Seminars

Solid Mechanics and Materials Engineering Group Seminars

Vikram Deshpande, University of Cambridge

A Coupled Framework For Climb-Assisted Glide In Discrete Dislocation Plasticity
When Jan 28, 2013
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865 283302
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It is now well established that the plastic deformation of crystalline solids is size dependent at the micron scale for a range of loading conditions. While there are many underlying reasons for these size effects, attention has been primarily focussed on situations where plastic strain gradients are generated. Models typically tend to over-predict the experimentally observed size effects as they neglect a range of dislocation relaxation mechanisms. These mechanisms include dislocation cross-slip and dislocation climb. Dislocation climb requires the diffusion of vacancies and hence significant amounts of dislocation climb only occur at temperatures above a third of the melting temperature - in these cases mass transport reduces the plastic strain gradients and thereby reducing the effect of specimen size. However, even at lower temperatures, dislocations can surmount small obstacles with the aid of small amounts of climb. This prevents the build-up of large dislocation pile-ups which consequently again relaxes stresses.

The coupling of vacancy diffusion with dislocation motion is a true "multi-scale" problem as vacancy/dislocation interaction is essentially a dislocation core effect. We present a two-dimensional discrete dislocation plasticity framework coupled with vacancy diffusion wherein dislocation motion occurs by both climb and glide. The effect of dislocation climb is explored for a range of problems including size effects in bending of crystals, metal-matrix composites and passivated films. Dislocation climb typically tends to reduce strength enhancements that occur with decreasing size but in some surprising cases can also result in strength increases.

Seminars

Solid Mechanics and Materials Engineering Group Seminars. Term Time: Mondays (2.00-3.00pm) in LR8, IEB Building, Engineering Science Department, Oxford University. For more Information please contact: Please contact jin-chong.tan@eng.ox.ac.uk Tel: +44(0)1865 2 73925

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Prof. Daniele Dini (Imperial College London, UK) Feb 20, 2017 from 02:00 PM to 03:00 PM LR8,
Modelling in Tribology: a Multidisciplinary Journey from Molecules to Engineering Applications
Prof. Jim Woodhouse (University of Cambridge, UK) Feb 28, 2017 from 02:00 PM to 03:00 PM LR1,
Can we Predict Friction-Driven Vibration?
Prof. Stephen Hallett (University of Bristol, UK) Mar 06, 2017 from 02:00 PM to 03:00 PM LR8,
High Fidelity Modelling of Low Velocity Impact Damage in Laminated Composites

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Previous Seminars
Seminars which have already happened.

Anna Pandolfi

Metaconcrete: Engineered aggregates for enhanced dynamic performance
When Mar 08, 2016
from 05:00 PM to 06:00 PM
Where LR1
Contact Name
Contact Phone 01865-273172
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Anna Pandoli CV

Recent progress in the field of metamaterials science has led to a range of novel composites which display unusual properties when interacting with electromagnetic, acoustic, and elastic waves. Metaconcrete is a new structural metamaterial with enhanced wave attenuation properties for dynamic loading applications. In this new composite material the standard stone and gravel aggregates of regular concrete are replaced with spherical engineered inclusions. Each metaconcrete aggregate has a layered structure, consisting of a heavy core and a thin compliant outer coating. This structure allows for resonance at or near the eigenfrequencies of the inclusions, and the aggregates can be tuned so that resonant oscillations will be activated by particular frequencies of an applied dynamic loading. The activation of resonance within the aggregates causes the overall system to exhibit negative effective mass, which leads to attenuation of the applied wave motion.

To investigate numerically the behavior of metaconcrete slabs under a variety of different loading conditions a finite element slab model containing a periodic array of aggregates is utilized. The various analyses presented in our studies provide the theoretical and numerical background necessary for the informed design and development of metaconcrete aggregates for dynamic loading applications, such as blast shielding, impact protection, and seismic mitigation. The work has been developed in collaboration with Stephanie J. Mitchell (Caltech), Michael Ortiz (Caltech) and Deborah Briccola (Polimi).

 

 

Metaconcrete Metaconcrete

 

 

 References

 

[1]    S. J. Mitchell, A. Pandofi, and M. Ortiz. Effect of brittle fracture in a metaconcrete slab under shock loading. Journal of Engineering Mechanics, ASCE, Accepted:1-28, 2015.

[2]    S. J. Mitchell, A. Pandolfi, and M. Ortiz. Investigation of elastic wave transmission in a metaconcrete slab. Mechanics of Materials, 91:295-303, 2015.

[3]    S. J. Mitchell, A. Pandolfi, and M. Ortiz. Metaconcrete: designed aggregates to enhance dynamic performance. Journal of the Mechanics and Physics of Solids, 65:69-81, 2014.

**CANCELLED - to be rescheduled** Prof. Benoit Devincre (LEM, CNRS/ONERA, France)

Improving crystal plasticity models with dislocation dynamics simulations
When Nov 21, 2016
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-283446
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Plastic deformation of crystalline materials is mainly the result of the collective movement of dislocations, in the response of their mutual interactions, external applied loading and interactions with boundaries such as free surfaces, interfaces or grain boundaries. The dislocation microstructures emerging from such dynamics are intrinsically heterogeneous and the way they affect the mechanical properties is a puzzling problem. Therefore, the development of constitutive equations for the modeling of this multiscale problem is a challenging issue confronting materials science. After two decades of developments, three dimensional Dislocation Dynamics (3D-DD) simulation comes out as a remarkable tool to investigate such problem. In this presentation, investigation of strain hardening in FCC crystals is presented and discussed from the viewpoint of scale transitions. This analysis is based on the storage recovery framework expanded at the scale of slip systems. Use of 3D-DD simulations is systematically made to guide and justify an improved formulation of a crystal plasticity model and to calculate the latter parameter values. Attention is focused on the forest interactions believed to control isotropic hardening and the internal stress (backstress) associated with dislocation patterning that is believed to control kinematic hardening. Among the different elementary features controlling both strain hardening phenomena, we show that junction strength and mobile dislocation mean free path are key physical parameters to understand the dislocation microstructure asymmetry upon load path changes. A model accounting for the dislocation short-range properties observed in DD simulation is proposed. This model captures quantitatively many details of the existing experiments on monotonic tensile tests, Baushinger tests and cyclic tests made on FCC single crystals.

devincre@onera.fr

Professor Bjoern Kiefer, TU Dortmund, Institute of Mechanics, Germany

Continuum Modeling of Magnetostriction on Different Length Scales
When Mar 13, 2015
from 02:00 PM to 03:00 PM
Where LR7, IEB Building, Engineering Science
Contact Name
Contact Phone 01865-273925
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Active and multifunctional materials have drawn considerable interest in recent years, as they show
great potential for enabling novel sensing, actuation, transduction, energy harvesting and biomimetic
applications, to be employed in aerospace, the automotive industry, microelectronics, the biomedical
and other fields. In addition to the exploration, synthesis and characterization of new material systems,
the accurate modeling and simulation of their constitutive response is of key importance in the
endeavor of leading the application design beyond purely conceptual ideas.
Here, we focus on the modeling of active materials exhibiting magneto-mechanical coupling, e.g. giant
magnetostrictives and magnetic shape memory alloys. From a modeling standpoint, great challenges
stem from the complex coupled, nonlinear, and inelastic nature of the response exhibited by these
materials. The macroscopic behavior is thereby driven by microstructural changes, such as phase
transformations or twin-boundary and magnetic domain wall motion. The presented work is concerned
with capturing these various magneto-mechanical mechanisms causing changes in deformation and
magnetization on different length scales via continuum thermodynamics based modeling.

Bjoern Kiefer

Figure 1: Prediction of magnetic-field-biased pseudoelasticity in the magnetic shape memory alloy
Ni2MnGa [3] based on the energy minimizing evolution of laminated microstructure.
Three particular modeling approaches are discussed and illustrated with numerical examples. First,
as an extension of classical computational inelasticity, a macroscopic model for magnetic shape memory
behavior is presented. Secondly, a general macroscopic variational-based modeling and simulation
framework for dissipative magnetostriction, is introduced, and some details on its algorithmic treatment
are given. The third part of the lecture discusses approaches to the meso-scale modeling of
microstructure evolution in magnetizable solids based on concepts of energy relaxation, particularly
in the context of extensions to the constrained theory of magnetoelasticity proposed by DeSimone and
James. Finally, the extendibility of these general concepts—e.g. to the phase field modeling of domain
evolution or to finite deformation theories for magnetoactive polymers—is briefly addressed.
References
[1] B. Kiefer and D. C. Lagoudas, Modeling the Coupled Strain and Magnetization Response of Magnetic
Shape Memory Alloys under Magnetomechanical Loading, Journal of Intelligent Material Systems and
Structures Vol. 20, 143–170, 2009.
[2] C. Miehe, B. Kiefer and D. Rosato, An Incremental Variational Formulation of Dissipative Magnetostriction
at the Macroscopic Continuum Level, International Journal of Solids and Structures Vol. 48,
1846–1866, 2011.
[3] B. Kiefer, K. Buckmann, T. Bartel, Numerical Energy Relaxation to Model Microstructure Evolution
in Functional Magnetic Materials, GAMM-Mitteilungen Vol. 21(1), 171–195, 2015.

http://www.mechbau.uni-stuttgart.de/ls1/members/former/kiefer/

Dr. Daniel Mulvihill (University of Glasgow, UK)

Potential Routes to Stronger Carbon Nanotube (CNT) Fibres via Carbon Ion Irradiation and Deposition
When Nov 28, 2016
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-283446
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Fibres made from carbon nanotubes (CNTs) have not yet achieved strengths approaching that of individual CNTs. The problem is that load is not effectively transferred between the constituent, discontinuous CNTs. High energy irradiation has shown promise on small CNT bundles, in creating covalent crosslinks to enhance load transfer, but cannot sufficiently penetrate real CNT fibres, which typically contain 106 or more CNTs. Here, we suggest that the “draw-twist” process for producing CNT fibres from forests offers an opportunity for CNT bundles to be individually treated with irradiation before being twisted to form a fibre. We use molecular dynamics to examine the effectiveness of low energy (1 eV) carbon ion irradiation (or deposition) in this context. We find that very small amounts of deposition can significantly enhance both intra-bundle and inter-bundle load transfer. Within bundles, deposition atoms mediate covalent links between both the sides and ends of neighbouring CNTs. Inter-bundle load transfer is improved as under-coordinated carbon adatom branches formed during deposition, spontaneously form inter-bundle cross-links as the bundles are forced together by the twisting action. The effects of varying fluence and twisting angles are examined, and the potential to add a prior higher energy irradiation step to penetrate larger bundles is explored. The possibility to produce an amorphous carbon/CNT composite fibre is also discussed.

Daniel.Mulvihill@glasgow.ac.uk 

David Gustafsson, Siemens

Crack Propagation Challenges in Gas Turbine Engine
When May 06, 2015
from 03:00 PM to 04:00 PM
Where LR8, IEB Building, Engineering Science
Contact Name
Contact Phone 01865-613069
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In gas turbines it is important to design for as high gas temperatures as possible in order to attain a high thermal efficiency. In the case of power generating gas turbines, the increase of temperature leads to lower fuel consumption, reduced pollution and thus lower costs. The high-temperature load carrying ability of critical components is therefore one of the most important factors that set the limits in gas turbine design.  Even though high temperature resistant superalloys are used, hot components are usually designed to run near their temperature and load limits. Uncertainties in models and methods used for fatigue life prediction under these circumstances are thus very problematic. 

 

Another complicating factor is the shift in the power generation market more focusing on distributed energy generation for a local energy market compared to the classic focus on lager power plants generating power for a larger geographical area. This is due to the larger part of renewable energy sources, such as wind and solar power, available today. Unfortunately, the wind don’t blow and the sun don’t shine twenty four seven. Thus, a reliable backup solution is needed.  The industrial gas turbine fills this role rather well but this type of operation means that the gas turbine will be forced to start daily which will have a huge impact on component cyclic life.

 

Among the most important questions in gas turbine design today is therefore how to predict the fatigue life of critical components. In many cases designing against fatigue crack initiation is not enough but designers must rely on a damage tolerant design where stable crack growth must be allowed. Component failure is extremely costly and can potentially lead to the loss of human life. Thus, avoiding failure is always a first priority. This put huge demands on accurate crack growth calculation methods and high quality material data

 

This seminar will discuss some of the more difficult challenges related to crack propagation in gas turbine design such as intergranular cracking of gas turbine disc materials, crack growth in single crystal material, complex loading cycles and complex geometries.

 

 

 

 

 

Dr Alessandro Mottura, School of Metallurgy & Materials, University of Birmingham

From Ni-based to Co-based superalloys: planar faults energies to order
When Nov 11, 2013
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-283302
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In 2006, an intermetallic phase based on the L12 crystal structure was observed in the Co-Al-W ternary system. This new intermetallic phase (γʹ), together with the high-temperature phase of pure Co (γ), can form the typical γ/γʹ microstructure observed in single-crystal Ni-based superalloys. Thanks to higher melting temperatures, this new class of γʹ-strengthened Co-based superalloys may become suitable substitutes for conventional Ni-based superalloys in the hottest sections of gas turbines and jet engines. Initial investigations of their creep properties revealed a pronounced anomalous yield strength at high temperatures, and examination of dislocation structures within the γʹ precipitates indicates dramatic differences when compared to their Ni-based relatives. This can, in part, be attributed to the various planar fault energies in the γʹ phase. The possible presence of a continuous γʹ phase field stretching from the Ni- to the Co-based systems may lead to new hybrid alloys with diverse properties. In this work, ab initio methods coupled with the axial next-nearest-neighbour Ising model and special quasi-random structures are used to study the effect of wider chemical changes on the relevant planar fault energies, with the objective of exploring the possibility of hybrid Ni-Co-based superalloys.

 

Dr Alexei Maznev, MIT

Non-diffusive thermal transport and THz ultrasonics at room temperature: in search for the phonon mean free path
When May 28, 2015
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
Contact Name
Contact Phone 01865-273925
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In non-metallic solids, heat is carried primarily by acoustic phonons. At cryogenic temperatures, the phonon mean free path (MFP) can be large and the connection between heat transport and acoustics is well established as exemplified by the studies of “phonon imaging”. At room temperature (RT) phonon frequencies significantly contributing to heat transport (typically above 1 THz) have been out of reach of acoustics research. The two fields are now converging thanks to two recent developments. On one hand, studies of the fundamentals of thermal conductivity showed that low-frequency phonons with long MFP play a much larger role in heat transport at RT than previously thought. On the other hand, acoustic frequencies ~1 THz and above are becoming accessible in laser-based picosecond ultrasonics experiments. In this talk, I will give an overview of our recent research on both thermal transport and high-frequency acoustics aimed at bridging the gap between the two fields. We will discuss thermal transport measurements of thin Si membranes and bulk GaAs with the laser-induced thermal grating technique where we detected a significant deviation from the diffusive transport at distances exceeding 1 mm, in stark contrast with the 40 nm average phonon MFP in Si at RT typically cited in textbooks. Furthermore, I will describe experiments with laser-generated coherent phonons at frequencies 0.3-1.5 THz in GaAs and GaN-based structures as well as Si membranes aimed at direct measurements of the phonon lifetime. We will also see how methods used in these studies can be employed by materials scientists for non-contract characterization of thermal and elastic properties of materials.

Dr. Alice Cicirello (University of Oxford, UK)

Uncertainty Models in Structural Dynamics
When Feb 06, 2017
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-283446
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Much industrial interest has been focused on rapidly explore the performance of a design to dynamic loading by building virtual prototypes. However, at the design stage there are uncertainties associated with (i) an inherent variability of the properties of the system because of the manufacturing process, and with (ii) a lack of knowledge of the analyst with respect to the system properties which are fixed. Therefore, the challenge is not only to develop a mathematical model able to capture the physics of the problem, but also to account for these uncertainties, which might be associated with the geometry and mechanical properties, loading, boundary conditions and structural joints.
The most direct approach to modelling uncertainties is to describe system parameters by means of a probability density function (pdf) and to propagate the uncertainties to yield the reliability of a system and/or the response moments. However, a limited set of data or vague information are often available, therefore specifying a single pdf for describing the uncertain variable becomes a very challenging task. This has led to the widespread of non-probabilistic uncertainty frameworks, such as intervals, convex and fuzzy descriptions, and several strategies have been developed to efficiently propagate these uncertainty models through the equations of motion to yield a bounded description of the system response. Other approaches introduce uncertainties in the probabilistic assignments by considering sets of probability distributions instead of a single distribution. However the application of these approaches to structural dynamics is very limited, mainly because of the computational burden associated to their propagation through the equations of motion. Alternatively, non-parametric models of uncertainty have been used to predict the response statistics of random structures as in Statistical Energy Analysis (SEA) method, Hybrid Finite Element/SEA method, and Random Matrix Theory.
In this talk, I will give an overview of these uncertainty models, with particular focus on recent advances on (i) the combination of parametric and non-parametric uncertainty models, and (ii) on imprecise probability.

Dr Arnaud Marmier, Exeter

Negative Poisson's ratio and other unusual elastic properties in pure silica zeolites: a DFT and force field study
When Mar 09, 2015
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
Contact Name
Contact Phone 01865-273925
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Poisson’s ratio can be a complex, highly anisotropic property that can have interesting and useful negative values. This phenomenon, referred to as auxeticity, is usually limited to a small range of directions, but can occasionally occur in a larger range, or even for all directions. Beside the minimum value there are presently no indicators of the “amount” of auxeticity of a material, and we propose a carefully derived typology to overcome this limitation. To establish a ”base-line” for auxeticity, we characterise the auxeticity of numerous single crystals using previously published elastic tensors. We demonstrate that occasional auxeticity occurs in around 37% of the cases, while “average” auxeticity is much less prevalent, and limited to the silica α-cristobalite. With the help of this scheme, we consider pure silica zeolites, previously thought of as good candidates due to their low density and chemical similarity to cristobalite. Due to the lack of experimental elastic data for pure silica zeolite (limited to MFI), we calculate the elastic tensors with Density Functional Theory and several force-fields methods. We find that the different models are in excellent agreement and that on average pure silica zeolite are not more auxetic than the reference crystals. On the other hand, we also identify with great confidence that the framework JST is not only averagely auxetic, but in fact completely auxetic, the first crystal with this property. Finally, we propose a mechanism, based on stiff bonds and flexible angles, which mimics the behaviour of JST under stress and explain its generalised auxeticity.

http://empslocal.ex.ac.uk/people/staff/ashm201/

Dr Axel Zeitler, Cambridge University

Engineering applications of Terahertz radiation: From probing supramolecular structure to non-destructive testing
When Jan 25, 2016
from 02:00 PM to 03:00 PM
Where LR8
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 Materials Characterisation: 

My main interest is in the characterisation of condensed matter samples with a direct link into chemical engineering. In our work we are exploiting the sensitivity of terahertz radiation to the interaction of molecules and the resulting structure and dynamics the molecules form in solids and liquids. One of my particular areas of interest is in the solid state physicochemical properties of pharmaceutical materials. In addition to this field I am working towards a better understanding of catalysts, fundamental properties of glasses and the dynamics of biomolecules. 

Non-destructive Imaging: 

By exploiting the fact that terahertz radiation is able to penetrate a number of materials that are opaque at visible frequencies we are able to quantitatively probe the structure of a wide variety of samples such as polymer coatings of pharmaceutical tablets. I am interested in how this information can be used to develop better processes and products, again with a particular interest in pharmaceutical manufacturing. Furthermore, in collaboration with the Semiconductor Physics Group I am interested in evaluating the potential of new, more powerful, sources of terahertz radiation such as quantum cascade lasers (THz-QCL) for imaging applications.

Industrial Sensor Applications: 

At the moment terahertz technology is predominantly used in laboratories in a research and development environment. In order to advance the applications into process control and manufacturing we are working together with industry towards a new generation of terahertz sensors which are capable of providing real-time data and operating under industrial conditions.

 http://thz.ceb.cam.ac.uk/group/axel-zeitler

Dr Camille Petit, Faculty of Engineering, Imperial College, London

Towards the Design of Multi-Purpose Nanomaterials for Sustainability
When Oct 27, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
Contact Name
Contact Phone 01865-283302
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Through examples from our research studies, this talk will highlight the pivotal role of hybrid nanomaterials in the development of energy- and environment-related technologies. Specifically, the use of hybrid materials based on Metal-Organic Frameworks (MOFs) and graphite oxide (GO) for toxic gases removal; along with the evaluation of novel organic-inorganic solvents for CO2 capture will be discussed.
The former type of composites exploits the complementary strengths of MOF and GO for separation. Their nanostructure and chemistry are designed to provide strong adsorption forces that allow these adsorbents to greatly surpass the efficiency of conventional physical adsorbents. The second type of hybrid materials consists of a polymeric canopy grafted onto an inorganic core. Here, it is shown that CO2 capture using these materials arises from an enthalpic effect via reaction with task-specific groups, as well as an entropic effect via the specific structural arrangement of the polymer chains. This unique feature in concert with the negligible vapour pressure and high thermal stability of these materials make them potential candidates for the next generation of capture materials.
Beyond enhancing separation capabilities, it is crucial to build integrated and sustainable technologies. Multifunctional materials have a critical role to play towards this goal. This aspect will be illustrated in the area of CO2 capture and conversion to chemicals. Here, we currently explore the use of nanocolloids to serve as dual-purpose materials for simultaneous CO2 capture and electrochemical conversion.

Dr Claudio Lopes, IMDEA Materials

Coupled experimental-numerical mechanical characterisation of composites at several scales
When Feb 22, 2016
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-273925
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A coupled experimental-numerical multiscale approach to design composite materials and structures is proposed. A virtual analysis strategy that physically describes the material behaviour at different length scales from ply to laminate, and to composite structure is being developed and validated at IMDEA Materials. This approach is then applied to design and optimize novel microstructures, non-conventional laminates, and next-generation structural composites that take full advantage of the potential design space and manufacturing possibilities in composites, such as steered-fibre composites.

The cornerstone of this approach is the characterization of the three elementary composite constituents (fibre, matrix and fibre/matrix interface) by means of experimental micromechanics. Then, the numerical simulation of the micromechanical behaviour of the composite plies is achieved by means of Representative Volume Element and Embedded Cell Element techniques that describe the elastic, plastic and fracture behaviour of the different phases by means of appropriate constitutive equations. At mesoscale level, laminates are simulated by means of continuum models for plies and ply-interfaces that take into account the mechanisms of deformation and failure as predicted by computational micromechanics and observed experimentally. At structural level, the composite is modelled by means of single shells that implicitly describe the behaviour of the plies in the laminate.

Besides the simulation of the actual physical mechanisms of deformation and damage in composites, another advantage of this bottom-up multiscale approach is that changes in the properties of the constituents, in microstructure, laminate lay-up or fibre architecture can be easily taken into account to perform reliable predictions of the macroscopic behaviour of the composite. Hence, multiscale design of next-generation composites is within reach.

 

http://www.materials.imdea.org/people/researchers/dr-claudio-lopes

Dr Daniel S Balint, Senior Lecturer in Mechanics of Materials, Imperial College London

Dynamic Discrete Dislocation Plasticity for Extremely High Strain Rates
When Jun 16, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Abstract: Traditionally, the study of plastic relaxation processes under weak shock loading and high strain rates in crystalline materials has been based on direct experimental measurement of the macroscopic response of the material. Using this data, well-known macroscopic constitutive laws and equations of state have been formulated. However, direct simulation of dislocations as the dynamic agents of plastic relaxation in those circumstances remains a challenge. Current Discrete Dislocation Plasticity (DDP) methods, where dislocations are modeled as discrete line singularities in an elastic continuum, are unable to adequately simulate plastic relaxation because they treat dislocation motion quasi-statically, thus neglecting the time-dependent nature of the elastic fields and assuming that they instantaneously acquire the shape and magnitude predicted by elastostatics. Under shock loading, this assumption leads to artifacts that can only be overcome with a fully time-dependent formulation of the elastic fields. The first part of this talk will be an overview of planar quasi-static discrete dislocation plasticity, including a summary of studies on size effects conducted over the last ten years. It will then be shown that the quasi-static approximation is unsuitable for very high strain rates (~10^6 and higher). Finally, a truly dynamic formulation for the creation, annihilation and arbitrary motion of straight edge dislocations will be presented. The Dynamic Discrete Dislocation Plasticity (D3P) method will be applied in a two-dimensional model of time-dependent plastic relaxation under shock loading, and some relevant results on the decay of the elastic precursor will be presented.

Dr Debdulal Roy, National Physical Laboratory, London

Nano-optics for nanostructures and nanochemistry
When Mar 02, 2015
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Heterogeneity and inhomogeneity are integral parts of most functional materials. Transmission electron microscopy (TEM), generally the technique of choice due to its atomic resolution, works only in vacuum. Near-field optical tool such as tip-enhanced Raman spectroscopy, however, allows conducting measurements on local nanoscale surfaces in ambient environments. In this talk two examples of measurements using nano-optics will be presented: one that measures chemistry on single electro-chemically active sites on a catalytic surface, paving the path for real-time investigation of catalytic reactions, and the other maps individual defects in 2D  materials such as  graphene.

http://www.npl.co.uk/people/deb-roy

Dr Digby Symons, Mechanical Engineering, Cambridge University

Design guidelines for granular particles in a conical centrifugal filter
When Oct 06, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Centrifugal filters are commonly used in the food processing and chemical industries in order to separate the liquid and

solid phases of a mixture.  Essential design criteria for successful drying of granular particles in a conical continuous centrifugal filter are proposed. Four criteria are considered: minimum flow thickness (to ensure sliding bulk flow rather than particulate flow), desaturation position, output dryness and basket failure. The criteria are based on idealised physical models of the machine operation and are written explicitly as functions of the basket size, spin velocity and input flow rate. The separation of sugar crystals from liquid molasses is taken as a case study and the regime of potential operating points is plotted. 

Dr Fehmi Cirak, Department of Engineering, University of Cambridge

Multiresolution shape and topology optimisation of shells and solids
When Nov 25, 2013
from 02:00 PM to 03:00 PM
Where LR8
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The widespread availability of computational engineering software and ­recent advances in fabrication are enabling the design of optimised structures with ever increasing geometric complexity. This talk will review our recent work on structural optimisation using multiresolution shape editing and immersed finite element techniques. The geometry of the to be optimised structure is represented with a sequence of increasingly finer surface meshes that can capture geometric details at various resolutions. Using an immersed finite element technique the structure is analysed without the burden and cost of creating high-quality conforming meshes. Relative to traditional approaches, the fine-grained geometry control provided by multiresolution optimisation results in finding much better performing designs. Moreover, the b-splines employed in multiresolution editing can readily be utilised in computer aided design software for downstream applications.

Dr Garth N Wells, Department of Engineering, University of Cambridge

Extreme scale simulation for solid mechanics
When Feb 24, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Technologies for moving to extreme scale simulations in solid mechanics will be presented and discussed. Technology for very large scale solid mechanics simulations has generally lagged other fields, and this has limited the scope for system (versus component) level design and optimisation, and has led to simulation capability mis-matches for multi-physics problems. Moreover, three-dimensional simulation is still not the norm in solid mechanics research, despite investigations into problems with inherently three-dimensional microstructures being widespread. I will address simulation issues through a discussion of the necessary end-to-end computational technology for extreme scale simulation, and in particular linear solvers. Examples applications with hundred of millions of degrees of freedom will range from complex turbomachinery to geophysical problems. I will also address how this technology can be made accessible to domain expert researchers with limited high performance computing expertise.

Dr Gianfelice Cinque, Diamond Light Source - Rutherford Appleton Laboratory

Multimode InfraRed Imaging and Microspectroscopy at the Diamond Synchrotron
When Jun 08, 2015
from 02:00 PM to 03:00 PM
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Infrared (IR) MicroSpectroscopy is a quantitative analytical method extensively applied for studying soft condensed matter because of its high molecular sensitivity and specificity. Specifically, Fourier Transform IR (FTIR) technique effectively probes optically-active vibrational modes of molecules, or IR fingerprint of molecular groups, at the microscopic scale. mFTIR combination with Synchrotron Radiation (SR) broadband and brightness provides an unique diffraction limited IR microprobe. In fact, SRIR photon flux density is up to 103 times higher than conventional sources and extends simultaneously from the near-IR (l > 1 mm) up to the far-IR (l < 1 cm). At MIRIAM beamline of Diamond such advantages are fully exploited to allow both the highest spatial resolution optically attainable in IR microscopy (practically dx ~ l fwhm), and an excellent spectral quality (figure of merit signal/noise>5000 rms in 30 sec) across the whole IR range for absorption spectroscopy.
 
The initial Life Science driver for MIRIAM of biochemical analysis of fixed cell cultures and tissue sections relevant to cancer, stem cell research and pathology, is now routine in confocal IR microscopy [1]. The new research in the field is ex vivo and real time IR microanalysis of living single cell, i.e. the study of subcellular metabolism via full field IR imaging e.g. imaging isotopic gradient around/inside living fibroblasts [2]. A new class of experiments have been pioneered at MIRIAM in the last couple of years, namely the in situ microanalysis of gas-solid interaction controlled by temperature which are specifically important for the chemistry of catalysis at single crystal level or functionalized Metal-Organic-Frames [3]. The recent optimization of the Coherent Synchrotron Radiation has expanded the MIRIAM experimental capability for absorption spectroscopy specifically in the “THz gap” domain. This is particular relevant in the study of large molecule collective modes e.g. the physics of MOFs [4], as well as the study of the water interaction with protein in solution [5].
 
1 A.J. Deegan et al. Analytical and Bioanalytical Chemistry 407 (2015) 1097, Tracking calcification in tissue-engineered bone using synchrotron micro-FTIR and SEM
2 L. Quaroni et al, Biophysical Chemistry 189 (2014) 40, Synchrotron based infrared imaging and spectroscopy via focal plane array on live fibroblasts in D2O enriched medium
3 A. Greenaway et al. Angewandte Chemie 126 (2014) 13701, In situ Synchrotron IR Microspectroscopy of CO2 Adsorption on Single Crystals of the Functionalized MOF Sc2(BDC-NH2)3†
4 M. R. Ryder et al.  Phys. Rev. Lett. 113 (2014) 215502, Identifying the Role of Terahertz Vibrations in Metal-Organic Frameworks: From Gate-Opening Phenomenon to Shear-Driven Structural Destabilization
5 J.W. Bye et al. J. Phys. Chem. A 118 (2014) 83, Analysis of the Hydration Water around Bovine Serum Albumin Using Terahertz Coherent Synchrotron Radiation

Speaker’s website: www.diamond.ac.uk/Beamlines/Soft-Condensed-Matter/B22/Staff/Cinque.html

Dr Guillaume Charras, UCL, London

Long and short time-scale rheology of living cell monolayers
When Feb 01, 2016
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-273925
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 One-cell thick monolayers are the simplest tissues in multi-cellular organisms, yet they fulfil critical mechanical roles in development and normal physiology. To study their mechanics, we use an experimental system for tensile testing of freely suspended cultured monolayers that enables the examination of their mechanical behaviour at multi-, uni-, and sub-cellular scales. Using uniaxial stress relaxation experiments, we examined the rheology of cell monolayers on time-scales of seconds, minutes, and hours. At the shortest time-scales, ATP-independent processes dominated relaxation and appeared to result from intracellular water redistribution in response to the large imposed deformation. At minute time-scales, relaxation was ATP-dependent and due to junctional protein turnover. At hour time-scales, oriented cell divisions drove relaxation of tissue and the return to resting cell packing. As the application of a stretch naturally elongates cells within the monolayer along the stretch axis, oriented divisions in our system are a direct consequence of the propensity of cells to divide along their interphase long axis and does not require cells to detect mechanical cues other than their own shape.

 

https://www.london-nano.com/our-people/%5Bfield_people_section-raw%5D/guillaume-charras

Dr. Hannah Joyce, Cambridge Engineering

Semiconductor Nanowires: From Growth to Device Applications
When Jun 13, 2016
from 02:00 PM to 03:00 PM
Where LR8
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Semiconductor nanowires exhibit outstanding potential as nano-building blocks for the next generation of electronic devices, ranging from solar cells to nanoscale lasers. A variety of innovative fabrication techniques can be employed to "grow" these nanowires with tight control over the nanowire geometry and crystallographic properties. Electrical characterisation of these tiny nanowires can be achieved with high accuracy and high throughput without requiring any electrical contacts, using a contact-free technique known as terahertz conductivity spectroscopy. This talk will discuss how the detailed nanowire growth studies together with terahertz conductivity spectroscopy are guiding the development of novel nanowire-based devices.    

http://www3.eng.cam.ac.uk/~hjj28/

Dr Ingo Munch, Institute for Structural Analysis, Karlsruhe Institute of Technology

Domain engineering of ferroelectric thin films for energy conversion
When Oct 10, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Contact Phone 01865-283488
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We investigate the behaviour of epitaxial sputtered ferroelectric thin films with uniform lattice orientation for the design of nano-generators to convert mechanical into electrical energy. Therefore, we use a phase field model to simulate the domain topology. For the efficiency of the generator, it is

important to consider an appropriate substrate layer to pre-stresses the ferroelectric material. Further, the BaTiO3 film needs a structured field of top electrodes. Both, pre-stress and structured electrodes enforce a domain topology which allows for energy conversion.

Alternating interfacial strain between the substrate and the ferroelectric, e.g. by bending the substrate, leads to alternating domain configurations. Thus, opposite surface charges between the electrodes generate an electric flux. Due to the mechanical cycle load there is need for an electrical circuit to

transform single-phase alternating current into co-current flow. It is essential to store the generated electric energy within an accumulator or capacitor. However, the contact and charge status of the electric storage medium strongly influences the performance of the generator.

Dr Jia Min Chin, University of Hull

Mixing Work and Play - Metal-Organic Framework Functionalized Materials and Beyond
When Jun 01, 2015
from 02:00 PM to 03:00 PM
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This talk addresses some of the work by my group on MOF and other types of colloidal materials, whereby we take a quirky, tongue-in-cheek approach to Chemistry. The coordination modulation of MOFs to tune crystal morphology so as to produce micro and nanoparticles will be discussed. Use of this strategy to produce MOF grass, microflower and micro-mushroom structures for imparting omniphobicity to a surface will also be examined.  Fabrication of microstructures for surface functionalization often requires lithographic techniques and specialized equipment. We report instead a simple strategy for the synthesis of microstructured surfaces via metal−organic framework (MOF) self-assembly. Expansion of our work into MOF composites, and dry liquids will also be touched upon.

 References:

[1] Tan, T.T. Y.; Cham, J. T. M.; Reithofer, M. R.; Hor, T. S. A.*; Chin, J. M.* "Motorized Janus metal organic framework crystals", Chem. Commun., 2014, 50, 15175

[2] Tan, T. T. Y.; Reithofer, M. R.; Chen, E. Y.; Menon, A. G.; Hor, T. S. A.; Xu, J.*; Chin, J. M.* “Tuning Omniphobicity via Morphological Control of Metal-Organic Framework Functionalized Surfaces”, J. Am. Chem. Soc. 2013, 135, 16272.

[3] Chin, J. M.*; Reithofer, M. R.; Tan, T. Y. T.; Menon, A. G.; Chen, E. Y.; Chow, C. A.;  Hor, T. S. A.; Xu, J.* “Supergluing MOF Liquid Marbles” Chem. Commun. 2013, 49, 493.

[4] Chin, J. M.; Chen, E. Y.; Menon, A. G.; Tan, H. Y.; Hor, T. S. A.; Schreyer, M. K.*; Xu, J.* “Tuning the aspect ratio of NH2-MIL-53(Al) microneedles and nanorods via coordination modulation” CrystEngComm, 2013, 15, 654.

 

Dr Ling Wu, University of Liege, Belgium

Probabilistic prediction of the quality factor of micro-resonator using a stochastic thermo-mechanical multi-scale approach
When May 23, 2016
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-273925
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As the size of the device is only one or two orders of magnitude higher than the size of the grains, the structural properties, such as the thermo-elastic quality factor (Q), of micro-electro-mechanical systems (MEMS) made of poly- crystalline materials exhibit a scatter, due to the existing randomness in the grain size, grain orientation, surface roughness. In order to predict the probabilistic behavior of micro-resonators, the authors extend herein a previously developed stochastic 3-scale approach to the case of thermoelastic damping. In this method, stochastic volume elements (SVEs) are defined by considering random grain orientations in a tessellation. For each SVE realization, the mesoscopic apparent elasticity tensor, thermal conductivity tensor, and thermal dilatation tensor can be obtained using thermo-mechanical computational homogenization theory. The extracted mesoscopic apparent properties tensors can then be used to define a spatially correlated mesoscale random field, which is in turn used as input for stochastic finite element simulations. As a result, the probabilistic distribution of the quality factor of micro-resonator can be extracted by considering Monte-Carlo simulations of coarse-meshed micro-resonators, accounting implicitly for the random microstructure of the poly-silicon material.

http://www.ltas-cm3.ulg.ac.be/staff.htm

Dr Lisbeth Garbrecht Thygesen, Copenhagen

Dislocations in plant cell walls
When Feb 02, 2015
from 02:00 PM to 03:00 PM
Where Audrey Wood Seminar Room, Clarendon Laboratory, Parks Road
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Contact Phone 01865-273925
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Senior Researcher Lisbeth G. Thygesen has worked within plant biomass science for more than 20 years, mostly within academia, in Denmark as well as abroad. Her research interests lie within plant biomass characterisation with the scope of linking physical and chemical characteristics to function and performance both within traditional wood science (for example wood modification and wood-water interactions) and within plant biomass utilisation via processes such as enzymatic degradation as part of biorefining schemes. She has worked extensively within plant cell wall–water-enzyme interactions, plant cell wall microstructure (especially cellulose) and in-situ testing. One of her research interest is the structural and functional characteristics of irregular plant cell wall regions known as dislocations or slip planes. One of the outcomes of these efforts is the result that elongated thick-walled plant walls break more easily at dislocations than at other locations during processing and that dislocations therefore are instrumental in the important fibre attrition taking place during the initial liquefaction step within biorefining.

 In the presentation, an overview of plant cell wall dislocations will be given. The talk will include subjects such as how common dislocations are, in which situations they are known to be formed during plant growth and after harvest, the structural characteristics of dislocations (to the extent these are known) as derived from different types of microscopy and spectroscopy, and the role of dislocations in plant biomass degradation during biorefining.

http://ign.ku.dk/english/employees/forest-nature-biomass/?pure=en/persons/310195

Dr Lucia Scardia, School of Mathematics & Statistics, University of Glasgow

Multiscale problems in dislocation theory
When Jan 20, 2014
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-283302
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One of the hard open problems in mechanical engineering is the upscaling of large numbers of dislocations.
In this talk I will discuss the rigorous derivation of continuum dislocation density models and strain-gradient plasticity models from discrete and semi-discrete theories via Gamma-convergence.
I will also discuss progress and open problems in modelling dislocation interactions and dynamics.

This is based on work in collaboration with Marc Geers, Ron Peerlings, Mark Peletier and Maria Giovanna Mora.

Dr Majid Malboubi, Department of Engineering, Oxford University

Pressure propagation in living cells
When Oct 20, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Contact Phone 01865-283302
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Pressure equilibration within the cytoplasm of living cells has been the focus of much interest over the past 10 years. Indeed, such an understanding is key to investigating the biophysical mechanisms that give rise to cellular protrusions, cell migration, as well as understanding how cells may sense applied mechanical stresses and transduce them into biochemical signals. At a tissue level, the rate of pressure propagation may affect the mechanisms through which cells coordinate their action to generate tissue-scale deformation during morphogenesis. To date, experimental work on the topic has yielded contradictory results. Some experiments have taken advantage of blebs – protrusions whose generation is thought to depend on intracellular hydrostatic pressure due to myosin contractility- to examine pressure propagation. Recent experiments showed that the cytoplasm behaved as a poroelastic material and yielded estimates for diffusion of intracellular water. Despite this, how hydrostatic pressure equilibrates within living cells remains poorly understood partly because the experimental setups employed could not be used to manipulate intracellular pressure. We utilise whole-cell patch clamp electrophysiology and atomic force microscopy to investigate pressure propagation in living cells. We show that pressure propagates fast in living cells but stresses equilibrate more slowly.

Dr. Maxime Dupraz (Paul Scherrer Institute, Switzerland)

Characterization of the Microstructure of Small Crystals using Coherent X-ray Diffraction and Atomistic Simulations
When Jan 16, 2017
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-283446
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Physical properties in small length scale objects, typically below the micrometer, deviate strongly from their bulk
counterpart. For instance, mechanical strength increases with reducing size and large residual strain due to
processing are present in nanostructures. Thus a better understanding of the physical properties in relationship with the
microstructure is needed for sub-micrometer materials. Coherent X-ray Diffraction (CXD) is an emerging synchrotron
technique highly sensitive to strain fields and structural defects that allows the detailed measurement of the crystal
structure, including strain field and defects, of micro/nano-objects.
The calculation of CXD patterns from atomic structures modelled with interatomic potentials allows to evidence the
unique character of the signature induced by a single crystal defect at the vicinity of a Bragg reflection. The study of
CXD patterns at several chosen Bragg reflections enables to determine all the defect characteristics (edge or screw type,
slip plane, dissociation in partials, position in the particle, …). This approach is illustrated on th e case of a gold
nanocrystal undergoing simulated indentation. It is shown that the defects generated in the early stages of indentation
can in principle be identified, provided that their number remains low. The interpretation of CXD patterns from more
complex systems with multiple defects nucleated on several slip planes proves to be far more challenging. In this case,
the reconstruction of the displacement field of the sample provides a more comprehensive picture.
To characterize the first stages of plastic deformation in a realistic structure, the in-situ nanoindentation of a submicrometer
gold island grown on a sapphire substrate is carried out in a synchrotron facility. The in-situ loading is
performed by a recently developed compact scanning force microscope for in situ nanofocused X-ray diffraction studies
(SFINX), designed to be mounted on a diffractometer, while the particle is illuminated in Bragg conditions with a
coherent X-ray beam. Using 3D reconstructions at different stages of the mechanical loading, nucleation of a prismatic
dislocation loop is clearly identified. In addition, a strain relaxation equivalent to a “mechanical annealing” process is
observed during the successive indentation steps, as well as an evolution of the particle shape and strain distribution
after 24h ageing under the X-ray beam.

Dr Mithila Achintha University of Southampton

Structural Glass: From Brittle Glass to Ductile Glass–GFRP Hybrids
When Jun 06, 2016
from 02:00 PM to 03:00 PM
Where LR8
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The brittle material behaviour of glass together with the lack of a reliable model to incorporate the effect of internal residual stresses in structural designs pose challenges to structural engineers when designing of load-bearing glass structural members, such as large panels, roofs, floors, staircases and partitions. The current industry practice is to overdesign the glass elements.

 

This talk will introduce the idea that a hybrid contour method-eigenstrain model can be used to incorporate the effect of residual stress in float glass. The application of the contour method to glass, or to any nonconductive material for that matter, is novel. This is also the first use of waterjet cutting for the contour method. The talk describes the experimental effort made to make a good cut for glass in contour method analysis. In the second half of the talk, the use of glass–GFRP (Glass Fibre Reinforced Polymers) hybrid as a strong and ductile composite is exploited. Using the results of a combined experimental/numerical investigation into the load response and failure behaviour of float glass–GFRP hybrid beams, in which a layer of GFRP bonded between two horizontal glass sheets, it is shown that strong and ductile structural members can be made from float glass.

 http://www.southampton.ac.uk/engineering/about/staff/mapm1e11.page

Dr Patrick Farrell, Oxford Mathematical Institute

Computing Multiple Equilibria of Structures via Deflation
When Apr 25, 2016
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-273925
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Nonlinear equations can permit multiple solutions. In solid mechanics, this manifests itself as distinct equilibria (stable or unstable) that satisfy the governing equations. For example, it is well known that a slender beam subject to a compressive force along its axis undergoes buckling: below the critical load, only one solution exists; above the critical load, three solutions exist. This multiplicity and multistability of structures is extremely useful to designers --- if the multiple solutions can be found.

The standard approach to computing multiple solutions of differential equations combines arclength continuation and branch switching, as invented by Keller in 1977 and implemented in popular software packages such as AUTO. This algorithm has been extremely successful and useful in engineering, but suffers from two major drawbacks. The first is that it only computes connected fragments of bifurcation diagrams: for example, if the symmetry of a slender beam is broken by gravity, the standard approach will only identify one solution, even though the equation permits three solutions. The second is that the algorithm relies on the solution of expensive subproblems, which typically limits its applicability to systems of ODEs.

In this talk I will present a new algorithm, called deflated continuation, that overcomes both of these drawbacks. It allows for the computation of disconnected bifurcation diagrams, and scales as far as the underlying equation solver (up to discretisations of PDEs with billions of degrees of freedom). I will demonstrate its applicability to several nonlinear problems in mechanics including the deformation of a hyperelastic beam, nematic and cholesteric liquid crystals, and the optimal design of pipes

https://www.maths.ox.ac.uk/people/patrick.farrell

Dr Peiman Hosseini, Oxford Materials

A new optoelectronic framework using chalcogenide based phase change materials
When Nov 16, 2015
from 02:00 PM to 03:00 PM
Where LR8
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https://www.linkedin.com/pub/peiman-hosseini/99/1a0/b6b

Traditional thin film optical filters rely on interference being generated at the boundaries between two materials with different refractive indexes. A multitude of well understood, easy to use, design techniques are widely available since the end of the 19th century. What often accomunates these techniques together is the use of highly transparent materials, usually insulators, with thicknesses multiples of 1/2 or 1/4 of the wavelength (i.e. hundreds of nanometres or more), to create filters with optical properties fixed at the initial design stage. 
In this seminar we will present an entirely new optoelectronic framework that not only employs thin films in the range of 1/50 to 1/100 of the visible wavelength but also uses mixed structures of metals and semiconductors to create ultra-fast, electrically tuneable optical films. Unprecedented applications such as ultra-high resolution flexible displays, smart windows for infrared management and security coatings will be presented and discussed.

Dr Robert Style, Mathematical Institute, Oxford University

The mechanics of soft solids - breaking classical laws
When Jan 26, 2015
from 02:00 PM to 03:00 PM
Where Materials Department Annexe, 21 Banbury RoadEngineering Science
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Soft solids make up the bulk of biological material, and are increasingly being used for new technology like wearable electronics, and soft robotics. However, despite their importance, experiments show that many classical laws fail to describe them. For example, I'll show how classical theories of composite mechanics and contact mechanics significantly break down at a critical `elastocapillary' lengthscale -- due to solid surface tension. The results highlight the existence of a swathe of new, small-scale behaviour in soft materials.

http://people.maths.ox.ac.uk/style/

Dr. Raymond Veness & Dr Rudiger Schmidt (CERN)

The Large Hadron Collider: Extreme Mechanics and Materials Challenges
When Nov 01, 2016
from 02:00 PM to 03:00 PM
Where LR1
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Contact Phone 01865-283446
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The Large Hadron Collider (LHC) at CERN operates today at 6.5 TeV, close to the nominal value of 7 TeV. The energy stored in each of the two high intensity proton beams is close to 300 MJ, two orders of magnitude beyond previous accelerators. With such parameters, various unwanted effects due to high intensity beams are observed (instabilities, beam-beam effects, e-cloud effects, unidentified falling objects). A particular challenge is to operate such a collider safely. Already a small fraction of the stored energy is sufficient to damage accelerator equipment or experiments in case of uncontrolled beam loss. Although the beams circulate in ultra-high vacuum, interception of protons must be carefully managed by the highly efficient collimation system that limits beam losses around the ring, by absorbers that ensure protection in case of beam trajectory errors and by beam monitors to measure beam parameters. Material and mechanical design of such devices need to be highly optimised to cope with the beam power. The presentations will introduce the LHC, its challenges and risks. In particular, the experience gained with beam intercepting devices in the LHC as well as in high energy beam test installations will be discussed.

Dr. Sam Vinko (University of Oxford, UK)

Investigating Hot-Dense Plasmas with X-ray Free-Electron Lasers
When Feb 13, 2017
from 02:00 PM to 03:00 PM
Where LR8
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The past few years have seen a revolution in the field of X-ray science. The advent of the world’s first hard X-ray free-electron laser (FEL) – the Linac Coherent Light Source free-electron laser at SLAC – in one step in 2009 increased the spectral brightness of X-ray sources over that of any synchrotron on the planet by a factor of a billion. Spatially coherent, monochromatic, femtosecond X-ray pulses can now be routinely produced over a wide spectral range, enabling access to spatial and temporal scales of atomic processes in plasmas simultaneously for the first time. Importantly, focused FEL pulses are intense enough to create solid-density plasmas at temperatures of several 100 eV on ultra-short, inertially confined timescales, akin to the conditions found half-way into the centre of the sun [1]. The ability to create such systems in a controlled manner has allowed for the first detailed measurements of several fundamental properties of dense plasmas, such as the ionization potential depression [2,3] and electron collisional ionization rates [4], and has driven renewed theoretical interest [5]. Here I will discuss some of these advances and show how obtaining accurate experimental data in the notoriously challenging dense-plasma regime is needed to advance our understanding of systems of crucial importance to a range of astrophysical and inertial confinement fusion applications.
 
 [1] Vinko et al., Nature 482, 59–62 (2012).
 [2] Ciricosta et al., Phys. Rev. Lett. 109, 065002 (2012).
 [3] Ciricosta et al., Nature Communications 7, 11713 (2016).
 [4] Vinko et al., Nature Communications 6, 6397 (2015).
 [5] Vinko et al., Nature Communications 5, 3533 (2014).

Dr Sheena Hindocha, Johnson Matthey Technology Centre

Johnson Matthey: A leader in sustainable technologies
When Feb 29, 2016
from 02:00 PM to 03:00 PM
Where LR8
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This talk will give an overview of the company as a whole before focusing on research being completed within the New Applications Group at the Johnson Matthey Technology Centre.  Johnson Matthey is a leading speciality chemicals company, as a business, we always aim to deliver what we promise.  We work together, applying our expertise in advanced materials and technology to innovate and improve solutions that are valued by our customers; optimise the use of natural resources; and enhance the quality of life for the people of the world, both for today and for the future. Johnson Matthey is focused on developing value adding sustainable technologies to our customers and to society. Today, 89% of the group's sales represent products and services which provide sustainability benefits, i.e. through the positive impact they have on the environment, resource efficiency or human health. We focus on clean air, clean energy and low carbon technologies and are experts in the application and recycling of precious metals. Johnson Matthey has operations in over 30 countries and employs around 13,000 people. Our products and services are sold across the world to a wide range of advanced technology industries. Whether it is harnessing chemical properties at an atomic scale or applying our engineering skills to create new solutions, developing advanced materials and technology is what we do best.

ttp://www.matthey.com

Dr Simon Guest, Department of Engineering, Cambridge University

Counting with symmetry for structural mechanics
When Oct 12, 2015
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Abstract:

Counting components, and then comparing the number of constraints with number of degrees of freedom available to a structure, is a good first step in evaluating likely structural behaviour.  Maxwell first described this in 1864 when he stated that, in general, a structure with j joints would require 3j-6 bars to make it rigid.  Later Calladine generalised this idea by pointing out that the difference between the number of bars and 3j-6 counts the difference between the number of mechanisms and the number of states of self-stress. Sometimes, just simple counting can lead to profound insights, such as showing that any stiff repetitive structure must necessarily be overconstrained.

 

This talk will introduce the idea that any rule that involves counting components can be expanded to a more general symmetry version that involves counting the symmetries of sets of components, and that this counting can practically be done by simply considering the number of components that are unshifted by particular symmetry operations.  This provides useful insight into why certain symmetric structures are able to move despite apparently having enough members to make them rigid, or that tensengrity structures can be rigid without having enough members.

 

The talk will describe a recent result on 'auxetic' materials: a symmetry criterion that shows when a periodic system made up of bars, bodies and joints has an 'equiauxetic' mechanism, that is, show the limiting behaviour of Poisson ratio equal to -1, with equal expansion/contraction in all directions. Such systems can provide good models for the design of lattice materials with high, stretching-dominated, shear modulus, but low, bending-dominated, bulk modulus.

http://www3.eng.cam.ac.uk/~sdg/

Dr Sotirios Grammatikos, University of Bath

Anomalous behaviour of fibre reinforced polymers exposed to environmental aging
When Nov 02, 2015
from 02:00 PM to 03:00 PM
Where LR8
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In this seminar, the anomalous behaviour of polymer composites subjected to hygrothermal aging will be discussed in reference to the results of a characterization study on polyester matrix fibre reinforced polymer (FRP) composites. Hygrothermal aging, the combination of moisture and elevated temperatures, has been proven to affect the durability of FRPs. Significant structural degradation can be expected as a consequence of hygrothermal aging, however it has been shown that some material properties exhibit a strong non-linear and even beneficial behaviour as a consequence of hygrothermal exposure, indicating the presence of competing mechanisms. To study these different mechanisms which control the structural performance of the exposed material, moisture absorption characteristics and mechanical performance were assessed at prescribed time frames. With a view to examining any induced physical or chemical changes due to aging, Dynamic Mechanical Thermal Analysis (DMTA), Scanning Electron Microscopy (SEM) and Infrared spectroscopy were also employed. Lastly, impedance spectroscopy was used as a tool to follow hygrothermal aging/moisture absorption during exposure.

http://www.bath.ac.uk/ace/people/grammatikos/index.html

Dr. Tiina Roose (Southampton University, UK)

Multiscale Image Based Modelling in Biology
When Oct 10, 2016
from 02:00 PM to 03:00 PM
Where LR8
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In this talk I will describe a state of the art image based modelling in several seemingly different areas ranging from biomedical (lymphatic, vascular and lung system) to agricultural problems of plant soil interaction. I will describe the workflow from imaging (X-ray CT, XRF, SEM-EDX, histology), image reconstruction, image segmentation, computation and how to utilize this work stream to acquire new scientific knowledge. In particular I will also outline several challenges and bottle necks in this process to hopefully stimulate discussion about how these can be addressed.

T.Roose@soton.ac.uk

Professor Tiina Roose, Biological and Environmental Modelling, Southampton University

Multiscale modelling of biological branching structures
When Mar 03, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Contact Phone 01865-283302
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In this talk I will present our very recent work on two seemingly separate, but related, areas, i.e., modelling of plant root systems and lymphatic system in humans.  The common feature of these two systems is that the both function to transport fluid and solutes from their environment to other areas of their systems. I will show couple of case studies of x-ray computed tomography image based modelling of plant root function and then describe mathematical models we have developed to describe lymphatic system function and development that combine multiscale biomechanics of the tissues.

 Biography:

Tiina Roose is a Professor of Biological and Environmental Modelling working on modelling and technology development for biological systems at the Faculty of Engineering and Environment, University of Southampton. Her first degree was in Systems and Control Engineering from Tallinn Technical University, Estonia. She completed a DPhil on ŒMathematical Model for Plant Nutrient Uptake¹ at Oxford in 2000 under the supervision of Professor Andrew C. Fowler. She then spent 2 years as a postdoctoral research fellow at the Steele Laboratory for Tumor Biology at the Harvard Medical School/Massachusetts General Hospital working on the tissue mechanics of biological systems with particular emphasis on tissue growth and remodelling related problems. The Steele Lab at HMS/MGH was part of the Harvard-MIT Health Science and Technology (HST) program which in order to foster fast interactions between engineers and medics had all the PDRAs part of Harvard Medical School faculty and all the PhD students where registered at the MIT graduate school. The work in Boston involved personally undertaking in-vitro and in-vivo experiments including measurements of the mechanical properties of collagen gels and solid tumours. In 2003 she returned to Oxford to take up a postdoc position with Professors Jon Chapman and Philip Maini at the Oxford Centre for Industrial and Applied Mathematics. In 2004 she was appointed Royal Society University Research Fellow, which she held first in Oxford and from Oct 2009 in Southampton.

Dr Wesley Williams, Louisiana State University

Large-Scale Experimental Fluid Mechanics in the Oil and Gas Industry and Beyond
When Jun 07, 2016
from 02:00 PM to 02:00 PM
Contact Name
Contact Phone 01865-283302
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Abstract: Dr. Williams will discuss the capabilities and some of the unique aspects of the large-scale fluid mechanics experiments being performed at the LSU PERTT laboratory. These experiments impact the oil and gas industry in multiple applied areas: drilling, completions, production, and overall environmental safety.  Current research projects include assessment of regulatory required Worst Case Discharge calculations for the US Bureau of Ocean Energy Management. Results of some of the current research on Worst Case Discharge will be presented. The talk will cover important aspects of single and two-phase flows in long vertical test sections. The LSU PERTT laboratory is actively looking for new research collaborations that would synergize with our unique large-scale capabilities.

Bio: Dr. Williams is a professional in residence and director of the Petroleum Engineering Research and Technology Transfer Laboratory (PERTT Lab) as part of the Louisiana State University’s Craft and Hawkins Department of Petroleum Engineering. Prior to returning to academia, Dr. Williams spent several years as an engineering consultant and project manager in the petrochemical and nuclear power industries where he gained experience in solving applied engineering design and process problems. His doctorate was earned from the Massachusetts Institute of Technology where he founded the research area of heat transfer enhancement with nanoparticle colloids (nanofluids).

Enabling new biomedical and bioinspired mechatronic systems with electroactive polymer actuators

Federico Carpi, Queen Mary University of London, School of Engineering & Materials Science, London, UK
When Jan 14, 2013
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865 283302
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Abstract:

The development of a variety of new biomedical and bioinspired mechatronic systems poses challenges that share the need for innovative technologies for electromechanical transduction, so as to enable applications not feasible or even imaginable with conventional approaches. To address this need, new technologies based on electromechanically active polymer (EAP) transducers are progressively emerging as a promising solution. The idea is to use ‘active smart materials’ that exhibit a inherent mechanical response to an electrical stimulus, so as to design radically new electrical devices characterized by light weight, mechanical compliance, compact size, simple structure, low power consumption, acoustically silent operation, and low cost. EAPs offer such properties and are referred to as ‘artificial muscle materials’, because of their ability to undergo large and controllable deformations upon electrical stimulation. This seminar will be focused on the most versatile and performing EAP technology, known as dielectric elastomer actuators. Following a brief overview on the field and on the underpinning physical and engineering fundamentals, the seminar will present some devices and applications under development by the speaker’s group, including wearable haptic displays for vibro-tactile feedback in virtual reality systems, variable-stiffness orthotic systems for motor rehabilitation of the hand, refreshable Braille displays as portable tactile readers for the blind people, and bioinspired systems for artificial vision.

About the speaker:

Federico CarpiFederico Carpi was born in Pisa in 1975. He received the Laurea degree in Electronic Engineering, the Ph.D. degree in Bioengineering and a second Laurea degree in Biomedical Engineering from the University of Pisa, Italy, in 2001, 2005 and 2008, respectively. From 2000 to 2012, he has been with the University of Pisa, Interdepartmental Research Centre “E. Piaggio”, School of Engineering, Italy. Since 2012, he serves as an Associate Professor / Reader in Biomedical Engineering and Biomaterials at Queen Mary University of London, School of Engineering and Materials Science, UK. His research interests include smart material based biomedical and bioinspired mechatronic devices, polymer artificial muscles, as well as electrical and magnetic systems for non-invasive diagnostics. He coordinates the 'European Scientific Network for Artificial Muscles (ESNAM)', focused on transducers and artificial muscles based on electroactive polymers, and organizes the annual 'EuroEAP: International Conference on Electromechanically Active Polymer Artificial Muscles & Transducers'. He is an Editorial Board member of three international journals, and member of the scientific committees of several conferences. His publications include some 60 articles in international journals, 2 edited books and several contributions to books and conferences.

Enrique Alabort, Engineering Science Department, Oxford University

Studying Superplasticity in Ti-6Al-4V
When Feb 16, 2015
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
Contact Name
Contact Phone 01865-273925
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Superplasticity is vital in many industrial fields — medical, defense, aerospace, transport and sports among others — for the fabrication of metallic components of complex geometry. The inherent peculiarity of the effect has kept theorists fascinated for many years. But significant controversy exists concerning the deformation mechanism which is operative. In particular, unequivocal evidence that supports precise details of the accommodation process — whether it is diffusion, dislocation accommodated, a combination of both or some sort of cooperative grain boundary — is unavailable. Traditionally, surface studies have been used to characterise superplasticity. These studies have relied in post-mortem observations. However, novel in-situ testing techniques are potentially significantly more powerful for the study of high-temperature deformation mechanisms. In this talk, the mechanisms of superplasticity in Ti-6Al-4V are discussed. In addition, superplasticity is studied under constant strain-rate conditions; this has allowed the regime of superplasticity to be pinpointed. For design purposes, this understanding is translated into validated material laws which are accurate and which capture the relevant phenomena. Microstructurally explicit material laws are proposed based upon the micromechanical modes of deformation which are shown to be operating. In the final part of the presentation, modelling is used to simulate an industrially-relevant manufacturing process which is important for the construction of hollow, lightweight structures which are of significant practical importance for the aerospace sector.

G. M. Castelluccio, Sandia National Laboratories, NM, USA

Mesoscale-SENSITIVE crystal plasticity
When Jun 23, 2016
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-273184
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Constitutive formulations have increasingly focused on physically-based approaches that are less phenomenological but incorporate information from multiple scales. Most dislocation-based plasticity approaches reflect homogeneous many-body dislocation physics without considering the length scales introduced by the localization of dislocation densities. These mesoscopic length scales have a critical intermediary role between the effective and real stress at micro- and nano-scales. This work presents a crystal plasticity framework that explicitly incorporates length-scales and evolution laws associated with mesoscale structures such as cells and persistent slip bands in metallic materials under cyclic loading. The formulation is based on Eshelby inclusion approach and demonstrates that the back stress can be originated by localization of dislocations into wall structures. The results show agreement between models and experiments for a wide range of strains without introducing artificial fitting coefficients.

gmcaste@sandia.gov

Prof. Adrien Desjardins (UCL, UK)

Next Generation Medical Devices with Integrated Optical and Ultrasonic Sensors
When Oct 05, 2016
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-283302
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Recent advances in optical sensing techniques have the potential to transform medical devices and minimally invasive procedures.
Traditionally, medical devices such as needles and catheters have been passive conduits for accessing tissue targets. There is currently significant interest in integrating sensors that provide real-time information about the micro-structure and molecular composition of biological tissue. These new active medical devices can also interact with image guidance modalities such as ultrasound or X-ray to identify their locations within patients. I will present recent developments by my Interventional Devices Lab at the University College London, which include optical transmission and reception of ultrasound, photoacoustic imaging, and ultrasonic medical device tracking. I will highlight recent images acquired in vivo, and discuss opportunities and challenges with translating new imaging and sensing modalities into clinical practice.

J R Barber, Department of Mechanical Engineering, University of Michigan, MI

Frictional elastic systems under periodic loading History-dependence, non-uniqueness and energy dissipation
When May 19, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
Contact Name
Contact Phone 01865-283302
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Frictional systems exist throughout engineering, including applications in aero engines,
tectonic plates, bolted joints, etc. In many of these cases, the contact is nominally
stuck, but because of the elasticity of the material, there may be regions of microslip. If
the loading is periodic, this results in energy dissipation, reflected as apparent hysteretic
damping of the system, and also may cause the initiation of fretting fatigue cracks.
For many years, tribologists assumed that Melan’s theorem in plasticity could be extended
to frictional systems — i.e. that if there exists a state of residual stress associated
with frictional slip that is sufficient to prevent periodic slip in the steady state, then the
system will shake down, regardless of the initial conditions. However, we now know that
this is true if and only if there is no coupling between the normal and tangential loading
problems, as will arise notably in the case where contact occurs on a symmetry plane.
More recently, Ponter has shown that this is a special case of a more general theorem that
for uncoupled systems, the time-varying terms in the steady state are independent of initial
conditions, and this result applies even in the corresponding elastodynamic problem.
With sufficient clamping force, ‘complete’ contacts (i.e. those in which the contact
area is independent of the normal load) can theoretically be prevented from slipping, but
on the microscale, all contacts are incomplete because of surface roughness and some
microslip is inevitable. In this case, the local energy dissipation density can be estimated
from relatively coarse-scale roughness models, based on a solution of the corresponding
‘full stick’ problem.
If the system is coupled, the steady state generally depends on initial conditions and
hence the system must retain a ‘memory’ of these conditions, which resides in the tangential
displacements at nodes that are instantaneously stuck. When a stuck node starts to
slip, it can ‘exchange’ memory with one or more other nodes, but there is generally some
degradation. Thus, long term dependence on initial conditions even for coupled systems
generally depends on the existence of a set of nodes that never slip in the steady state.
However, we shall demonstrate simple cases where there exist more than one distinct
steady state even when this set is null.

Prof. Jim Woodhouse (University of Cambridge, UK)

Can we Predict Friction-Driven Vibration?
When Feb 28, 2017
from 02:00 PM to 03:00 PM
Where LR1
Contact Name
Contact Phone 01865-283446
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Vibration excited by friction can be unwanted, as in vehicle brake squeal, or it can be intentional, as in a bowed violin string.  In either case there are good reasons to want to be able to predict and control the vibration.  Modelling of the linear component of vibrating systems is well understood, but a sufficiently accurate model for the dynamic friction force at an interface is another matter.  Many models have been proposed, but none has a very good track record for reliable prediction.  This talk will describe recent work in which some useful progress has been made on this old and thorny problem.  Both the applications mentioned above will be discussed: they raise significantly different questions, and turn out to require different styles of modelling.  To predict the threshold of instability for a phenomenon like brake squeal, it will be argued that a kind of frequency response function of sliding friction is needed. Recent work to measure and model this function will be described.  For the violin string, the focus switches to the tougher challenge of predicting transient details of the vibration, not just the threshold. Various candidate models will be compared with measurements, and it will be argued that for friction mediated by violin rosin the most important controlling variable is the contact temperature.  However, a fully accurate model still eludes us at present.

Prof. K. Jimmy Hsia Departments of Mechanical Engineering & Biomedical Engineering Carnegie Mellon University, Pittsburgh, USA

Self-Assembled 3D Shape Formation Induced by Chemical Stimuli
When Sep 07, 2016
from 10:00 AM to 11:00 AM
Where LR7
Contact Name
Contact Phone 01865-283302
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Self-assembly processes, often driven by mechanical interactions between different parts in a material, can lead to formation of tubes, 3D structures, or devices with unique properties. The stimuli for these self-assembly processes can be thermal, chemical, or even by living cells or through intrinsic material properties such as lattice parameter differences. Here I present a few case studies of how chemical stimuli can be used to control shape formation of soft materials. The first case demonstrates mismatch strain-driven curvilinear shape formation by folding of polymer films induced by differential swelling upon chemical stimulation. Experiments with combined top-down and bottom-up approach demonstrate capabilities to form various curvilinear shapes. Finite element modeling of these systems is used to guide the shape formation processes, leading potentially to origami folding. Another case involves controlling the polymerization of a gel in a confined configuration leading to buckled 3D configurations. Such polymerization (growth) induced 3D shape may be used to study complex shape formation processes of biological systems such as organs of complex shapes. Based on the mechanics understanding of these mechanical mechanisms, one can design different 3D shapes including origami.

kjhsia@cmu.edu

Biography:

K Jimmy Hsia

K. Jimmy Hsia is Vice Provost for International Programs and Strategy, and Professor of Mechanical Engineering and Biomedical Engineering at Carnegie Mellon University. He received his B.S. from Tsinghua University, Beijing, China, and his Ph.D. from MIT. His research interests include deformation and failure mechanisms of materials, mciro/nanomechanics of nanoscale materials, and cell mechanics. He is a Fellow of American Association for the Advancement of Science (AAAS), a Fellow of American Society of Mechanical Engineers (ASME), and recipient of NSF Research Initiation Award, Max-Planck Society Scholarship, Japan Society for Promotion of Science Fellowship. Before joining CMU in 2015, he was W. Grafton and Lillian B. Wilkins Professor of Mechanical Science and Engineering at the University of Illinois at Urbana-Champaign (UIUC). From 2005-2007, Hsia served as Founding Director of Nano and Bio Mechanics Program in the Directorate for Engineering at NSF. He served as Associate Dean of Graduate College and Associate Vice Chancellor for Research for New Initiatives at UIUC. He is a co-Editor-in-Chief of a newly launched Elsevier journal, Extreme Mechanics Letters, whose announcement on iMechanica attracted more 10,000 hits within a week.

Lee E. Goldstein, M.D., Ph.D., Associate Professor of Psychiatry, Neurology, Ophthalmology, Pathology & Laboratory Medicine, Biomedical Engineering, Electrical & Computer Engineering, Boston University

Mechanisms, Neuropathology, Animal Models, and Computational Analysis of Blast and Impact Neurotrauma
When Oct 07, 2015
from 02:00 PM to 04:00 PM
Where LR8
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Contact Phone 01865-283302
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Research Interests 

Tbc

Dr. Leo Ma (Culham Centre for Fusion Energy, UK)

Fundamentals and Applications of Magnetic Spin-Lattice Dynamics
When Nov 14, 2016
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-283446
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Magnetic materials, such as iron, iron alloys and steels, are structural materials for nuclear fission and fusion applications, but the range of applications of these materials extends well beyond fission or fusion. Excitation of magnetic degrees of freedom in magnetics alloys contributes to the free energy and entropy, and influences the stability of crystal structures, defects configurations, defect production, migration rates, and various other properties. We developed spin-lattice dynamics simulation algorithms [1,2] that generalise molecular dynamics to the case of magnetic materials, and can simulate dynamic evolution involving non-collinear fluctuations of magnetic moments and translational motion of atoms on a million atom scale. Recently, we have released a spin-lattice dynamics code SPILADY [3], implemented for both CPU and GPU computers.

Spin-lattice dynamics simulations have been applied to a diverse range of applications in fission and fusion, and beyond. For example, simulations explain the anomalous variation of elastic constants, lattice structure, vacancy formation and migration energy near the Curie temperature. Other applications explored using spin-lattice dynamics simulations include the correlated dynamics of magnetic moments in thin films, ultrafast demagnetization of materials by a laser pulse, and ultra-high frequency magnetic refrigeration.

[1] Pui-Wai Ma, C. H. Woo, and S. L. Dudarev, Phys. Rev. B 78, 024434 (2008)

[2] Pui-Wai Ma, S. L. Dudarev, and C. H. Woo, Phys. Rev. B, 85, 184301 (2012)

 [3] Pui-Wai Ma, S. L. Dudarev and C. H. Woo, Comp. Phys. Comm., 207, 350 (2016); http://spilady.ccfe.ac.uk

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 under grant agreement No 633053 and from the RCUK Energy Programme [grant number EP/I501045]. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

Leon Govaert, Eindhoven University of Technology, The Netherlands

Current options for fast evaluation of the long-term performance of load-bearing thermoplastics
When Dec 08, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Failure under static or dynamic loading conditions is a major concern in the application of polymers in load-bearing components: it is not the question whether it will fail, but rather on what time-scale. It is imperative that the ability to estimate the lifetime of polymer components under design-load specifications is essential in their design and optimization. With the increasing social demand for durable products with long service life, and a gradual shift to more critical applications, e.g. involving high loads and temperatures (e.g. under-hood automotive applications, domestic hot water systems), the need for such predictive methods is even more vital.

 

In the absence of reliable models, one is left with no other option than to revert to prototyping and full-product tests. Unfortunately, such trial and error approaches are time-consuming, costly and hamper flexible product development.

 

In my presentation I will give an overview of current options for modelling and accelerated characterization protocols concerning the long-term performance of load-bearing polymer systems.

 

 

Professor Ludovic Noels, Aerospace science, Electromechanical Engineering, Mechanical Engineering & Physics, Liege University

Muti-scale methods with strain-softening: damage-enhanced MFH for composite materials and computational homogenization for cellular materials with micro-buckling
When Apr 28, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Materials used in the aerospace industry, as composite or foamed materials are multiscale in nature. To predict the macroscopic behaviour of structures made of such materials, the micro-scopic responses should also be computed within a nested scheme. This is particularly true when non-linear behaviours are modelled, or when the failure and post failure analyses are sought. In this work, multi-scale methods with strain softening are developed in the contexts of damage modelling for composite laminates and of buckling analyses in cellular materials.

 

First, an anisotropic gradient–enhanced continuum damage model is embedded in a mean–field homogenization (MFH) process for elasto-plastic composites. The homogenization procedure is based on the newly developed incremental secant mean-field homogenization formulation, for which the residual stress and strain states reached in the phases upon a fictitious elastic unloading are considered as starting point to apply the secant method. The mean stress fields in the phases are then computed using isotropic secant tensors, which are naturally used to define the Linear Comparison–Composite The resulting multi– scale model is then applied to study the damage process at the meso–scale of laminates, and in particular the damaging of plies in a composite stack. By using the gradient–enhanced continuum damage model, the problem of losing uniqueness upon strain softening is avoided.

 

Second, an efficient multi–scale finite element framework capturing the buckling instabilities in cellular materials is developed. As a classical multi–scale computational homogenization scheme loses accuracy with the apparition of the macroscopic localizations resulting from the micro–buckling, the second order multi–scale computational homogenization scheme is considered. This second–order computational framework is enhanced with the following novelties so that it can be used for cellular materials. At the microscopic scale, the periodic boundary condition is used because of its efficiency. As the meshes generated from cellular materials exhibit a large void part on the boundaries and are not conforming in general, the classical enforcement based on the matching nodes cannot be applied. A new method based on the polynomial

interpolation2 without the requirement of the matching mesh condition on opposite boundaries of the representative volume element (RVE) is developed.

Next, in order to solve the underlying macroscopic Mindlin strain gradient continuum of this second–order scheme by the displacement–based finite element framework, the treatment of high order terms is based on the discontinuous Galerkin (DG) method to weakly impose the C1-continuity.

Finally, as the instability phenomena are considered at both scales of the cellular materials, the path following technique is adopted to solve both the macroscopic and microscopic problems.

 

Prof. Marc FIVEL (SIMAP-GPM2, Universite Grenoble Alpes, France)

Crack Initiation and Propagation in Fatigues 216L Stainless Steels: A 3D Dislocation Dynamics Investigation
When Nov 07, 2016
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-283446
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 Since in most of metals, crack nucleation and crack growth are intimately related to dislocation plasticity, discrete dislocation dynamics simulation is a very promising numerical tool to address damage mechanics. The recent improvement of 3D discrete dislocation dynamics codes makes it now possible to perform realistic simulations of the intragranular crack propagations. In this presentation, three types of 3D discrete dislocation dynamics investigations are presented with the goal to better understand fatigue damage in 316L stainless steels. First, crack initiation mechanisms are investigated in a surface grain cyclically loaded under symmetric plastic strain amplitude. After few cycles, dislocations organize into a complex 3D microstructure made of persistent slip bands. Extrusions are evidenced at the surface, precisely where the bands are located. Calculations of the elastic energy stored within the simulated grain and the stress tensor inside the simulation box show that the first crack will initiate at the surface, most probably at the bottom of the extrusion. Secondly, the propagation of a fatigue crack is investigated using the same modeling technics in which a crack is now introduced. The role of the pre-existing slip band on the crack path is analyzed. The magnitude of the crack tip slip displacement is evaluated quantitatively for various distances between the tip and the grain boundary. This shows that grain boundaries systematically amplify the slip dispersion ahead of the crack tip and consequently, slow down the crack growth rate. Finally, the model is used to study crack transmission from one grain to the next one. Assuming the first grain is cracked, we investigate indirect crack transmission to the next grain i.e. a crack nucleation from the formation of a persistant slip band in the second grain. It is found that crack transmission strongly depends on grain disorientation. In some situation the pre-existing crack can accelerate the crack formation in the next grain. For other orientations, the crack can impose its own persistent slip band, different from the one that would develop without the presence of the cracked grain. For both cases, the crack imposes the extrusion growth rate in the second grain.

Marc.Fivel@simap.grenoble-inp.fr

 

 

Dr Mark Thompson, Institute of Biomedical Engineering, University of Oxford

Tendon mechanobiology
When Jun 10, 2013
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865 283302
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Understanding the microstructural deformation mechanisms of tendon, a hierarchical biopolymer fibre composite, is key to preventing painful and disabling tendon disease and rupture. It will also enable us to harness the biological ability of cells to respond to and adapt their surrounding tissue to the mechanical loading that they experience

Matthew R Begley, University of California, Santa Barbara

Dynamic Response of Coatings on Substrates Subjected to Impulse Loading
When Aug 27, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Femtosecond laser pulses with low energy can be used to precisely debond the interface between oxide films and substrates, which creates novel opportunities to characterize the properties of the film and the interface between the film and substrate.  This talk will describe theory and experiments that establish a framework that relates laser pulse parameters to film deformation, which can be used to infer coating/interface properties from experiments, or design experimental protocols that produce well defined interface flaws for subsequent testing.  First, an analytical model will be used to highlight two (apparently) underappreciated aspects of the problem: (i) the time-scale of the laser pulse is much shorter than that of the inertial time-scale of the film, such that the details of the pressure generated by the pulse are largely immaterial (greatly simplifying the problem) and (ii) the inertial contribution to the energy release rate can be substantial, with important implications for reliability in this and other dynamic scenarios (e.g. foreign object damage). The model will be used to generate regime maps that indicate failure modes as a function of pulse characteristics and film properties, which will be shown to be in good agreement with experiments. Second, the talk will present a brief discussion of several facets of dynamic debonding that can only be captured numerically, notably those relating to crack arrest after dynamic initiation. Illustrative simulations will be presented of such phenomena, which were generated using an explicit discrete element method with widely embedded cohesive zones.  The simulations demonstrate that it is possible to trigger dynamic kinking out of the interface (thus leading to a spalled section of film), even when quasi-static calculations suggest otherwise.

P J Tan, Department of Mechanical Engineering, University College London

Crack initiation and Fracture toughness of random Voronoi Honeycombs
When Feb 25, 2013
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865 283302
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The work presented here is motivated by a commonly held belief that architectural variations, in addition to a reduction in bone density, account for the age-related increase in fracture risk in trabecular bones, a class of materials known as cellular solids. However, there are significant uncertainties in the measurements of the mechanical properties of trabecular bones due, in part, to small sample sizes and to the large length scales of inhomogeneities. Since its 3D architecture is not easy to quantify, the fracture toughness of elastic-brittle Voronoi honeycombs is studied, instead, to gain insights into the influence of cell topological variations. Just like its 3D counterpart, 2D Voronoi honeycombs have isotropic overall mechanical properties and deform primarily by cell wall bending.
In this work, cell regularity in the Voronoi lattices is controlled using a global non-dimensional parameter that places a constraint on the minimum cell size and its distribution. Using a well-known scaling law, the resistance of the lattices to pure mode I and mode II loadings are evaluated and the effects of cell regularity on the global toughness of the lattices compared. The knock-down/enhancement in toughness due to imperfections is found to be different from existing studies that employed a node perturbation technique to introduce imperfections. Fracture loci for the lattices will be shown in combined mode I and II stress intensity factor (SIF) space and their critical effective SIF will be compared under different competing influences of cell-regularity, relative density and mode-mixity. The effects of T –stress (the non-singular stress parallel to the crack plane) upon the effective toughness of the lattices will also be presented. Fracture maps showing the location of initial cell wall fracture will be shown for lattices with different relative density under various mode-mixity conditions. The clustering of the wall fracture locations around the crack-tip in the random lattices is reminiscent of the plastic zone shapes in the linear elastic fracture mechanics of fully dense solids. 

Brief Biography:

PJ Tan obtained his first degree in Mechanical Engineering from the National University of Singapore and his PhD from UMIST/The University of Manchester. He worked as a post-doc in Manchester and Aberdeen before joining UCL as a lecturer in 2007. His research exploits theoretical, computational and experimental methods to characterise the structural and functional performance of engineering materials and structures. His research is multi-disciplinary and covers a number of fields including, but not exclusively, dynamic mechanical response and fracture of cellular solids; constitutive modeling of polycrystalline materials; designing protective functionality (against blast and impact loading) into lightweight sandwich systems; and, impact dynamics.


Prof. Pierre Thibault (Southampton University, UK)

X-ray ptychography
When Oct 17, 2016
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-283446
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X-ray ptychography is a synchrotron-based imaging technique that produces sub-micrometer quantitative maps of a transmission function through the combination of diffraction measurements from the illumination of multiple overlapping regions on the specimen. Thanks to the information gain provided by the diversity of the measurements, image reconstruction from ptychographic data is especially robust in comparison to other X-ray lensless imaging methods. The technique has been extended to three-dimensional imaging and is now used routinely at a few dedicated instruments. This talk will give an overview of recent developments in the field, from biomedical applications to extensions of the method for partial coherence, dynamical studies, and near-field imaging.

pierre.thibault@soton.ac.uk

Professor Harm Askes, Department of Civil and Structural Engineering, University of Sheffield

Gradient elasticity - concepts, micro-mechanics and finite element implementations
When Oct 26, 2015
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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In this seminar, the use of gradient elasticity in mechanical applications will be discussed. In gradient elasticity, higher-order spatial derivatives of relevant state variables are added to the governing equations. First, some fundamentals about gradient enrichment will be discussed, such as motivations for gradient enrichment, particular formats of gradient enrichment (including the widely used formulations of Mindlin, Eringen and Aifantis) and the infamous "sign paradox". Next, micro-mechanical motivations for the inclusion of higher-order gradient will be discussed; these will help in identifying and quantifying the additional constitutive constants. Finally, implementation of gradient elasticity theories will be discussed with a focus on using so-called C0 finite elements, so that standard finite element software can be used.

Prof Amy Zavatsky & Prof Paul Buckley, Oxford

Making sense of the stretching, squashing and twisting of tendon: One of nature's primary load-bearing materials
When Mar 07, 2016
from 02:00 PM to 03:00 PM
Where LR8
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Tendons transmit forces from muscles to bones. We and other mammals depend on them for moving our limbs. But, although they are so important, the mechanical properties of tendon tissue remain poorly understood. The constitutive behaviour is complex, with pronounced nonlinearity, viscoelasticity, and anisotropy; tendons’ irregular geometry complicates experiments on them; and they can show wide scatter in properties. Recent research has aimed at overcoming these problems, to achieve improved characterisation and understanding of the 3D deformation of tendon tissue. This has required progress in several areas: a new method for capturing the 3D geometry of tendons; tensile tests with a new approach to interpreting the results; a new approach to testing tendons in lateral compression; and the first testing of tendons in axial torsion, with and without superposed axial tension.  Combining results from all these tests provides improved knowledge of the 3D deformation of tendons, and helps interpret this in terms of what is known of their structure.

Prof Cathy Ye, Oxford Engineering Science (IBME)

Biomaterials for tissue engineering applications
When Nov 09, 2015
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-273925
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**THIS SEMINAR HAS BEEN CANCELLED AND WILL BE RE-SCHEUDLED**

To be able to culture cells in 3D, suitable extracellular matrix (ECM) are required to facilitate cell adhesion, growth, migration and differentiation (for stem cells). Different cell types may require ECM of different properties including composition, stiffness, surface properties, pore size, porosity and etc. One of the techniques established in the group, electrospinning, has been used to produce very fine fibres (diameters from hundreds of nano meters to several microns) of natural proteins and synthetic polymers. These fibres are a close mimic of the natural fibrous ECM in vivo in terms of dimensions and arrangement. Hydrogels are another category of biomaterials used to mimic ECM. These are jelly-like materials with high water content and they can have properties ranging from soft and weak to hard and tough based on the degree of cross-linking. Commonly used ones for cell culture are collagen, alginate, chitosan and hyaluronan.

http://www.ibme.ox.ac.uk/research/regenerative-medicine/tissue-engineering/people/dr-hua-cathy-ye

Prof Cathy Ye, Oxford Engineering Science (IBME)

Biomaterials for tissue engineering applications
When Feb 15, 2016
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-273925
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To be able to culture cells in 3D, suitable extracellular matrix (ECM) are required to facilitate cell adhesion, growth, migration and differentiation (for stem cells). Different cell types may require ECM of different properties including composition, stiffness, surface properties, pore size, porosity and etc. One of the techniques established in the group, electrospinning, has been used to produce very fine fibres (diameters from hundreds of nano meters to several microns) of natural proteins and synthetic polymers. These fibres are a close mimic of the natural fibrous ECM in vivo in terms of dimensions and arrangement. Hydrogels are another category of biomaterials used to mimic ECM. These are jelly-like materials with high water content and they can have properties ranging from soft and weak to hard and tough based on the degree of cross-linking. Commonly used ones for cell culture are collagen, alginate, chitosan and hyaluronan.

http://www.ibme.ox.ac.uk/research/regenerative-medicine/tissue-engineering/people/dr-hua-cathy-ye

Prof. Daniele Dini (Imperial College London, UK)

Modelling in Tribology: a Multidisciplinary Journey from Molecules to Engineering Applications
When Feb 20, 2017
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-283446
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Tribological phenomena are governed by events and mechanisms which find their roots at the small scales, even more so in environments where mechanical and chemical effects are intimately coupled.  For example, nano-scale thermal and particle emission events control the formation of antiwear additive films and oxidation; surface damage, such us crack initiation and wear, results from the accumulation of strain at dislocations level; corrosion events are triggered and controlled by molecular interactions. The key challenge addressed in this talk is the need for the development of robust methodologies for the integration of the skills and techniques recently developed by our modelling team at different scales to capture physical, chemical and mechanical processes and interactions across the scales via multi-physics modelling strategies. Example of modelling methodologies developed and employed to solve problems at specific length- and time-scales will be presented before concentrating on coupling strategies to be adopted to shed light on macro-scale tribological events while zooming-in to understand their governing mechanisms.  Various applications will be discussed ranging from automotive and aerospace to biomedical engineering.

Professor David Hills, Department of Engineering Science, Oxford University

Shakedown of Frictional Contacts
When Mar 04, 2013
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865 283302
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Shakedown is a process we associate with plasticity when components are subject to cyclic load - it is the self-generation of residual stresses which tend to supress subsequent excusrsions into the plastic state. A similar phenomeon is observed when a frictional contact is subject to oscillatory shear forces less than those needed to cause sliding. One question which arises is whether the shakedown theorems devised for plasticity may be applied to contact problems, and here I shall start to answer the question.

Prof. James Kermode (Warwick University, UK)

Multiscale Modelling of Materials Chemomechanics
When Jan 23, 2017
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-283446
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‘Chemomechanical’ processes involving complex and interrelated chemical and mechanical processes that originate at the atomic scale often determine the ultimate behaviour of materials. Fracture and dislocation creep are prominent examples, and remain some of the most challenging ‘multi-scale’ modelling problems, typically requiring both an accurate description of chemical processes and the inclusion of very large model systems.
I will explain how these requirements can be met simultaneously by combining a quantum mechanical description of crack tips and/or dislocation cores with a classical atomistic model that captures the long-range elastic behaviour of the surrounding crystal matrix, using a QM/MM (quantum mechanics/molecular mechanics) approach such as the `Learn on the Fly’ (LOTF) scheme.
The situation is further complicated when the relevant failure processes are also rare events, e.g. thermally activated crack propagation or dislocation climb. While QM/MM schemes help to address multiple length scales, they don’t do much to address the timescale issue. I will demonstrate that kinetic Monte Carlo models based on barriers computed from first principles offer one solution, and describe a novel machine learning implementation of the LOTF scheme which reduces the number of expensive QM calculations to provide another. Methodological aspects will be illustrated with applications, focussing in particular on slow processes such as thermally or chemically activated fracture.

Prof Javier Llorca, Polytechnic University of Madrid

High Temperature Nanomechanics
When Feb 18, 2013
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865 283302
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Nanomechanics and micromechanics experiments have become very popular in recent years as they provide unique evidence of the deformation and damage processes at the µm and nm scale. This information is of paramount importance to design novel materials with optimized properties, to develop physically-based models (as opposed to phenomenological ones) of the mechanical behavior and to explore size effects in the realm of nanotechnology. Basic experimental techniques to achieve these goals involve either in situ mechanical testing within a microscope (so the actual deformation and damage processes can be resolved at the submicron scale), instrumented nanoindentation (in which a very small volume is deformed) or a combination of both. In addition to the experimental challenges, nanomechanics often requires the use of sophisticated simulations tools (atomistics, dislocation dynamics, crystal plasticity, etc.) to interpret the results.

A further challenge in nanomechanics is the extension to high temperature. This is important from the theoretical point of view, as many deformation mechanisms are thermally-activated, as well as from the engineering side. In fact, many current or intended applications of nanostructured materials (metal-ceramic nanoscale multilayers in integrated circuits interconnects, high absorbance coatings in thermo-solar applications, radiation-resistant nanostructured metals, etc.) involve operation at high temperature. However, the field of high temperature nanomechanics is a rather unexplored area because of the challenges associated with thermal drift (while trying to measure nm), oxidation, chemical reactions and microstructure evolution in very small specimens or nanostructured materials.

In this seminar, the current activities at IMDEA Materials Institute on the area of high temperature nanomechanics will be reviewed. They include the determination of the size and temperature effects on deformation mechanisms of LiF [111] micropillars, Al/SiC metal-ceramic nanoscale multilayers and Cu/Nb metallic multilayers. Experimental results will be interpreted and understood to the light of dislocation dynamics and crystal plasticity simulations (in the case of LiF micropillars), finite element modeling (Al/SiC multilayers) o theoretical models (Cu/Nb multilayers).

Prof John Hutchinson, Harvard University

Advanced instability modes in soft material systems
When Dec 07, 2015
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-273925
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**THIS SEMINAR HAS BEEN CANCELLED**

http://www.seas.harvard.edu/hutchinson

Prof. Michael Preuss (University of Manchester, UK)

The Long Journey of Understanding Degraduation Mechanisms in Zr-based Nuclear Fuel Cladding
When Jan 30, 2017
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-283446
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In water-cooled reactors zirconium alloys have been the material of choice for fuel assemblies due to a combination of low neutron cross-section, excellent corrosion performance and good mechanical properties. However, fuel cladding performance, or our ability to predict its performance, remains the limiting factor in an effort to push for increased fuel burnup, i.e. the energy extracted from a fuel assembly before it is removed from the core. As the UK is expected to get a large fleet of civil light water reactors for the first time, it is important to develop an understanding that will enable us to optimise fuel assembly performance, maximise burnup while minimising fuel failures. During the last decade Zr cladding research in the UK has grown from almost not existent into a thriving world leading activity. During my presentation I will focus on progress we have made in understanding the effect of alloying elements on aqueous corrosion performance, hydrogen pick-up and irradiation damage in Zr-alloys while also highlighting the many remaining gaps in understanding. I will present results of detailed studies using a multiscale characterisation approach by employing diffraction and scattering techniques as well as novel electron microscopy techniques. These techniques have been employed to investigate in detail the oxide grown during autoclave testing or during in-reactor service and to characterise irradiation damage formed during accelerated proton irradiation to compare with neutron irradiated material. While state-of-the-art characterisation tools now allow us to make new observations and rethink previously proposed mechanisms, it is also clear that more modelling efforts are required in the future to fully explain the experimental observations.

Dr. Rüdiger Schmidt, CERN

Tbc
When Oct 01, 2016
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-283446
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Tbc

Prof Philippe Young, University of Exeter & Simpleware Ltd

On image based simulation: From cricket to crickets
When Feb 08, 2016
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-273925
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New developments in using 3D image data, such as MRI, CT, Microscopy, as the basis for finite element and computational fluid dynamics simulation will be described (image based modelling). In addition the presentation will focus on the use of these image-based model generation techniques across a range of industries including Oil and Gas, Automotive and Aerospace and Healthcare. Varied applications will be showcased such as modelling cavitation during head impacts, homogenisation of material properties, generation and optimisation of internal micro-structures for additive layer manufacturing and statistical modelling for medical implant design.

http://emps.exeter.ac.uk/engineering/staff/pgyoung

Prof Riyi Shi, Purdue University

A Multimodal Investigation of Traumatic Brain Injury: From Biomechanics to Beha
When May 31, 2016
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-273925
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Mild blast-induced traumatic brain injuries (mbTBIs) have been labeled as the “signature injury” of modern warfare, accounting for the majority of combat head trauma. Many reports show compelling evidence that, even in the absence of acute noticeable symptoms, mbTBI can cause long-term brain damage leading to dire mental and neurological consequences. The social impact is reflected by the disturbing phenomena of combat veterans struggling to reintegrate into civilian life and frightening rate of suicide among soldiers that has heightened public interest and the urgency to deter this unsettling trend. However, keen research effort has been hampered by the limitations of human studies, insufficient animal models, and technical difficulties. To this end, we have established a unique and clinical relevant rat blast injury model.  Further, we have obtained the first evidence in an animal mbTBI model of: 1) intracranial deformation, 2) a promising molecular diagnostic biomarker and target for treatment (acrolein) that correlates with nerve tissue deformation and mental abnormalities, and 3) psychosocial deficits recapitulating human struggles with societal reintegration following mbTBI.  Ongoing studies, using novel neuroengineering technologies and an array of cross-disciplinary approaches, are aiming at revealing the degree of tissue deformation, resultant primary (physical-instantaneous) and secondary (biochemical-delayed) injuries in the brain region known to be critical for the psychosocial behavior and other motor (basal ganglia) and sensory (auditory) functions. It is expected that this line of investigation will facilitate the understanding of key pathological mechanisms, identification of novel treatment strategies that will prove capable of preventing, mitigating, or reversing blast-induced brain damage and, in turn, reduce the ultimate incidence and impact of post-deployment neuropsychiatric dysfunction.

 http://www.purdue.edu/gradschool/pulse/groups/integrative-neuroscience/faculty/shir.html

Prof Robert Adams, Oxford

Composite materials: Are they good or bad for the environment?
When Jan 18, 2016
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-273925
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Composite materials have been with us for thousands of years. Multimaterial adhesively bonded bows using bamboo, horn, and leather were used by Mongol archers on horseback, but modern composites were not possible until polymers had been invented which could wet the fibres and cure to give a strong and stiff material. Today, several racing drivers owe their lives to the high strength carbon fibre monocoque in which they sit, and Boeing have made much of the extensive use of CFRP in the 787 Dreamliner to save fuel. So we have several questions to answer.

1 Why have the advantages of composites not been realised in mass production cars and why are we still mainly using aluminium and steel?

2 How do we assess what is good for the environment? Is it so complex that no-one can truly assess good and bad? How do we sensibly define the mysterious carbon footprint. Are there complex equations with slithery variables which seem to adapt to give whatever answer “the boss” expects?

3 Can we tilt the scales by being clever engineers, or are there some other parameters in the equation?

4 And how do we deal with end of life scenario?

Quidquid it est, timeo Danaos et dona ferentes

Virgil, Aeniad II, 49

Prof. Roel Dullens (University of Oxford, UK)

Dynamics of Grain Boundary Loops in Two-Dimensional Colloidal Crystals
When Oct 24, 2016
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-283446
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Many aspects of the dynamics of grain boundaries in polycrystalline metals and alloys remain unclear as direct observation of grain boundary migration at the atomic level is cumbersome with current techniques. In this respect, grain boundaries in colloidal crystals are convenient model systems since imaging their dynamics requires only simple optical microscopy and they can be manipulated using optical tweezers. In colloidal polycrystalline materials, grain boundaries form a network akin to that found in most metals but these systems remain very complex to study the details of grain boundary migration.    Here, we look at grain boundary dynamics in the simple case of an isolated circular grain boundary, called a grain boundary loop. In a colloidal crystal, we can create such loops on demand, using holographic optical tweezing. After creation, such grain boundary loops spontaneously shrink under the action of capillary forces. We find that complete shrinkage of the grain enclosed by the loop takes longer upon increasing the misorientation of the grain boundary. The rate of shrinkage also strongly depends on the initial radius of the loop. Importantly, we prove the existence of a transition between a regime of pure grain rotation and one dominated by shrinkage, which is specific to grain boundary loops. Finally we elucidate the shrinkage mechanism by directly visualising the dislocation reactions that enable the reduction of the grain boundary perimeter.

 roel.dullens@chem.ox.ac.uk

 

Prof. Mark Spearing, Pro Vice Chancellor, University of Southampton

High resolution tomography studies of composites: The data rich mechanics opportunity
When Apr 29, 2013
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865 283302
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ABSTRACT
High resolution X-ray tomography has been used to observe and quantify damage mechanisms in composite materials under load.  Using synchrotron and micro-focus X-ray sources resolutions of less than 2 µm have been routinely achieved.  This enables individual broken fibres to be observed and crack opening and shear displacements for delaminations and intra-laminar cracks to be measured.  Examples of the application of these techniques to transverse ply cracking, notch-tip splitting (see figure 1), delamination initiation and growth (see figure 2) and fibre fracture accumulation will be presented.  Quantitative data will be compared to model predictions.  The overall implications for using such high-resolution 3-D measurements to inform a “data-rich mechanics” approach to materials evaluation and modeling will be discussed.


Mark Spearing 
Fig. 1. An example of imaging with in situ loading of a split emanating from a notch tip (top left).  The left hand image is unloaded and the right hand loaded.
 

 

 

 


Mark SpearingFig. 2. A three-dimensional rendering of an early stage delamination, showing initial damage consisting of fibre-matrix debonding and subsequent coalescence


Professor TW Clyne, Department of Materials Science & Metallurgy, Cambridge

Thermo-mechanical Stability of Plasma Sprayed Thermal Barrier Coatings in Gas Turbines
When Nov 04, 2013
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865 283302
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Thermal barrier coatings (TBCs) are key elements in the development of improved gas turbine efficiency.  A brief summary will be given of the background to this.  Their stability, and particularly their resistance to spallation (debonding), is critical to wider and more effective usage.  A central problem is that they tend to undergo sintering at service temperatures (~1200-1400˚C), giving enhanced stiffness, which leads to higher stresses, and hence a greater driving force for spallation, during differential thermal contraction on cooling [1].  These changes can be accelerated by the presence of impurities that segregate to the grain boundaries, where they enhance the solid state diffusivity [2] or, at sufficiently high concentrations, produce a vitreous phase that can dramatically accelerate sintering.  The impurities that are most likely to have such effects are often termed CMAS (calcia-magnesia-alumina-silica), which are ingested into the engine in the form of particulates and may be deposited on the coating.  Environmental deposits, such as volcanic ash, with low melting points compared to typical gas turbine entry temperatures may pose a particular threat in this respect [3].  An outline will be given of the modelling and experimental work carried out to investigate these effects, and of the implications for creation of coatings with improved stability.  The experimental work relates mostly to plasma sprayed coatings, but the main conclusions are equally applicable to those produced using the other commonly-employed technique, which is physical vapour deposition.

References

[1]        Shinozaki, M and Clyne, TW, A methodology, based on sintering-induced stiffening, for prediction of the spallation lifetime of plasma-sprayed coatings, Acta Materialia, 61 (2012) p.579-588.

[2]        Cipitria, A, Golosnoy, IO, and Clyne, TW, A sintering model for plasma-sprayed zirconia TBCs. Part I: Free-standing coatings, Acta Materialia, 57 (2009) p.980-992.

[3]        Shinozaki, M and Clyne, TW, The effect of vermiculite on the degradation and spallation of plasma sprayed thermal barrier coatings, Surface and Coatings Technology, 216 (2013) p.172-177.

 

Professor Adib Becker, Department of Mechanical, Materials and Manufacturing Engineering, University of Nottingham

Computational Mechanics: The limits of Computer Simulations
When Nov 17, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Contact Phone 01865-283302
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Prof. Becker will present a number of computational mechanics problems which stretch the capabilities of  existing Finite Element and Boundary Element techniques.  The applications will include surgery simulations, welding processes, waterjet erosion and other problems

Professor Alain Goriely

Morphology and mechanics of evolving biological structures: neurons, seashells, rhubarbs, and chameleons too
When Feb 17, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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In this talk I will present a general mathematical framework to model and analyse the shape and mechanics of filamentary or cylindrical biological structures such as neurons, stems, and roots. This formulation leads, in the simplest case, to reduced models that can be analysed systematically. In particular, I will show how these ideas can be applied to understand morphological patterns appearing on seashell structures, how flowers and rhubarbs fight gravity, and reveal the elastic secrets of the chameleon.

Professor Alba Sofi, Department of Civil, Energy, Environmental & Materials Engineering, University Mediterranea of Reggio Calabria, Italy

Analysis of structures with UNCERTAIN parameters MODELED AS interval VARIABLES
When Feb 23, 2015
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Uncertainties affecting structural parameters, such as material or geometric properties, have traditionally been modeled in the context of the classical probability theory as random variables or random fields. Well-established probabilistic methods have been developed to analyze the effects of uncertainties on structural performance. However, available data are often insufficient to build credible probabilistic distributions of the uncertain parameters, especially in early design stages.

Over the last decade, several non-probabilistic approaches have gained much popularity as alternative tools for quantifying and processing uncertainties described by fragmentary or incomplete data. In this context, the interval model, originally developed on the basis of the interval analysis, is widely used when only bounds are known for the parameters involved in the engineering problem. This model does not provide any information on the frequency of occurrence of values between the lower and upper bounds. The analysis of structures with interval basic input parameters is thus oriented to estimate the range of variation of the response quantities. Solutions obtained by applying the classical interval analysis are often useless from an engineering point of view due to excessive conservatism. Therefore, much research effort has been devoted in the literature to the development of alternative procedures able to limit the overestimation affecting the classical interval analysis.

In this presentation, I will illustrate the main features of the so-called improved interval analysis recently developed by my research group for analyzing the behavior of linear structures with uncertain geometric and/or material properties modeled as interval variables. The improved interval analysis has proved to be a very efficient approach for obtaining sharp bounds on the interval response quantities useful for decision-making in practical engineering.

 E-mail: alba.sofi@unirc.it

Professor Carlos Levi, University of California, Santa Barbara

The Challenges of Higher Temperature Coatings for Gas Turbines
When Jul 14, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
Contact Name
Contact Phone 01865-283490
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Gas turbine technology is at a cross-roads, with demands for increased engine performance and fuel flexibility translating into higher material temperatures, ≥1300°C, and more chemically aggressive environments for the gas path components.  All potential materials solutions, including the current Ni-based superalloys, involve thermal and/or environmental barrier coatings to enable their use under these conditions.  This presentation will start with a broad perspective on the problem, focusing primarily on thermal barrier coatings (TBCs) for metallic components.  Zirconia with 7±1wt%Y2O3 (7YSZ) has been the standard thermal barrier oxide since the commercial insertion of TBCs but is now reaching its limit of applicability.  Candidate new materials are mostly based on ZrO2 with rare earth and/or or transition metal additions.  Two groups emerge, one based on the non-transformable tetragonal (t’) form of ZrO2, and the other on rare earth zirconates.  Unfortunately no candidate in either group meets all the requirements for the more advanced applications.  Tetragonal materials are endowed with toughening mechanisms that underpin their durability.  However, as the engine temperature increases they are compromised by sintering, destabilization of the t’ phase, and by penetration of molten silicate deposits.  In contrast, the zirconate materials are phase stable and offer improved resistance to sintering and silicate penetration, but are limited by the absence of intrinsic toughening mechanisms, thermochemical interactions with the thermally grown oxide that protects the underlying alloy, and often by processability.  This presentation will discuss the scientific foundation of the design strategies for these materials and the challenges ahead.

Acknowledgments: The presentation benefits from various extramural collaborations and includes contributions from past and present graduate students (E.M. Donohue, M.R. Fisch, S.G. Heinze, J.A. Krogstad, R.M. Leckie, C.A. Macauley, D.L. Poerschke, T.A. Schaedler, K.M. Wessels, E.M. Zaleski) and post-docs (S. Burk, C. Carbogno, S. Krämer, R.W. Jackson, J.S. Van Sluytman and J.Y. Yang).  Research support provided by programs from the National Science Foundation (DMR-1105672), the Office of Naval Research (N00014-08-1-0522), AFOSR, and the UCSB-Honeywell Alliance for TBCs. 

About the speaker:  C.G. Levi received a Ph.D. in Metallurgical Engineering from the University of Illinois at Urbana-Champaign in 1981 and has been in the faculty at UCSB since 1984, where he is Professor of Materials and Mechanical Engineering. The overarching theme of his research is the fundamental understanding of microstructure evolution in inorganic materials, and the application of this understanding to the design and synthesis of improved coatings, thin films, composites and monolithic systems, with emphasis on high temperature applications.  Current areas of work include thermal and environmental barrier coatings for advanced gas turbine components, self-healing matrices and fibers for CMCs, environmental barrier layers for advanced nuclear energy systems, novel high temperature alloys and multi-phase functional materials.  His professional contributions have been recognized with election as Fellow of the American Ceramic Society (2012), the TMS Morris Cohen award (2014), the 2008 NIMS Award, the DLR Wissenschaftspreis (2004), the Alexander von Humboldt Forschungspreis (2002), the 1989 Grossman Award and the 1982 Howe Medal from ASM International.  For additional information please see: http://www.materials.ucsb.edu/~levic/levi.html

Professor Chris Truman, Solid Mechanics, Bristol University

Simulation of high energy beam welding with validation
When May 12, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Contact Phone 01865-283302
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New methods for joining materials used in advanced nuclear power plants are of interest to increase efficiency and productivity. Optimised joints require narrow heat affected zones, low residual stress, strain and distortion. This requires research into a large range of aspects including the nature of the joining processes, characterisation of the joint materials and the integrity of joints in manufacture and service. Of particular interest is the high energy beam welding of P91 steel, used extensively in power plants. Laser beam welding (LBW) and electron beam welding (EBW) are both considered. The presentation will discuss recent advances in numerical modelling of LBW and EBW including the influence of solid state phase transformations and validation of the predictions through the use of neutron diffraction. Conclusions will be made which consider and highlight the key requirements for the provision of accurate predictions. 

Professor David Nowell, Department of Engineering Science, Oxford University

'The Kilmore East Bushfire'
When Feb 10, 2014
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865-283302
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In February 2009, a series of bushfires took place in the state of Victoria in Australia.  The Kilmore East/Kinglake fire was responsible for 120 deaths and approximately £1 billion in property damage.  The Victoria Bushfires Royal Commission concluded that the fire was started by the failure of a power distribution line close to Kilmore East.  The fire has recently become the subject of a civil case in the Supreme Court of Victoria.  The seminar will examine the technical background to the failure of the power line, but will also cover some legal aspects, including the nature of my involvement in the case during late 2013.

Professor Felix Hofmann, Oxford Engineering Science

Tungsten armour for fusion reactors: Ion-implantation damage and its effect on material properties
When Nov 23, 2015
from 02:00 PM to 03:00 PM
Where LR8
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http://hofmanngroup.org

Nuclear fusion has the potential to provide an environmentally friendly, sustainable energy source. A hurdle to the development of commercial fusion reactors is the availability of sufficiently resistant materials. Plasma-facing components will be exposed to high temperatures, intense neutron flux and bombardment with hydrogen and helium. Tungsten-based materials are the main candidates for divertor components, which will experience the harshest conditions. To assess the integrity of these components a detailed understanding of the influence of neutron irradiation and ion-implantation damage on material properties is essential. Unfortunately it is not yet possible to recreate the conditions that will be experienced by divertor components. Here we instead use ion-implantation as a proxy to study the interaction of injected helium with displacement damage. This approach avoids sample activation and allows large implanted doses to be reached quickly. However, due to limited ion penetration, implanted layers are only a few microns thick. Furthermore the defects generated are too small to be resolved by TEM and hence must be probed using alternative approaches.

 

In this talk I will discuss the use of synchrotron X-ray micro-diffraction to measure the lattice strains that arise due to helium-ion implantation in tungsten. By combining these measurements with density functional theory calculations we can elucidate the underlying defect microstructure, which appears to be dominated by helium-filled Frenkel pairs. Subtle changes in the elastic modulus of the implanted material, measured using the transient grating technique, are consistent with such a defect population. The transient grating method also allows the thermal transport properties of the implanted layer to be measured. We find that even a modest concentration of implanted helium leads to a substantial decrease in thermal diffusivity. Using a kinetic theory model this effect can be captured and shown to be consistent with the underlying defect microstructure. Importantly the changes in thermal properties are not a trivial function of implanted ion dose, but appear to depend on other factors, such as impurities in the material. Measurements of lattice strains in samples heat-treated after ion-implantation show significant evolution of the defect microstructure. Indeed they suggest that at elevated temperatures defects migrate deeper into the material bulk. Thus we can start to form a joined-up picture of ion-implantation-induced damage in tungsten and its diverse effects on mechanical and transport properties. This is a first step to assessing the anticipated evolution of properties in armor components of future fusion reactors.

Professor Toshimitsu Yokobori, Graduate School of Engineering, Tohoku University, Japan

Potential driven particle diffusion theory and its application to engineering problems
When Nov 18, 2013
from 02:00 PM to 03:00 PM
Where LR8
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There is an extensive range of practical engineering problems in which diffusion plays an important role.  This includes mechanisms of hydrogen diffusion which causes embrittlement, micro crack initiation and growth under high temperature creep conditions and electro-migration due to vacancy diffusion in LSI circuits.  In this presentation, numerical methods are described for the simulation of diffusion under a potential gradient.  Results are presented and discussed for the problems of hydrogen embrittlement, vacancy diffusion which results in creep deformation and damage development in the form of microcracks, and failure of interconnects in LSI circuits.  The effect of temperature and concentration (of hydrogen or vacancies) is discussed based on the proposed analysis.  Furthermore, the way in which hydrogen diffusion and concentration influence fatigue behaviour under cyclic loading conditions is discussed and some engineering significances are proposed.

 

Professor Hongbiao Dong, Department of Engineering, Leicester University

Effect of Post-Weld Heat-Treatment on Hydrogen Embrittlement
When Nov 03, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Dissimilar welds, such as those between ferritic and austenitic materials, are commonly employed in various industrial applications.  In the petrochemicals sector, subsea low alloy steel hubs are joined to micro-alloyed steel linepipes using nickel alloy fillers. The forging assembly is commonly given a post weld heat treatment (PWHT) to temper hard heat affected zones and redistribute residual stresses that form in the as-welded condition.  Whilst PWHT is successful in returning some ductility to the HAZ, a number of in-service failures warrant further investigation.

Whilst in-service, structures of this kind are subjected to cathodic protection (CP) via aluminium based anodes - a common method of mitigating corrosion of the ferritic parts.  CP has, however, been associated with the evolution of hydrogen on metallic surfaces, wherein diffusion, followed by hydrogen embrittlement (HE), may occur.  As a result, a programme of mechanical performance testing under CP, followed by fractographic examination has been designed.  Dissimilar weld specimens were given various PWHTs, and ranked according to resistance to embrittlement.  In this presentation, recommendations on forging chemistry and PWHT are made, based on phase evolution during PWHT and its effect on resistance to fracture.

Professor J. N. Reddy, Texas A & M University, College Station

On numerical simulations of physical phenomena and nonlocal and strain gradient theories
When Jun 09, 2016
from 02:00 PM to 03:00 PM
Where LR1
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Contact Phone 01865-273172
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In this lecture, two distinct topics are addressed. The first one deals with numerical simulations of biological cells, composite and functionally graded shells, and flows of viscous incompressible fluids. The second topic deals with the authors’ recent research on nonlocal elasticity and modified couple stress/strain gradient theories in formulating the governing equations of functionally graded material beams and plates. The importance of numerical simulations of biological cells is discussed briefly, followed by the development of a high-order spectral/hp continuum shell finite element that is free of all types of locking, and the least-squares finite element model for the numerical simulation of flows of viscous incompressible fluids. In the second part of the lecture, two different nonlinear gradient elasticity theories that account for geometric nonlinearity and microstructure-dependent size effects are revisited to establish the connection between the modified couple stress theory of Mindlin and the strain gradient theory of Srinivasa-Reddy. Some numerical examples are presented to bring out the salient features.

 

About the author:

Dr. Reddy is a Distinguished Professor, Regents’ Professor, and inaugural holder of the Oscar S. Wyatt Endowed Chair in Mechanical Engineering at Texas A&M University, College Station, Texas. Dr. Reddy earned a Ph.D. in Engineering Mechanics in 1974 from University of Alabama in Huntsville. He worked as a Post-Doctoral Fellow in Texas Institute for Computational Mechanics (TICOM) at the University of Texas at Austin, Research Scientist for Lockheed Missiles and Space Company, Huntsville, during l974-75, and taught at the University of Oklahoma from 1975 to 1980, Virginia Polytechnic Institute & State University from 1980 to 1992, and at Texas A&M University from 1992.

Dr. Reddy’s research has involved theory and applications of the finite element method to a broad range of problems, encompassing composite structures, numerical heat transfer, computational fluid dynamics, and biology and medicine.  His shear deformation plate and shell theories and their finite element models have been implemented into commercial finite element computer programs like ABAQUS, NISA, and HyperXtrude. Dr. Reddy is one of the original top 100 ISI Highly Cited Researchers in Engineering around world with over 19,640 citations  with h-index of over 66 as per Web of Science; the number of citations is over 47,000 with h-index of 89 and i10-index of 405 as per Google Scholar. Dr. Reddy is a member of the US National Academy of Engineering and a Foreign Fellow of the Indian National Academy of Engineering.

 

**SEMINAR CANCELLED** Professor J.S. Wettlaufer, Mathematical Institute, Oxford University

DUE TO UNFORESEEN CIRCUMSTANCES THIS SEMINAR HAS HAD TO BE BEEN CANCELLED AND WILL BE RE-SCHEDULED FOR A LATER DATE
When Jan 27, 2014
from 02:00 PM to 03:00 PM
Where Seminar
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Professor Jianguo Ling, Department of Mechanical Engineering, Imperial College, London

Evolutionary Theories and Applications in Plasticity Manufacturing Technologies
When Nov 24, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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The presentation will include the following aspects:

 

(i)     Evolutionary theories  and industrial needs

 

(ii)    Engineering applications

  •  High temperature creep damage/failure
  •  Fatigue failure
  •  Metal forming applications

 

(iii)  Unified theories and applications in metal forming

 

(iv)  Case studies

  •  Forming of lightweight automotive body structures
  •  Creep age forming
  •  Micro-forming

 

(v)    Challenges and Future research trends

 

 

 

Professor Johan Moverare, Department of Engineering, Linkorping University, Sweden

Thermomechnical fatigue and dwell time effects in high temperature materials.
When Nov 19, 2013
from 05:00 PM to 07:00 PM
Where LR7
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Contact Phone 01865-273023
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 The combined effect of thermomechanical fatigue (TMF) and dwell times are growing in importance since changes in demand, and competition within sectors such as the power generation market and the transportation industry forces many components to operate under  ever more arduous conditions, to reduce fuel usage and CO2 emissions and at the same time maximize performance. A brief overview of the common test method for TMF as well as some results from recent studies on single crystal alloys, wrought superalloys and cast iron will be given.

 

Professor John Lambros, University of Illinois

Three dimensional studies of particle fracture in Si- and Sn-based Li+ composite electrodes
When Jun 19, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Silicon or tin based electrodes for lithium ion (Li+) batteries are of significant interest because of their potential to be high capacity alternatives to the commonly used graphitic carbon anodes. A drawback to their use, however, is the inevitable particle debonding and fracture that occurs as a result of the volumetric expansion by the host particles upon lithiation of the anode. In this work we use X-ray microcomputed tomography to visualize the evolution of the internal microstructure of both Si-based and Sn-based electrodes before and after lithiation either in complete cycles or in partial lithiation steps. Using X-ray microtomography, we are able to visualize with image resolution around 1 μm/voxel material evolution, generating 3D images of up to 1 billion voxels. From these images we develop a threshold edge detect method to perform 3D volumetric measurements of particle expansion and we measure up to 290% volume expansion in Si after 100% theoretical lithiation. Similar, though less, volume expansion is seen in Sn particles after lithiation. However, the details of particle fracture and particle/matrix debonding differ between the two materials, with Si particles fragmenting whereas Sn particles crack radially. In addition to image analysis we wish to perform internal measurements of the electrode composites’ strain using three dimensional (3D) digital volume correlation (DVC). 3D-DVC measures displacement and strain inside a material subjected to deformation, which in this case is caused by the cyclic lithiation-delithiation process of the battery charging and discharging. Our high-performance DVC algorithm uses parallel computing to help with computation time and memory management issues so that large scale DVC problems can be run. The algorithm is first implemented in surrogate composite materials of glass particles in a PDMS matrix, and is then extended to the electrochemically active Si-based and Sn-based electrodes.

 

Short bio – John Lambros: In 1988 I received a B.Eng. degree in Aeronautics from the Imperial College of Science and Technology of the University of London. I then spent approximately 7 years at Caltech obtaining an M.S. degree in 1988, a Ph.D. degree in 1994 and, finally, one year as a postdoctoral research fellow, all at the Graduate Aeronautical Laboratories (GALCIT). In August of 1995 I joined the Mechanical Engineering department of the University of Delaware as an Assistant Professor. I moved to the Aerospace Engineering department of the University of Illinois at Urbana-Champaign in 2000 as an Associate Professor, and became a full professor in this department in 2007. In 1999 I received an NSF CAREER award. Between 1999-2005 I was Associate Editor for Experimental Mechanics, in 2009 I became a fellow of the American Society of Mechanical Engineers, and in 2013 a fellow of the Society for Experimental Mechanics. Currently I serve as an Associate Editor for the ASME Journal of Applied Mechanics, am directing the Army funded MURI on Nonlinear Stress Wave Mitigation, and am serving as the Associate Department Head for Graduate Studies in the Aerospace Engineering Department at UIUC.

Professor Leon Abelmann, University of Twente, The Netherlands

Shaky nanomagnets: Magnetic reversal of sub-micron Co/Pt multilayered islands studied by Anomalous Hall Effect
When Oct 17, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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When magnets become sufficiently small, they will have only two magnetisation states (up/down, left/right). These states are separated by an energy barrier. If the barrier is much higher than the thermal energy, the magnetisation state is stable, and we can for instance store information. The energy barrier can be overcome by external field, temperature or simply by waiting sufficiently long. By means of Anomolous Hall Effect, we have studied the reversal of individual sub-micron magnetic elements with an out-of-plane easy axis. From thousands of hysteresis loops, we tried to extract the height of the energy barrier at zero field and switching field at zero temperature. All depends however on the exact relation beween the external field and the height of the energy barrier. Here the fun starts…

 

Leon Abelmann, University of Twente, Netherlands and KIST Europe, Germany

 

Leon Abelmann is working at the Korean Institute of Science and Technology in Saarbrücken, Germany, and hold professorships at the University of Twente and Saarland University. His expertise is on magnetostatics on the micro- and nanoscale. As a PhD student and postdoc, he worked mainly in the area of magnetic data storage, moving from tape recording to magnetic force microscopy, magnetic patterned media and probe storage. When the magnetic recording industry left Europe entirely, he changed his research field towards magnetostatics in combination with fluids. He currently address magnetism in life sciences and magnetically assisted three-dimensional self-assembly.

 

http://leon.manucodiata.org

Professor Marco Sebastiani, University of Rome

Measurement of fracture toughness by nanoindentation methods: recent advances and future challenges
When May 11, 2015
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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The analysis of deformation and failure mechanisms in small-scale devices and thin films is a critical issue, not yet solved.

In this presentation, we describe recent advances and developments for the measurement of fracture toughness at small scales by the use of nanoindentation-based methods including techniques based on micro-cantilever, beam bending and micro-pillar splitting. A critical comparison of the techniques is made by testing a selected group of bulk and thin film materials.

For pillar splitting, cohesive finite element simulations are used for analysis and development of a simple relationship between the critical load at failure, pillar radius, and fracture toughness for a given material. The minimum pillar diameter required for nucleation and growth of a crack during indentation is also estimated. An analysis of pillar splitting for a film on a dissimilar substrate material shows that the critical load for splitting is relatively insensitive to the substrate compliance for a large range of material properties.

Micro-pillars are then produced by Focused Ion Beam (FIB) ring milling, being the pillar diameter approximately equal to its length; this ensures full relaxation of pre-existing residual stress in the upper portion of the specimen. Nanoindentation splitting tests are performed in-situ and the deformation mechanisms corresponding to each class of materials have been investigated.

Experimental results from a selected group of materials show good agreement between single cantilever and pillar splitting methods, while a discrepancy of ~25% is found between the pillar splitting technique and double-cantilever testing.

The limitations of the method are finally discussed. In particular, a minimum pillar’s diameter for the nucleation and growth of a crack during indentation is identified and quantified for a wide range of materials properties. It is concluded that both the micro-cantilever and pillar splitting techniques are valuable methods for micro-scale assessment of fracture toughness of brittle ceramics, provided the underlying assumptions can be validated. Although the pillar splitting method has some advantages because of the simplicity of sample preparation and testing, it is not applicable to most metals because their higher toughness prevents splitting, and in this case, micro-cantilever bend testing is preferred.

Marco Sebastiani – Short biography

Dr. Marco Sebastiani received a PhD in mechanical and industrial engineering in 2008 from university of Rome “Roma TRE”, studying the architecture design and nano-mechanical behavior of thin coatings for advanced mechanical applications, with prof. Edoardo Bemporad.

He was then appointed as an assistant professor of Materials Science at the University of “Roma TRE”, working on the development of innovative methodologies for residual stress assessment in thin films and small-scale devices, by the use of Focused Ion Beam (FIB) and nanoindentation techniques.

He is lecturer of Materials Science and Engineering for Bioengineering at the University of “Roma TRE” and has been advisor or co-advisor of about ten master/bachelor degree theses.

During the last years, the achieved results turned out to be extremely successful making him today an expert in the research fields of thin films technology, residual stress analysis in nanostructured or amorphous materials and nano-mechanical characterization of small-scale structures and micro-devices.

 His research results in fact led to:

  • the award of a Faculty Position as assistant professor (RTD) at university of “Roma TRE”, which was renewed in 2014;
  • the role of Coordinator and Principal Investigator of the large collaborative project ISTRESS (FP7-NMP-2013-LARGE-7, Grant Agreement N. 604646, starting Jan 1st 2014);
  • the role of Associate Editor of the Elsevier Journal “Materials and Design”;
  • the award of a Fulbright Research Scholarship which was spent at the university of Tennessee-Knoxville (USA), March to August 2014;
  • the publication of 52 papers on high-impact international journals with an h-index of 12 and more than 420 citations (Scopus: author ID: 7005846216);
  • Two best paper Awards at international conferences (ICACC 2007 and ITSC 2009)
  • Invitations to be member of the organizing committees of International Conferences and Meetings (among them: ICMCTF 2013, ICMCTF 2014 and NANOMEASURE 2014);
  • Member of the Editorial Board of “Surface Engineering” (peer reviewed journal, IF 1.6);
  • Invitations as reviewer for International Journals (more than 30 reviews up to now);

 

MAJOR COLLABORATIONS

Only those proved by common publications and/or official partnerships in European projects:

Prof. William D. Nix (University of Stanford, Materials Science and Engineering, CA, USA); Prof. George M. Pharr (University of Tennessee and ORNL, TN, USA); Prof. Alexander M. Korsunsky (University of Oxford, UK); Dr. Warren Oliver (president, Nanomechanics inc, Oak Ridge, TN, USA); Prof. Chris Eberl (Fraunhofer Institute, IWM, and University of Freiburg, Germany); Prof. Alberto Diaspro (Italian Institute of Technology, IIT, NAPH / Nanobiophotonics); Prof. Y. T. Cheng (University of Kentucky, USA); Prof. Mathias Goeken (University Erlangen-Nuremberg, Germany); Prof. Erik Bitzek (University Erlangen-Nuremberg, Germany); Prof. Ralph Spolenak (ETH Zurich, Switzerland); Dr. Nigel Jennett and Dr Jerry Lord (National Physics Laboratory, NPL, UK); Prof. Karsten Durst (Technical University Darmstadt, Germany); Prof. Reinhard Pippan, Prof. Jozef Keckes and Prof. Rostislav Daniel (University of Leoben, Austria); Dr. Dietmar Vogel (Fraunhofer Institute, ENAS); Dr. Martin Gall (Fraunhofer institute, IKTS); Prof. Mariana H. Staia and Prof. Eli  Puchi-Cabrera (Central University of Venezuela);

 

Professor Martyn Pavier, University of Bristol

Failure of Cast Iron Railway Bridges
When Oct 21, 2013
from 02:00 PM to 03:00 PM
Where LR8
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In the early days of the railways cast iron girder bridges were commonly used to take railways across roads and rivers. These bridges could be substantial structures. Following a series of failures a Royal Commission was appointed which reported in 1847 with the main conclusion that cast iron bridges should be replaced.  Nevertheless, the famous Staplehurst accident in 1865, almost 20 years later, was aggravated by the failure of the cast iron bridge the train was crossing when it derailed. In this talk I'll describe the history of the use of cast iron in railway bridges, look at the design calculations used at the time and comment on their validity in the light of modern understanding.

Professor Mehrdad Negahban, Mechanical & Materials Engineering University of Nebraska-Lincoln, USA

Molecular dynamics to continuum modeling: Some tools for bridging the gap
When Jun 05, 2014
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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We have been studying the relation between molecular dynamic (MD) simulations and continuum models for polymer systems. This was motivated by the affine and non-affine motion of molecules in most polymer systems and trying to find its relation to internal variable modeling at the continuum level.

 This effort has resulted in the development of new measures of deformation and stress. Some of these measures are based on minimizing the difference between the concepts of deformation and traction on a continuum level and their counterparts in the MD simulations. These new measures are fairly general and may have applications in other studies of the relation between MD and macroscopic response.

 Mehrdad Negahban is a Professor of Mechanical & Materials Engineering at the University of Nebraska-Lincoln specializing in the characterization and modeling of large deformation thermodynamic response of materials and their numerical simulation. He graduated with a B.S. in Mechanical Engineering from Iowa State University of Science and Technology, and an M.S. and Ph.D. in Applied Mechanics from the University of Michigan. His research has focused on continuum mechanics and thermodynamics of solids, constitutive theory, characterization and modeling of glassy polymers and crystallizing polymers, finite deformation plasticity, numerical simulation at large deformations, finite elements for generalized shells, dynamic loading of materials and stereo-optical measurements of strains. His work combines theoretical, experimental, and computational methods. A new applied area of work is in 3D printing and the possibility of printing material properties on a molecular scale.  (mnegahban@unl.edu)

 New Book: The Mechanical and Thermodynamical Theory of Plasticity, Mehrdad Negahban, 2012, CRC Press, Taylor & Francis Group

Professor Moritz Riede, Oxford Physics

Flexible Solar Power
When Apr 27, 2015
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Organic solar cells (OSC) have attracted increasing attention in recent years from science and industry. Although OSC have lower power conversion efficiencies than most of their inorganic counterparts, they can have cost advantages, due to low material consumption, simple processing methods as well as the possibility for flexible and light-weight devices. One very promising approach for OSC is based on the thermal evaporation of small molecules in vacuum to create an organic stack in the p-i-n concept, i.e. using molecular p- and n-doping. The result is a very versatile platform both for investigation of fundamental processes and device optimisation. The current state of the art as well as an outlook towards commercialisation will be presented.

**CANCELLED** Professor Paul Buckley & Professor Amy Zavatsky, Department of Engineering Science, Oxford

Making sense of the stretching, squashing and twisting of tendon: one of nature’s primary load-bearing materials
When Oct 19, 2015
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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***PLEASE NOTE THAT THIS SEMINAR IS CANCELLED DUE TO A DEPARTMENT SEMINAR BEING HELD AT 1:00-2:30 (including lunch) in LR2 by Frank Haselbach, Rolls-Royce. Please use link below to register for this event***

https://weblearn.ox.ac.uk/portal/hierarchy/mpls/eng/events

 

Tendons transmit forces from muscles to bones. We and other mammals depend on them for moving our limbs. But, although they are so important, the mechanical properties of tendon tissue remain poorly understood. The constitutive behaviour is complex, with pronounced nonlinearity, viscoelasticity, and anisotropy; tendons’ irregular geometry complicates experiments on them; and they can show wide scatter in properties. Recent research has aimed at overcoming these problems, to achieve improved characterisation and understanding of the 3D deformation of tendon tissue. This has required progress in several areas: a new method for capturing the 3D geometry of tendons; tensile tests with a new approach to interpreting the results; a new approach to testing tendons in lateral compression; and the first testing of tendons in axial torsion, with and without superposed axial tension.  Combining results from all these tests provides improved knowledge of the 3D deformation of tendons, and helps interpret this in terms of what is known of their structure.

Professor Paul Wilcox, Bristol

Recent advances in ultrasonic imaging for non-destructive evaluation
When Feb 09, 2015
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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ABSTRACT: Quantitative Non-Destructive Evaluation (NDE) is essential for ensuring the safe operation of safety-critical structures. NDE invariably exploits the interaction of material anomalies with some form of wave, whether the wave be elastic, electromagnetic, optical or thermal. Of these, elastic waves usually in the 1-10MHz ultrasonic range and x-rays are the only modalities that can penetrate the interior of common engineering materials. Computed x-ray tomography remains the gold standard against which other NDE techniques are often compared, but it cannot be applied in situ. Increasingly stringent health and safety requirements also mean that single-exposure x-ray techniques are becoming less popular for in situ inspection. Hence, there is a drive to exploit ultrasonic elastic waves to extract more and more information about the interior of a structure. In this talk, I will focus on several developments using ultrasonic arrays. I will show how modern instrumentation allows the diffraction limit of resolution to be reached and, in some cases exceeded. Extensions to "difficult" materials such as carbon-fibre composites will also be examined. Classical ultrasonic NDE is based on linear elasticity, but elastic nonlinearity is well known to be extremely sensitive to the early stage of material degradation processes such as fatigue and creep. This regime is of critical importance to operators of ageing plant but measurement of elastic nonlinearity is notoriously diifficult. I will show initial results from a new technique, which has the potential for imaging elastic nonlinearity within the interior of a component. 

http://www.bristol.ac.uk/engineering/research/ndt/

Professor Pedro Ponte Castaneda, University of Pennsylvania

Constitutive Models for Magneto-Active Elastomers at Finite Strains: Dipolar Interactions versus Magnetic Torques
When May 20, 2013
from 02:00 PM to 03:00 PM
Where LR8
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Contact Phone 01865 283302
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This presentation is concerned with the application of a finite-strain homogenization framework to develop constitutive models for magneto-active elastomers (MAEs) consisting of initially aligned, rigid magnetic particles distributed randomly in an elastomeric matrix. For this purpose, a novel strategy is proposed to partially decouple the mechanical and magnetostatic effects in the composite. Thus, the effective electro-elastic energy of the composite is written in terms of a purely mechanical component, together with a magnetostatic component evaluated in the deformed configuration of the composite, as estimated by means of the purely mechanical solution of the problem. The theory predicts the existence of certain “extra” stresses—arising in the composite beyond the purely mechanical and magnetic (Maxwell) stresses—which can be directly linked to changes in the effective magnetic permittivity of the composite with the deformation. For the special case of isotropic distributions of magnetically isotropic, spherical particles, the extra stresses are due to changes in the particle two-point distribution function with the deformation, and are of order volume fraction squared, arising from dipole interactions between the particles. On the other hand, for the case of aligned, ellipsoidal particles, the effect can be of order volume fraction, when changes are induced in the orientation of the particles, as a consequence of magnetic torques on individual particles. The theory is capable of handling the strongly nonlinear effects associated with finite strains and magnetic saturation of the particles at sufficiently high deformations and magnetic fields, respectively. It will be shown that particle rotations can be used to produce relatively large field-induced strains and actuation stresses, as well as to control the instantaneous stiffness of the material.

Professor Philip Withers FREng, University of Manchester

Correlative Tomography - Spanning Length and Timescales
When May 05, 2015
from 02:00 PM to 03:00 PM
Where LR8, IEB Building, Engineering Science
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Together non destructive (X-ray) and destructive (serial section electron microscopy) enable us to probe materials behavior across a very wide range of lengthscales and timescales.  In this presentation I will describe the multi-scale 3D characterization workflows combining macroscale X-ray computed tomography (CT), micro X-ray CT, nanoscale serial section FIB/SEM imaging and analysis, and scanning transmission electron microscopy to study a range of materials degradation and repair processes. This approach allows us to travel down the scales to better understand macroscale damage in terms of the underlying microstructure and to co-visualise structural, stress fields, crystallographic (EBSD) and chemical (EDS) information.  This approach is particularly powerful when trying to develop a multiscale understanding of degradation form fatigue cracking, through creep cavitation to corrosion.  Examples will be drawn from the aerospace, nuclear and oil and gas sectors. Future workflows and visualization software advances will enable the materials scientist to bring together multiple scales and information or undertake high resolution imaging with a high degree of knowledge of the local context.

Finally, the three pillars of materials science (microstructure-chemistry-performance) are traditionally studied separately in the microscopy suite, the chemistry lab and the mechanical test facility on different samples.  Correlative techniques currently allow one to bring them all into registry in three dimensions.

 

Professor Ray Ogden, School of Mathematics and Statistics, University of Glasgow

Modelling non-symmetry of collagen fibre dispersion in the elasticity of arterial wall tissue
When Nov 30, 2015
from 02:00 PM to 03:00 PM
Where LR8
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In [1] we developed a rotationally symmetric model to describe collagen fibre dispersion in soft biological tissues based on a generalized structure tensor. This has been highly successful, but recent experimental results [2] on the collagen fibre dispersion in human arterial layers have shown that the dispersion in the tangential plane is more significant than that out-of-plane. A rotationally symmetric dispersion model is not therefore able to capture this distinction. For this reason we introduce a new non-symmetric dispersion model, based on the bi-variate von Mises distribution, which is used to construct a new structure tensor. The latter is incorporated in a strain-energy function that accommodates both the mechanical and structures features of the material, extending our rotationally symmetric dispersion model [1]. Material and structural parameters were obtained by fitting predictions of the model to experimental data obtained from a human abdominal aortic adventitia. In a finite element example the non-homogeneous stress distribution is obtained for circumferential and axial strips under fixed extension.  

 

[1] TC Gasser, RW Ogden, GA Holzapfel. Hyperelastic modelling of arterial layers with distributed collagen fibre orientations. J. R. Soc. Interface 3:15-35, 2006.

[2] AJ Schriefl, G Zeindlinger, DM Pierce, P Regitnig, GA Holzapfel. Determination of the layer-specific distributed collagen fiber orientations in human thoracic and abdominal aortas and common iliac arteries. J. R. Soc. Interface 9:1275-1286, 2012.

Transfer Talks

To be confirmed
When Oct 14, 2013
from 02:00 PM to 03:00 PM
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Professor René de Borst, University of Glasgow

Multiscale Mechanics and Cohesive-Surface Models
When Feb 04, 2013
from 02:00 PM to 03:00 PM
Where LR8
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In this lecture, we will start by a concise classification of multi-scale computational methods. We will concentrate on computational methods that allow for concurrent computing at multiple scales. Difficulties that relate to the efficient and accurate coupling between the various subdomains will be highlighted, with an emphasis on the coupling of domains that are modelled by dissimilar field equations, such as continuum mechanics and molecular dynamics. Two main approaches can be distinguished for resolving interfaces and evolving discontinuities. Within the class of discrete models, cohesive-surface approaches are probably the most versatile, in particular for heterogeneous materials. However, limitations exist, in particular related to stress triaxiality, which cannot be captured well in standard cohesive-surface models. In this lecture, we will present an elegant enhancement of the cohesive-surface model to include stress triaxiality, which preserves the discrete character of cohesive-surface models.

Among the recent developments in continuum approaches we mention the phase-field theories, and we will relate them to gradient damage models. In particular, we will elaborate a phase-field approach for cohesive-surface models, which, although being a continuum approach, results in a well-posed boundary value problem, and is therefore free of mesh dependence.

Whether a discontinuity is modelled via a continuum model, or in a discrete manner, advanced discretisation methods are needed to model the internal free boundary. The most powerful methods are finite element methods that exploit the partition-of-unity property of the shape functions, and isogeometric analysis. Examples will be given, including analyses that include coupling of evolving discontinuities with non-mechanical fields such as moisture and thermal flow.