, Elec & Manufac Eng

Abstract

Nickel-based alloys are widely used in power generation, nuclear and aerospace industries due to their superior mechanical properties at high temperature. As structural materials, a strong resistance to crack initiation and propagation is particularly required for safe-life design and assessment of their components. At elevated temperature, crack growth rates in such alloys exposed to air can be drastically accelerated, by two and even three orders of magnitude, due to the attack of oxidation. Over time, significant effort has been made to investigate the crack tip oxidation mechanism in order to provide a basis for the development of quantitative models that predict crack growth under operational temperatures and loading conditions. However, this problem has been neither fundamentally nor fully understood, and current lifing practice in industries is still predominantly empirical and relies on expensive and extensive experimental data on crack growth.

This research aims to investigate the physical process of oxidation damage at a crack tip and the associated crack growth behaviour for nickel alloys, which will provide a direct insight, for the first time, into the oxidation-embrittlement phenomenon at crack tip. Oxidation damage at a crack tip is a combined effect of time, temperature, local deformation and material microstructure. Knowledge of this process is vital to assess crack propagation behaviour under the attack of oxidation. In the proposed work, single crystal, directionally solidified and polycrystal nickel alloys will be used for crack growth testing under fatigue-oxidation conditions in controlled environments (vacuum, air, oxygen-18). Advanced microscopy analyses will be carried out to characterise and measure the oxygen penetration and microstructural damage at a crack tip, and the results will be used to calibrate important diffusion and damage parameters during oxidation. Numerical analyses will be carried out to model such processes at a microscopic scale using a coupled mechanical-diffusion model. Effects of loading condition and grain boundary character on oxygen diffusion will be fully investigated, especially the connection between oxidation damage and crack growth. A crack propagation model will be ultimately developed and validated for accurate fatigue-oxidation life prediction.

The work draws together three established groups to tackle these fundamental problems in a collaborative, systematic and multi-scale manner. Interaction between oxidation damage and crack tip deformation requires carefully designed specialist testing on fatigue crack growth in a controlled environment, which is the expertise of UoS. The problem also requires advanced microscopy characterisation and physical measurements of the phenomena using the established techniques at IC. The new models will be developed, with validation against these experimental results, by UoP who has a strong background in material and crack growth modelling. Owing to our complementary skills, this joint project should establish a physically based connection between oxidation damage and crack growth for fatigue design and safe life prediction of nickel alloy components.

The research will generate unique and practically-useful data and models which can be quickly exploited through our committed industrial collaborators including E.On, Alstom, NASA and Dstl. The results will also be of generic use to other industries striving to achieve maximum service life and temperature capabilities of critical high-temperature components. Researchers and academics working on high-temperature materials and related areas will also directly benefit from our targeted dissemination activities including workshops, conferences and journal papers. A wider audience will be reached via specially designed public engagement programmes and continuously updated web sites.

Planned Impact

The research has a direct impact on power generation and gas turbine propulsion systems. The fast growing energy demand and concerns about climate changes drive industries to implement higher operating temperature and longer maintenance intervals to achieve high-efficiency power and energy systems. Fatigue-oxidation behaviour of high temperature materials is a key issue in the performance of high temperature plant. This project aims to address this critical problem for nickel alloys, an important class of high temperature materials, through development of accurate models for the design, performance prediction and lifing calculations for successful commercial applications. Exploitation of the research outcomes will be carried out with our project partners in the energy and aerospace industries (E.ON, ALSTOM, NASA, DSTL) via the bi-annual review meetings and contacts over the project period. The research outcomes will provide scientific guidance and support for industries to gain a maximum service life through the optimisation of service conditions and material microstructures. The model developments will enable industries to conduct "numerical experiments", in place of expensive, risky and time-consuming experimental tests wherever possible, and save significant costs in product development. This will contribute to ensuring the structural integrity and safety in terms of fatigue design and life management of critical nickel alloy components under service conditions. Our research team will also take a lead in going out and promoting the impact of the work with the assistance of our existing collaborative network and the experienced Knowledge Transfer Network teams at three institutions. Support from regional organisations, such as Solent Local Enterprise Partnership (www.solentlep.org.uk), will also be sought to additionally exploit the research outcomes for local business growth and prosperity.

Key scientific findings generated from the research will have a direct impact on research communities working on high temperature materials, particularly the physical measurements of oxidation damage near a crack tip and the predictive modelling tools for crack growth under fatigue-oxidation conditions. The research also delivers an underlying generic contribution which should also benefit research communities in physics, mathematics, biomechanical and petrochemical engineering. A number of methods will be used to maximise the impact across communities, including publication in journals for different audiences and presentation at a variety of conferences, seminars and workshops. Additionally all research data produced from the project will be archived on the Materials Data Centre hosted at UoS, which will allow other researchers to access the relevant data and models linked to our published findings.

The proposed research will enable the three PDRAs to establish themselves with further significant skills and experience in the areas of fatigue, fracture, characterisation and modelling. The PDRAs, as well as the pledged PhD student at UoS, will supply a critical engineering skills shortage in the UK. This joint research grant will also be an important step for the investigators to develop and expand their leadership roles in respective fields.

Wider public audiences will be easily engaged due to the direct relevance of this research to power generation and air travel. We will provide a number of infrastructure opportunities for researchers to develop research-led engagement activities. Also key summaries about significant findings and developments will be published on our continuously updated websites. In addition, all three institutions have school visits where the issues regarding oxidation and mechanical behaviour will be directly linked to the content of current chemistry and physics GCSE and A-level syllabi.

Publications

10 25 50
 
Description This project aimed to test and measure oxygen diffusion and oxidation-induced damage at a crack tip in nickel superalloys under industrially relevant conditions, ultimately leading to the provision of a predictive model for crack growth. New methodologies were developed and interesting results were produced through the joint efforts of three institutions and four industrial partners. Firstly, critical results of crack growth behaviour have been generated for polycrystalline (PM LSHR; 725?C) and SX (MD2; 825?C) nickel alloys, in controlled environments, including vacuum and isotopically 18O2 enriched, at a range of dwells at elevated temperature. Secondly, a direct examination of oxy-gen distributions in and around the crack has been delivered, including a quantification of the ex-tent of oxygen-induced damage during fatigue. Study revealed the importance of oxygen diffusion along crack path, controlled by temperature, dwell at maximum load and environment. Static oxidation tests were also carried out to calibrate the damage mechanisms and oxygen diffusion parameters to feed into modelling work. Thirdly, a new computer model has been developed to simulate the fully-coupled interaction between oxygen diffusion and crack-tip deformation. The mod-el was utilised for prediction of oxygen penetration into the crack tip, and also for quantification of the compressive stress field near the crack tip generated by oxygen diffusion or oxidation process which was the first ever effort. Finally, a two-parameter criterion, in terms of mechanical deformation and oxygen penetration, has been proposed to control crack growth. The criterion was combined with XFEM to develop an efficient numerical procedure for crack growth prediction under fatigue-oxidation conditions. The model was able to predict the crack growth behaviour ob-served and measured experimentally. These significant outcomes are anticipated to provide further scientific support for ensuring the structural integrity of high-temperature alloy components in terms of fatigue design and life management of critical alloy components under typical service conditions.
Exploitation Route The experimental data, parameters and models produced out of this project are of high value to academics and researchers working worldwide on high temperature metallic materials. The development of fully coupled diffusion-deformation and crack growth models is new and of immediate interest to researchers in the international context. Particularly, we have been exchanging the results with our contacts in US, China, France, Ireland and Germany. The impact of key scientific findings is facilitated by publications in appropriate journals and major international conferences in the relevant fields. Dissemination within the UK includes attendance of industry-orientated national conferences on high temperature fatigue, creep and materials. Our researchers also have been presenting the work at regional and national seminars and workshops to promote the uptake of our research findings by users. Certainly, these activities will be continued well beyond the completion of the project. This research studied critical mechanical and environmental variables that influence crack growth rates of advanced alloys in arduous high temperature environments. Specifically, it is fundamental research that tackles the issue of oxidation-assisted crack growth. The research outcomes have important applications in major industries including power generation plants, nuclear reactors and aerospace striving to achieve high-efficiency power and energy systems in response to climate change. This includes optimisation of service conditions and material microstructures for the achievement of maximum service life. The models and numerical tools that have been developed should also enable cost and resource reduction in product development stage. Again, exploitation of the project outcomes will be continued through our established industrial network.
Sectors Aerospace, Defence and Marine,Energy

 
Description Exploitation of our research outcomes has been attempted by our project partners in the energy and aerospace industries (Uniper Energy, GE Power, dstl and NASA). GE power has circulated our results within their relevant departments, and also collected and stored our test data in their internal system to support component designers, material developers and structural integrity assessment teams. Uniper energy is planning to exploit the modelling approach for life management and performance assessment of their turbine components. A follow-up Royal-Society industrial fellowship is under consideration for Prof Zhao, the lead investigator of the project, in terms of carrying out one or two-year secondment at Uniper energy to further promote such activities. Also dstl and Rolls-Royce have played active roles in dissemination and exploitation of our research findings for other end users, particularly the physical examination of oxidation damage near a crack tip and the predictive modelling tools for crack growth under fatigue-oxidation conditions. These activities will be continued through extended collaborations with these key industrial partners.
Sector Aerospace, Defence and Marine,Energy
Impact Types Economic

 
Title Fully coupled deformation-diffusion model 
Description The developed modelling tool can: (1) simulate the full interaction between oxygen diffusion and crack-tip deformation; (2) predict oxygen penetration into the crack tip; (3) quantify the stress field near the crack tip generated by oxygen diffusion or oxidation process; (4) Predict crack growth under fatigue-oxidation conditions. 
Type Of Material Computer model/algorithm 
Year Produced 2017 
Provided To Others? Yes  
Impact The tool is able to certify the life extension of existing power plants, thus with a potential of saving millions of pounds by extending the pre-designed service life of power plants.The model can be used to replace expensive laboratory-based tests and full-scale experiments, speed up the design cycle, save cost and improve UK competitiveness. The numerical tools will also help, more effectively, with service-life management of critical components, thus preventing unexpected outage of the system and heavy economic suffer caused by component damage and failure. The tool has generic feature and can be applied to other materials systems involving fatigue-environment damage such as aeroengines, oil and gas industries. 
URL http://www.sciencedirect.com/science/article/pii/S0167844217304585
 
Description E.On 
Organisation E ON
Country Germany 
Sector Private 
PI Contribution Provide a fundamental understanding of fatigue-oxidation damage mechanism for gas turbine blades and discs.
Collaborator Contribution Supply of technical advice and attendance of project review meetings.
Impact Journal and conference publications
Start Year 2013
 
Description GE power 
Organisation General Electric Power
Country United Kingdom 
Sector Private 
PI Contribution Experimental and computational study of fatigue-oxidation damage and crack growth of single crystal and directionally solidified superalloys for gas turbines in power generation.
Collaborator Contribution Supply of materials and technical advice; attendance of project review meetings.
Impact Conference and journal publications
Start Year 2013
 
Description NASA 
Organisation National Aeronautics and Space Administration (NASA)
Country United States 
Sector Public 
PI Contribution Experimental and computational study of critical fatigue-oxidation damage of NASA superalloy LSHR for application in gas turbines.
Collaborator Contribution Supply of materials and technical advice.
Impact Journal and conference publications.
Start Year 2013
 
Description dstl 
Organisation Defence Science & Technology Laboratory (DSTL)
Country United Kingdom 
Sector Public 
PI Contribution Provide guidance for life assessment of gas turbines
Collaborator Contribution Providing technical advice and attending project review meetings.
Impact Journal and conference publications
Start Year 2013
 
Description Open day 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Public audience showed interest and impress in our research

Promoted the profile of School, University and UK research
Year(s) Of Engagement Activity 2014,2015,2016,2017
 
Description Website 
Form Of Engagement Activity A magazine, newsletter or online publication
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Public/other audiences
Results and Impact Website promoted a share of our research activities, methodologes and results

Promoted global awareness of the research achievements
Year(s) Of Engagement Activity 2015
URL http://www.lsw-oxidation.com/