Micromechanics and Materials Modelling

Hydrogen in metals: from fundamentals to the design of new steels (HEmS)

HEmS (Hydrogen in metals - from fundamentals to the design of new steels) is a major initiative to investigate the process of embrittlement of metals from hydrogen. The research is funded by the Engineering and Physical Sciences Research Council (EPSRC) and is a joint collaboration between the Universities of Oxford, Cambridge, Sheffield and Imperial and King's Colleges, London.

It has been known for over a hundred years that hydrogen causes catastrophic failure in high strength steels. The overall aim of HEmS is to provide a comprehensive understanding of the process of hydrogen embrittlement in steels, through an integration of advanced characterisation, testing and modelling techniques. This new understanding will be used to develop new ultra-high strength steels that are resistant to embrittlement in the presence of hydrogen.

Potential applications of this research are envisaged across a wide range of key UK industries, including the automotive and energy sectors, and it is anticipated that this research will impact directly on the fuel efficiency of the next generation of cars, and the service lifetimes of wind turbines and energy pipelines.

People:  Alan Cocks, Olga Barrera, Charles Harper

Sponsor:  EPSRC

Date: December 2013–June 2019

Website:  http://www.hems.ox.ac.uk/

Precision Guided Flexible Forming: Closed-loop Control of Geometry and Properties for High Value Metal Metal Component Manufacture

Manufacturing involves only three types of processes - adding, changing or removing material. 'Metal Bashing' - changing the shape of metal components without removal or additions - is easily over-looked but continues to be central to UK manufacturing: jet engines, medical scanners, cars, high-rise offices and contemporary industrial equipment all depend on metal forming, both to define component geometries and to create the properties such as strength and toughness which determine product performance. Metal forming processes are central to the production of a third of all manufactured exports from the UK which are in total worth over £75bn. However, the tools required for forming metal components are custom-made for each application at great cost, so metal forming is often expensive unless used in mass production, yet the drivers for development of future high-value UK manufacturing require increased flexibility and smaller batch sizes without sacrificing either the accuracy or properties of metal parts. In the past twenty years, several research labs around the world have responded to this challenge and explored the design and development of novel flexible metal forming equipment. However these processes have largely failed to move from the lab into industrial use, due to a lack of precision and a failure to guarantee product microstructure and properties. Recent developments in sensors, actuators, control theory and mathematical modelling suggest that both problems could potentially be overcome by use of closed-loop control. The project brings together four disciplines, previously un-connected in the area of flexible forming, to develop the key knowledge underpinning future development of commercially valuable flexible metal forming equipment: mechanical design of novel equipment; control-engineering; materials science of metal forming; fast mathematical process modelling. At the heart of the project is the ambition to link design, metallurgy and modelling to control engineering, in order to develop and apply flexible forming, and to demonstrate it in practice in four well focused case-studies.

Partners: Department of Material Science, University of Oxford; Department of Engineering and DAMTP, University of Cambridge; Siemens VAI; Firth Rixon; Jaguar Land Rover

People: Stephen Duncan, Roger Reed, Matthew Arthington, Jianglin Huang, Vahid Jenkouk, Evros Loukadies, Julian Allwood, Chris Cawthorne, Ed Brambley

Sponsor:  EPSRC

Duration: July 2013-June 2017

MAST: Modelling of advanced materials for simulation of transformative manufacturing processes

An international research project dedicated to bring a transition to the next step in high-value manufacturing in the 21st century. To achieve this, modern manufacturing necessitates a broader use of higher temperatures and loading rates. In practice, development of modelling and simulation tools are the only practical in which these challenge will be met, particularly for new transformative manufacturing processes. Material-removing processes such as solid-state joining processes using linear friction welding, rotary friction welding and ultrasonically-assisted high speed machining are of particular this project research interests. The project involves mathematical modelling including constitutive modelling and experimental validation for high grade alloys such as nickel-based superalloys, titanium and magnesium alloys.

Partners: Loughborough University, MTC, Thompson Friction Welding, Indian Institute of Science Bangalore, Indian Institute of Technology New Delhi, Indian Institute of Technology Guwahati

People: Roger Reed, Nik Petrinic, Clive Siviour, Alan Cocks, Fauzan Adziman

Sponsor: EPSRC

Date: February 2014 – February 2017

Lightweight Fan System Technology Development - SILOET II Project 2

This project focuses on the development of predictive numerical modelling capability for design of lightweight composite materials and systems for components of large aircraft gas turbine fan systems threatened by impact loading.  The research covers a spectrum of activities from experimental observation and quantification of relevant strain rate dependent deformation and failure mechanisms, through to development and validation of multi-scale modelling algorithms aimed at engineering of composite material architectures for optimised performance of engine components.  Topology optimisation based on the capability to simulate strain rate dependent behaviour of materials is an integral part of the materials engineering efforts.

People: Nik Petrinic, Robert Gerlach, Justus Hoffmann

Sponsor: TSB, Rolls-Royce plc

Date: October 2012-September 2016

Development of multi-scale modelling methodology for simulation of rate dependent behaviour of titanium alloys

This project focuses on the understanding of the effect of geometric and physical aspects of material microstructure upon the strain rate dependent behaviour of titanium alloys at macroscopic scale.  Observation and quantification of the response to well controlled dynamic loading regimes and detailed modelling of thus characterised behaviour are carried out in close collaboration with material manufacturers in order to enable design of alloys with advanced/controlled response to impact loading.

People: Nik Petrinic, Benjamin Cousins


Date: October 2010-March 2015

Micro-mechanical modelling techniques for forming texture, non-porportionality and failure in auto materials

A well-defined programme of biaxial forming tests will be carried out by our collaborators at BMW in which the steel microstructures, both before and after straining, are fully characterised using optical and scanning electron (with EBSD) microscopy in order to quantify micro- and macro-level texture and its evolution and to provide the material morphology and crystallography as input to the computational work. Additionally, TEM and x-ray/neutron diffraction work will be carried out in order to investigate dislocation structures established and whether particular forms develop during non-proportional straining. The non-proportional 'Nakajima' tests will be carried out on two materials; namely, a conventional 'forming grade' steel and a high strength steel for which currently, formability is a problem. 3D representative volume elements, RVEs, with appropriate periodic boundary conditions will enable texture development, non-proportionality of straining and localisation and necking to be studied and direct comparisons may be made with the experimental data. The key features of the crystal slip model - the form of the evolution of statistically stored dislocations, the development of geometrically necessary dislocations due to plastic strain gradients, and the establishment of dislocation structures - can be refined by use of the TEM and the experimental localisation results. Once established and validated, the RVE technique becomes powerful and enables parametric studies of the effects of non-proportionality to be carried out in a way that is simply impossible with an equivalent experimental programme.The computational models will naturally take into account the full range of length scales that occur in this problem: at the dislocation and grain levels as well as length scales related to the formation of the localized band of deformation. The resulting simulations will be used to guide the development of simplified models based on the Marciniak and Kuczynski approach. This will be undertaken at a number of different levels to aid the development of tools that can be readily used within an industrial environment. Also the full range of simulations will be used to aid the development of design rules which account for non-proportional loading and which can be used to guide the initial development of a processing route.

People: Tomiwa Erinosho, Fionn Dunne, Alan Cocks, Angus Wilkinson, Richard Todd

Sponsor: EPSRC, BMW

Date: October 2011-October 2014

Fundamentals of Deformation: Slip, Slip Transfer, and Twinnings

Slip is an important mechanism of deformation but there are many aspects of it for which full understanding does not exist, and more so for which good modelling techniques are required to be able to determine accurate stress and strain fields. For example, the role of grain boundaries and the nature of slip transfer; the development of dislocation structures and textures and planar slip. This project, in collaboration with colleagues in Materials Science, aims to employ micro- and nano-mechanical testing combined with new model development in crystal plasticity in order to develop good simulation techniques at the relevant length scales. In particular, we aim to address slip transfer by use of two-grain micro-mechanical cantilever bend testing combined with discrete dislocation and crystal plasticity modelling.

People: Bo Lan, Fionn Dunne, Angus Wikinson

Sponsor: Clarendon Scholarship

Date: October 2011-October 2014

Micro-mechanical Crystal Plasticity of Diffusion-bonded Structures

Micro-mechanical crystal plasticity studies are to be carried out in pseudo-single phase, polycrystal titanium alloy in order to investigate in detail the stress states generated local to a crack tip and in particular, their dependence on local combinations of crystallographic orientation, but more broadly, the effects of texture. In addition, further studies are to address multi-cracked polycrystals; that is, containing a distribution of facets orientated parallel to the primary load direction.

People: Alan Cocks, Mehmet Kartal, Fionn Dunne

Sponsor: Rolls-Royce

Date: November 2011-September 2014

Creep deformation and failure of 316H stainless steels

 The Generation II nuclear power plants are approaching the designed life. EDF Energy is seeking to extend the life of some Generation II power stations for more than 20 years. The creep and failure behaviour of Type 316H stainless steels at elevated temperatures, mainly used in the advanced gas-cooled reactors (AGR) in the Generation II nuclear systems, is under investigation in EDF Energy. The traditional empirical strain hardening model has been shown to be insufficient in predicting all the creep behaviours of the material.

This research is aimed at improving our understanding of the creep behaviour of Type 316H stainless steel by developing and verifying a mechanistically motivated integrated creep deformation model, which will allow more realistic lifetime predictions to be made for such power plant components. In particular, we aim to establish a multi-scale self-consistent polycrystal plasticity model and investigate how the evolution of the microstructural state, e.g. dislocations, precipitations and solid solutions influences the creep behaviour of the material. Meanwhile, in collaboration with researchers at EDF Energy and Bristol University, we seek to explore how the modelling can evaluate the experimental data, including the creep data and the neutron diffraction measurement of the crystallographic lattice strain evolution during high temperature creep deformation.

 People: Jianan Hu, Alan Cocks

Sponsor: EDF Energy

Date: January 2011–September 2014

Modelling type IV failure of welds (MHI)

This project focuses on the development of numerical modelling tools for the prediction of deformation and damage in welded components operating at high temperatures. At elevated temperature, failure of engineering materials generally results from the nucleation and growth of voids on grain-boundary facets which coalesce to form microcracks, whose size scales with the size of the grains in P91 ferritic steels. Failure in these welded components occurs in the fine grained heat affected zone (FGHAZ). Deformation data and number of cavities (as a measure of damage) is being analysed in samples reproducing the microstructure found in the FGHAZ of P91 ferritic steels. Further information about nucleation and growth can be gained by designing notched bar specimens with a range of geometries, each resulting in a different multi-axial stress state under remotely applied uniaxial tension. We have then used these deformation and stress states, as computed by Finite Elements (FE), to predict the nucleation and growth of cavities. We explore whether damage development can be assessed by using: the stress field; strain-rate field; a suitable combination of the stress and strain rate fields - all determined from a finite element simulation which includes the effect of damage on deformation. The continuum model of deformation is being coupled with the local damage development represented as a cohesive zone which can be extended to micromechanical models.

People: Mitsubishi Heavy Industries

Sponsor: Alan Cocks, David Gonzalez

Date: June 2012-June 2014

Micro-mechanical Modelling and Experimentation

This is an EPSRC Platform Grant (PI Professor Alan Cocks) which is enabling support of research in to residual stress determination in single and polycrystal metals at the length scale of second-phase particles and grains respectively. High-resolution EBSD is an excellent technique for determination of 3D strain fields from free surfaces; what it is not able to do, however, is to provide information about the strain state existing on embedded surfaces subsequently sectioned. This work builds on eigenstrain and crystal plasticity techniques in order to determine sub-surface strain and stress distributions from free-surface EBSD measurements. These stresses are needed particularly in understanding sub-surface fatigue crack nucleation often at, for example, second phase particles.
Good progress has been made and new eigenstrain techniques validated against independent finite element calculations. The new techniques have been brought to bear on a single crystal nickel alloy containing large sub-surface carbide particles. Stress and strain fields which existed sub-surface local to the particle have been determined from the sectioned surface from high-res EBSD. The results are currently being verified by crystal plasticity finite element modelling.

People: Alan Cocks, Mehmet Kartal, Olga Barrera, Fionn Dunne, Angus Wilkinson

Sponsor: EPSRC Platform grant (PI Professor Alan Cocks)

Date: February 2011-March 2014

Strain Scanning For Engineering Applications Using Synchrotron X-ray Radiation

The advent of the third generation synchrotron source at the European Synchrotron Radiation Facility (ESRF) in Grenoble opened up new possibilities for efficient use of X-rays to map internal stresses in engineering components non-destructively. The photon flux furnished by the modern insertion devices exceed that available in the lab by the factor of 109, making measurements through over a centimetre of Al to the strain accuracy of 10-4 possible during seconds. Joint development projects are carried out in collaboration with Manchester Materials Science Centre (Prof. P. J. Withers) and University of Salford (Prof. P. J. Webster) . Several beamlines are involved at the ESRF (ID31 Powder Diffraction, ID11 Materials Science) and SRS (16.3 Materials) and development of hardware and software solutions for engineering purposes is carried out.


Sponsor:ESRF, EU TMR


Internal Stress Evaluation Using Multiple Peak Laboratory X-ray Diffraction Analysis

The main thrust of the project is to develop the application of multiple peak diffraction analysis to stress determination in polycrystals using modern laboratory X-ray equipment. While significant amount of effort over the last decade has been devoted to synchrotron X-ray and neutron work in this area, the distinct and important implications for laboratory X-ray analysis only begin to be systematically explored. In situ monotonic and cyclic loading devices will be used in order to develop novel methods allowing the determination of the sample's deformation history, and its residual strength.


Sponsor: Oxford, EPSRC


Design of coatings and coated systems

Coatings such as carbides, nitrides, dry film lubricants and multilayers are used in industry to produce drastic improvements in the hardness, low friction properties and hence performance of contacting components. Due to the extremely small layer thicknesses used the modelling and characterisation methods must be developed specifically for this class of problems. Instrumentation such as nanoindenters and atomic force microscopes are used to study the surface response and condition. Interpretation of the results requires understanding of the contact process, careful instrument calibration, and extrapolation towards the lowest loads, so that the coating-only behaviour can be extracted.

Lifeng Ma is working on his doctoral project concerning characterisation of contact deformation of coated systems, including the conditions of fretting fatigue and wear.


Sponsor: EPSRC


In situ investigation of microstructure under load

The aim is to develop a system for in situ observation of microstructural evolution, using Atomic Force Microscopy. The principal thrust of the project is the examination of ferroelectric surfaces to address key questions about the evolution of microstructure under mechanical and electrical loads. It is well understood that the material behaviour is governed by processes occurring at the microstructural length scale, such as domain wall motion and pinning.  However, relatively little has been done to model the evolution of domain structures under load.  In order to understand this process, it is necessary to observe how the domain structure of the materials changes under load.  This is a challenging task because conventional observation of the microstructure relies on etching of polished surfaces. However, several methods of scanning probe microscopy can directly identify the microstructure of ferroelectrics. This raises the possibility of scanning a ferroelectric surface while the material is loaded to observe the microstructural events that control the material response.

People: John Huber, Alan Cocks

Sponsor: John Fell Fund


Transferability of Small-Specimen Data to Large-Scale Component Fracture Assesment (EPSRC)

The integrity of the piping components of a nuclear reactor is vital to ensure the supply of coolant to the core at all times during the plant life span. One of the inputs to demonstrating integrity is a fracture mechanics assessment to demonstrate defect tolerance. A difficulty in ensuring structural integrity using the higher level methods is the demonstration of transferability of fracture parameters determined from specimens to the application at component level. A large number of tests have been performed on reactor grade piping components of the Indian Pressurized Heavy Water Reactor (PHWR) to improve understanding of transferability. This project will assess these tests using a range of defect assessment approaches to demonstrate transferability for practical piping components and computational models will be developed to reproduce the complex, large-scale component response.

In this context, microstructural models are crucial to reflect the underlying physics of the fracture mechanisms. Therefore, microstructurally motivated ductile fracture models will be developed, based on the available experimental data which will provide both microscopic and macroscopic information about the deformation and crack growth processes. Initially, a continuum type damage model will be established, which could capture the details of cavity nucleation, growth and localization processes. Then an enriched cohezive zone model of the ductile failure will be incorporated into the continuum damage model. In this way, both the initiation and the propagation of the cracks will be captured and the results will be validated through the experimental data.

People: Tuncay Yalcinkaya



Self-consistent Modelling And Diffraction Study Of BCC And HCP Polycrystals

A coordinated experimental and modelling study of polycrystalline deformation and internal stress development in polycrystals of important structural materials will be carried out. Diffraction of beams of penetrating radiation (neutrons at ISIS and ILL, and synchrotron X-rays at the ESRF) on samples and components subjected to residual and live in-situ stresses will be used to collect detailed data on the internal stress evolution during straining. The data will be used to develop and validate elasto-plastic self-consistent (EPSC) models for these materials.

The systems targeted in the present study are ferritic steels (as representative of bcc structure) and titanium alloys (hcp structure). The approach will be used to characterise the evolution of intergranular stresses during monotonic and cyclic loading of polycrystalline samples, which will give new and improved insight into the influence exerted by the internal stresses on the performance and service life of structural materials.


Sponsor: EPSRC


Internal Stress Evaluation Using Multiple Peak Laboratory X-ray Diffraction Analysis

The main thrust of the project is to develop the application of multiple peak diffraction analysis to stress determination in polycrystals using modern laboratory X-ray equipment. While significant amount of effort over the last decade has been devoted to synchrotron X-ray and neutron work in this area, the distinct and important implications for laboratory X-ray analysis only begin to be systematically explored. In situ monotonic and cyclic loading devices will be used in order to develop novel methods allowing the determination of the sample's deformation history, and its residual strength.


Sponsor:Oxford, EPSRC