PhD projects 2009 cohort
- Novel bio-inspired materials and structures for future aerospace concepts
- Time-dependent bistable morphing structures
- Optimisation of wind turbine blade structural topology
- Self-healing composites via metal triflate catalytic curing agents
- Modelling textile deformation and its effects in 3D woven composite
- Mechanisms of defect formation in carbon fibre composites
- High temperature composite materials and structures
- An investigation into the fatigue damage development in open- and bolted-hole composite laminates
- Vibration isolation utilising the nonlinear deformation of anisotropic plates
- Plasma treatment of carbon nanotubes and carbon fibre for use in composite materials
Novel bio-inspired materials and structures for future aerospace concepts
Student: Desislava Bacheva
Supervisors: Richard Trask, Katharine Robson Brown, Stuart Alexander (Airbus), Chris Lynas (Airbus) and Norman Wijker (Airbus)
Biological composites have long been recognized as potential sources for fostering new ideas. The aim of the current research is to investigate the potential of damage tolerant lattice structures, inspired by the hierarchical skeleton of hexactinellid Euplectella aspergillum.
This PhD study investigates the structural efficiency of the skeletal lattice of E. aspergillum by studying the function-property relations and evaluating factors such as adaption to specific environment and loading conditions, evolution and age. It also concentrates on characterisation of the skeletal lattice and quantification of its mechanical properties taking into account the interactions between the different levels of hierarchy. The aim is to translate this methodology to develop advanced composite materials and structures for the commercial aerospace environment, employing micro-computed tomography, FE modelling and rapid prototyping techniques.
Time-dependent bistable morphing structures
Student: Alex Brinkmeyer
Supervisors: Paul Weaver, Alberto Pirrera, Matthew Santer (Imperial College London) and Paul Curtis (dstl)
This project aims to develop a new concept of morphing structures using composite materials. In conventional bistable structures, actuation is required both ways to deform the structure from one state to the other. In this concept, material viscoelasticity is intentionally used to create a pseudo-bistable structure. When the load is removed, the structure is able to remain in its second stable state for a determined period of time, before quickly snapping back to its original configuration. The objectives are first to demonstrate the existence of pseudo-bistability in simple composite structures and to devise simple design rules to obtain this behaviour. The second main objective is to develop an aerospace-based application which could benefit from this concept.
Optimisation of wind turbine blade structural topology
Student: Neil Buckney
Supervisors: Paul Weaver, Giuliano Allegri, Alberto Pirrera and Tomas Vronsky (Vestas)
Wind turbines become more cost effective as they grow larger; however the blade mass increases at a greater rate than the power. For a continued size increase, reducing the mass of the blades is necessary. Additionally, lighter blades lower overall turbine costs because the loads on the rest of the structure are decreased. Therefore, the use of lightweight blades can have a significant impact on the cost of wind energy. To achieve blade mass reductions, an alternative structural layout is generated using topology optimisation. The result is a topology which varies along the blade length, transitioning from a structure with trailing edge reinforcement to one with offset spar caps. An alternative beam topology optimisation method is developed that enabled a buckling constraint to be applied. The structural efficiency of the topologically optimised blade is then assessed using shape factors and performance indices, measures which have been expanded to account for asymmetric bending of beams with multiple materials. The utility of shape factors is first demonstrated on six example beam sections before being applied to the blade. To demonstrate application to a more refined design, the performance of a 100m wind turbine blade is assessed, using maps to visualise the structural efficiency. The effect of using carbon fibre and offsetting the spar caps is evaluated, providing a greater understanding of the improved designs. Overall, the results show that wind turbine blades can be improved with structural layouts that take advantage of favourable bend-bend coupling between the out-of-plane and in-plane directions. Because traditional design concepts do not account for bending coupling, a missed opportunity for further mass reduction exists. To this day, the structural topology of the blades has remained fixed despite increasing length and changing loads. Topology optimisation and structural efficiency analysis are shown as methods used to challenge this design convention and reduce blade mass, thereby lowering the cost of wind energy.
Neil was awarded a Faculty of Engineering Commendation for his thesis.
Self-healing composites via metal triflate catalytic curing agents
Student: Timothy Coope
Supervisors: Ian Bond, Richard Trask, Duncan Wass and Stewart Brewer (dstl)
This research focuses on the development and implementation of novel self-healing chemistries for fibre reinforced polymer (FRP) composite materials that are cost effective, compatible with the host epoxy-based matrix material, stable to environmental conditions (i.e. air and moisture) and achieve high healing efficiencies after the initial fracture event.
The initial ‘proof of concept' study involved identifying a suitable catalytic curing agent that was capable of initiating the ring-opening polymerisation (ROP) of epoxides, to restore the fracture toughness of the host epoxy matrix after a fracture event had occurred. The solid-state Lewis acid catalytic curing agent, scandium(III) triflate (Sc(OTf)3), was selected as the healing agent and the healing efficiency was quantitatively evaluated using a tapered double cantilever beam (TDCB) test specimen geometry. To provide an efficient delivery method for the epoxide constituent, microcapsules containing diglycidyl ether bisphenol A (DGEBA) epoxy resin and a non-toxic solvent, ethyl phenylacetate (EPA), were synthesised via an in situ urea-formaldehyde microencapsulation procedure. Autonomous (AUTO) test specimens containing embedded Sc(OTf)3 particles and DGEBA-EPA microcapsules, resulted in >80% recovery of the host matrix fracture toughness value. Heating at moderately raised temperatures accelerated the ring-opening polymerisation (ROP) of the epoxy resin.
Due to the inherent volume limitations of a microcapsule-based system, a bio-inspired microvascular geometry for self-healing agent delivery was implemented into glass and carbon FRP composites, to facilitate the repair of large internal damage volumes. Thermal cure analysis and mechanical characterisation of the metal triflate-initiated self-healing polymers were also investigated to optimise cure conditions at low temperature and to investigate the influence of reagent loadings on the mechanical properties.
Suitable self-healing agents were externally injected into a double cantilever beam (DCB) test specimen geometry via 0.5 mm microvascule channels located on the composite laminate mid-plane, where the healing agent was left to autonomously wet out the exposed fractured crack planes. Initially, pre-mixed and non-mixed self-healing agents were evaluated in glass FRP (GFRP) DCB test specimens and left to cure at moderately raised temperatures, to provide healing efficiency values of >99% for fracture toughness and load, at the point of crack initiation. To assess the transfer of this technology to carbon FRP composites, DCB test specimens were manufactured using out of autoclave (OOA) techniques that contained embedded Sc(OTf)3 catalyst on the composite laminate mid-plane. The catalyst was subsequently exposed during testing via the propagating crack. Fractured specimens were injected with an epoxy-EPA solution to initiate healing and provide a 60% restoration of the initial fracture toughness value after 24 hours at 80°C. This research, therefore, demonstrates the tailorability of the underpinning chemistry of this self-healing system, across a variety of delivery systems, in epoxy-based polymer composite materials.
Modelling textile deformation and its effects in 3D woven composites
Student: Steven Green
Supervisors: Stephen Hallett and Dmitry Ivanov
The use of composite materials is growing rapidly in a wide range of applications, most notably in the aerospace sector. There are however still drawbacks and barriers to their usage, in particular low through-thickness strength, high cost of production and lack of design tools. 3D woven composites offer solutions to the first two of these by introducing though-thickness fibres during the weaving process. This results in increased damage resistance due to the additional through-thickness reinforcement and also the ability to produce near net shaped preforms directly from the loom, significantly reducing part count and layup time. The increased complexity in fibre architecture however means that current design tools, e.g. numerical models for stiffness and strength prediction are no longer applicable. This project aims to develop new tools for the prediction of realistic fibre architectures in complex preforms. The output from this work will be used to assess the influence of internal tow architecture deformations on the numerical predictions for mechanical properties, a crucial aspect in the design of composite components.
Mechanisms of defect formation in carbon fibre composites
Student: James Lightfoot
Supervisors: Kevin Potter, Michael Wisnom and Gordon Kelly (GE Aviation)
Laminate compression failure is a critical driver in the design of composite propeller blades. It is known that the presence of tow wrinkles or fibre waviness within a laminate can considerably reduce the compression performance. While there is a good theoretical understanding of the influence of a wrinkle or fibre waviness on the compression strength, the mechanisms responsible for creating the fibre misalignment are not well understood. While the method development for structural analysis and manufacturing methods have seen significant growth during the last 10 years, the gap still exists linking the manufacturing techniques variation and defect generation within the resulting parts.
Growth in the composite market will depend upon the ability of manufacturing companies to efficiently manufacture composite parts. This research is key to improve the understanding of manufacturing parameters and achieving a goal of increasing the produceability of composite structures. The aim of this project is to investigate the effect of key material and processing variables on the generation of fibre misalignment. The work is aimed at textile composites, which can be manufactured using out of autoclave techniques. The project initially considered the mechanisms responsible for creating the fibre misalignment, followed by development of methodology to analyse the key parameters involved. Theory will then be validated based on controlled experimental technique, and will finally be related the factors to full scale parts.
High temperature composite materials and structures
Student: Joseph Mills-Brown
Supervisors: Kevin Potter and Paul Weaver
Currently, aerodynamic surfaces in motorsport and aerospace applications can experience exhaust gas impingement giving rise to temperatures of up to and in excess of 1000°C. This is not a phenomenon exclusive to aerodynamic surfaces, as the compaction of components and systems closer to hest sources and the associated performance gains, highlights similarly high temperature regions in which thermally resistant material systems could be exploited. Furthermore, such high temperatures demonstrate the large amounts of wasted heat energy which could potentially be usefully recovered.
There is, therefore, a primary need to develop high temperature capable material and structural solutions where thermal protection can be expanded and combined within a structural component, potentially offering weight saving advantages as well as design flexibility. Current solutions employed are excessively costly, offer poor durability and propose problems with manufacturing. The high temperature environment also offers scope to recover and make use of currently wasted heat energy.
Two general concepts are currently employed in high temperature material systems; low density with poor structural performance, high density with good structural performance. This study will investigate a divergence from such approaches by seeking to exploit innovative composite materials and structures to create 'hot' structures and systems with low density and good structural performance. The study also offers scope to expand on these novel concepts by seeking to incorporate 'smart' materials and structures to capture the heat energy from the deployed thermal protection systems and make use of this energy for improvements in mechanical performance.
An investigation into the fatigue damage development in open- and bolted-hole composite laminates
Student: Oliver Nixon-Pearson
Supervisors: Stephen Hallett and Michael Wisnom
Notched strength is an important design criterion for composite components since it greatly reduces the load carrying capacity from that of pristine laminates. Much of the assessment of notched strength in industrial applications is done by empirical methods, which requires significant experimental testing. Such methods are dependent on the specific layup tested so require re-calibration to new experimental data for any changes to the laminate configuration. Significant progress has been made in the understanding of the driving mechanisms for quasi-static open hole tension failure. In order to advance such methodologies further, they need to become more robust and capable of predicting more advanced cases, representative of those seen in composite component design. The first important case to consider is that of fatigue loading.
Specimens were fatigue loaded at 5Hz with various amplitudes to 1E6 cycles or catastrophic failure, which ever occurred first. A number of tests were interrupted at various points as the stiffness dropped with increasing cycles in order to accurately determine a sequence of damage events leading to failure.
The interrupted test specimens were evaluated using X-ray Computed Tomography (CT) scanning in order to determine the level of damage in each specimen. CT reconstructions are carried out on each sample after scanning. This enables visualisation of a given sample as a 3D map in which features of interest such as delaminations and matrix cracking could be identified.
Design methods for composite components are moving towards damage tolerant design. There is thus a need to be able to take account of progressive damage growth which might occur over the service life of a component. Work done in this project will address this need by investigating the damage mechanisms occurring and how this is influenced by choice of layup, laminate thickness, laminate width, hole diameter, and gauge length.
Finite element modelling techniques which take account of fatigue degradation in cohesive interface elements will be applied to the experimental results in order to develop and validate a predictive capability. The second important case to consider is that of bolted holes. The use of fasteners is an inevitable necessity for the joining of composite components. Whilst there is much research into fully bonded joints, barriers remain for the adoption for aerospace structural applications due to difficulties in ensuring the integrity of the bonded joint.
Vibration isolation utilising the nonlinear deformation of anisotropic plates
Student: Alexander Shaw
Supervisors: Simon Neild and Paul Weaver
Previous work into bistable composites has focused on their potential exploitation in morphing structures. However, this work exploits a bistable composite plate held near its unstable equilibrium position, in the field of vibration isolation. The resulting negative stiffness region is used to tailor the nonlinear force/displacement response of a mount, so that it has High Static but Low Dynamic Stiffness, giving vibration isolation whilst maintaining a load carrying capacity. The project will investigate the design of such a mount, and also produce new insights into the static and dynamic response of composite plates.
Plasma treatment of carbon nanotubes and carbon fibre for use in composite materials
Student: John Williams
Supervisors: Sameer Rahatekar, Hua-Xin Peng and Ian Walters (Haydale)
The performance of components, structures and vehicles are always in some way limited by the materials they are made from, often leading to compromises. This drives development to invent and discover new materials or processes to improve upon the current state of the art. In this respect the research contained in this thesis has investigated the use of plasma treatments as a method for modifying the properties of carbon nanotubes and carbon fibres to improve upon current composite materials technology.
The initial study focused on functionalising relatively large quantities of carbon nanotubes using a unique and scalable technique suitable for industrialisation. The research began with a standard oxygen and ammonia treatment, which led to the development of an oxygen plasma treatment for increased carboxyl functionality. The important discoveries were that processing time and gas pressure had a large impact upon the agglomerate size, bulk density, surface energy, solvent stability and the quantity of carboxyl functionalisation. The treated carbon nanotubes developed in the initial study were investigated for their use within an epoxy system. The treated carbon nanotubes were shown to disperse better, reduced resin viscosity, increase resistivity, but had little effect upon mechanical properties, degree of cure or glass transition temperature compared to the untreated carbon nanotubes.
The development of the oxygen treated carbon nanotubes led the research to investigate if these treated carbon nanotubes could be used to improve the fracture toughness of a pre-preg system. The results showed that it was possible to improve initiation and propagation mode I fracture toughness significantly. Mode II results also showed increased initiation but relatively unchanged propagation toughness at lower areal densities, however in both mode I and II at the highest carbon nanotube coating density the fracture propagation resistance was reduced.
The final study looked into the use of plasma treating carbon fibres as a method to modify the fibre matrix interface. The interface strength was found to improve for short oxygen and ammonia treatments, while reduce for the tetrafluoromethane treatment on the unsized fibres. However for the commercially sized fibres each plasma treatment appeared to damage the propriety treatment in terms of interface strength. A further investigation into attaching carbon nanotubes to carbon fibres showed a dramatically reduced interface strength. The research demonstrates a variety of methods which could be used to tailor the interface for improved strength or damage tolerance.