Department of Engineering - Annual Report 1999/2000

Micromechanics and Materials

CAMBRIDGE CENTRE for MICROMECHANICS

COMPOSITE MATERIALS
Compressive Failure of Composites
Designing with Composite Materials
Composite Sandwich Panels
Draping of Woven Materials
Braided Composites
Anisotropic Elasicity
Adhesive Joints
Singularity Analysis

CELLULAR SOLIDS
Mechanical Properties of Metallic Foams and Lattice Materials
Acoustic and Thermal Properties of Metal Foams
Sound Absorption in Metal Foams with Semi-open Cells
Heat Transfer in Lattice-Frame Materials
Micropolar Elasticity

TRIBOLOGY
Friction and Wear of Dry and Lubricated Contact
Metal Working Tribology
Indentation

PROCESSING
Modelling Compaction of Composites
Powder Compaction of Composites

THERMAL MATERIALS
Thermal Conductivity of Thermal Barrier Coatings

SMART MATERIALS/STRUCTURES
Selection of Sensors
Fracture and Damage Mechanics of Piezoelectric Ceramics
Optimal Design of Micro-Mechanical Clips

OTHER TOPICS
Adiabatic Shear
The Design and Performance of Ferroelectric Actuators
Road Materials
Thermal Shock Resistance of Solids
Thermal Management with Phase Change Cooling
Cracking of Zirconia Refractories Under Thermal Shock
Thermal Shock Resistanc of Solids
Two-dimensional Cellular Metals as Heat Dissipation Media
Optimal Design of Flexural Actuators
Optimal Design of Cold-Formed Steel Sections
Mechanics of Corrugated Packaging Boards
Mechanics of Celklular Metals
Failure Analysis
Tendonitis

INTERDISCIPLINARY RESEARCH CENTRE IN SUPERONDUCTIVITY
Force Free Configurations
Superconducting Multilayers
Micro-Hall Probe Investigations
AC Losses in Superconducting Tapes
Superconducting Fault Current Limiters
Superconducting Magnetic Bearings
Ceramic Powder Processing by Evaporative Decomposition of Solution (EDS)
Melt Processed Bulk High Temperature Superconductors for Engineering Applications
Fundamental of the Reversible Magnetisation in YCBO

References

Cambridge Centre for Micromechanics

An inter-Departmental Centre for research in Micromechanics was opened in March 1996. Micromechanics is "the application of the principles of mechanics, thermodynamics and kinetics to the modelling of material response at length scales ranging from the atomistic to the macroscopic". The Centre is housed in Engineering and hosts joint projects with the Departments of Applied Mathematics and Theoretical Physics, and Materials Science and Metallurgy. The current research thrusts of the Centre are (a) mechanics of composites, (b) processing (c) cellular solids and (d) tribology. The Centre has financial support from a number of EPSRC programmes and also from US and EU funding. A weekly workshop stimulates inter-disciplinary discussion on defined problems in micromechanics. A major aim of the Centre is to stimulate closer interaction with U.K. industry and with the best international researchers, and to co-ordinate research on fundamental and important problems.

Composite Materials

Compressive Failure of Composites

The practical design of composite components is often governed by their poor compressive strength. The Cambridge group has identified fibre waviness and matrix shear yield as the underlying reason for low compressive strength. Theoretical models are used to predict the effect of fibre waviness on strength. The effect of structural features such as holes and ply drops is being examined both theoretically and experimentally. The work is funded by EPSRC and is being done in collaboration with the Materials Science and Metallurgy Department in Cambridge, British Aerospace and Hexcel Composites. A novel method for measuring fibre waviness in composite specimens has been developed, using a simple image analysis program which identifies fibres in a photographic image of the specimen. Initiation of failure has been investigated using a model composite made of alternate layers of epoxy and steel. The effects of Z-pins and ply drops on compressive strength have been explored^{(E11,E12,E20,E21,E22)} 

Designing with Composite Materials

A predictive design methodology has been developed based explicitly on analytical models of the physical microscopic cracking phenomena by which damage accumulates with time in carbon fibre and glass fibre epoxy-based composite materials^(E5,E7). These models of damage growth can be manipulated in a format suitable for the conceptual design of crack-insensitive, damage-tolerant composite material systems over a range of stress, temperature, and environment. This work includes a new micro-mechanical theory of stress-corrosion cracking of glass fibre-epoxy composites^(E6,E59,E60). The model defines the conditions of operational stress and environment for which the microstructure of the composite can be optimised and the resistance of glass fibre-epoxy component to stress corrosion cracking and corrosion fatigue can be maximised. 

There are three new studies. The first is based on the reformulation of such physical models applied to the repair of ageing (fatigue cracked) metallic aircraft structures using bonded composite patches. The second is concerned with raising the resistance of aluminium ally components to foreign object damage. The approach is to adhesively bond a thin composite skin to the metallic substrate. The third new project is the direct observation of the deformation and failure mechanisms of natural fibres and natural fibre composites under stress by in-situ dynamic scanning electron microscopy. 

Composite Sandwich Panels

Dr M.P.F. Sutcliffe
Professor N.A. Fleck 

A project has been initiated with Hexcel Limited on the behaviour of sandwich panels containing a polymeric foam core, subjected to bend loading and end loading. Design charts have been produced to optimise sandwich panel construction. 

Draping of Woven Materials

A collaborative programme with Nottingham University and seven industrial partners has started, considering the way in which woven material drapes over curved structures. The research at Cambridge focuses on the micromechanical aspect, including the effect of shear in the fabric on the resulting composite stiffness and strength. 

Braided Composites

A mechanics analysis has been compared with measurements on the deformation and fracture of braided tubes in tension, compression and shear. Such composites have high energy absorption capacity, particularly when foam filled^{(E32,E33,E34)}.

Anisotropic Elasticity

Composites have anisotropic material properties that affect the stresses around discontinuities or points of stress concentration. In an anisotropic material the affect of a stress concentration is more widespread in the direction of large elastic modulus. Quantitative estimates of the rate of decay around stress concentrations have been obtained as a function of certain modulus ratios for generic patterns of stress distribution. The rate of decay depends on the complexity of the traction distribution rather than on the moment of the applied tractions^(E65). 

Analytical methods have been developed for calculating contact stresses near a rigid indentor pressed into an elastic material that contains either voids or inclusions (e.g. stiff fibres). These methods that employ distributions of dislocations on interfaces, are useful when different sources of stress variation are sufficiently close so that they interact^(E25).

Adhesive Joints

Dr M.P.F. Sutcliffe
Dr H.R. Shercliff 

A joint project with TWI has been studying the degradation of adhesive joints due to a hot-wet environment. Material tests have characterised acrylic adhesive properties under these conditions. These measurements have been used to develop a Finite Element program to model progressive failure in the joint under increasing load, validated with measurements on lap shear joints.

A collaborative post-graduate training partnership (PTP) project with The Welding Institute (TWI) and British Aerospace has used finite element analysis to predict the onset of damage and delamination failure of adhesive fillet joints in CFRP, as a function of joint geometry and layup^(E17,E18). Fundamental studies on property degradation models have revealed sensitivities to mesh orientation and the number of elements allowed to fail per iteration^(E16). 

Singularity Analysis

A singularity analysis of scarf joints has been performed in order to deduce the local stress state at the free edge between two adjoining solids^(E44). 

Cellular Solids

Mechanical Properties of Metallic Foams and Lattice Materials

A co-ordinated activity between Cambridge, Harvard, MIT and UVA continues on measuring and modelling the mechanical properties of metallic foams. These new engineering materials show great potential for use in energy absorption in automotive applications, as the foam cores for sandwich panels, and in heat transfer and acoustic attenuation applications. If they are to be used more widely, research is needed on their deformation and fracture properties, including fatigue and creep. A broad range of experiments have been completed, including multi-axial loading tests, high strain rate tests, fatigue and fracture studies, and sandwich panel testing. A comprehensive Design Guide for Metal foams was published by Butterworth-Heinemann in June 2000.

A new class of engineering materials has been designed and manufactured - lattice materials. These are miniaturised truss structures with exceptionally high values of stiffness and strength when compared with cellular materials of comparable density^{(E4,E14,E15,E35,E52,E54)}.

Acoustic and Thermal Properties of Metal Foams

Metal foams with open cells have application as heat-exchange elements. The heat-transfer characteristics of metal foams are analysed. Predictive models, adapted to the optimisation of density, pore size, etc. are developed.

Metal foams offer a combination of attractive properties. Among these is the ability to absorb sound, suggesting their use for acoustic insulation as well as mechanical strength at low weight. Experiments to characterise sound absorption in metal foams are in progress. 

Sound Absorption in Metal Foams with Semi-open Cells

The sound absorption capability of aluminium foams with semi-open cells is studied, with support from the Chinese National Science Foundation. The foams are processed via negative-pressure infiltration, using a preform consisting of soluble spherical particles. Normal sound absorption coefficient and static flow resistance are measured for samples having different porosity, pore size and pore opening. A theory is developed for idealised semi-open metallic foams, and the predicted sound absorption coefficients are compared with those measured. Focus has been placed on revealing the correlation between sound absorption and morphological parameters such as pore size, pore opening, and porosity.

Heat Transfer in Lattice-Frame Materials

Lattice-frame structured materials are a new development made possible by computer based design and control of numerically controlled processing. They consist of a 3-D network of struts or plates. The microstructural morphologies of a LFM resemble those of cellular aluminium foam. Hence the overall heat transfer performance of both types of material is expected to be similar. However, a precise computer-designed LFM does not contain processing-induced geometrical imperfections widely found in metal foams. Work has started to determine relationships between thermal properties and microstructure for LFMs by combining experimental measurement with analytical modelling and numerical simulation, and develop an implementation methodology that enables design engineers to select LFMs with microstructures optimised for best heat transfer performance at affordable costs.

Micropolar Elasticity

Micropolar elasticity theory has been used to analyse planar boundary value problems for stresses and deformations. This theory employs couple stresses to represent cell wall bending in cellular solids; these stresses are in addition to the direct and shear stresses acting on a representative element of a foam or honeycomb. Around any particular stress concentration, the rate of decay of couple stresses has been shown to be one order larger than that of direct and shear stresses. For dynamic loads, couple stresses give surface waves which are persistent or far-reaching. These micropolar effects are large at free surfaces and have relatively small influence at interior points^(E69). 

Tribology

Friction and Wear of Dry and Lubricated Contact

A review paper on wear mechanisms and diagnostics was published^(E57) while work on boundary lubrication continued with papers presented at a meeting of the OECD International Research Group on Wear in Cambridge and the 2000 Gordon Conference in the U.S. Further work is to be presented at an interdisciplinary meeting at Oxford in April 2001 and the World Tribology Congress in Vienna in September 2001. Two invited book chapters have been completed, one on contact mechanics and the second on mechanisms of abrasive wear in hydraulic systems; both will appear in early 2001. Environmental concerns, market forces and legislative requirements are combining to lead to a search for metal working and forming lubricants with reduced environmental impact^(E43,E70) and more industrially supported work in this area is anticipated. The annual Cambridge Tribology Course was held at the end of September 2000 when a record number of delegates from both industry and academia attended.

Metal Working Tribology

Work in this area focuses on measurement and modelling the interaction between tool and workpiece surfaces, in lubricated cold rolling of metals. Projects are in collaboration with Corus, Avesta Sheffield, Alcan and Alstom. The effect of considering the true stochastic nature of rough surfaces has been modelled and compared with measurements both on experimental mills and from industrial mills. The development of pits and voids through the pass schedule in stainless steel had been examined and work is progressing to help model this.

Indentation

A finite element analysis of indentation between two dissimilar elastic-plastic spheres has been performed. This analysis is a fundamental solution for a wide range of contact problems.

Processing

Modelling Materials Processing

Dr H.R. Shercliff 

Two projects on modelling of materials processing have been completed, under the post-graduate training partnership (PTP) with The Welding Institute (TWI). Analytical methods have been used to predict the thermal response and microstructure in friction stir welds, while finite element analysis has been used to predict residual stresses in TIG-welded aerospace aluminium alloys, in collaboration with British Aerospace and the University of Manchester. The predictions have been validated against detailed synchrotron diffraction data. Aspects of both projects are continuing via an EU 5th framework programme on welding of dissimilar light alloys. 

Modelling of processing of aluminium alloys continues with Alcan International and the University of Manchester. The advanced statistical methods of Gaussian process modelling and dynamic neural networks have been applied to recrystallisation of hot deformed alloys^(E61), and age hardening of aluminium-copper alloys. 

Powder Compaction of Composites

Experiments and modelling have been performed to elucidate the mechanisms by which mixed powders, one ductile and the other rigid, consolidate under pressure at both low and high temperatures. The pressure-density characteristics are measured under purely hydrostatic and more complex triaxial loadings. Micromechanical models capture the constitutive response based on indentation theory. A study has been completed on the compaction of particulate-reinforced powder composites, as part of an initiative to use SiC particle-reinforced aluminium alloy compacts in automotive applications. The multi-axial yield surface has been measured and the stage I compaction of composites has been modelled^(E56,E62).

Thermal Materials

Thermal Conductivity of Thermal Barrier Coatings

Highly porous zirconia based thermal barrier coatings have recently been synthesised with zig-zag morphology pores which appear to impede heat flow through the thickness of the coating. A combined analytical/numerical study of heat conduction across these microstructures is presented and compared with thermal conductivity measurements. The effects of pore volume fraction, pore type, pore orientation and pore spacing, together with the wave length and the amplitude of zig-zag pore microstructures on overall thermal performance are quantified. The results indicate that even a few volume per cent of zig-zag inter-column pores oriented normal to the substrate surface reduce the overall thermal conductivity of the coatings by more than 50%.

Smart Materials/Structures

Selection of Sensors

A systematic design procedure has been developed for the selection of sensors. The characteristics of sensors have been mapped and particular applications have been plotted with the same axes, in order to select the appropriate classes of sensor^(E62).

Fracture and Damage Mechanics of Piezoelectric Ceramics

This work deals with the microcrack damage in anisotropic piezoelectric materials. The interpretation and evaluation of the two components of the J_k-vector along specially introduced closed contours in two-dimensional piezoelectric materials with strongly interacting microcracks are presented. The interaction problem for arbitrary arrays of microcracks with different oriented angles and located angles with respect to a global coordinate system is studied in detail. It is found that the total contributions induced from the formation of the arbitrary arrays of microcracks to both components of the J_k-vector in the global system vanish, provided that the closed contour for calculating the vector encloses all the microcracks or there are no other discontinuities outside of the contours. In addition, variable tendencies of the crack tip stress intensity factors and the electric displacement intensity factor as influenced by the remote electric displacement loading (may be positive or negative) in two microcrack interacting configurations are discussed in detail.

Optimal Design of Micro-Mechanical Clips

Silicon or silicon nitride based microclips have recently been developed to hold optical fibres in micro-optical systems for high optical coupling efficiency. Single mode optical fibres are inserted and held in position in V-shaped grooves etched in silicon substrates by the cantilever clips protruding from the edges of the V grooves. As the fibre core is in general either level with or above the silicon surface, the clips are deflected by the fibre and act as springs holding the fibres kinematically in place. In this work, the optical thickness, mass and shape of fibre-holding clips are studied with an effective optimisation procedure, called "Metamorphic Development" (MD). This procedure, coupled with the method of finite elements, aims at finding structural shapes and topologies of the microclips that minimise their structural compliance and weight subject to stress and deflection constraints.

Other Topics

Adiabatic Shear

The theory of adiabatic shear in polymers has been reviewed.

The Design and Performance of Ferroelectric Actuators

Professor N.A. Fleck 

Ferroelectric materials have the ability to strain under an electric field and to produce an electrical charge under a mechanical stress. Recently, robust constitutive laws have been determined to describe the behaviour of these materials, supported by multi-axial experiments. Experiments on the cyclic behaviour reveal electrical and mechanical fatigue phenomena. A finite element program has been written in order to design practical devices (actuators and sensors) with these materials^(E38). 

Road Materials

Work on the damage mechanisms of asphalt paving materials has continued^(E13). A project concerned with the mechanics of thin bitumen films, tested in tension, had led to greatly improved understanding of the wide range of behaviour exhibited by asphalts. These include the ductile to brittle transition, as well as voiding and flow mechanisms^(E36). 

Thermal Shock Resistance of Solids

Dr T.J. Lu
Professor N.A. Fleck 

Work continued on predicting the thermal shock resistance of solids. It is found that a pre-existing, mode-I edge crack in a finite plate subjected to cold shock may grow in one of three different modes: continued extension in plane strain, channelling and spalling. The thermal shock conditions governing each failure mode are quantified. Failure maps for the various cracking patterns are constructed in terms of the critical temperature jump and Biot number, and merit indices are identified for materials selection against failure by thermal shock^(E71). In another closely related project, the evolution of stresses and strains in a zirconia-containing refractory tube subjected to a hot shock on the outer surface and convective cooling at the inner surface is analysed. The parameters that control the time-dependent stress and strain responses are identified by performing finite element calculations that span the range of variables relevant to steel making. The effects of tube thickness, hot shock duration, initial temperature, temperature dependence of elastic modulus, and transformation amplitude on stress and strain distributions are discussed, and heating and process strategies to eliminate surface cracking are suggested^(E37).

Thermal Management with Phase Change Cooling

This ONR sponsored work focuses on the prospect of designing high power electronic packages with phase change cooling. An analytical model is developed for a finite slab suddenly exposed to a uniform heat flux at the top surface and cooled by convective medium at the bottom. The phase change problem is divided into sub-problems and solved progressively, and the solutions are analysed to guide the design of advanced packages with optimised cooling strategies^(E46).

Cracking of Zirconia Refractories under Thermal Shock

Zirconia (ZrO₂) is widely used in refractory applications (e.g. slag line sleeves for steel making) due to its high corrosion resistance. However, zirconia is unstable when subjected to thermal transients, which may cause cracking in the refractory. The evolution of stresses and strains in a zirconia-containing refractory tube subjected to a hot shock on the outer surface and convective cooling at the inner surface is analysed. To account for the temperature-induced phase transformation in the zirconia as well as the overall thermal expansion, a coefficient of total dilatation is introduced. The parameters that control the time-dependent stress and strain responses are identified by performing finite element calculations that span the range of variables relevant to steel making. The effects of tube thickness, hot shock duration, initial temperature, temperature dependence of elastic modulus, and transformation amplitude on stress and strain distributions are discussed, and heating and process strategies to eliminate surface cracking are suggested.

Thermal Shock Resistance of Solids

Work continued on predicting the thermal shock resistance of solids. The propagation of a pre-existing edge crack across a finite plate subjected to cold shock has been studied. The plate, initially at uniform temperature, is exposed to a cold shock on one surface whilst three different types of heat transfer boundary condition are separately considered for the opposing face. For all three boundary conditions, the plate experiences tensile stressing near the cold-shocked surface and compressive stressing near the mid-plane. Consequently, a mode I edge crack extending into the compressive region may grow in one of three different modes: continued extension in plane strain, channelling and spalling. The thermal shock conditions governing each failure mode are quantified, with a focus on crack channelling and spalling. Failure maps for the various cracking patterns are constructed in terms of the critical temperature jump and Biot number, and merit indices are identified for materials selection against failure by thermal shock.

Two-dimensional Cellular Metals as Heat Dissipation Media

Stiff sandwich panels with metal honeycomb core can be used to carry structural load as well as dissipate heat through solid conduction and forced convection. This work attempts to uncover the nature of heat transfer in these lightweight systems, with emphasis on the effects of varying cell morphologies and cell arrangements on their heat transfer and load carrying efficiency. The types of cell considered include regular hexagon, square and triangle. Isothermal conditions are imposed on the sandwich walls. Overall heat transfer coefficient and pressure drop are obtained as functions of relative density, cell shape, cell arrangement, fluid properties, and overall dimensions of the heat sink. 

Optimal Design of Flexural Actuators

A minimum-weight flexural actuator is designed. The actuator comprises a triangular corrugated core with shape memory alloy (SMA) faces. It is clamped at one end and free at the other. For design and optimization, the temperature history of the face sheets upon heating and subsequent cooling is first obtained as a function of the cooling efficiency (Biot number) and the operational frequency deduced. Based upon this response, a phenomenological model is employed to represent the martensite evolution. Thereafter, the end deflection is calculated as a function of temperature. The minimum weight is calculated subject to the provisos that: (i) the end deflection attains a specified value; (ii) the power consumed is less than the upper limit of the supply; and failure is averted by (iii) face/core yielding and (iv) face/core buckling; (v) the operational frequency of the panels achieves a specified limit. 

Optimal Design of Cold-Formed Steel Sections

This is a new programme funded by EPSRC to improve the strength, rigidity, fire resistance and acoustic properties of cold-formed steel sections without compromising the process speed or increasing the material content. Basic methods based on minimum weight have been developed to optimise the cross-sectional shape of sheet sections. An acoustic model has been developed to study the sound transmission loss across partition walls and the influence of various stud designs.

Mechanics of Corrugated Packaging Boards

This work, supported by the Swedish packaging firm SCA, has in the past year focused on the deformation and failure mechanisms of corrugated sandwich panels under compressive loading. Extensive finite element calculations are performed to obtain their compressive strength as a function of a variety of geometrical and material parameters, and to study the global and local plastic collapse mechanisms during the deformation process. Experimental measurements are conducted on two types of commercially available corrugated board panels to verify the accuracy of theoretical predictions. In addition, four different experimental methods are used to evaluate the elastic properties of a board panel: Chladni-pattern based vibration test, uniform lateral compression, three-point bending, and simple shear. 

Mechanics of Cellular Metals

Works on cellular metals continue on studying their mechanical properties, including yielding and fracture toughness. A phenomenological model is developed for plastic yielding and compared with experimental measurements. The model-I crack growth behaviour is studied by using the finite element method. The effects of inclusions and holes on elastic module and yield strength are studied in detail.

Failure Analysis

Numerous investigations of engineering and metallurgical failures have been carried out in collaboration with a wide range of industrial sponsors. Specific projects have ranged from the investigation and elimination of joint failures in electronic devices to the statistical analysis of structural failure in supertankers. As a result, an extensive knowledge base of information on the causes, mechanisms and implications of failures has been developed. The knowledge base has important applications to the design, manufacture, assessment and maintenance of safe and robust components and structures. Emphasis is placed on the generic nature of failure analysis and the transfer of technology and practice between different fields of application. A number of significant spin-offs have resulted from this work. Dr Jones is founder Editor-in-Chief of the Elsevier Science Journal Engineering Failure Analysis. This has developed into the prime international journal in the field and now acts as a major focus for international collaboration. A project with The Open University will lead to the production and use of distance learning materials in the field of forensic engineering. A project with the Instrumentation Group of the Institute for Manufacturing is investigating problems in the performance of automotive transmission components.

Tendonitis

A programme has just started to look at Engineering aspects of tendonitis. The work will focus on in-situ measurement and characterisation of damage. These may be used to develop appropriate micromechanical models.

Interdisciplinary Research Centre In Superconductivity

The IRC continues to be funded mainly by responsive mode grants and correlates research in five departments. Dr Campbell is the present director. Considerable progress has been made in developing the applications of these materials and the Engineering group plays a major role in the activities of the IRC.

Force Free Configurations

Dr A.M. Campbell 

This is a computing project in collaboration with Earth Sciences to investigate how flux lines can cut each other when there is a current flowing parallel to the magnetic field. It required a three dimensional solution to the time dependent Ginzburg Landau equations, which proved very difficult and certain simplifications had to be made. Nevertheless we were able to reproduce the cutting process and obtain pictures of entangled vortices.

Superconducting Multilayers

Dr A.M. Campbell 

This work is being done in collaboration with Materials Science. We are using thin film multilayers of low Tc materials as a model system to investigate the irreversibility line in High Tc materials. It shows clearly that the dimensionality of the transition changes as the layers become more widely spaced, just as is observed in high Tc layered structures. 

Micro-Hall Probe Investigations

Miniature Hall probes are being used to map the magnetic field distribution in crystals of heavy fermion semiconductors. These show interesting phase transitions which are reflected in unusual structures in the vortex lattice. The dome shape associated with the geometric barrier is clearly seen.

AC Losses in Superconducting Tapes

This is being carried out in collaboration with Materials Science. The 50Hz losses of a series of commercial tapes are being measured and the results compared with theory. Finite element analysis is being used to predict the properties of various tape geometries. It has been found both theoretically and experimentally that a ferromagnetic shield round the superconductor can reduce the losses. 

Superconducting Fault Current Limiters

Dr A.M. Campbell
Dr T.A. Coombs 

The LINK scheme with Materials Science and five companies ended with a reasonably successful test of a single phase, 400 Amp, 2kV limiter. The main problem has proved to be variability between elements which causes all the voltage to be dropped along a small number of elements which then fail. A numerical code has been written which can model this effect and this is being used to develop a new design of limiter and show how uniform the materials have to be.

Superconducting Magnetic Bearings

This project is in collaboration with URENCO and is to develop a superconducting bearing for an energy storage flywheel. A 40kg rotor has been successfully levitated but it was found to be out of balance. Since there is almost no damping, large oscillations built up at low speeds so we have constructed an alternative rotor out of metal and this is now being tested.

Ceramic Powder Processing by Evaporative Decomposition of Solution (EDS)

Dr D.A. Cardwell 

The mechanism for the decompositon of powders formed during evaporative decomposition of solution (EDS) has been determined and used to account for the variety of product powder morphologies observed in this process. This has enabled optimised calcination conditions to be derived for a range of high temperature superconductor precursor powders. 

The processing conditions for Au-doped NdBCO precursor powder prepared by EDS have been optimised. The effect of Au addition is to reduce the peritectic temperature of NdBa₂Cu₃O₇ compared with the undoped compound. This powder has subsequently been used successfully to fabricate large grain NdBCO by a self-seeded melt growth process with the seed crystal added to the samples surface at room temperature for the first time. 

Melt Processed Bulk High Temperature Superconductors for Engineering Applications

Medium scale (4 samples simultaneously) fabrication of large grain YBCO up to 6cm in diameter by top seeded melt growth under thermal gradients has been achieved. Four high quality samples can now be produced by this technique in a period of 2-3 days. The porosity in bulk YBCO crystals, which results in degraded flux trapping properties and poor mechanical strength, has been reduced by pre-sintering the precursor pellet under vacuum prior to melt growth.

The effect of dopant addition to (RE)BCO high temperature superconductors has been investigated. In particular, Ca, Zn and U have been found to have beneficial effects on the properties of fully melt grown YBCO. Ca-doped samples, which potentially offer enhanced flux pinning properties at 77K, have been grown in large grain form for up to 10 wt.% Ca for the first time. Zn enhances significantly flux pinning in the bulk material whereas U generates a fine distribution of second phase material on a scale of around 300-500nm. This is the first time a second phase distribution on this length scale has been observed for a solid state reaction technique. Subsequent slow neutron irradiation of samples doped with natural uranium (0.718% U²³⁵) at the Atom Institüt, Vienna, yields a 7-fold increase in critical current density and trapped magnetic field at 77K to greater than 1.2T in a small specimen (~2cm diameter). Further studies of the effect of adding enriched (up to 3% % U²³⁵) and depleted (0.284% % U²³⁵) uranium on the sample properties are underway. 

NdBCO has been grown in the form of large grains using both MgO and NdBCO seed crystals. In the latter, the seed has been added at both room temperature (using Au-doped precursor powder) and at elevated temperature (using undoped powder). The Nd_(l-x)Y_(x)BaCuO phase diagram has been investigated thoroughly and used to identify limitations on the NdBCO grain growth rate for air and reduced oxygen processing atmospheres. The latter technique enables control of single crystal formation and has been used to develop an effective process to suppress solid solution formation and the resultant non-steady state solidification. A simple growth model has been developed to account for differences between the morphologies of NdBCO and YBCO crystals grown under similar conditions.     

Fundamental Studies of the Reversible Magnetisation in YBCO

The reversible magnetisation of large grain melt processed YBCO has been measured by both magnetometry and calorimetry. Good agreement between these techniques has been observed for the first time and offers significant potential for future studies of this parameter in materials which contain strong flux pinning centres.

E1. Ahmed, R., Sutcliffe, M.P.F. An analysis of micro-plasto-hydrodynamic lubrication using a strip drawing rig. 16th International Conference on Computer-Aided Production Engineering (CAPE 2000), Edinburgh (August 2000); Edited by J.A. McGeough, 329-342. IMechE Conference Transactions 2000-5 (Professional Engineering Publishing, 2000). ISBN 1 86058 263 X.

E2. Ahmed, R., Sutcliffe, M.P.F. Evolution of surface roughness within the roll bite during cold rolling of stainless steel. Proceedings, Institute of Materials 3rd International Conference on Modelling of Metal Rolling Processes, London (December 1999), 390-399 (IoM Communications, 2000). ISBN 1-86125-105 X.

E3. Ahmed, R., Sutcliffe, M.P.F. Identification of surface features on cold-rolled stainless steel strip. Wear, 244, 60-70 (September 2000).

E4. Ashby, M.F., Evans, A.G., Fleck, N.A., Gibson, L.J., Hutchinson, J.W., Wadley, H.N.G. Metal Foams: a Design Guide. (Butterworth-Heinemann, 2000). ISBN 0750672196.

E5. Beaumont, P.W.R., Sekine, H. Physical modelling of engineering problems of composites and structures. Applied Composite Materials, 7, (1), 13-37 (January 2000).

E6. Beaumont, P.W.R., Sekine, H. Physical modelling of problems in the design of Engineering composites and structures. 5th Research Seminar on Experimental Techniques and Design in Composite Materials, Cagliari, Sardinia, Italy (September 2000).

E7. Beaumont, P.W.R., Shercliff, H., Dimant, R., Sekine, H. Physical modelling of engineering problems of composites and structures. European Conference on Composite Materials (ECCM-9), Brighton (June 2000).

E8. Bradley, A.D., Doyle, R.A., Lo, W., Cardwell, D.A., Campbell, A.M. Large transport critical currents across boundaries in artificially joined large-grain YBCO. Superconductor Science and Technology, 12, (12), 1054-1058 (December 1999).

E9. Cardwell, D.A. Hari Babu, N., Lo, W., Campbell, A.M. Processing microstructure and irreversibility of large grain Nd-Ba-Cu-O. Superconductor Science and Technology, 13, (6), 646-654 (June 2000).

E10. Cocks, A.C.F., Ashby, M.F. Creep-buckling of cellular solids. Acta Materialia, 48, (13), 3395-3400 (August 2000).

E11. Creighton, C.J., Ellis, J., Sutcliffe, M.P.F., Clyne, T.W. The influence of processing variables on fibre alignment and compressive strength of unidirectional laminates. Poster session, European Conference on Composite Materials (ECCM-9), Brighton (June 2000).

E12. Creighton, C.J., Sutcliffe, M.P.F., Clyne, T.W. Characterisation and control of fibre alignment during prepreg manufacture. Proceedings, ICCM12, 12th International Conference on Composite Materials, Paris, France (July 1999).

E13. Deshpande, V.S., Cebon, D. Uniaxial experiments on idealised asphalt mixes. Journal of Materials in Civil Engineering (ASCE), 12, (3), 262-271 (August 2000).

E14. Deshpande, V.S., Fleck, N.A. High strain rate compressive behaviour of aluminium alloy foams. International Journal of Impact Engineering, 24, (3), 277-298 (March 2000).

E15. Deshpande, V.S., Fleck, N.A. Isotropic constitutive models for metallic foams. Journal of the Mechanics and Physics of Solids, 48, (6-7), 1253-1283 (June/July 2000).

E16. Feih, S., Shercliff, H.R., McGrath, G. 3D progressive damage modelling for adhesively bonded composite peel joints: implementation in a UMAT and related compuational issues. Proceedings, 14th ABAQUS User Group UK Conference, Warrington (September 2000).

E17. Feih, S., Shercliff, H.R., McGrath, G. Design of composite adhesive joints. Cambridge University Engineering Department Technical Report CUED/C-MICROMECH/TR.31 (March 2000).

E18. Feih, S., Shercliff, H.R., McGrath, G. Optimisation of peel joint geometrics in CFRP laminates under static loading with regard to first and final failure. European Conference on Composite Materials (ECCM-9), Brighton (June 2000).

E19. Fleck, N.A. Adiabatic shear instability: theory. In: Mechanical Properties and Testing of Polymers; Edited by G.M. Swallowe, Chapter 4, 15-19 (Kluwer Academic Publishing, 1999). ISBN 0412801701.

E20. Fleck, N.A. Micromechanisms of compressive failure in composites. Proceedings, 3rd Nordic Meeting on Materials and Mechanics, Aalborg, Denmark; Edited by R. Pyrz (May 2000).

E21. Fleck, N.A., Liu, D., Shu, J.Y. Microbuckle initiation from a hole and from the free edge of a fibre composite. International Journal of Solids and Structures, 37, (20), 2757-2775 (May 2000).

E22. Fleck, N.A., Zhao, L.G. Microbuckle tunnelling in fibre composites. Journal of the Mechanics and Physics of Solids, 48, (9), 1865-1891 (September 2000).

E23. Fortes, M.A., Ashby, M.F. The effect of non-uniformity on the in-plane modulus of honeycombs. Acta Materialia, 47, (12), 3469-3473 (September 1999).

E24. French, G.J., Foulds, S.A.L., Abell, J.S., Bradley, A.D., Lo, W., Doyle, R.A., Cardwell, D.A. Magneto-optic characterisation of artificial grain boundaries in melt textured YBCO. Applied Superconductivity 1999, 4th European Conference (EUCAS 1999), Sitges, Spain (September 1999); Edited by X. Obradors, et al, 195-198. Institute of Physics Conference Series 167 (IoP Publishing, 2000). ISBN 0750306947/0750307463.

E25. Fujimoto, K., Stronge, W.J. Computational method for elastic contact near rigid inclusion. Computational Methods in Contact Mechanics IV: 4th International Conference, Stuttgart, Germany (July 1999); Edited by L. Gaul, C.A. Brebbia, 153-162 (Wessex Institute of Technology Press, 1999). ISBN 1853126942.

E26. Hari Babu, N., Cardwell, D.A., Lo, W., Campbell, A.M. Flux pinning in large NdBa₂Cu₃O_7-(delta) grains fabricated by seeded-melt growth. Physical Review B - Condensed Matter, 61, (1), 735-740 (January 2000).

E27. Hari Babu, N., Kambara, M., Smith, P.J., Cardwell, D.A., Shi, Y.H. Fabrication and microstructure of large single grain Y-Ba-Cu-O through infiltration and seeded growth processing. Journal of Materials Research, 15, (6), 1235-1238 (June 2000).

E28. Hari Babu, N., Lo, W., Cardwell, D.A., Campbell, A.M. The irreversibility behaviour of NdBCO fabricated by top seeded melt processing. Applied Physics Letters, 75, (19), 2981-2983 (November 1999).

E29. Hari Babu, N., Lo, W., Cardwell, D.A., Shi, Y.H. The effect of undercooling and Nd422 phase content on the nucleation of large NdBCO grains fabricated by top seeded melt processing. Journal of Materials Research, 14, (10), 3859-3863 (October 1999).

E30. Hari Babu, N., Lo, W., Cardwell, D.A., Shi, Y.H. Fabrication and microstructure of large grain Nd-Ba-Cu-O . Superconductor Science and Technology, 13, (5), 468-472 (May 2000).

E31. Hari Babu, N., Lo, W., Cardwell, D.A., Shi, Y.H. Fabrication and microstructure of large grain Nd-Ba-Cu-O. Applied Superconductivity 1999, 4th European Conference (EUCAS 1999), Sitges, Spain (September 1999); Edited by X. Obradors, et al, 1, 41-46. Institute of Physics Conference Series 167 (IoP Publishing, 2000). ISBN 0750306947/0750207463.

E32. Harte, A-M., Fleck, N.A. Deformation and failure mechanisms of braided composite tubes in compression and torsion. Acta Materialia, 48, (6), 1259-1271 (April 2000).

E33. Harte, A-M., Fleck, N.A. On the mechanics of braided composites in tension. European Journal of Mechanics A - Solids, 19, (2), 259-276 (March/April 2000).

E34. Harte, A-M., Fleck, N.A., Ashby, M.F. Energy absorption of foam-filled circular tubes with braided composite walls. European Journal of Mechanics A - Solids, 19, (1), 31-50 (January/February 2000).

E35. Harte, A-M., Fleck, N.A., Ashby, M.F. Sandwich panel design using aluminium alloy foam. Advanced Engineering Materials, 2, (4), 219-222 (April 2000).

E36. Harvey, J.A.F., Cebon, D. Bitumen films in tension. Society of Chemical Industry, Young Researchers Forum,London (April 2000). Published as: SCI Lecture Paper Series LPS 105/2000 (SCI, 2000).

E37. He, P.F, Lu, T.J., Clegg, W.J. The cracking of zirconia refractory tubes under hot shock. Journal of Materials Science, 35, (10), 2443-2449 (May 2000).

E38. Huber, J.E., Fleck, N.A. Multiaxial models and experiments with ferroelectrics. Smart Structures and Materials 2000 - Active Materials: Behavior and Mechanics Conference, Newport Beach, CA, USA (March 2000); Edited by C.S. Lynch, 288-295. Proceedings of SPIE - the International Society of Optical Engineering 3992 (SPIE, 2000). ISBN 0819436100.

E39. James, S.S., Dewhurst, C.D., Doyle, R.A., Paul, D.M., Paltiel, Y., Zeldov, E., Campbell, A.M. Flux pinning, surface and geometrical barriers in YNi₂B₂C. Physica C: Superconductivity and Its Applications, 332, (1-4), 173-177 (May 2000).

E40. Jones, D.R.H. Failures of structures and components which fracture mechanics would have prevented. (Invited paper). Fracture Mechanics: Applications and Challenges, Invited Papers Presented at the 13th European Conference on Fracture, San Sebastian, Spain (September 2000); Edited by M. Fuentes, M. Elices, A. Martin-Meizoso, J.M. Martinez-Esnaola, 29-46. ESIS Publication 26 (Elsevier, Oxford, 2000).

E41. Le, H., Sutcliffe, M.P.F. Analysis of surface roughness of cold-rolled aluminium foil. Wear, 244, 71-78 (September 2000).

E42. Le, H., Sutcliffe, M.P.F. Surface finish of cold rolled aluminium foil. Proceedings, Conference on Modelling of Metal Rolling Processes 3, London (December 1999) (IOM Communications Ltd, 2000). ISBN 1-86125-105 X.

E43. Liew, W.Y.H., Hutchings, I.M., Williams, J.A. The interaction between tool material, environment and process conditions in the machining of aluminium alloys. Machining Science and Technology, 3, (2), 273-286 (1999).

E44. Liu, D., Fleck, N.A. Scale effects in the initiation of cracking of a scarf joint. International Journal of Fracture, 95, 67-88 (1999).

E45. Lo, W., Hari Babu, N., Cardwell, D.A., Shi, Y.H., Astill, D.M. Fabrication and characterisation of large Nd-Ba-Cu-O grains prepared under low oxygen pressure. Journal of Materials Research, 15, (1), 33-39 (January 2000).

E46. Lu, T.J. Thermal management of high power electronics with phase change cooling. International Journal of Heat and Mass Transfer, 43, (13), 2245-2256 (July 2000).

E47. Lu, T.J., Chen, C. Thermal transport and fire retardance properties of celluar metals. Metal Foams and Porous Metal Structures: International Conference, Bremen, Germany (June 1999); Edited by J. Banhart, M.F. Ashby, N.A. Fleck, 391-398 (Verlag MIT Publishing, 1999). ISBN 3980574873.

E48. Maine, E.M.A., Ashby, M.F. Cost estimation and the viability of metal foams. Advanced Engineering Materials, 2, (4), 205-209 (April 2000).

E49. Majoros, M., Glowacki, B.A., Campbell, A.M. Magnetic screening as a possible way to decrease transport AC losses in multifilamentary superconductors - basic theoretical considerations. Physica C: Superconductivity and Its Applications, 334, (3-4), 129-140 (June 2000).

E50. Majoros, M., Glowacki, B.A., Campbell, A.M. Transport losses and screening properties of multifilamentary tapes covered with magnetic materials. Physica C: Superconductivity and Its Applications, 338, (4), 251-262 (September 2000).

E51. Majoros, M., Glowacki, B.A., Campbell, A.M., Apperley, M., Darmann, F. Transport ac losses in (Bi, Pb) SrCaCuO₂₂₂₃/Ag multifilamentary tapes with different filament arrangements. Physica C: Superconductivity and Its Applications, 323, (3-4), 125-136 (October 1999).

E52. McCullough, K.Y.G., Fleck, N.A., Ashby, M.F. The stress-life fatigue behaviour of aluminium alloy foams. Fatigue and Fracture of Engineering Materials and Structures, 23, (3), 199-208 (March 2000).

E53. Murakami, M., Cardwell, D.A., Dew-Hughes, D., Obradors, X., Salama, K. (Editors) Papers Presented at the 2nd International Workshop on the Processing and Applications of Superconducting (RE)CBO Large Grain Materials, Morioka, Japan (October 1999). Published as: Superconductor Science and Technology, 13, (6) (Special Issue) (June 2000).

E54. Olurin, O.B., Fleck, N.A., Ashby, M.F. Joining of aluminium foams with fasteners and adhesives. Journal of Materials Science, 35, (5), 1079-1085 (March 2000).

E55. Petras, A., Sutcliffe, M.P.F. Indentation resistance of sandwich beams. Composite Structures, 46, (4), 413-424 (December 1999).

E56. Redanz, P., Fleck, N.A. On discrete element modeling of powder compaction. IUTAM Symposium on Theoretical and Numerical Methods in Continuum Mechanics of Porous Materials, Stuttgart, Germany (September 1999).

E57. Roylance, B.J., Williams, J.A., Dwyer-Joyce, R.W. Wear debris and associated wear phenomena - fundamental research and practice. Proceedings of the Institution of Mechanical Engineers Part J, Journal Engineering Tribology, 214, (J1), 79-105 (2000).

E58. Rycroft, S.F.W.R, Doyle, R.A., Fuchs, D.T., Zeldov, E., Drost, R.J., Kes, P.H., Tamegai, T., Ooi, S., Campbell, A.M., Liang, W.Y., Foord, D.T. Effect of surface barriers on transport properties of Bi₂Sr₂CaCu₂O₈ single crystals using the Corbino disc configuration. Superconductor Science and Technology, 12, (12), 1067-1070 (December 1999).

E59. Sekine, H., Beaumont, P.W.R., et al. An experimental study of the repair efficiency of composite patches bonded to aircraft structural panels. 9th US-Japan Conference on Composite Materials, Toray Human Resources Development Centre, Mishima, Shizuoka, Japan (July 2000).

E60. Sekine, H., Beaumont, P.W.R., et al. On a simple power law for macroscopic crack propagation rate due to stress corrosion cracking in unidirectional GFRP composites. Materials Science and Engineering A, 285, (1), 298-302 (June 2000).

E61. Shercliff, H.R., Withers, P.J., Lovatt, A.M., Barat, P. Advanced statistical modelling applied to recrystallisation of hot-worked Al-Mg alloy. Proceedings, 21st Risø International Symposium on Materials Science, Roskilde, Denmark; Edited by N. Hansen, X. Huang, D. Juul Jenson, E.M. Lauridsen, T. Leffers, W. Pantleon, T.J. Sabin, J.A. Wert, 575-580 (September 2000).

E62. Shieh, J., Huber, J.E., Fleck, N.A., Ashby, M.F. The selection of sensors. Cambridge University Engineering Department Technical Report CUED/C-MATS/TR.252 (2000) and CUED/C-MICROMECH/TR.36 (2000).

E63. Smith, P.J., Cardwell, D.A., Hari Babu, N., Shi, Y.H. The effect of the addition of gold on secondary phase formation in RE-Ba-Cu-O (RE=Y, Nd). Applied Superconductivity 1999, 4th European Conference (EUCAS 1999), Sitges, Spain (September 1999); Edited by X. Obradors, et al, 1, 75-78. Institute of Physics Conference Series 167 (IoP Publishing, 2000). ISBN 0750306947/0750307463.

E64. Sridhar, I., Fleck, N.A. Yield behaviour of cold compacted composite powders. Acta Materialia, 48, (13), 3341-3352 (August 2000).

E65. Stronge, W.J., Kashtalyan, M. Saint Venant's principle in orthotropic planar elasticity: rates-of-diffusion for stress . Quarterly of Applied Mathematics, LVII, (4), 741-755 (December 1999).

E66. Sutcliffe, M.P.F., Le, H. Measurements of surface roughness in cold metal rolling in the mixed lubrication regime. Tribology Transactions, 43, (1), 39-44 (January 2000).

E67. Sutcliffe, M.P.F., Montmitonnet, P. A coupled tribological and mechanical model for thin foil rolling in the mixed lubrication regime. Proceedings, Institute of Materials 3rd International Conference on Modelling of Metal Rolling Processes, London (December 1999) (IOM Communications Ltd, 2000). ISBN 1-86125-105 X.

E68. Sutcliffe, M.P.F., Xin, X.J., Fleck, N.A., Curtis, P.T. Composite compressive strength modeller, version 1.4 1999. Cambridge University Engineering Department Technical Report CUED/C-MICROMECH/TR.35 (2000).

E69. Wang, X., Stronge, W.J. Micropolar theory for elastodynamics of honeycomb. ICTAM 2000: International Congress of Theoretical and Applied Mechanics, 20th IUTAM Congress, Chicago, IL, USA (August/September 2000).

E70. Williams, J.A., Hutchings, I.M., Liew, W.Y.H. The influence of environmental process conditions on the machining of aluminium alloys in tribology in environmental design 2000 . Proceedings, 1st International Conference on Tribology in Environmental Design 2000, Bournemouth (September 2000); Edited by M. Hadfield, C. Ciantar, 175-184 (Professional Engineering Publications, 2000). ISBN 1 86058 266 4.

E71. Zhao, L.G., Lu, T.J., Fleck, N.A. Crack channelling and spalling in a plate due to thermal shock. Journal of the Mechanics and Physics of Solids, 48, (5), 867-898 (May 2000).

E72. Zheng, D.N., Ingle, N.J.C., Campbell, A.M. Irreversibility fields of superconducting niobium alloys. Physical Review B - Condensed Matter, 61, (22), 15429-15435 (June 2000).

E73. Zheng, D.N., Zhao, Z.X., Campbell, A.M. Determination of irreversibility fields and superconducting parameters in BaPb0. Physical Review B - Condensed Matter, 61, (21), 14804-14809 (June 2000).

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Last modified: September 2001