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

Lead Research Organisation: University of Southampton
Department Name: Faculty of Engineering & the Environment

Abstract

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.
 
Description Early stage fatigue crack growth in aeroengine turbine disc and blade materials is very hard to predict. We are imaging the complex crack paths being taken - these are affected by crystal orientations (how the atoms are lined up) as well as the temperature and environment. Understanding these complex crack paths by imaging of the local strain accumulation across grain and twin boundaries is feeding into modelling work at other Universities. We are using scanning electron microscopy-digital image correlation (SEM-DIC) to measure these local strains and X-ray tomography to visualise the 3D structure of very small fatigue cracks
Exploitation Route The understanding of the factors controlling early stage crack growth can be used by aeroengine and power generation turbine manufacturers to extend the safe operation of these turbines
Sectors Aerospace, Defence and Marine,Energy,Manufacturing, including Industrial Biotechology

 
Description Schlumberger Faculty for the Future
Amount $50,000 (USD)
Organisation Schlumberger Limited 
Sector Private
Country United States
Start 09/2014 
End 09/2017
 
Description Alstom power 
Organisation Alstom
Department Alstom UK
Country United Kingdom 
Sector Private 
PI Contribution Experimental and computational study of fatigue-oxidation damage and crack growth in single crystal and directionally solidified turbine blade superalloys for gas turbines in power generation applications.
Collaborator Contribution Supply of materials and technical advice; attendance at project review meetings.
Impact Conference and journal publications
Start Year 2013
 
Description Manchester 
Organisation University of Manchester
Department Division of Cell Matrix Biology and Regenerative Medicine
Country United Kingdom 
Sector Academic/University 
PI Contribution We are working jointly on an EPSRC grant, We are providing machined samples and organising materials and fatigue testing samples we provide to them
Collaborator Contribution We are working jointly on an EPSRC grant, Manchester are providing heat treatments for our materials and will be analysing the deformation processes in tested samples we provide to them
Impact ongoing, we expect there to be joint publications
Start Year 2015
 
Description Rolls Royce 
Organisation Rolls Royce Group Plc
Country United Kingdom 
Sector Private 
PI Contribution We are providing information on crack growth behaviour on alloys of interest to Rolls Royce, this has recently extended to looking at effects of shot peening on fatigue life predictions as well as crack growth behaviour in different alloys at high and low temperature
Collaborator Contribution Rolls Royce have provided alloys for us to test and attend technical review meetings to discuss our results
Impact joint publications are planned
Start Year 2014
 
Description Swansea 
Organisation University of Rwanda
Country Rwanda 
Sector Academic/University 
PI Contribution We are working on fatigue testing at high temperatures - we provide crack propagation results
Collaborator Contribution Swansea have great expertise on strain controlled testing, something we cannot contribute
Impact joint publications are planned
Start Year 2014
 
Description Warwick 
Organisation University of Warwick
Department Department of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution Collaboration on an EPSRC programme that has now finished, but we have tested samples from other programmes that are of mutual interest that will lead to joint publications and Warwick is allowing us access to advanced characterisation facilities and input from their staff
Collaborator Contribution Collaboration on an EPSRC programme that has now finished, but we have tested samples from other programmes that are of mutual interest that will lead to joint publications and Warwick is allowing us access to advanced characterisation facilities and input from their staff
Impact Joint publications are planned
Start Year 2015
 
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 Science and Engineering Day, UOS 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact We ran a stand at our University's award winning Science and Engineering Day on 22nd March 2014, a large number of people from the local region attended. We ran it again in March 2015

increased visibility of materials science applied to turbine systems for the general public
Year(s) Of Engagement Activity 2014,2015