|
Professor Fionn Dunne
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
- Start Date
- October 20011
top
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
- Mehmet Kartal, Fionn Dunne, Angus Wilkinson
- Sponsor
- EPSRC platform grant (PI Professor Alan Cocks)
- Start Date
- February 2011
top
The overall aim of the work is to provide computational
techniques, coupled with the Deform finite element code, to enable the
prediction of crystallographic textures arising during the deformation
processing of components fabricated from hcp alloys. The approach to be
adopted is a coupled Mises – crystal plasticity technique in which
macro-scale analysis of component deformation is carried out using
Mises plasticity and at appropriately identified locations, crystal
modelling is employed using representative volume elements in order to
determine local macro-and micro-texture.
Progress so far includes the development of texture software which
enables the automated determination of pole figures from ebsd texture
data and the generation of representative volume elements for
implementation in to ABAQUS. In addition, crystal plasticity modelling
techniques for fcc, bcc and hcp have been developed for large
deformations (which includes the rigid body rotations) and new
techniques have been developed for determination of geometrically
necessary dislocation densities. Software has now been developed
for the determination of Kearn’s factors (to characterise texture) from
both directly obtained EBSD measurements and from crystal plasticity
simulations. The new models will shortly be tested against a range of
experimental data for Zr.
- People
- Robert Kiwanuka, Fionn Dune
- Sponsor
- Rolls-Royce Aero, Rolls-Royce Marine
- Start Date
- October 2009
top
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
- Start Date
- October 2011
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
- Mehmet Kartal, Fionn Dunne
- Sponsor
- Rolls-Royce
- Start Date
- November 2011
|
