Computational Mechanics Group
Tom Bellamy - The role of pores with regards to plastic deformation and fatigue crack initiation in single crystal nickel superalloys.
Title: The role of pores with regards to plastic deformation and fatigue crack initiation in single crystal nickel superalloys.
Investigator: Tom Bellamy
Supervisor: Professor Fionn Dunne
Industrial Partner: Beijing Institute of Aeronautical Materials (BIAM)
Abstract: For a gas turbine engine, the demands of the turbine blades require the high temperature stability that single crystal nickel alloys can bring. The high service temperature required of turbine blades, along with large stresses and a corrosive operating environment result in complex fatigue behaviours. The aim of this research is to help provide new understanding into the mechanisms of plastic deformation for single crystal nickel superalloys and hence the mechanistic drivers for fatigue crack nucleation and growth. In particular, the role of notches to represent both internal casting defects and surface damage will be addressed. Initial work will be carried out to analyse the effect of notches on the fatigue performance of small cylindrical samples. Appropriate modelling methods will be used in combination with independent experimental studies to understand the deformation and degradation processes taking place around the notch.
Alexander Bergsmo - Experimental and Modelling Methodologies for Investigation of Fatigue Crack Nucleation
Title: Experimental and Modelling Methodologies for Investigation of Fatigue Crack Nucleation in Agglomerate-containing Poly-crystalline Nickel Superalloys Fabricated by Powder Metallurgy.
Investigator: Alexander Bergsmo
Supervisor: Prof Fionn Dunne
Duration: 36 Months
Powder metallurgy has recently become the preferred method for producing nickel-superalloy turbine disks. An unavoidable side-effect of powder metallurgy is the propensity to include non-metallic agglomerates in the nickel matrix. These inclusions may create a locally active stress field which may drive the nucleation of cracks. Recent studies have investigated different criteria for crack nucleation at room temperature. The real working conditions of a jet engine turbine are at high temperature and work remains to be done on crack nucleation in such an environment. This project aims to investigate the criteria for which cracks appear around an inclusion subject to high temperature conditions by using experimental and modelling techniques.
Daniel Long - Modelling Delayed Hydride Cracking and Crack Growth in Zr Cladding
Modelling Delayed Hydride Cracking and Crack Growth in Zr Cladding
Delayed hydride cracking (DHC) is a phenomenon which can occur in metallic alloys subjected to thermal cycling. At elevated temperatures, hydrogen diffuses preferentially to crack tips and precipitates at high concentrations, forming hydride phases. These hydride precipitates give rise to stress concentration at crack tips and can influence crack growth-rates significantly. DHC is of particular interest in zirconium alloys, which are used as cladding material in nuclear reactors. This project aims to develop novel crystal plasticity models, coupled with hydrogen diffusion models (accounting for hydride formation and dissolution), building upon a computational modelling framework developed by Dr David Wilson, which enabled mechanistic understanding of fatigue crack growth in zirconium alloys. The influence of hydrogen content on crack growth rate and direction in zirconium alloys under cyclic loading will be investigated, and hence, a more mechanistic understanding of cyclic crack growth characteristics in real materials will be gained. More generally, this work aims to provide an understanding of the influence of microstructural characteristics on crack growth-rate, acceleration, deceleration, and tortuosity etc. Another key challenge of this work is to expand the current 2D modelling capabilities and develop a 3D crack-growth modelling framework. The crystallographic plane along which a crack grows can influence the growth-rate by up to 4 to 5 times. This will enable more accurate prediction of crack-growth characteristics in zirconium alloys; however, as existing methodologies are already very computationally expensive, this is a key obstacle to be overcome.
Vassilios Karamitros - Modelling of Microstructure Sensitive Short Crack Growth in Gas Turbine Alloys
Vassilios Karamitros – Modelling of Microstructure Sensitive Short Crack Growth in Gas Turbine Alloys
Investigator: Vassilios V. Karamitros
Supervisor: Prof. Fionn PE Dunne
Collaborator: Dr. Duncan Maclachlan, Rolls – Royce plc. / Royal Society
Duration: 01/10/18 – 30/09/22 (PhD Studentship)
Short Crack Growth (SCG) in metals and alloys refers to the process of crack growth from nucleation of a small crack approximately the size of a single grain, up to the size of some tens of grains, typically 100 to 200 microns. The process of short crack growth depends on the local microstructure which is significant in determining the useful service life of aero-engine components. In this research project new methods for modelling the growth of short cracks in advanced nickel based superalloys are to be developed. The project combines 3D crystal plasticity modelling and crack growth rate calculations with explicit modelling of crack propagation using the eXtended Finite Element Method (XFEM). Micro-mechanical parameters that are affected by the material’s local microstructure such as the stored elastic energy will be explored to attempt to quantify SCG. Through state of the art modelling techniques and characterization methods, effect of grains, the grain orientations and grain boundaries on crack growth rates, crack branching and crack arrest can potentially be decoded.
Chaitanya Paramatmuni - Studies of deformation and twinning in magnesium alloys.
Studies of deformation and twinning in magnesium alloys.
Chaitanya Paramatmuni, Prof. Fionn Dunne
The plastic behaviour of a material during forming is influenced by the extent of its anisotropy. Magnesium (Mg) alloys show high plastic anisotropy caused by the activation of different deformation modes depending on the loading direction. One of the main contributors to this anisotropy is deformation twinning. It is found to be detrimental to the formability of Mg alloys as it abruptly reorients the texture, preferentially activates hard slip systems and also acts as hot spots in the microstructure. While these hot spots aid in the generation of desirable weak texture during heat treatment, in performance they act as high stress concentration spots for crack nucleation and facilitate crack growth.
This project utilises integrated experimental and numerical investigation of twin nucleation and growth on the free-surface of the sample and in the 3D microstructure. Twin-assisted and slip-assisted twin nucleation in wrought Mg alloy AZ31 and rare-earth Mg alloy WE43 respectively are investigated in detail on the free surfaces of samples. The spatial variation of field variables inside and at the vicinity of twins is explicitly measured using high resolution electron backscatter diffraction (HR-EBSD) to study the twin growth in Mg alloy AZ31. The experimental observations are then integrated with crystal plasticity strain-gradient modelling to investigate twin growth in Mg alloys. Finally, these free-surface investigations are extended to three-dimensions by conducting diffraction contrast tomography (DCT) and 3D-EBSD experiments to obtain 3D grain and twin morphologies, which are further analysed using mesoscale modelling techniques.
Ben Poole - The Temperature Sensitivity of Galling in Hard-facing Alloys
Project title: The Temperature Sensitivity of Galling in Hard-facing Alloys
Investigator: Benjamin Poole
Supervisors: Prof. Fionn Dunne and Prof Daniele Dini (Mechanical Engineering)
Collaborators: Dr David Stewart, Rolls Royce plc.
Duration: 01/10/2016 – 30/09/2020
Funded by the ICO CDT in Nuclear Energy (EPSRC) and Rolls Royce plc.
Description: Hard-facings are hard surface coatings applied to components to provide resistance from wear and corrosion. Within a pressurised water reactor, hard-facings are typically applied to the internal surfaces of primary loop control valves. These coatings are crucial in order to resist galling, a severe wear process associated with large amounts of plastic deformation and leading to operability issues. Cobalt based Stellite alloys provide excellent galling resistance but are easily activated by neutron fluxes within the core. Work has been proceeding on the development of Co-free, Fe-based hard-facings for nuclear applications but most have been shown to display unacceptable and poorly-understood temperature sensitivity
Through the use of representative crystal plasticity finite element models and elevated temperature mechanical testing, this project aims to study the plastic deformation of Fe-based hard-facings at elevated temperatures. This understanding will feed into the development of future Co-free hard-facing alloys.
David Shepherd - Development of Non-Destructive Ultrasonic Detection Methods of Macrozones within Titanium Alloys
Title: Development of Non-Destructive Ultrasonic Detection Methods of Macrozones within Titanium Alloys
Investigator: David Shepherd
Supervisors: Prof Fionn Dunne and Prof Mike Lowe
Collaborators: Mr Koichi Inagaki, IHI Corporation
The project aims to establish ultrasonic wave speed measurement methods in order to extract information about macrozones in aero-engine titanium compressor discs. Macrozones are relatively large regions of uniform crystallographic grain orientation which are believed to lead to a life debit in engine components through dwell fatigue leading to crack nucleation. Hence a non-destructive method using ultrasound waves to detect their presence in engineering components is eagerly sought.
The project aims to investigate ultrasonic normal and shear wave speed measurements in titanium alloys for this purpose. Because titanium grains are elastically anisotropic, reflecting their orientations, wave speed methods offer the potential to detect large regions of uniform crystallographic orientation by inferring their presence from measured wave speed changes.
Thomas Porter - Modelling methods for optimal functionally graded materials by novel processing
Project title: Modelling methods for optimal functionally graded materials by novel processing.
Investigator: Thomas J Porter
Supervisors: Prof. Fionn Dunne and Dr David Stewart
Duration: 01/10/2019 – 30/09/2022
Funded by the ICO CDT in Nuclear Energy (EPSRC) and Rolls Royce plc.
Project Description: Powder Hot Isostatic Pressing (HIP) is a metallurgy process for consolidating powder particles. High temperature and pressure are applied simultaneously to form a component of complete theoretical density. The application of pressure allows for elimination of porosity and mitigation of grain growth. A near net shape component is produced, with better chemical homogeneity in comparison to traditional casting methods.
The aim of the project is to develop functionally graded components through the HIP process. So far, components are in general made from a single type of material. However, the possibility of hot isostatically pressing two different types of material together would address the need of varying performance along a component. An example is in graded structures coupling pressure vessels and piping in a nuclear power plant.
This project addresses the establishment of computational material modelling methods to facilitate optimally designed HIPped materials and structures, and will likely be partnered with an experimental programme, in collaboration with Rolls-Royce.
Xiaoxian Zhang - Fatigue crack nucleation around annealing twin boundary in nickel superalloy
Project title: Fatigue crack nucleation around annealing twin boundary in nickel superalloy
Investigator: Xiaoxian Zhang
Supervisors: Prof. Fionn Dunne
Collaborators: Dr Jean Charles Stinville, University of California Santa Barbara
Duration: 01/10/2019 – 01/10/2020
Funded by CSC and RAEng
Description: For polycrystal nickel superalloy, fatigue crack nucleation (FCN) is well known around annealing twin boundary except for non-metallic inclusions. The mechanism about this nucleation is associated with the special crystal orientation and microstructure/morphology of twin. Even the slip behavior (parallel and inclined) happened around twin has been investigated systemically, an intrinsic scientific question remains elusive, that is why fatigue crack preferentially nucleate around twin boundary rather than other grain boundary and grain interior.
This project employ a crystal plasticity finite element analysis coupled a HR-DIC characterization, to investigate the micro-behavior appended around twin boundary during a cyclic loading, providing an understanding about the FCN around twin from an energy approach associated with microstructure.
Dr Yilun Xu - Crystal Plasticity Modelling of Electronic Solder Joint Performance
Investigator: Dr Yilun Xu
Supervisor / Line Manger: Prof Fionn Dunne
Title: Crystal Plasticity Modelling of Electronic Solder Joint Performance
Duration: 01/07/2018 - 30/06/2021
This project aims to predict and control of solder joint reliability in electronics using high fidelity crystal plasticity finite element (CPFE) modelling and discrete dislocation dynamics (DD) techniques. The research involves the establishment of mechanistic crystal-level models for slip and failure (in conjunction with micro-mechanical experimentalists) and the development of solder bead micro-structural models. The output of the modelling guides the design of solder bead microstructure and provides the optimal performance for particular loading regimes, e.g. thermo-mechanical and mechanical shock loading. The project also includes working with colleagues utilising the models through to simulating solder arrays under in-service loading.
Yang Liu - Towards validated predictive capability for microstructure-sensitive fatigue and delayed hydride cracking in Zr
Investigator: Dr Yang Liu
Supervisor / Line Manger: Prof Fionn Dunne
Title: Towards validated predictive capability for microstructure-sensitive fatigue and delayed hydride cracking in Zr
Duration: 31/03/2020 - 14/09/2021
The aim of this work is to establish microstructure-level dislocation-based crystal plasticity (CP) modelling coupled with atomistic hydrogen diffusion for spatially resolved analysis of hydride precipitation and dissolution, hydride cracking and interaction with fatigue-driven crack growth in (proton) irradiated Zr-alloy. The CP modelling method facilitates explicit representation of microstructure (grain and second-phase morphology, crystallography, slip, grain boundary interactions). The work will rely on property extraction and quantitative characterisation (HR-DIC, HR-EBSD) at Manchester and Oxford, discrete dislocation modelling at Imperial and Oxford. New experimental work at Imperial could be conducted to investigate hydride precipitation and dissolution under representative in-service thermo-mechanical loading in (proton) irradiated samples in order to validate the new CP modelling framework developed.
Chong Zhao - Study of Micro Mechanical Deformation in Hardfacing Alloy
Student: Chong Zhao
Supervisor: Prof. Fionn Dunne, Dr.Jun Jiang
Funding: Rolls Royce (55%), Imperial College London (45%)
Hardfacing alloys have excellent corrosion and wear resistance. Cobalt base hardfacing alloys have good hard facing capacity and are widely used in nuclear applications. However, radiation from Cobalt is hazardous for both workers and plant materials so a non-radiative iron base hardfacing alloy is designed to replace traditional cobalt base alloys. This project will observe the sliding wear behaviour in this material. Mechanical properties, such as shear strength, elastic strain and galling resistance in increasing temperatures will be studied. These mechanical properties shall be compared with other hardfacing alloys and the relationship between chemical compositions and mechanical properties shall be investigated.