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Solid Mechanics & Materials Engineering Group |
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Current Research Projects of Prof. Paul Buckley
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Deformation of solid polymers with tailored molecular architecturesThe aim is to determine how the constitutive response and
crazing of polymers in the solid state are controlled by structural
variations at the molecular level. The work forms part of a major UK
initiative in polymer science and engineering: two consecutive
EPSRC-funded projects on different aspects of “Microscale Polymer
Processing” MuPP1 and MuPP2, also involving the universities of
Leeds, Durham, Sheffield, Bradford and Cambridge and numerous
industrial companies. The overall objective of the MuPP projects has
been to exploit recent developments in synthetic polymer chemistry and
in theoretical polymer modelling at the various length scales, to study
and predictively model the passage of polymers with well-defined
molecular structures, right through melt processing, into the final
solid state, and finally into use in a load-bearing product. At Oxford,
our focus has been on the “Solid State” strand of the project,
investigating solid state mechanical properties. We have made a
detailed study of the effects of varying molecular length and length
distribution on the yield, plastic flow and crazing, including
inhomogeneous deformation (via Finite Element modeling), of some model
glassy polymers – atactic polystyrenes with various, known, molecular
lengths and length distributions. We have studied and developed
physically-based models for predicting: the variations of yield stress
and crazing stress with molecular chain length; the development of
frozen-in molecular orientation during processing, and the consequent
development of optical anisotropy and anisotropic glassy state
deformation, yield, flow and crazing.
Deformation of thermoplastic polyester and its nano-composites during forming processesA continuing interest of the group is the rheology of the most
important thermoplastic polyester – PET - in the temperature range
80-120C where it is processed in the manufacture of numerous
commercially important products – e.g. liquid crystal displays and
plastic bottles. Our laboratory houses a high-speed biaxial stretching
machine, which was developed by us specifically for studying PET under
process conditions, and results obtained with it have been used to
assist development of a 3D constitutive model for this material.
Recently there have been two strands to this work. (1) We have
contributed to a collaborative European project ‘Apt-Pak’, aimed at
improving the modeling of manufacturing processes for production of
packaging such as bottles and thermoformed trays, to assist product and
process design to achieve ever higher standards of performance with
less material. Our contribution has been to carry out a more
comprehensive study of the biaxial deformation of PET under relevant
conditions than had ever been attempted previously, and to use the data
obtained to improve further the ‘Oxford’ constitutive model for PET,
designed for use in Finite Element simulations of packaging
manufacturing processes. (2) We have also contributed to an
EPSRC-funded project in collaboration with the University of Bradford
and Queen’s University Belfast (QUB), to develop the capability to
predictively model the constitutive response of PET under conditions as
above, but when containing nano-particles. The focus has been
especially on ‘nanoclays’, produced by ‘exfoliation’ of clay particles.
Information from electron microscopy at QUB is used to create 3D
representative volume elements representing the distribution and
geometry of nano-particles in the PET, and Finite Element analysis,
incorporating our latest PET constitutive model, is then used to
predict deformation of the microstructure and the homogenized
macroscopic constitutive response.
Mechanical integrity of moulded Ultra-High Molecular Weight PolyethyleneUltra-high molecular weight polyethylene (UHMWPE) is usually
used for one of the two bearing surfaces in hip and knee-joint
prostheses. There is a long-standing problem of early failures of some
knee prostheses, know to be associated with the presence of ‘fusion
defects’ remaining from the manufacturing process. Junjie Wu and Paul
Buckley, in collaboration with Professor John O'Connor (former
Director, Oxford Orthopaedic Engineering Centre), some years ago
identified the origin of fusion defects in apparently defect-free
UHMWPE as sites of incomplete molecular diffusion across particle
boundaries during moulding of solid samples of the polymer. In
subsequent work we have studied, and modelled, the development of
interface integrity in moulded UHMWPE. This work revealed that, because
of the exceptionally high molecular weight, full integrity can never be
reached on realistic processing timescales. Diffusion-deficient
interfaces are always present in UHMWPE mouldings. Current work (within
the context of an international collaborative study of UHMWPE supported
by IUPAC) aims to investigate further the practical consequences of
these, in the mechanical performance of UHMWPE with varying average
molecular lengths.
Development and validation of 3D mechanical models for tendons and ligamentsA study is being made of the deformation of tendons and
ligaments, with the ultimate aim of developing an improved finite
element model for simulating the deformation of human cruciate
ligaments, and of their replacement in damaged knees by tendons. The
model will be used in existing knee joint and lower limb finite element
models, for use in the design and implementation of knee prostheses,
and injury prediction, prevention and repair. While there has been
previous research on tendon and ligament modelling, our work aims to
improve on the realism with which the three-dimensional and time
dependent properties of these tissues can be simulated. In the work to
data we have used bovine digital extensor tendons in the following
studies. We have developed an economical, non-contact, optical method
for determining the cross-section geometry of tendons and ligaments. We
have studied and modelled the transverse compression and axial
extension of tendons. We have developed a technique for measuring, and
have carried out an extensive study of, the deformation of tendons
under combined axial tensile and torsional loading. The latter work has
revealed for the first time the exceptionally high degree of coupling
that exists between them in tendons. This is currently being explained
and modelled in terms of their distinctive geometry and anisotropic
mechanical properties.
Prediction of deformation in thermoplastic matrix composite materialsContinuous fibre, thermoplastic matrix, composite materials
are attractive for high volume products because they combine attractive
manufacturing economics with some of the stiffness, strength and
density advantages of the more widely used thermoset matrix composites.
However, these materials offer new challenges for the prediction of
their properties in-use, arising from the pronounced viscoelasticity
and plasticity of the matrix polymer. Their strain rate-dependence is
of particular interest, since some of the potential applications may
involve impacts (e.g. in car bodies). We have studied and modelled the
deformation of polypropylene/glass composite laminates, and also of the
polypropylene matrix material and of reference polypropylenes for
comparison. Deformation studies have been carried out over an
exceptionally wide range of strain-rate from ca 10-4 to ca 104 s-1, and
a physically-based constitutive model has been developed for
polypropylene based on the data obtained, and parameterized for each of
the polypropylenes studied. Microscopy of the moulded
polypropylene/glass laminate has been used to create representative
volume elements (RVEs) for the composite. The matrix constitutive model
and RVE are then combined to create a 3D Finite Element model for
predicting the anisotropic constitutive response of a single lamina of
the composite. This can then be used via laminate theory to predict
deformation of a laminated sheet of the composite. The objective is to
make possible the predictive modeling of in-use performance of these
materials at the stage of laminate and component design.
Optimising performance of polyurethanes via control of chemical structureThe project is a collaborative study of the relationship
between mechanical performance and chemical structure, for a uniquely
versatile family of synthetic polymers: polyurethane copolymers. The
goal is to enable molecular tailoring of these materials, to optimise
their performance in a variety of practical situations. The approach is
interdisciplinary, combining measurements and quantitative modelling of
mechanical properties at Oxford University with synthesis of a large
number of model copolyurethanes (C.Prisacariu and A.Caraculacu of the
Romanian Academy Institute of Macromolecular Chemistry, Iasi, Romania),
supported by microstructural studies by small and wide-angle x-ray
scattering (C.Martin of the University of Manchester) and neutron
scattering (D.Bucknall of Georgia Institute of Technology). The project
is concerned primarily with two contexts where polyurethanes are highly
competitive materials: thermoplastic elastomers and “shape memory”
plastics. In the former case, a uniquely detailed study is being made
of the effects on the complex constitutive response, of varying the
chemistry of hard segment, soft segment and chain extender in the
copolyurethane systems. With the aid of the information on molecular
structure and nm-scale physical structure, work is under way to develop
a quantitative understanding of the major features of the macroscopic
mechanical response, in terms of material composition and
microstructure.
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