Dislocation-Microstructure Interaction at a Crack Tip - In Search of a Driving Force for Short Crack Growth

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


Nickel-based superalloys are particularly used in applications involving high temperatures and stresses, such as the critical gas-turbine blades and discs in aerospace and power-generation industries. The behaviour of short cracks in nickel superalloys is of particular importance for component design and life prediction, as a large proportion of service life is spent in the growth of small cracks before final failure. Due to the strong influence of local microstructure and heterogeneous stress/strain fields, short cracks are known to grow anomalously under fatigue and tend to exhibit high, irregular and scattered growth rates. The physical driving force for short crack growth is still not well understood yet despite intensive research effort, mainly due to the limited understanding of crack-tip behaviour.

This proposal aims to investigate the fundamental deformation mechanism at the tip of a short crack for nickel-based superalloys under fatigue at a range of temperatures. The research will focus on the influence of evolving local plasticity, induced by dislocation dynamics at the crack tip, on short crack growth. The interaction between dislocation and material microstructure is the major source for heterogeneous plasticity and internal stress concentration, leading to initiation and growth of short cracks. Short crack growth testing in a controlled environment will be carried out to study the anomalous behaviour of short crack growth in these alloys under fatigue, which is the expertise of UoS. Temperature will be varied in order to observe the critical effect of temperature change on the slip behaviour near the crack tip. Following crack growth tests, post-mortem transmission-electron-microscopy analyses of crack-tip zone will be performed to reveal the detailed mechanisms for nucleation and multiplication of dislocations, pile-up and penetration of dislocations at phase/grain boundaries and the influence of grain misorientations on dislocation behaviour. In particular, match-stick samples will be extracted from the crack-tip fracture process zone of fatigue-tested specimens to allow in-situ measurements of crack tip deformation under fatigue, which are the established techniques at UoM. In this case, high resolution digital image correlation, with the assistance of grain orientation mapping and scanning-electron-microscopy imaging of gold remodelled surfaces, will be used to quantify shear strain in slip traces formed near the crack tip during fatigue loading. In addition, high energy synchrotron X-ray diffraction studies will be carried out to measure the elastic strain response and load transfer between different phases around the crack tip, which will provide insight regarding the penetration of dislocations into the gamma-prime precipitates.

To physically simulate the material plasticity behaviour, a three-dimensional discrete-dislocation-dynamics (DDD) approach will be developed to model the interaction between dislocations and material microstructures, which is the strength of LU, based on experimental results. The DDD model will be interfaced with viscoplasticity and crystal plasticity models, and further applied to investigate the role of dislocation dynamics in depicting short crack growth. A multi-scale finite element method will be established for the crack-tip deformation analyses, which aims to identify a micromechanics-based driving force for short crack growth. Computational simulations will be thoroughly validated against local strain measurements (at both mesoscale and microscale), in-situ and post-mortem measurements as well as X-ray tomography of extracted match-stick samples. The ultimate goal is to deliver an efficient finite element procedure to predict short crack growth, with full validation against the experimental data, for end users.

Planned Impact

The research will have a direct impact on power generation and aero engine industries, as it addresses the fatigue behaviour of high temperature materials, a critical issue in the performance of gas turbine systems. Nickel-base superalloys, the materials studied in this proposal, are an important class of high temperature materials, and currently irreplaceable for application as gas turbine discs and blades. In response to fast growing energy demand and climate change concerns, gas turbine industries are striving to implement even higher operating temperature and longer maintenance intervals to produce power and energy systems with high efficiency. This can only be achieved through improved understanding of materials behaviour and development of accurate models for fatigue design and life prediction of modern gas turbine systems, which will be delivered by this research.

Through close collaboration with our project partners in energy and aerospace sectors (Alstom, Rolls-Royce and Dstl), exploitation of the research outcomes will be carried out in terms of optimisation of service conditions and material microstructures to achieve a maximal service life. This will contribute to fatigue design and life management of critical gas turbine components, with ensured structural integrity and safety, under service conditions. Furthermore, the developments of materials and fatigue life models will allow industries to use "numerical experiments" in product development wherever possible, and save costs by reducing the number of expensive, risky and time-consuming experimental tests. In addition, all three institutions have extensive collaborative research networks and experienced knowledge transfer services, which will be fully utilised by our research team to further promote the impact.

This research will deliver new scientific findings, particularly the physical measurements of micro-deformation near a crack tip and the predictive modelling tools for short crack growth under fatigue conditions. This will strengthen the international competitiveness of UK research in advanced metals and alloys, and also create impact in research communities worldwide who are working on high temperature materials. The underlying generic outcomes, delivered by this research, also provide benefit to researchers in mathematics, physics, manufacturing, petrochemical, biomedical and nuclear engineering. To maximise the impact across disciplines, publications and presentations will be sought for a range of journals, conferences, seminars and workshops, both nationally and internationally. In addition, the Southampton Heterogeneous Data Centre will be used to archive all research data and findings generated from this research for easy access by wider researchers and audiences.

Through this research programme, the three post-doctoral research associates (PDRAs), as well as the externally funded PhDs, will equip themselves with advanced knowledge, skills and experience in fatigue, characterisation and numerical modelling. This will provide a remedy to the critical shortage of engineering skills in the UK. The multi-institutional research project will also provide an important platform for the investigators to further establish their expertise in respective research fields, and develop and expand their leadership roles.

The research has a direct relevance to power generation and air travel, and can easily engage wider public audiences. This activity will be mainly led by UoS who will design specific, further engagement programmes for this research, with all researchers being involved. Significant findings and developments will be summarised and published in a timely manner on our project website which will be set up from the beginning and continuously updated throughout the project period. In addition, public engagement with this research programme will be promoted using the school visit and open day opportunities at each institution.


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Description This project developed dislocation dynamics-based and microstructure-sensitive predictive tools for modelling deformation and propagation of short cracks in nickel superalloys. Through the joint efforts of three institutions and industrial partners, new methodologies and interesting results were produced out this research project. Firstly, 3D discrete dislocation dynamics code has been developed, for the first time, to model fully-cyclic deformation for MD2 (single crystal alloy with cubic precipitates), calibrated and validated with experimental results. Secondly, cyclic stress-strain behaviour has been obtained for alloy MD2 under low cycle fatigue, complemented with microscopy (transmission electron microscopy) examinations. The study revealed the decisive role of dislocation-precipitate interactions in controlling the mechanical behaviour of nickel superalloys. More importantly, the experimental results have been utilized to calibrate a crystal plasticity model for prediction of crack initiation. Thirdly, cutting-edge in-situ SEM (scanning electron microscopy) experiments of short crack initiation and growth have been carried out for alloy MD2, and short crack initiation and growth were found to be totally controlled by crystallographic slip behaviour. The in-situ experiments generated unique and pilot data and new insights into the anomalous short crack growth behaviour in single-crystal superalloys under low cycle fatigue. Finally, an additional 2D DDD code has been developed to model the formation of discrete slip bands at a crack tip by considering precipitate phases and alloy anisotropy; followed by prediction of slip-controlled crack initiation and growth using the crystal plasticity model already calibrated, which is also new. These research outcomes are new and highly valuable for maintaining the structural integrity of critical gas turbine systems from a fundamental micro-mechanics point of view
Exploitation Route This research provided a more physical and scientific understanding of crack-tip micromechanics and its significance in short crack growth, which is critical to formulate the slip-based driving force for crack propagation at early stages. The results have been published in well-recognised international journals in the field, and also presented at relevant national/international conferences as invited and keynote speeches (over 13 journal papers/conference presentations). Specifically, it includes the delivery of the 3D dislocation-microstructure interaction code and short crack growth model, as well as novel techniques for crack growth testing and micro-deformation measurement. Experimental data and parameters generated out the project are of high value and great benefit to researchers worldwide studying the micro-mechanical behaviour of high performance alloys. In particular, the results have been shared with researchers worldwide including Australia, Ukraine, China, Switzerland, USA, India, Russia, Ireland and Germany. Through our industrial partners including Rolls-Royce, GE power and dstl, the research out-comes have also been disseminated to power generation, nuclear energy and aerospace sectors to assist with structural integrity management of gas turbine systems. This offers an opportunity for industry to use the crystal plasticity model, as opposed to conventional continuum plasticity theory, for predicting slip-controlled crack initiation and growth under fatigue. The results are also used to support the development of new-generation nickel superalloys in terms of microstructure optimization, aiming to improve the critical "damage tolerance" behaviour of the alloys.
Sectors Aerospace, Defence and Marine,Energy

Description A 3D discrete dislocation dynamics (DDD) code has been developed and used to model mechanical deformation of single crystal alloy MD2 with cubic precipitates under fatigue, and the model has been validated with experimental results. This pioneering code allows our project partners in the energy and aerospace industries, i.e. GE power and Rolls-Royce, to understand the underlying mechanism of plastic deformation of high-performance alloys. Essential stress-strain data has been obtained for alloy MD2 under low cycle fatigue, and the behaviour was linked to dislocation-precipitate interaction in the alloy, and the results are passed on to GE power to support their activities in alloy development. This includes the crystal plasticity model calibrated with the low cycle fatigue test data. Cutting-edge in-situ SEM (scanning electron microscopy) experiments have been carried out for alloy MD2, and short crack initiation and growth were found to be totally controlled by slip behaviour. The experiments generated unique data and new insights into the anomalous short crack growth behaviour in single-crystal nickel superalloy under low cycle fatigue. Both GE Power and Rolls-Royce are highly interested in the results which should allow for a more physically-based approach to be used for predicting initiation and growth of short fatigue cracks. Particularly, the data have been collected and circulated within GE power for consideration in their structural integrity assessment. The formation of discrete slip bands at a crack tip was further verified by 2D DDD simulations, which led to the development of a crystal plasticity-XFEM approach in prediction of slip-controlled crack initiation and growth, with validation against experimental data. These research outcomes are generated using a fundamental micro-mechanics approach, and they are highly valuable for ensuring the structural integrity of critical gas turbine systems. Also, dstl has taken a proactive role in disseminating our research findings to other end users, through their extensive industrial network.
Sector Aerospace, Defence and Marine,Energy
Impact Types Economic

Title 3D DDD model 
Description 3D discrete dislocation dynamics code has been developed to model cyclic deformation for a single crystal alloy, with calibration against experimental results. 
Type Of Material Computer model/algorithm 
Year Produced 2018 
Provided To Others? Yes  
Impact The model allows a fundamental understanding of mechanical behaviour of materials from a micromechanics point of view , and help with microstructure optimisation of high performance alloys. 
URL https://www.tandfonline.com/doi/full/10.1080/14786435.2018.1447159
Title Slip-controlled short crack growth data 
Description In situ SEM data of short crack initiation and growth in a single crystal alloy. 
Type Of Material Database/Collection of data 
Year Produced 2019 
Provided To Others? Yes  
Impact The data has significance in understanding and quantifying crack initiation and growth in critical turbine blade components in gas turbine engines. 
URL https://www.sciencedirect.com/science/article/pii/S0921509318315569
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 Rolls-Royce plc 
Organisation Rolls Royce Group Plc
Country United Kingdom 
Sector Private 
PI Contribution Outcome data submitted to EPSRC on a Final Report
Collaborator Contribution Supply of materials, specimens and technical advice; Attendance of project review meetings.
Impact Joint journal publications and conference presentations; Share of test data, results and models.
Start Year 2007
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 2 
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 A website was produced to increase visibility and public/professional awareness of this research.
Year(s) Of Engagement Activity 2016
URL http://lms-dislocation.com/