• Covalent adaptable networks: from dynamic bonds to sustainable
• Vacuum-packed particles as a switchable stiffness mechanism for morphing
• Active thermal management in FRP composites via embedded vascular networks
• Stacking-sequence design of composite laminates with many plies
• Optical control of electrostatic actuators using amorphous silicon
• Novel tape termination for delamination suppression
• Embedded actuation of reconfigurable origami
• The combined influence of through-thickness compression and shear on the longitudinal fibre dominated tensile
• The effects of interleaf architecture on composite toughness
• WrapToR composite truss structures: analysis, manufacturing, characterisation
• Toughening composite laminates using hierarchical electrospun fibrous veils
• Autonomous and objective characterisation of composite precursors in manufacturing
• Pulsed-DC plasma-assisted CVD: construction, plasma analysis, diamond growth, and possible alternative in nanocomposite
• Optimal aeroelastic tailoring of wind turbines
• Composite compliant shell mechanisms: tailoring and characterisation
• Physical, mechanical and rheological behaviour of prepregs

Covalent adaptable networks: from dynamic bonds to sustainable

Student: Callum Branfoot
Supervisors: Ian Bond, Paul Pringle, Duncan Wass (Cardiff), and Tim Coope (NCC)

Covalent adaptable networks (CANs) are polymers that exploit dynamic bonds (crosslinks) to reversibly transition between thermoset and thermoplastic materials. There are two main categories of CAN, dissociative and associative, which behave subtly differently. The crosslinks of dissociative systems—epitomized by Diels-Alder–materials—break upon stimulus (e.g. heating) to form a melted state akin to that observed with thermoplastics. In contrast, associative CANs do not go through a strict transition of ‘depolymerisation’, but rather maintain a constant crosslink density as bonds are broken and formed in a concerted manner. In recent years, considerable attention has been given to CANs, which have excellent potential in terms of reformability, recyclability and repair. These features derive from the large increase in chain mobility observed when the crosslinks are activated/broken; chain mobility is otherwise greatly restricted in crosslinked (thermoset) materials.

Fibre-reinforced polymer composites (FRPs), exhibit a range of useful material properties—notably specific strength and stiffness—whilst affording a rich design flexibility. Together, these characteristics make FRPs attractive materials for numerous structural applications. However, despite their widespread deployment, FRPs are still hindered by drawbacks that present outstanding challenges. Among these are challenges concerning the polymer matrix: toughness, repair and recyclability. At present, although there are several methods to toughen FRPs, including Z-pinning and the inclusion of thermoplastic phases, they are generally much less tough than competing metallic materials because they lack efficient energy absorption mechanisms. This brittleness means that FRPs are easily damaged, with even low energy impacts often leading to matrix cracking and delamination. This problem is compounded by the difficulties associated with the repair of FRPs; to repair a crack requires the contact of the opposing crack faces and this is almost impossible to achieve without a significant amount of polymer chain mobility (which is not usually accessible in common FRPs).

The use of CANs in FRPs could help to achieve technological outcomes such as:

• Self-healing composites: recovery of material properties after a damage event
• Reformable thermoset composites and 3D-printable thermoset composites
• Recyclable thermoset composites: reinforcement/matrix separation, multi-material disassembly and reuse
• Toughened composites: reversible crosslinks to provide a mechanism of mechanical energy absorption
• Tailorable mechanical properties: localised variation in degree of crosslinking

In theory, CAN/FRP composites could be applied as a substitute material for any structural part which is subject to moderately high loading conditions and is not subject to high temperatures during operation. These materials would be particularly attractive for parts which are at high risk of localised impact damage, with this technology presenting an opportunity for repeated repair. Otherwise, these materials could find employment in any application meeting the aforementioned criteria (moderate temperature and loading) and necessitating end-of-life recycling of the material. This is because the thermoset-thermoplastic transition could facilitate efficient matrix-reinforcement separation, which is not generally possible with conventional thermoset composites. Consequently, these features may make these materials particularly useful in the automotive, marine and sports sectors.

The aims of this research are:

• To investigate the potential application of current (Diels-Alder) CAN technology as FRP matrices; creating and characterising new composite materials
• To investigate novel crosslink chemistry for potential application in CANs; including main-group inorganic linkages such as S–S, Se–Se and P–P bonds

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Vacuum-packed particles as a switchable stiffness mechanism for morphing

Student: David Brigido
Supervisors: Ben Woods and Stephen Burrow

Background

One of the most persistent challenges hindering the development of effective morphing aerostructures is the need to have material/structural solutions which provide a viable compromise to the directly competing design drivers of low actuation energy requirements and high resistance to deformation under external loading. This work is proposing an entirely new solution using a novel adaptation of phase transformation mechanism with switchable stiffness. Granular jamming is the mechanism for state change transition, from liquid-like state to solid-like state and vice versa. It is also able of immediately shift among a range of stiffness or variable stiffness. The design and analysis of such a mechanism is related to the stress field application and the physical characteristics of the soil materials. The bulk behaviour of the loose material which is usually built of grains enclosed inside an impermeable and elastic membrane is controlled by vacuum.

Granular jamming has been applied successfully in soft robotic applications for variable stiffness mechanism, most of those applications have been designed using only the bulk behaviour of the granular material. Morphing structures can use jammed systems not only to vary stiffness but also for changing shape, this kind of properties are quite dependent of the flexural properties of the jamming behaviour. The mechanical behaviour of granular jamming materials has been studied using different kind of tests, most of those tests are based in the analyse of the bulk behaviour. Triaxial compression and shear tests are the most common. Unfortunately, the flexural mechanics of granular jammed are not studied yet.

Main Research Challenge/ Question

The aim of this work is to develop a better understanding of the flexural behaviour of the jammed systems from a micro to macro scale. The material characterization will be conducted using four-point bending test to obtain the Flexural rigidity and Bending Stiffness of different type of grains (e.g. shape, size, material, etc). Finally, a novel concept of Morphing structure wing will be designed with the objective to reduce noise and drag resistance due to the discontinuities of the control surface and the wing.

A numerical design scheme will be necessary to correctly implement the aero morphing mechanism. It would be necessary to integrate 3 different types of numerical methods. The morphing mechanics will be designed using Finite Element Analysis. The granular jamming properties will be modelled using Discrete Element Method and the Aerodynamics forces will be modelled using numerical aerodynamic analysis methods, either low, mid or high fidelity as required to sufficiently capture the underlying physics. The integration of these three types of numerical simulations would be a great basis that could lead to implementing an ideal methodology for morphing structures design based on granular jamming.

Objectives

The objectives of this project are split into several primary work streams:

Stream 1: Material characterisation

• Build a 'switchable brick' unit cell of the metamaterial
• Stiffness characterisation of the unit cell
  • 4-point bending tests

Stream 2: Numerical modelling of the structural properties of the granular jamming structures

• Finding a numerical methodology to simulate the flexural mechanics using Discrete element methods
  • Continuum theory
  • Statistical physics
  • Deterministic or stochastic models

Stream 3: Integration of FEA (Finite Element Analysis), aerodynamic modelling and DEM

• Morphing Demonstrator using FEA 
• Granular Jamming using DEM
• Aerodynamic forces using VLM/panel methods/CFD

Stream 4: Morphing wing

• Detailed design
• Manufacturing
• Experimental testing
• Stiffness tailoring
• Wind tunnel tests

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Active thermal management in FRP composites via embedded vascular networks

Student: Jim Cole
Supervisors: Ian Bond, Andrew Lawrie and Adam Bishop (Rolls-Royce)

Fibre-reinforced polymer composites are currently limited in high temperature applications by the glass transition temperature, and thermo-oxidative ageing of the polymer matrix. For example, typical aerospace epoxy resins may not be used at temperatures greater than 150°C - 200°C, whereas metals and ceramics have much higher operating limits. Composite materials are, however, becoming stronger candidates for higher temperature applications due to their comparatively higher specific mechanical performance, which permits weight savings and efficiency gains.

To address this limitation, this project aims to investigate the application of embedded vascular cooling networks. These networks of narrow conduits can circulate a cooling fluid within the laminate, thereby absorbing heat and maintaining the matrix at an acceptable temperature, retaining mechanical performance and slowing the ageing process.

This project is inherently multi-physical, having aspects of thermodynamics, fluid mechanics, laminate mechanical analysis, manufacturing and optimisation. All of these aspects interact with one another; for example, vascule manufacturing quality will affect mechanical performance due to defect formation, and fluid flow due to flow irregularities, which will in turn influence thermodynamic performance. These interactions present a complex optimisation problem, with many trade-offs to be made. A trade-off of primary interest in this project is that of thermal and mechanical performance; the structural influence of displacing load-bearing material and disturbing local laminate architecture by embedding vascules must be better understood.

This project has several novel aspects when compared to existing understanding. Firstly, air is being investigated as the cooling fluid, despite its modest performance in this role compared to higher density liquids. Air offers several advantages over liquids; it is abundantly available (in aerospace), does not interact with the polymer matrix, and can potentially be safely exhausted into the surrounding atmosphere, rather than recirculated, without affecting the wider system. Secondly, emphasis is placed on applications with flow over the external surface. This is typical of external skins of moving vehicles, and components of systems with internal gas flows. Lastly, it seeks to approach both the thermal and mechanical performance aspects of the problem simultaneously. A successful design must retain the required mechanical performance at the desired operating temperature, so the network must yield maximal thermal performance and a minimal impact on mechanical performance. Addressing these aspects in isolation may fail to capture any subtle interactions between them.

A key outcome, which is currently under development, is a numerical tool for analysing and predicting the thermal and mechanical response of a laminate containing a given embedded network design. This model will be continually updated and validated by experimentation throughout the project, improving its predictive capabilities in both physical regimes. It is hoped that this tool will propose optimal laminate and network designs based on a set of thermal and mechanical requirements. Such a tool will be a fundamental part of any framework for the design and optimisation of composite components containing vascular cooling networks.

It is envisaged that this active thermal management technology could extend the operating envelope of traditional, low-cost resins, such as epoxy, to temperatures previously only attainable with more exotic resin systems such as polyimide, reducing cost and manufacturing complexity. It could also provide an additional margin of performance for those exotic high temperature resins, further expanding the range of application of fibre reinforced polymer composite materials. Such applications include turbomachinery, space vehicles, battery containment cells and hypersonic aircraft.

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Stacking-sequence design of composite laminates with many plies

Student: Noémie Fedon
Supervisors: Terence Macquart, Paul Weaver and Alberto Pirrera

Composite laminates with stiff fibres have high strength-to-weight and stiffness-to-weight properties and hence are widely used in industries where minimising the mass of structures is of paramount importance. However, to this day, optimising structures made of composite laminates remains challenging. This is due to the high number of possible design solutions, especially for structures divided into many zones, i.e. panels, and where panels have many plies. Limited computational resources restrict the number of design solutions that can be evaluated, especially when laminate structural responses are assessed by costly numerical analyses. Therefore, laminate optimisation methods that do not require a prohibitive number of structural analyses are preferred. This is the case for multi-step laminate optimisation methods in which the first steps optimise laminate thicknesses and homogenised stiffness properties (e.g. Lamination Parameters (LPs)), and a final step retrieves laminate lay-ups, with the objective to match target thicknesses and homogenised stiffness properties. Because laminate fibre orientations are usually limited to a set of discrete values (e.g. 0, ±45 and 90°), the retrieval step must solve a combinatorial problem that is intractable for large-scale structures. Current lay-up retrieval methods fail to sufficiently explore the laminate design space within reasonable computational times and therefore yield sub-optimal solutions, e.g. heavier than necessary. Designing laminates is made even more challenging when aerospace lay-up and ply-drop design guidelines are applied to prevent undesirable stiffness couplings and promote strength.

In this thesis, four algorithms are proposed to solve three of the key challenges in laminate design: the enforcement of design guidelines, efficient design-space exploration, and scalability of the design method.

• RELAY and RELAY -MP are novel deterministic repair algorithms for single- and multi-panel laminate structures. In order to maximise design-guideline satisfaction, RELAY modifies laminate ply angles while RELAY-MP modifies both ply angles and ply-drop positions. The repair algorithms also minimise the changes in stiffness between initial and repaired laminates. Both repair algorithms are shown to be robust, and RELAY is fast, even for laminates with more than 100 plies.
• LAYLA is a new optimisation algorithm that retrieves the lay-ups of single-panel laminate structures best matching LP targets. The main novelty of the proposed algorithm is the use of beam searches and novel heuristic cost functions to design laminates. Employing beam searches results in a fast and scalable algorithm. The heuristic cost functions ensure guidance in the exploration of the laminate design-space such that the difference between the LPs of optimised laminates and their targets is either optimal or near-optimal. An iterative redesign of the lay-ups using ply-group beam searches also improves the convergence of the method. LAYLA enforces the feasibility of design solutions using the RELAY repair method. LAYLA has been shown to optimise laminate lay-ups with up to 300 plies within seconds or minutes and to have superior efficiency in matching LP targets compared to previous state-of-the-art algorithms.
• BELLA is a new optimisation algorithm that retrieves multi-panel laminate structures which must best match LP targets. The laminate-design method is divided into several steps to simplify the optimisation problem and allow for the use of beam searches, the elements of success for design-space exploration in LAYLA. BELLA starts determining the ply-drop distribution and then optimises ply angles. Finally, the best design solution is repaired employing the RELAY-MP repair method. BELLA efficiently returns feasible design solutions to large problems. However, BELLA is outperformed by an evolutionary algorithm at design-space exploration. This other algorithm is demonstrated to have an excellent performance, even for designing structures with many panels and plies.

The performance of the proposed algorithms has been assessed by novel test cases that can be used in the future to benchmark laminate design and repair methods. Moreover, the algorithms are open-source and their codes are available to download from GitHub repositories on the web page https://github.com/noemiefedon.

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Optical control of electrostatic actuators using amorphous silicon

Student: Calum Gillespie
Supervisors: Andrew Conn, Jonathan Rossiter, Fabrizio Scarpa and Asier Marzo

Soft artificial muscles (SAMs) are an emerging subset of smart materials that exhibit high percentage strains very similar to biological systems. This allows for novel designs of actuators with multiple degrees of freedom. This project aims to design a photo-responsive SAM system by implementation of novel nano-composite materials. The focus of this project is to achieve high resolution response and high-speed switching using local photonic stimulation, allowing for this technology to be implemented across a number of fields including morphing skins in wing designs as well as haptic displays and visual technologies.

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Novel tape termination for delamination suppression

Student: Tharan Gordon
Supervisors: BC Eric Kim, Stephen Hallett and Michael Wisnom

With an ever-increasing drive towards rapid production of more efficient structures, composites manufacture is progressively being automated. The automated fibre placement (AFP) technique is a process that has reached maturity for aerospace components, owing to its ability to rapidly and reliably laminate complex composite structures. However, there remain problems with the introduction of critical defects during the AFP process near to ply drop-off regions. These defects, often referred to as resin pockets, can be critical in nature as they become crack initiation zones upon loading. Conventionally such sites have been accepted as an artefact of manufacture, with much work undertaken to accurately predict and account for their detrimental effect in the design stage. This is at great expense both computationally and in efficiency.

The aim of the proposed research is to study the feasibility of a novel tow termination method. The project will focus on developing a tow scarfing mechanism that automatically tapers the tow ends during the AFP lay-up process, analysis of its potential effect on composite structural performance, and experimental validation.

Successful implementation will allow for the avoidance of complex post-manufacture analysis and offer improved structural performance of AFP produced laminates with thickness variations. Furthermore, the proposed method will confer the ability to control the geometry of the tow ends. This novel ability opens the horizon to a broad range of research in the field of automated composites manufacturing, analysis and optimisation.

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Embedded actuation of reconfigurable origami

Student: Steven Grey
Supervisors: Mark Schenk and Fabrizio Scarpa

Origami inspired structures, which began as a method of packaging solar arrays on spacecraft, provide a method to achieve complex three-dimensional geometries from flat sheets. These could be used as novel manufacturing techniques, allowing simple manufacturing in a sheet form before self-assembly. Alternatively, origami could be the philosophy behind the design of a deployable structure, folding structures can be designed to have a negative Poisson’s ratio and have a single degree of freedom. This means a large area can be packed into a small space and deployed easily. Furthermore, origami structures can change their gaussian curvature, forming saddles or even spheres from flat sheets, making them ideal for morphing structures.

To design origami structures first they must be understood. Early work in this area focussed on the geometry and kinematics of origami, where the patterns are modelled as rigid panels connected by frictionless hinges. Structures modelled in this way can have a single degree of freedom, meaning that an actuation anywhere will cause a homogenous response across the structure. Adding a torsional stiffness along the fold lines turned origami from mechanisms into structures, with homogenised elastic moduli. An important development has been to incorporate bending of the material between the fold lines. This means that additional degrees of freedom, arising from the bending of the facets, can be captured. The consequence of this on design is that a single actuator cannot be used to deform an entire structure, as the deformation from any single actuator will decay away from the source, instead actuators must be distributed. Contrariwise, this does allow for the possibility of distributed actuators being used to morph a two-dimensional sheet into a complex three-dimensional object.

The aim of this project is to actively control the configuration of a folded structure using embedded actuators. To achieve this, an accurate model of the ‘elastic decay’, describing the distance over which an actuation extends through the folded structure, could be used as a design tool for developing actively controlled origami structures. This decay will be modelled using both analytical and numerical models with experimental results providing a validation of the accuracy of these tools. To be generally applicable to a wide variety of engineering systems it is important that the behaviour of actuation on a range of length scales is explored. Finally, the understanding gained from these models could be used to construct a prototype folded structure with embedded actuation.

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The combined influence of through-thickness compression and shear on the longitudinal fibre dominated tensile

Student: Kilian Grüebler
Supervisors: Stephen Hallett, Michael Wisnom and Aleixo Gonzalez (Rolls-Royce)

The application of loads to composite components can lead to complex three-dimensional stress states, especially in the through-thickness direction within the material. Classic analysis of composite structures only covers common stress states in two dimensions, where only the in-plane stresses and properties are considered. Laminated composites are mainly reinforced in-plane which leads to weak through-thickness properties, but there has been much less investigation of through-thickness stress states.

Typical load application and transmission situations which lead to complex three-dimensional stress states and significant through-thickness stresses are thick composite components with high contact loading. The critical stress state can be characterised by high through-thickness and interlaminar shear stresses in combination with longitudinal in-plane tensile stresses.

In previous research, bespoke test methods were developed for testing the material properties of carbon fibre reinforced polymer (CFRP) composites in the presence of high through-thickness compression. It was shown that the interlaminar shear strength of CFRP rises with increased through-thickness compression stress. It was also shown that the longitudinal tensile strength of CFRP decreases significantly with increased through thickness compression stress. However, the interaction and influence of combined through-thickness and interlaminar shear stress on the longitudinal tensile strength was not investigated.

The aim of the project is to investigate the influence of combined through-thickness direct stresses and interlaminar shear stress on the longitudinal fibre tensile strength. The objectives are the development of a bespoke test method to generate and measure the complex stress state for different load levels. Furthermore, a validated modelling tool and failure criteria will be developed.

With such a novel failure criteria composites can be designed more efficiently and reliably. The biggest advantage of a well-developed failure criteria is the increased safety and the mass saving opportunity. These two aspects are of high importance for applications in the aerospace and other industries.

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The effects of interleaf architecture on composite toughness

Student: Robin Hartley
Supervisors: James Kratz,  Ivana Partridge, Ian Hamerton, Alex Baidak (Hexcel) and David Tilbrook (Hexcel)

Composite structures are overdesigned because failure is defined at the onset of damage. Therefore, we either need to improve our ability to predict damage propagation to enable damage tolerant design or increase the toughness of the matrix to increase crack initiation values. The current linear-elastic fracture equations applied to composite materials suggest that incorporating thick resin layers in the interleaf region increases the size of the plastic zone ahead of the crack tip. However the inclusion of particles in this region may change the toughening mechanisms. Materials using this particle toughening strategy are used on the Boeing 787 and Airbus A350. At present, limited understanding exists as to whether the particles act as spacers or increase the length of the crack front by crack pinning, however the particles do increase damage mitigation.

The thermo-mechanical evolution of the particle matrix interface that arises during cure is an unexplored area of critical importance. Adhesion between the particle and matrix will influence the energy required to fracture or pull-out particles in an energy absorbing event. If a thermal and elastic mismatch is developed during manufacture, a residual stress state will change the level of adhesion in the final composite. The processing history of these systems can influence final cured properties and the crack propagation behaviour.

The focus of this project is to improve the understanding of the interleaf region by examining the interaction between the interleaf particles, the surrounding epoxy matrix, and the carbon fibre reinforcement. This understanding of why particles improve the fracture behaviour will enable the next generation of interleaf materials. This project aims to study the material, processing, and performance relationship of the interleaf constituents in a model aerospace matrix system. Rigid particles will be dispersed into a suitable epoxy matrix; this material will have a 180°C cure temperature, a high cured Tg, and a thermoplastic phase-separating phase resulting in a high Vf carbon fibre prepreg. The following objectives might be considered in the project:

1. Investigate the fracture toughness and strain energy release rate of interleaf systems made using particles with different physical characteristics (hardness, modulus, coefficient of thermal expansion, size, loading) and chemical characteristics (e.g. surface treatments).

1. Identify a suitable model matrix-particle system from for the manufacture of neat resin samples. Manufacture particle reinforced model-matrix plates, test samples for fracture toughness, and analyse the fracture surface for damage mechanisms and surface energy.
2. Investigate the fracture mechanisms in fibre reinforced samples using ASTM standard mode I, II, and mixed-mode testing techniques. Consider real-time testing techniques to capture the onset of damage in-situ by testing samples in a micro-CT scanner.
3. Explore small-scale fracture testing to investigate the failure at the particle length-scale. Bending of neat resin or composite samples with particles could, with micro-DIC, capture the strain field and fracture at the particle-resin interface.

 2. Develop a thermo-mechanical model to study the residual stress state between the particle and matrix during processing, and compare the predicted stress state to fracture performance.

1. Obtain the thermo-mechanical properties of the particle, matrix, and fibres from literature or through material characterisation. Models required include cure kinetics, specific heat capacity, coefficient of thermal expansion, thermal conductivity, chemical cure shrinkage, glass transition temperature, gelation, and modulus development.
2. Create a 3D numerical model to predict the interleaf composite stress-state during processing and validate the model simulations. Constant force rheometry with a floating gap could be used using the heated glass peltier plate rheometer with microscope.

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WrapToR composite truss structures: analysis, manufacturing, characterisation

‌‌Student: Chris Hunt
Supervisors: Ben Woods and Michael Wisnom

The wrapped tow reinforced truss is a patented concept that combines the benefits of a truss geometry with the impressive material properties of composite materials to produce highly efficient structural members. While composite trusses are not a new phenomenon, this particular concept uses a novel manufacturing process to produce a unique wound truss configuration. The manufacturing method, which is a modification of the filament winding process, involves wrapping wet fibre tow around rigid longitudinal members forming a truss geometry. Using this manufacturing method greatly reduces labour demands when compared with traditional trusses which require assembling of a large number of parts. The wrapped tow concept therefore offers a low-cost method for producing structural members which could offer significant weight savings in a number of applications.

The Gamera II human powered helicopter project used the truss concept extensively in its design which led to large reductions in weight and the breaking of several FIA world records. After further development the technology is likely to be most beneficial in weight critical applications such as aircraft, spacecraft and wind turbines. However, the offered low-cost of production may result in the trusses also being of use in lower-end applications.

Before the technology can be used in commercial applications further development and understanding is required. To achieve this the PhD will look at four different sections of work: analysis, manufacturing, characterisation and optimisation.

Currently all analysis conducted on the trusses has been low fidelity and has largely concentrated on stiffness properties. For the PhD, higher fidelity analysis techniques will be considered and implemented concentrating on strength. The aim of this analysis is to develop a better understanding of how the trusses distribute load and fail. Any analysis will be verified by mechanical testing which forms the characterisation section of the project. Testing will require the development of a number of test methods to investigate different loading scenarios.

The manufacturing section of the project will be looking at the development of the manufacturing process. This begins with the design and build of an automated winding machine for production of the trusses. The next main aspect of this section will then be focused on scaling. So far trusses produced with this method have been relatively small and being able to produce larger scale trusses will be crucial to their use in a number of applications. Increasing the size is likely to create challenges in the manufacturing process and developing solutions to the challenges will form a part of the work.

The optimisation section will use the developed analysis methods coupled with optimisation tools to design trusses that maximise the weight saving for specific applications. This will result in a useful design tools for future applications of the trusses.

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Toughening composite laminates using hierarchical electrospun fibrous veils

‌‌Student: Konstantina Kanari
Supervisors: Stephen Eichhorn and Michael Wisnom

Sectors like the aerospace and automotive industry use laminated composite materials to achieve lighter and, at the same time, toughened structures. This approach will result in reduced fuel consumption and therefore fewer emissions, hence it is more environmentally friendly. However, one of the biggest issues with the use of composites in load-bearing applications is that laminated composites suffer from delamination and low fracture toughness. Various approaches are currently pursued on the improvement of these properties, with one approach being the use of micro- and nanofibrous interleaves as plies. The interleaves have been shown to improve the toughness of laminated composites, mostly due to their bridging of crack zones. Furthermore, nanoparticles have been added in laminated composites to enhance the stiffness of the material and increase its fracture toughness. The incorporation of nanoparticles leads to the enhancement of local strain to failure within fracture zones due to a toughening mechanism called crazing.

This project aims to improve laminated composites’ toughness by combining all the above mechanisms. Therefore, in this research electrospun/ electrosprayed nanofibrous interleaves are to be used, nanoparticles are to be included in the composite material and different sizes of fibres and particles are to be tested. This research project is inspired by what is seen in nature, and aims to use different nanomaterials in a hierarchical way to form improved materials. Nevertheless, the improved toughness of the composites should be achieved without adding excessive weight to the final product.

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Autonomous and objective characterisation of composite precursors in manufacturing

Student: Anatoly Koptelov
Supervisors: Dmitry Ivanov, Stephen Hallett, Jonathan Belnoue and Ioannis Georgilas (University of Bath)

From a mathematical perspective, composite manufacturing processes present a serious challenge: all of the processes are highly non-linear, occur concurrently on multiple scales, exhibit intrinsic coupling of different mechanisms, and all of the material and processing parameters are subject to large variations. As a result, the description of these processes is particularly difficult. A characteristic example of process coupling is a consolidation problem.

The compressibility of the material is determined by flow characteristic of prepregs. Compaction leads to changes in the fibre volume fraction which causes evolution of the heat distribution and affects the viscosity hence flow characteristics, making this process a closed loop process.

The main question of this research is to understand if we can substitute the current techniques of studying parameters in isolation for a model of an entire system of coupled parameters? Conventional techniques do not offer such capabilities; therefore, this project is focused on the development of different philosophy of modelling. Instead of trying to describe the material in terms of isolated mathematical models assembled into a single complicated mathematical framework, the approach to modelling will be based on a self-developing adaptable system capable of capturing the main characteristics of the physical system through absorbing a large volume of data coming from well-instrumented manufacturing trials.

The main objective is to develop a prototype of an adaptable learning system, departing from one dimensional consolidation and cure problems, and assess the efficiency of such an approach using fully deterministic reference models and simulating the experimental input for the model. Such a system can be applied to in-situ real-time processing of sensor data, noise filtering, and formulating control parameters for fast curing resins. The next step will be to extend the predictions to include the data supplied from actual experiments and formulate a case study showing the efficiency, limitations, and potential of the technique. For instance, based on variety of tests conducted using DSC, predicting degree of cure for an arbitrary thermal history without using a cure kinetics model.

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Pulsed-DC plasma-assisted CVD: construction, plasma analysis, diamond growth, and possible alternative in nanocomposite

Student: Dominic Palubiski
Supervisors: Fabrizio Scarpa, Neil Fox and Tom Scott

Research is presented for expanding the capabilities of a pulsed-DC PA-CVD reactor and increased understanding of diamond growth parameters within. Expanding operational capabilities involved upgrades to the electrode and cooling set up, which was achieved through experimentation and modelling. The original design was replaced with smaller electrodes and improved cooling, allowing for higher power inputs and more reliable and stable plasma production. This resulted in long, stable growths producing high-quality diamond films. Each upgrade to the electrode design required operational Taguchi optimization in order to understand the new limitations as well as increase understanding of parameter influence on the grown diamond. The final design incorporated novel asymmetric electrode cooling and a gas/water cooled cathode. The improved plasma stability allowed for spectral plasma analysis, providing insight to H_β and C₂* radical distribution and primary plasma reaction pathways. Spectral and growth analysis of closed chamber, zero gas flow growth was also explored, a first with this type of instrument. Diamond quality does not appear to be affected by the closed chamber growth, but plasma stability severely limited extended testing. With a new, cooler anode/substrate complex, low melting point substrate growth was trialled. Silicon growth produced limited success, reflecting the recent literature's limited discussion of the material, but a novel copper substrate produced rapid growth of free-standing high-quality diamond films. The production of these films produced interest in the exploration of diamond/epoxy nanocomposites. DND and HPHT diamond powder nanocomposites were manufactured and mechanically tested for a variety of physical properties. The HPHT nanocomposites showed similar moduli to the DND equivalent with less reduction in failure strain or UTS. Overall, this showcased promising variations between materials that could be used for future applications.

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Optimal aeroelastic tailoring of wind turbines

Student: Sam Scott
Supervisors: Terence Macquart, Alberto Pirrera, Paul Weaver, Peter Greaves (ORE Catapult) and Chong Ng (ORE Catapult)

Context: Given current global carbon-emission targets/climate plans, it is desirable to increase the economic viability of wind energy, as it has a lower environmental impact than conventional energy sources. The challenge for the wind turbine designer is thus to reduce the cost of energy (CoE). Wind turbines are highly coupled systems and thus sub-systems cannot be designed in isolation, leading many researchers to apply multi-disciplinary optimisation (MDO) techniques to aid in design. Further, much research has been devoted to the investigation of aeroelastic blade couplings, where load alleviation and CoE improvements have been displayed.

Aim: The aim of this PhD is to evaluate the potential benefits of aeroelastic tailoring within wind turbine design. A multi-disciplinary optimisation framework will be specifically developed to broaden the conventional design space and find cost-of-energy-optimal solutions. Additionally, novel optimisation framework architectures, and non-conventional modelling techniques—applied to wind turbines—will be investigated and compared with traditional methods.

Objectives:

1. Develop and verify the relevant modelling tools/techniques required for successfully representing aeroelastic couplings.

1. Assess the accuracy of traditional BEM-based aerodynamic models against more physics-based models such as Lifting Line, for swept/prebent/highly flexible large blades.
2. Implement efficient structural constraint computations.

2. Develop a multi-disciplinary optimisation tool capable of considering previously investigated sources of aeroelastic coupling that have illustrated possible benefits for CoE.

1. Assess choice of optimisation architecture on efficiency and accuracy, an alternative option may include the monolithic optimisation architecture.
2. Investigate use of gradient vs meta-heuristic optimisation algorithms.

3. Apply the optimisation tool to assess what cost of energy benefits can be offered by novel aeroelastic couplings – using the Levenmouth 7MW wind turbine as a baseline for optimisation and comparison.

1. Aeroelastic couplings from three main sources will be considered: coupling through external geometry (i.e. sweep), coupling from internal geometry (i.e. laminate/structural asymmetry), and material anisotropy. Various degrees of design freedom will be considered using lamination parameters.

Potential applications/benefits: In the long term, the concepts investigated in this research have the potential to drive down the cost of wind energy and thus increase its economic viability. Whilst some commercial designs are beginning to incorporate aeroelastic coupling through small amounts of blade sweep, the use of multiple sources of aeroelastic coupling presents a reasonable deviation from conventional designs and much detailed design work is needed to fully assess the viability. Therefore, work like this, that incorporates state-of-the-art design tools, is able to further confirm the benefits of such designs and guide industry in commercial exploitation of these designs.

Novel engineering: The novel aspects of this research are as follows:

• Consideration of a multitude of aeroelastic wind turbine couplings in an optimisation framework has not been attempted before.
• Lamination parameters have not been used before to parameterise the design of a wind turbine.
• Whilst many of the optimisation/modelling techniques are not novel in their own right, many of them have not been applied to wind turbine design before. In addition to the two key bullet points above, these include: combining linearised and non-linear load computations, constraint aggregation.

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Composite compliant shell mechanisms: tailoring and characterisation

Student: Jonathan Stacey
Supervisors: Mark Schenk, Matt O’Donnell, Carwyn Ward and Just Herder (TU Delft)

Compliant shell mechanisms are thin shell structures designed to undergo large elastic deformations. These have advantages over classical mechanisms by reducing the need to include (and maintain) joints. As well as this, compliant shell mechanisms offer increased design freedom due to the anisotropic nature of different deformation modes.

There is great potential to further enhance compliant shell mechanisms by using composite materials. Inclusion of these anisotropic materials further enhances the ability to tune the mechanism kinetics and kinematics. Moreover, composite materials make the manufacture of complex geometries more straightforward, and allow these mechanisms to possess superior specific properties than equivalent metallic alternatives.

Inclusion of composite materials in compliant shell mechanisms isn’t without its challenges however. The stiffness (and hence deformation behaviour) of composite structures is usually controlled via the structure shape, although when challenging geometry requirements already exist this can be unsuitable. Furthermore, a compliant composite shell mechanism would have to exhibit a specific force-displacement response through large deformations, potentially for many cycles. Such responses are sensitive to small changes in geometry and material distribution, meaning successful manufacture could be challenging. Classical composites issues such as fatigue, creep, stress relaxation and delamination also necessitate the study of long-term behavioural changes in any such mechanism.

As such, the objective of this project is to determine to what extent the benefits of composite-based compliant shell mechanisms are 1) realisable, and 2) maintainable over long time periods or cyclic loading. The project aims to investigate different combinations of prestress and variable stiffness (thickness, layup, variable fibre angles etc.), and their effects on mechanism behaviour. The impact on design spaces and manufacturing feasibility will also be studied. Additionally, the project aims to investigate the various potential risks that composite materials pose to characteristics such as fatigue life, creep and durability in the high-strain environment of compliant mechanisms.

Tailored composite complaint shell mechanisms could have potential applications across many sectors. One area could be the development of passive shell-based exoskeletons and medical supports, where shape and space constraints provide a challenging design space for compliant mechanisms. Elsewhere new concepts for morphing aerostructures rely on the interplay between aerodynamic loading and bend-twist coupling in composite skins. Further enhancement of these structures using prestressed skins or auxiliary members could help lead to more optimal designs. Finally, the need for space and energy efficiency in spacecraft architectures provides opportunities to develop novel deployable composite structures. Being able to fully understand and tailor the stiffnesses and stress states within the composite shells could enable control of their stable configurations and deployment behaviour.

Working within the Bristol Composites Institute (ACCIS), and in partnership with researchers at TU Delft, this project aims to deliver novel analytical models, finite element simulations and experimental insights into prestressed thin composite shells, as well as manufacturing and design optimisation considerations for several application areas.

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Physical, mechanical and rheological behaviour of prepregs

Student: Yi Wang
Supervisors: Stephen Hallett, Dmitry Ivanov, Jonathan Belnoue and James Kratz

Automated fibre placement (AFP) is becoming one of the mainstream composites manufacture techniques in commercial aerospace. It provides a both promising and practical way to increase the production rates and reduce the manufacturing costs. It can also create a larger design space through tow steering. However, it is limited by defects generated during the steering process on curved surfaces, which will much degrade the mechanical properties of the finished cured structures.

The critical radius defines the minimum radius attainable before the forming of defects (such as wrinkles, tow pull-off, fibre misalignment). Studies indicate that it is somehow dependent on the in- and out-of-plane properties of the prepreg material (e.g. shear, bending, tackiness and friction). However, the influence still lacks a clear understanding and remains to be properly quantified. Therefore, whilst they are both costly and time consuming, trial-and-error methods are still the favoured technique used in industry nowadays to determine the value of the critical steering radius for a prescribed set of deposition parameters. This research project aims to gain more understandings of the influence of the prepreg material properties on the magnitude of the critical radius and the defects’ severity. Building on this, new numerical tools able to predict the critical radius and help to mitigate against defects formation will then be generated.

The objectives of the project are as follows:

1. To establish new material characterisation techniques needed for prepreg, with emphasis on in-plane shear, in-plane bending and adhesion properties.
2. To establish, based on the experimental data produced, bespoke material models that can be implemented as material subroutines within a finite element package.
3. To create benchmarking manufacturing trial cases and validate the numerical model proposed.
4. To create the experimental and modelling protocols showing a complete procedure, including experimental methods, material parameter derivations, deposition process models and routine validations.
5. To present PhD results in journal papers and meetings with industrial partners. Engage in dissemination of the project’s results.

The potential applications and benefits

A good understanding of defects formation in AFP process can be achieved through this research. The project can set a good foundation for accurate numerical prediction of the as-manufactured geometry and tow trajectories, making virtual manufacturing data achievable at the early stage of the design process. In the meantime, it can better ensure the parts can be manufactured with a minimum of defects and help the optimization of AFP deposition processes.

The novel Engineering and Physical sciences part of the research

The research focuses on investigating defects generation during AFP processes which, although the technique is widely used in industry practices, remains not well understood. Defects are largely dependent on the material characteristics of the materials’ precursors and can greatly influence the mechanical properties of the finished part. It is aimed at gaining better understandings of the physics controlling the deformation of prepreg material in order to predict and mitigate against defects generation. A combination of knowledge, covering the physical, mechanical and rheological subjects will be involved in the research. A bespoke model will be proposed to optimize the AFP steering process and early design of the structure.

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