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Cambridge Centre for Micromechanics
Powder Compaction of Composites
Mechanical Behaviour of Metallic Foams
The Thermal Properties of Metal Foams
Metal Matrix Composites
Wood Composites
Composite Materials
Deformation and Fracture
Failure Analysis
Materials Teaching
Melt Processed Bulk High Temperature Superconductors for Engineering Applications
Modelling Materials Processing
Mechanics of Woven Composites
Interfacial Fracture
Strain Gradient Plasticity Theory
Smart Materials
Metal Rolling
Compressive Failure of Composites
Optimal Design of Composite Structures
References
Dr N.A. Fleck
Prof. M.F. Ashby
The Cambridge Centre for Micromechanics is directed by Dr N.A. Fleck and co-directed by Professor M.F. Ashby, Dr T.W. Clyne (Materials Science and Metallurgy Department) and Professor J.R. Willis (Department of Applied Mathematics and Theoretical Physics). 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 macroscropic". 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 and (c) metallic foams. The Centre has financial support from a number of EPSRC programmes and also from U.S. 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.
Dr. N.A. Fleck
Prof. M.F. Ashby
Experiments and modelling are designed 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 both purely hydrostatic and more complex triaxial loadings. Micromechanical models capture the constitutive response. A study has begun 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. An international symposium sponsored by IUTAM was held in Cambridge on 15-17 July 1996 to review the current status of the Mechanics of Powder Compaction and Granular Flow.
Prof. M.F. Ashby
Dr. N.A. Fleck
Low-density aluminium foams, derived from several different sources, are characterised structurally and mechanically. The tensile and compressive stress-strain response is measured, and fracture and fatigue behaviour is explored. Micromechanical models for the properties are developed.
Dr. T.J. Lu
Prof. M.F. Ashby
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.
Dr C.Y. Barlow
Work has continued on microscopical aspects of deformation structures in aluminium-based metal-matrix composites. The particular focus has been to understand the local strain fields around reinforcing particles or whiskers, comparing experimentally measured strain distributions with theoretical models. An EC-funded multinational collaborative project on forging of MMC components was completed during the year. It resulted in the production of a viable product, a brake drum which is expected to be used on a production basis by Fiat.
Dr C.Y. Barlow
In collaboration with groups from France and the University of Bath, studies have continued on the interrelation of manufacturing process and mechanical properties of wood composite materials such as fibre-board. Work is also in progress on the design and fabrication of glue joints in load-bearing laminated or jointed wood structures.
Dr P.W.R. Beaumont
The combined effects of fatigue and environmental degradation on the failure of composites have been studied(D10,D11,D12). Physical models of damage growth have been developed based on observations using in-situ scanning electron microscopy. Failure mechanisms have been plotted in diagrams taking stress and temperature or environment as axes. Work is done in collaboration with the University of Tohuku, Japan, and the Department of Aeronautical Engineering, MIT, USA.
Dr D.R.H. Jones
An experimental study of the effect of a liquid-metal environment on fatigue-crack propagation is continuing(D18,D19). Results to date indicate that there is a competition between the processes of fatigue crack propagation and crack-tip dissolution in the liquid metal. The modelling of creep and hot isostatic pressing (HIP) using metal-type organic compounds is continuing(D13,D35). Current theories of HIP have been modified to include back-stress effects. When known or measured values for the physical parameters of the model materials are inserted into the modified theory the equations predict the experimental results accurately without the use of variable "fitting" parameters. The approach appears to have promise for predicting the HIP processing conditions of metallic dispersion-strengthened alloy systems.
Dr D.R.H. Jones
A project which involves a series of detailed investigations of specific engineering failures is continuing(D23,D24). The results should be of relevance to professionals involved with the design, manufacture and maintenance of safe and reliable engineering artifacts. The project has resulted in the launch of a new journal, Engineering Failure Analysis (editor-in-chief: D.R.H. Jones). Now in its third year of publication by Elsevier Science, the journal is the first to be devoted wholly to the subject of Failure Analysis.
Dr D.R.H. Jones
A second edition of the textbook "Engineering Materials 1" has recently been published(D2). Polish translations of the textbooks "Engineering Materials 1"(D3) and "Engineering Materials 2"(D4) have also been published.
Dr D.A. Cardwell
A process for the fabrication of large grain YBa2Cu307 HTS has been developed based on a single crystal seeding technique. Samples of up to 3 cm diameter can now be fabricated routinely by this method and trap a magnetic field of up to 0.4 T at 77 k and 1 T at 64 k. The magnetic moment of these samples has been observed to scale with applied field over a wide temperature range. The distribution of flux pinning centres in this material has been studied extensively and has yielded a number of publications.
Dr H.R. Shercliff
Research in processing of aluminium alloys continues in collaboration with several universities and aluminium producers(D5,D17,D22,D30,D31,D33). A joint EPSRC funded project with the University of Sheffield has been completed, demonstrating the importance of transients in deformation conditions during hot working, and developing models to describe the behaviour. New projects include a study of the effect of processing on toughness of aluminium alloys, a study with the Welding Institute on friction stir welding, and the development of a methodology to integrate process modelling with selection of material and process in design. Work has also started on a review of materials modelling in the light of the Technology Foresight recommendations, for the Office of Science and Technology and the Institute of Materials.
Dr N.A. Fleck
Experimental and theoretical work continues on the failure mechanisms of braided tubes. Energy absorption in tension and in compression has been addressed, and the optimal geometry and material properties of foam-filled tubes have been selected to maximise energy absorption.
Dr N.A. Fleck
A study has been completed on the cracking patterns in metal-ceramic laminates, as a function of relative thickness of metal and ceramic layer. Failure is either by single cracking (low energy mode) or by multiple cracking (high energy mode).
Dr N.A. Fleck
A new theory of plasticity has been developed with the inclusion of a size effect, associated with strain gradients. The theory has been used to model a range of plasticity phenomena (such as indentation) where size effects are observed.
Dr N.A. Fleck
A systematic materials selection procedure has been developed for the use of active materials in particular applications. A micromechanics model has been developed for phase transformation in ferroelectrics and in shape memory alloys.
Dr M.P.F. Sutcliffe
Experimental work (sponsored by Alcan International) has verified the foil rolling model originally developed at Cambridge by Dr N.A. Fleck and Professor K.L. Johnson. Models of the deformation behaviour of random rough surfaces in metal working have been developed, in collaboration with Professor W.R.D. Wilson at Northwestern, U.S.A. The spectrum of the surface is used to identify the behaviour of short and long wavelength components of roughness. It is found that the short wavelengths are much more resistant to crushing and so determine the frictional behaviour in the bite.
Dr N.A. Fleck
Dr M.P.F. Sutcliffe
Compressive failure in carbon fibre composites has been further explored. Experiments have supported earlier work showing that microbuckle propagation can be modelled as a sliding crack. The ignition toughness has been modelled using a finite element model.
Large scale bridging models for compressive failure of notched carbon fibre have been developed. The transition between linear-elastic fracture mechanics and notch-insensitive plastic behaviour has been described using a map. Design criteria have been developed for the initiation and growth of damage from notches, and the effects of multi-axial loading have been included.
Theoretical and experimental work is taking place on the mechanisms of failure in carbon-fibre-epoxy composites. Compressive failure is generally by plastic microbuckling. Experiments have investigated the propagation zone at the tip of such a microbuckle. An analysis treating the microbuckle as a crack and applying conventional fracture mechanics has been fruitful. In-situ scanning electron microscope experiments have been performed to study the microbuckle propagation phase. The collapse response is modelled by a simple fracture mechanics model.
The fibre microbuckling phenomenon has been investigated for a stochastic distribution of fibre waviness and for a composite under multiaxial loading. A PC program has been written to predict notched compressive strength for a range of laminates including woven composites.
Dr M.P.F. Sutcliffe
Dr N.A. Fleck
The methodology of Professor M.F. Ashby for materials selection is applied to the optimal design of composite structures, for example stiffened and sandwich structures. Detailed failure mechanisms for honeycomb panels are being explored, in collaboration with Hexcel.
D1. ASHBY, M.F., BRECHET, Y., DUPEUX, M., LOUCHET, F. Choix et usage des matériaux, Techniques de l'Ingenieur, T-5-100, 1-23 (Editions Maurice Postel, SA, Paris, France, 1966).
D2. ASHBY, M.F., JONES, D.R.H. Engineering Materials 1 - An Introduction to their Properties and Applications. 2nd edition. (Butterworth-Heinemann, 1996).
D3. ASHBY, M.F., JONES, D.R.H. Engineering Materials 1 - An Introduction to their Properties and Applications. Polish edition, translated by W.I. Zastosowania (Wydawnictwa Naukowo-Techniczne, Warsaw, 1995).
D4. ASHBY, M.F., JONES, D.R.H. Engineering Materials 2 - An Introduction to Microstructures, Processing and Design. Polish edition, translated by S.M. Wojciechowskiego (Wydawnictwa Naukowo-Techniczne, Warsaw, 1996).
D5. BAXTER, G.J., FURU, T., WHITEMAN, J.A., SELLARS, C.M. The influence of transient deformation conditions on strength, microstructure and recrystallisation during thermomechanical processing of aluminium alloy Al-1%Mg. Proceedings, 5th International Conference on Aluminium Alloys, Grenoble, France, 459-464. Materials Science Forum, 217-222, (July 1996) (Transtec Publications, Zurich, 1996).
D6. CARDWELL, D.A. HTS materials - requirements for magnets. IEE Colloquium on High Tc Superconducting Materials as `Magnets', Cambridge, 1/1-1/3. IEE Digester 1995/223 (December 1995).
D7. CARDWELL, D.A., CAMPBELL, A.M. The design of electrical machines using high temperature superconductors. IEE Colloquium on the Impact of New Materials on Design, London, 2/1-2/5. IEE Digest 1995/234 (December 1995).
D8. CARDWELL, D.A., WEAVER, G. I.R. T.V. by the pyroelectric effect: materials selection and deployment. In: Case Studies in Manufacturing with Advanced Materials; Edited by A. Demaid, J.H.W. de Wit, 99-136 (Elsevier, 1995).
D9. CLARKE, A.R., ARCHENHOLD, G., DAVIDSON, N., SLAUGHTER, W.S., FLECK, N.A. Determining the power spectral density of the waviness of unidirectional glass fibres in polymer composites. Applied Composite Materials, 2, (4), 233-243 (1995).
D10. COWLEY, K.D., BEAUMONT, P.W.R. Fracture and damage mechanics of composite materials at elevated temperatures, Vol. 1 (Parts 1-3). Cambridge University Engineering Department Report CUED/C-MATS/TR.232 (1996).
D11. COWLEY, K.D., BEAUMONT, P.W.R. Fracture and damage mechanics of composite materials at elevated temperatures, Vol. 2 (Parts 4-6). Cambridge University Engineering Department Report CUED/C-MATS/TR.233 (1996).
D12. COWLEY, K.D., BEAUMONT, P.W.R. The micromechanics of crack growth in fibre composites: observing and modelling the failure processes. Proceedings, ICCM-10, 10th International Conference on Composite Materials, Whistler, British Columbia, Canada (August 1995); Edited by A. Poursatip and K. Street, 1, 787-801 (Woodhead Publishing, Abington, Cambridge, 1995).
D13. DAVIES, G.C., JONES, D.R.H. Creep of metal-type organic compounds - III: friction stress models for creep in particle-hardened systems. Scripta Materialia, 35, (4), 523-528 (1996).
D14. DESHPANDE, V.S., CEBON, D. Plane strain deformation of a regular array of hexagonal particles with a bitumen binder. Cambridge University Engineering Department Report CUED/C-MATS/TR.231 (May 1996).
D15. DEWHURST, C.D., DOYLE, R.A., CARDWELL, D.A., CAMPBELL, A.M., BALAKRISHNAN, G. `Arrowhead'
magnetisation anomaly, hysteresis loop scaling properties and the irreversibility line in
Bi2Sr2CaCu2O8+
. Applied Superconductivity,
Edinburgh, 291-294. Institute of Physics Conference Series No 148, 291-294 (1995).
D16. DEWHURST, C.D., DOYLE, R.A., CARDWELL, D.A., CAMPBELL, A.M., BALAKRISHNAN, G., PAUL, D.M.
Determination of the onset of bulk pinning and the low temperature irreversibility line in
Bi2Sr2CaCu2O8+. Physical Review B,
53, (21), 14594-14599 (1996).
D17. DULY, D., BAXTER, G.J., SHERCLIFF, H.R., WHITEMAN, J.A., SELLARS, C.M., ASHBY, M.F. Microstructure and local crystallographic evolution in an Al-1 wt% Mg alloy deformed at intermediate strain-rate and high temperature. Acta Materialia, 44, (7), 2947-2962 (1996).
D18. FERNANDES, P.J.L., JONES, D.R.H. The effect of crack blunting in liquid metal environments on KIEAC determined by the rising load test. Engineering Failure Analysis, 3, (3), 227-230 (1996).
D19. FERNANDES, P.J.L., JONES D.R.H. The effect of temperature on fatigue crack growth in liquid metal environments. Corrosion Science, 38, (5), 745-754 (1996).
D20. FLECK, N.A., KANG, K.J., WILLIAMS, J.A.. The machining of sintered bronze. International Journal of Mechanical Sciences, 38, (2), 141-155 (1996).
D21. FLECK, N.A., SUTCLIFFE, M.P.F., SIVASHANKER, S., XIN, X.J. Compressive R-curve of a carbon fibre-epoxy matrix composite. Composites Part B, 27B, (6), 531-541 (1996).
D22. FURU, T., SHERCLIFF, H.R., SELLARS, C.M., ASHBY, M.F. Physically-based modelling of strength, microstructure and recrystallisation during thermomechanical processing of Al-Mg alloys. Proceedings of 5th International Conference on Aluminium Alloys, Grenoble, France, 1, 453-458. Materials Science Forum, 217-222, (July 1966) (Transtech Publications, Zurich, 1996)
D23. JONES, D.R.H. Whirling failure in a woodworking lathe. Engineering Failure Analysis, 3, (1), 71-76 (1996).
D24. JONES, D.R.H., MacDONALD, K.A. Fatigue failure of a rotating chemical vessel. Engineering Failure Analysis, 3, (2), 77-93 (1996).
D25. KANG, K.J., FLECK, N.A. Analysis of the machining process of sintered bronze. KSME Journal, 10, (3), 314-320 (1996).
D26. LO, W., CARDWELL, D.A. The growth of large grain YBCO by seeded peritectic solidification. Applied Superconductivity, Edinburgh, 139-142. Institute of Physics Conference Series 148, July (1995) (IOP Publishing, 1995).
D27. LO, W., CARDWELL, D.A., DEWHURST, C.D., DUNG, S-L. Fabrication of large grain YBCO by seeded peritectic solidification. Journal of Materials Research, 11, (4), 786-794 (1996).
D28. LO, W., CARDWELL, D.A., DUNG, S-L., BARTER. R.G. Preparation and properties of spray dried precursor powder for melt processed bulk YBCO ceramics. Journal of Materials Research, 11, (1), 39-49 (1996).
D29. McKELVEY, A.L., CEBON, D., COLLOP, A.D., ASHBY, M.F. An investigation of fracture of bituminous composites. Cambridge University Engineering Department Report, CUED/C-MATS/TR.228 (May 1996).
D30. MYHR, O.R., SHERCLIFF, H.R., FURU, T. The effect of hot deformation on the ageing response and notched strength of aluminium alloy AA6082. Cambridge University Engineering Department Report, CUED/C-MATS/TR.230 (March 1996).
D31. MYHR, O.R., SHERCLIFF, H.R., FURU, T. The influence of hot working and ageing on notched strength and ductility of Al alloy AA6082. Proceedings, 5th International Conference on Aluminium Alloys, Grenoble, France, 2, 1221-1226. Materials Science Forum, 217-222 (July 1996) (Transtech Publications, Zurich, 1996).
D32. POOLE, W.J., ASHBY, M.F., FLECK, N.A. Micro-hardness of annealed and work-hardened copper polycrystals. Scripta Materialia, 34, (4), 559-564 (1996).