University home > Advanced Composites Centre for Innovation and Science > Research > Multifunctional composites and novel microstructures
Advanced composites provide unique opportunities to create structural materials with added functionality e.g. for sensing, or self-repair, and new material architectures incorporating novel fibres and nanomaterials. Select from the options above for an overview of work undertaken in ACCIS under this theme.
The properties of composite materials can be tailored through microstructural design at different length scales such as the micro- and nano-structural level. At the micro-structural level, our novel approach creates microstructures with controlled inhomogeneous reinforcement distributions. These microstructures effectively contain more than one structural hierarchy. This has the potential to create whole new classes of composite materials with superior single properties and property combinations. Research also involves tailoring the nano-structures of micro-wires/ribbons for macro-composites. A network distribution of TiB whisker within Ti matrix is shown right.
Research has shown that shaped fibres can be an effective means of improving the through thickness properties. A set of guidelines for fibre shape and a preferred ‘family’ of fibres have been generated from qualitative analysis for the role of reinforcing fibres in composites. Methods have also been developed to produce such shaped fibres from glass in order to form reinforced laminates in sufficient quantity for materials property testing using standard methods. Fibre shape has been shown to play a key role in contributing to the bonding force between fibre and matrix, with significant increases in fracture toughness possible. Our results suggest that the shaped fibre specimens have a greater throughthickness strength than the circular fibre composites that are currently used.
Impact damage to composite structures can result in a drastic reduction in mechanical properties. We are adopting a bio-inspired approach to effect selfhealing which can be described as mechanical, thermal or chemically induced damage that is autonomically repaired by materials already contained within the structure. We are utilising our ability to manufacture and incorporate multifunctional hollow fibres to generate healing and vascular networks within both composite laminates and sandwich structures. The release of repair agent from these embedded storage reservoirs mimics the bleeding mechanism in biological organisms. Once cured, the healing resin provides crack arrest and recovery of mechanical integrity. It is also possible to introduce UV fluorescent dye into the resin, which will illuminate any damage/healing events that the structure has undergone, thereby simplifying the inspection process for subsequent permanent repair.
The material most commonly used in the construction of dentures is poly (methyl methacrylate) and although few would dispute that satisfactory aesthetics can be achieved with this material, in terms of mechanical properties it is still far from ideal. The fracture of acrylic resin dentures remains an unresolved problem and may result from impact failure, for example dropping the denture accidentally, or from fatigue failure due to repeated flexing under masticatory forces. Over the years there have been various attempts to improve the mechanical properties of the resin including the search for an alternative material, such as nylon, the chemical modification of the resin through the incorporation of butadiene styrene as in the "high impact resins" and the incorporation of fibres such as carbon, glass and polyethylene. The use of self-healing technology within dental resins is a novel and exciting approach to solve the problems of the failing dental resins. Methods are currently being developed to translate the self healing resin technology into dental and biomaterials science.
With recent advances in nanotechnology, various nano-scaled materials are becoming increasingly available. Motivated by their unique properties, researches into the development of nanomaterials and their composites (so-called nanocomposites) with specific engineering applications are being actively pursued, these include: Carbon nanotube fibres and their nanocomposites (Dr. Rahatekar); Nanocomposites Coatings for CFRP Composites (Dr. H-X Peng); Nanofibres and nanocomposites for biomedical applications (Drs. Rahatekar and Su); Modelling and design of nanoinclusions (Prof. Scarpa); Clay nanocomposites (Dr. van Duijneveldt); Dispersion and macro-scale properties of nanocomposites (Prof. Geoff Allen).
The main aim of this work is to examine methods ofincluding magnetic materials within a composite whilst maintaining structural performance. This has been achieved by filling hollow fibres with a suspension of magnetic materials after manufacture of the composite component. Research is continuing to tailor the magnetic properties of the composite to other applications. In another approach, magnetic microribbons and microwires are being tailored and embedded into macrocomposite materials to provide magnetic sensing functions.
Auxetic solids expand in all directions when pulled in only one, therefore exhibiting a negative Poisson’s ratio. We are developing new concepts for composite materials, foams and elastomers with auxetic characteristics for aerospace, maritime and ergonomics applications. The use of smart material technologies and negative Poisson’s ratio solids has also led to the development of smart auxetics for active sound management, vibroacoustics and structural health monitoring.
Mechanical and electromagnetic design Increasing importance is currently placed on developing cellular structures and honeycombs to design microwave absorbers for electromagnetic compatibility applications. Usually the electromagnetic design of the materials is kept separate from the mechanical one. We develop, model and prototype honeycomb topologies for sandwich panels, Salisbury screens and general microwave absorbers in a multidomain optimisation framework. Design and modelling is performed using analytical, finite element and finite difference time domain simulations using optimisation methods, like differential evolution. We also design honeycomb topologies with embedded sensing and actuation to provide cores for novel sandwich panels, structural health monitoring and active microwave applications.
Researchers in the CVD Diamond Film Lab based in the School of Chemistry are investigating ways to make diamond fibre reinforced composites. The diamond fibres are made by coating thin (100 mm diameter) tungsten wires with a uniform coating of polycrystalline diamond using hot filament chemical vapour deposition. The diamond-coated wires are extremely stiff and rigid, and can be embedded into a matrix material (such as a metal or plastic) to make a stiff but lightweight composite material with anisotropic properties. Such materials may have applications in the aerospace industry.
Further information: Bristol University CVD diamond group
New types of composites with a combination of strength, toughness and functionality are being prepared by combining research in the self-assembly of inorganic nano-particles with the synthesis of organic polymers. This interdisciplinary approach, which is based on bio-inspired strategies, has been used to produce flexible fibres of magnetic spider silk as shown in the photograph (right). Silk fibres are coated by a dipping procedure using dilute suspensions of magnetic iron oxide nano-particles that are prepared with specific surface properties. Similar methods are being investigated with swellable polymer gels and bacterial supercellular fibres to produce novel hybrid composites.
Although non-covalent interactions have been widely used in supramolecular chemistry, ionic interactions have largely been neglected. Research in the Faul group therefore focuses on the use of ionic interactions for the production of highly organised nanostructured materials. More specifically, the process of Ionic Self-Assembly (ISA), i.e., the assembly of charged amphiphilic molecules and oppositely charged oligoelectrolytic building blocks (or tectons), is investigated. Owing to the wide range of charged tectons available, ISA presents a facile way to produce a wide variety of functional (conductive) and switchable (photo-switchable) crystalline or liquid-crystalline (LC) materials.
Progress in new types of materials will fuel the inventive revolution in the next 20 years. Technology is driven not only by applications but advances in technology itself which opens new possibilities for application. It is important to step away from the mindset which separates actuators, sensors, controllers and power generators. It would be better to consider a continuum where at one end these elements are quite separate (the current state of the art) and at the other extreme one can imagine the fusion of these capabilities embodied in a new ‘composite’smart material (the goal). Much of today’s engineering employs the use of rigid structures. However, the advent of smart materials (such as artificial muscles, undulating swimming fins and smart surfaces) which are soft or semi-rigid offers the prospect for a step-change in the way we use and make the material world around us. New smart materials will insinuate themselves into every aspect of technology as is the case of electronics today. At the Bristol Robotics Laboratory (BRL) we want to exploit such novel ideas in a new generation of robots and although the technology is in its infancy we are keen to build devices which are pliant and will require different approaches to their control; robots can be much more than 19th century mechanical engineering with 21st century electronics ‘stuck on the top’.
Further information: Bristol Robotics Laboratory
Soil materials are widely used to build geotechnical systems such as railway and highway embankments, road and rail subgrades, dams for water resources storage, embankments for flood protection, and foundation for buildings.
Under certain loading conditions, resulting for example from earthquakes and dynamic actions, storms, heavy rainfall or flooding, but also from erosion, soils may become unstable and can flow like a liquid across very large distances, giving rise to catastrophic failures. In an era in which climate change is taking place (dramatic changes in precipitation levels, temperature and sea-level rise), the vulnerability of these structures to such natural hazards is anticipated to be even higher.
One way to improve their behaviour is to reinforce the soil over the directions of weakness by using tensile resisting elements like short, flexible fibres, randomly distributed throughout the soil mass and possibly with a random distribution of orientation. Our recent research conducted at the University of Bristol shows that this technique has a real potential for use in Civil Engineering applications and can provide an effective environmental protection of geotechnical systems. Further research will also consider the development of adequate design tools for predictive capabilities, and validation at real structural scale.
Composite materials have been increasingly used in biomedical applications. Current orthopaedic implants utilising polymers, metals and ceramics are not without problems. Concerns regarding the wear particles of polymers, increased ion release of metals and fracture of ceramics call for new composite materials which combine the advantages of individual components. We have been exploring the fabrication and application of ceramic/metal and ceramic/polymer composites with aligned, graded and co-continuous phase structure for load-bearing orthopaedic and dental prosthesis and implants. Different ceramic fabrication processes, including freeze casting, rapid prototyping, and direct foaming, have been investigated to produce ceramic preforms followed by infiltration of polymers and metals for the fabrication of new composites. A range of non-degradable and degradable ceramics (alumina, zirconia, hydroxyapatite, bioglass), polymers (PEEK, epoxy, PCL) and metals (CoCr, Mg) have been investigated.
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