PhD projects: 2018 cohort
- Extending the range of applications for Z-pins in composites
- Composite metamaterials for lightweight sound insulation
- Hygromorph materials for 4D printing shape change and environmental sensing
- A 'recycling map' for end-of-life sails: end-of-life sails as a feedstock for circular manufacturing
- Eastomeric matrix composites for morphing skins
- Path to aircraft electrification
- Sustainability of composite materials delivered using HiPerDiF
- Highly aligned discontinuous fibre thermoplastic filaments as feedstock for additive manufacturing
- Imperfection insensitive Continuous Tow Sheared rocket launch structures
- Modelling the behaviour of toughened prepreg during Automated Fibre Placement process
- Infusion of integrated structures with semi-cured elements
- Framework for life cycle engineering assessment of structural laminates for application to the marine environment
- Advanced continuous tow shearing process for manufacturing defect-free complex composite parts
- Improving the Mode II delamination bridging behaviour of Z-Pins through materials solutions
Extending the range of applications for Z-pins in composites
Student: Mudan Chen
Supervisors: Stephen Hallett, Bing Zhang, Giuliano Allegri and Sven Friedemann
Inserting small discrete rods in the thickness direction of fibre-reinforced polymer (FRP) composite laminates has been proved to be an effective approach to improve the out-of-plane mechanical performance of composites. The reinforcing rod is usually called a Z-pin, and the reinforcing technology is called Z-pinning in the composites community. Z-pinning has been well demonstrated in the literature regarding its capability to improve the delamination tolerance, including static, dynamic and cyclic loading cases. However, almost all the mechanical applications of Z-pins were demonstrated on resisting delamination propagation in flat laminate, and there is much less research on the application of Z-pins for preventing or delaying the initiation of damage in irregular geometries and non-laminate composite structures.
Apart from the mechanical enhancement, there is an increasing interest of exploring the multi-functionalities of Z-pins. In the literature, a limited number of studies have been conducted regarding the delamination monitoring, electrical conductivity enhancing and thermal conductivity enhancing functions of Z-pins. There is still no research on the application of Z-pins for tailoring the magnetic properties of composites. Also, there is still no structural applications for demonstrating the benefits of improving the physical properties of composites by Z-pins.
This PhD project aims to explore the potential of Z-pins for some unconventional applications. Specifically, the objectives of this PhD project cover:
- Investigating the effects of metal and carbon-fibre Z-pins on the magnetic properties of carbon fibre reinforced laminates (CFRL) through experimental measurements and numerical modelling at different volume fractions (extended on the previous XP project).
- Experimentally exploring the effectivity of using Z-pins for resisting delamination initiation in irregular shapes of composite specimens, e.g. L-shape laminates.
- Building robust and effective finite element models to understand the mechanical mechanisms for these untraditional Z-pinned composite structures.
- Studying the feasibility of decreasing the lightning strike induced damage by conductive Z-pins for composite structures, in combination with in-plane conductivity enhancements.
Composite metamaterials for lightweight sound insulation
Student: Gianni Comandini
Supervisors: Valeska Ting, Fabrizio Scarpa, Mahdi Azarpeyvand, Robin Neville, Giuliano Allegri, Johnny Deschamps (ENSTA, ParisTech)
Noise insulation is an important feature required for health, safety and protection of human occupants in aerospace and automotive structures. It is a difficult problem to tackle with light and thin layers at low frequencies. This can be solved using traditional strategies like heavy materials, but it requires space and weight to be effective. In order to achieve the same results with less weight and consequently wasting less energy a new approach is needed, and metamaterials for acoustic applications seem to be the likely solution. Studying the effect of cavities shaped as fractals or other geometries in traditional materials show unexpected physical behaviour of acoustic waves that can be exploited.
To develop this study several steps are needed.
- Measuring the effectiveness of the metamaterials manufactured, recording the Transmission Loss, Absorption and Reflection Coefficients, using different test rigs like impedance tube and waveguide
- Use a statistical approach like the full factorial design, to validate results
- Experiment with different infill materials with macro-meso-micro porosity
- Investigate additive manufacturing methods to produce composite metamaterials
- Exploring space infilling curves like Peano and Lebesque and other geometries, to understand their effect
- Perform Finite Element simulations in order to increase the efficiency during testing and predict which geometries will be more effective, hereby reducing waste
- Test the vibrational transmissibility of the designed geometries with and without infilling materials, to understand their damping characteristics
The potential impact of this research could apply composite metamaterials in real-world applications for passive devices in technology used for sound insulation.
Hygromorph materials for 4D printing shape change and environmental sensing
Student: Charles de Kergariou
Supervisors: Giuliano Allegri, Fabrizio Scarpa, Adam Perriman, Hind Saidani-Scott, Antoine Le Duigou (IRDL France
3D printing is currently extensively evaluated for battlefield and in-situ operations. The applications of 3D printing cover different topics, from logistics and repair, operational drones and biomaterial imprints for tissue repair. 4D printing is also attracting significant interest, because of the possibility or embedding shape change capabilities over time to create self-aware drones/robots, reconfigurable surfaces for antennas and communications, and deployable structures. Current 3D/4D printing materials used for potential battlefield operations involve the use of either polymeric (fossil) or metal constituents, that need to be imported in the battlefield environment. Reducing the overall operational carbon footprint is a target for 21st Century Armies, also in view of climate change implications. Moreover, there are issues about accessing in-situ the materials in different operational theatres, with critical implications on the logistics of maintaining a local additive manufacturing capability. A potential alternative would be to use locally sourced materials for 3D/4D printing, and plant fibres represent a commonly found material asset around the globe. In a recent collaborative work between ACCIS and IRDL we have demonstrated that biocomposites made of thermoplastic matrix and different types of plant fibres (flax, jute, kenaf and choir) can be engineered to make artificial structural actuators that deform in a controlled way at different humidity conditions. These hygromorph biocomposites also possess very high specific stiffness/strain values, with actuation authorities between the ones of shape memory alloys and hydraulic actuators. The use of plant fibres also make these hygromorph biocomposites (HBCs) a suitable material for enhanced life cycle characteristics, and low manufacturing costs. In this project we want to examine the use and development of novel HBCs materials for in-situ, locally sourced and sustainable 4D printing.
The main objectives of the project will be to develop materials based on the use of recyclable, on-site and environmental friendly for 4D printing that are able to undergo shape change and provide sensing capabilities based on mechanisms like selective liquid and moisture absorption and electrical stimulus. It will also aim at developing a suitable 4D printing rig able to exploit the hygromorph materials to build complex structures, with particular emphasis on scalability and in-situ operations. Finally, will be performed a mechanical and multifunctional (thermal, dielectric, transport) characterisation of the hygromorph materials based on a design of experiments approach to build predictive surface response methods for exploring the design space of these materials. Build prototypes of complex architected structures made of the 4D hygromorph printing rig and evaluate the mechanical, shape change and multifunctional properties of the configurations.
The use of relative humidity and controlled electrical/thermal stimulus for 4D printing via hybrid hygromorph materials is a novel approach to make use available local resources (water vapour, fluids, natural fibres) to produce shape morphing structures. Reinforcements like flax and hemp are relatively common and low-cost in several geographical locations, from continental Europe to Asia; the availability of these raw materials would make an in-situ and locally sourced 3D/4D printing process a reality. The materials developed in this project could be also potentially upscaled in terms of manufacturing to produce larger structures for applications like sun-shading or evapotranspiration for shelters; autonomous sun-shading systems could reduce building input by 90%, leading to a significant reduction of heat signature and power consumption for new built and infrastructure logistics. The hybrid hygromorph materials could also be used as a platform for low cost environmental self-sensing structures, able to trigger deformation and actuation authority at controlled thresholds of dual humidity/external stimulus (temperature and/or electricity). These hybrid hygromorph materials could also find applications as underwater morphing structures, as we have shown in our demonstrator.
A 'recycling map' for end-of-life sails: end-of-life sails as a feedstock for circular manufacturing
Student: Marcelle Hecker
Supervisors: Ian Hamerton, Marco Longana, Ole Thomsen
The sailing industry currently has no end-of-life plan for sailcloth. Once sails have lost their ideal flying shape, been torn beyond repair, or become unsightly from mildew, they have nowhere to go. This mismanaged ‘waste’ is not only detrimental to our environment, but also a loss of valuable materials.
While there are many constructions of sailcloth available today, most are a combination of synthetic fibres, coatings, adhesives, films, and other chemical additives. The most advanced sails produced by the world’s largest sailmaker, North Sails, for example, are largely made of carbon and aramid fibres.
The aim of this project is to develop economically and environmentally feasible methods to recover the valuable fibres from North Sails’ “3Di” range of sails and remanufacture these into secondary composite products (either for use on the yacht or for other structural uses). The feasibility of all methods will be quantified utilising accepted numerical life cycle assessment/life cycle engineering methods. Using this approach in the sailing industry is novel and will facilitate the industry making informed waste-management decisions.
Alongside this main remanufacturing/recycling aim, a non-destructive (spectroscopic) method will be investigated to determine when the sails are nearing the end of their useful life. The expected outcome is a method/algorithm that does not need expert interpretation and that will be based on a calibration curve involving sailcloth mechanical strength data and spectroscopic (fibre degradation) data, thus allowing the determination of remnant mechanical properties by employing a hand-held detector in the field (i.e. on the water).
This project will be undertaken with the support of North Sails.
Eastomeric matrix composites for morphing skins
Student: Rafael Heeb
Supervisors: Ben Woods, Michael Dicker, Fabrizio Scarpa
To lead the aerospace industry into a more sustainable future, greenhouse gas emissions and noise need to be reduced and aircraft manufactured from recyclable materials. Morphing wings are a promising solution to make wings aerodynamically more efficient. They allow aircraft to continuously change their shape to adapt to changing operating conditions – reducing drag and therefore fuel burned. In order to achieve these radical changes in shape in a smooth and continuous manner, morphing wings require new materials and structural design philosophies that allow for flexibility and compliance. In particular, the skins covering these devices must be flexible in the morphing direction to keep the actuation forces low while also being stiff in the out-of-plane direction to resist aerodynamic loading. Research has shown that the conflicting material properties for morphing skins can be achieved by combining a flexible membrane with a core structure which is made from a much stiffer material taking advantage of the effect of the second moment of area, effectively generating a flexible sandwich panel. Thus far the main research effort has been focused on core structures and the underlaying and load carrying morphing mechanism rather than the entire aerodynamic morphing skin.
This project will explore novel methods to develop morphing skins, taking into account the underlaying morphing technology and as well as factors such as various loading cases and aerodynamic performance, allowing the skins mechanical properties to be tailored to the specific application. Conventional core materials used in aerospace, such as the honeycomb based on a 2.5-dimensional geometry, have primarily been developed with the geometry due to the ease of manufacturing. Additive manufacturing will provide a new freedom of design where cores can be tailored to the specific requirement in the 3rd dimension. This allows for example the core member thickness to be varied through the height of the part. Using multi material additive manufacturing materials with a lower stiffness to be utilised in areas where bending is preferred and stiffer materials where rigidity is required. The multi material 3D printing has also shown that the core can be printed directly onto the outer skin membrane, where the membrane can have a variable thickness. This allows the skins to be manufactured in a single process, providing a perfect adhesion between the skin and the core. The materials used in this research are different formulations of Thermoplastic Polyurethanes which are optimised for additive manufacturing. Using thermoplastic elastomers has various advantages that makes them more sustainable than thermosets used in aircraft manufacturing. A component made from thermoplastics can be repaired in service using thermal and chemical welding, it is less likely to get damaged in service and when at the end-of-life it can relatively easily be recycled by melting the polymer.
This morphing skin concept has its significant scientific complexity. The proposed morphing skins are made from thermoplastic elastomers which have a highly non-linear behaviour when strained. Furthermore, the core structures themselves when subjected to a large in-plane deformation show a non-linear behaviour. Current models described in literature only hold true for relatively small displacements, but morphing works best with larger change in shape. In this work we aim to capture the complex interaction between the flexible core and skin membrane to develop analytical and numerical methods to determine their mechanical properties, in the in-plane as well as out-of-plane direction. These models can in return be utilised to design and optimise bespoke skin solutions for a variety of different morphing applications.
Path to aircraft electrification
Student: Callum Hill
Supervisors: Richard Trask, Ian Hamerton, Giuliano Allegri, Jason Yon, Patrick Norman (Strathclyde), Catherine Jones (Strathclyde), Jameel Khan (Rolls-Royce)
The electrification of aircraft requires significant improvements in thermal management, weight reduction, energy storage and electrical distribution. The use of multifunctional composites is essential in these endeavours and provides an effective way to eliminate the mass of components by using existing structural elements to perform the same function. This methodology has the potential to drastically increase the specific performance of the aircraft and its individual systems. Conventional carbon fibre reinforced composites have exceptional mechanical properties, but often exhibit poor electrical, thermal and magnetic properties. By tailoring the combination of fibre, resin system and nano-reinforcement used; these three properties can be fundamentally optimised and significantly improved.
In fibre-reinforced composites, the mechanical and physical properties are highly anisotropic – with superior tensile strength, stiffness, thermal and electrical conductivities occurring in the in-plane fibre direction rather than in the out-of-plane direction. However, the electrical and thermal behaviours of composites are poorly understood, particularly for complex geometries with different stacking sequences. To more fully understand this behaviour, a multi-physics model will be produced to characterise the electrical and thermal conductivity at the micro-scale. Through a series of homogenisations, this will build into a high-fidelity model of macro-scale components. Each scale will be validated with representative samples of composite. The final model will be capable of substituting in a variety of material combinations and ply stacking sequences to accurately determine their electrical and thermal behaviour in a geometrically complex component. The knowledge of a composite’s electrical and thermal behaviour is essential to develop the next generation of multifunctional materials. Without this knowledge, efficient design will be difficult and certification near impossible.
The authors wish to acknowledge the support of Rolls-Royce plc through the Composites University Technology Centre (UTC) at the University of Bristol and the EPSRC through the ACCIS Centre for Doctoral Training grant, no. EP/G036772/1.
Sustainability of composite materials delivered using HiPerDiF
Student: Ali Kandemir
Supervisors: Steve Eichhorn, Ian Hamerton, Marco Longana
There is an expanding societal concern about the environment relating to sustainability in many engineering fields, including fibre reinforced polymer matrix composites. Due to their non-environmentally friendly constituents, i.e. thermosets and synthetic fibres, this sector is committed to researching sustainable solutions. Two different approaches can be considered for possible solutions, and the first approach is reducing the environmental impact of constituent materials by using sustainable raw materials, such as natural fibres and bio-based matrices. The second approach is reducing landfill waste by considering “repair, reuse or recycle” for composites. Because the composite industry uses mostly thermoset matrices, which are irreversible when cured, it is difficult to apply the second approach. However, thermoplastic matrices or Covalent Adaptable Network (CAN) polymers overcome this and make it easier to apply the second approach. Furthermore, changing the fibre reinforcement geometry from continuous to discontinuous promotes and facilitates the concept of repair and reuse, which are quite difficult to achieve with continuous fibre reinforced composites. Because after the possible repair or reuse process, factors such as manufacturing defects, flaws and bottlenecks will be altered and all assumptions will become invalid, alternatively, those of discontinuous fibre composites remain relatively the same. Moreover, highly aligned discontinuous fibre composites can show the same performance as those of continuous fibres, and a new way to manufacture composites, the High Performance Discontinuous Fibre (HiPerDiF) method provides highly aligned fibre preforms. Remarkably, it was shown that the mechanical performance of composites processed via HiPerDiF preforms is comparable to standard composite materials (Compos. Part A, 65 (2014) 175). This project will focus on processing materials where all components are sustainable and can be repairable, reusable or recyclable.
Highly aligned discontinuous fibre thermoplastic filaments as feedstock for additive manufacturing
Student: Narongkorn (Knight) Krajangsawasdi
Supervisors: Dmitry Ivanov, Marco Longana, Ian Hamerton, Ben Woods
Fused deposition modelling (FDM), a widely used additive manufacturing (AM) method, allows for production of complex geometries in an efficient way. However, the mechanical properties of the FDM products are limited by the materials, i.e. thermoplastic polymers, used in this process. Adding fibrous reinforcement to the polymer to generate a composite filament leads to substantial performance improvement. Highly aligned discontinuous carbon fibre with a length above critical is an optimal reinforcement as it combines the high mechanical performance of continuous fibre and the high processability/formability of short fibre.
This work aims to create a novel composite manufacturing process using aligned discontinuous fibres produced with the High Performance Discontinuous Fibre (HiPerDiF) impregnated with thermoplastic as a feedstock for a general FDM machine. To achieve this goal, a continuous process to produce HiPerDiF-thermoplastic filament must be developed; no autoclave or complex tools should be required. When obtaining the proper filament, printing parameters will be investigated by trial printing and various mechanical tests to determine the relationship between the parameters, part dimension and mechanical performance. The flow and deposition of material extruded from the FDM nozzle can be simulated using computational fluid dynamic (CFD) and finite element modelling. Valid simulations should reduce the number of printing trials and could predict the behaviour of other fibre-matrix combinations. Considering the current literature landscape, this is the first highly aligned discontinuous fibre thermoplastic placement using additive manufacturing method.
The FDM process offer the possibility to design and produce complex composite geometries with a low manufacturing cost, in a short time and limiting the human error. Moreover, the benefits of the discontinuous fibre can minimise intrinsic continuous composite manufacturing defects such as wrinkles and fibreless areas. Even if fibre reinforced FDM products cannot replace traditional composites, the improved mechanical performance could expand the usage of FDM from hobbystic, aesthetic and low-end applications to more structural and functional ones such as be one-off products and prototypes, highly tailored secondary structures, tooling, or biomedical devices.
Imperfection insensitive Continuous Tow Sheared rocket launch structures
Student: Reece Lincoln
Supervisors: Rainer Groh, Alberto Pirrera, Paul Weaver, Evangelos Zympeloudis (iCOMAT), Chauncey Wu (NASA LaRC)
A large proportion of the dry-mass of a rocket launch structure is made of up the fuel and oxidiser cylindrical tanks. Using composite materials instead of current generation Lithium-Aluminium fuel tanks is calculated to save up to 30% in mass and 25% in reoccurring manufacturing costs. In recent years, these structures have been blade-stiffened shells or foam/honeycomb structures. These architectures are often manufacturing multi-stage processes, which increases time and cost to the end user. Using monocoque cylindrical structures instead would combat these issues. However, monocoque cylinders are very imperfection sensitive under axial compression – small variations in the geometry of the wall reduce the maximum load carrying capacity. To account for the difference in maximum load carrying capacity in theory and in experimental, knockdown factors are applied to the structure. This means that more material must be added, increasing weight and cost, because the structure is imperfection sensitive. This PhD project aims to design, build and test an inherently imperfection insensitive rocket launch structure.
To achieve this, a novel carbon fibre tow-shearing process will be used – Continuous Tow Shearing. By shearing the dry fibre tow instead of bending it, this automated manufacturing technique counteracts the common in-process defects associated with Automated Fibre Placement such as fibre wrinkling, fibre bridging, tow gaps and tow overlaps. In addition, there is a coupling between the fibre angle variation and the local thickness of the tow. This enables pseudo-stringers and -hoops to be embedded within the structure. By creating such stiffening elements, the imperfection sensitivity of these cylindrical structures will be greatly reduced.
The aim of this project is to design, build and test an inherently imperfection insensitive rocket launch structure. Python-based scripting of the commercial Finite Element package Abaqus will form the basis for understanding the linear and nonlinear behaviour of these structures. This will also be the platform for a Genetic Algorithm optimisation to generate an optimum structure to manufacture. The optimum design will be manufactured and tested in axial compression. This will be compared against simulations to validate the models.
- Create Finite Element models that represent the variable-angle composite structures and allow for exploration of the design space
- Perform an optimisation of design variables to maximise buckling performance and minimise imperfection sensitivity
- Manufacture an optimised structure using state-of-the-art Continuous Tow Shearing manufacturing process
- Test the manufactured structure in axial compression and validate test against Finite Element models
Application and Benefits
By designing imperfection insensitive composite rocket launch structures, the mass of the structure will be greatly decreased. Not only because of the specific properties of composite materials, but also due to smaller knockdown factors applied to account for the difference between experimental and theoretical results. This will reduce cost of manufacture, cost of use, reduce amount of fuel used and create an extremely efficient, safe structure.
- Numerical models will be created of the Continuous Tow Sheared cylinders and optimised to decrease imperfection sensitivity and increase buckling performance
- A relationship between variable-angle composite cylinders and imperfection sensitivity will be established
- The first ever Continuous Tow Sheared cylinder will be manufacture and tested
Modelling the behaviour of toughened prepreg during Automated Fibre Placement process
Student: Sarthak Mahapatra
Supervisors: Jonathan Belnoue, Stephen Hallett, James Kratz, Dmitry Ivanov
Manufacturing 4.0, also known as smart manufacturing, refers to an overarching concept in which machines are augmented with sensors and connected to a system that intelligently controls and adapts production. Currently, one of the most widely used automated manufacturing processes for composite parts is automated fibre placement (AFP). The deposition process involves the simultaneous warming, lay-up and consolidation of prepreg consisting of multitude of process parameters. Majority of the process parameters for AFP are derived by expensive and time-consuming trial-and-error approaches to ensure part conformance. The aim of this project is to be able use finite element simulations to predict the as-manufactured geometry of a preform deposited by AFP and with in-process sensing, adapt process parameters on the fly to mitigate the formation of defects. The focus of this work is to use process modelling to capture the various behaviours of toughened prepreg during the deposition process and validate it against experimental data for various processing conditions. The behaviours include, prepreg consolidation, tow path deviation, bridging and wrinkling.
A refined and accurate quasi-coupled thermo-mechanical model of the AFP head , fully capturing interaction of all elements of prepreg deposition is used a baseline model, shown in Figure 1 (below).
Within the model, a fully characterised viscoelastic material definition  is used for the prepreg tape along with a hyperelastic material for compaction roller to accurately represent the physical process. Various lay-up speeds, heater powers and compaction forces are simulated and compared with results derived experimentally. This work also highlights the impact of variabilities in material, process parameters and sensing on the measured response of the preform. To reduce the empiricism present in the manufacturing process, the viability of incorporating the numerical models into existing statistical relationships between process parameters and manufactured geometry is examined. Finally, a future framework to incorporate the developed models into a feedback control system, to enable more deterministic manufacturing, required for manufacturing 4.0 is also laid out.
 R. Lichtinger, P. Hormann, D. Stelzl, R. Hinterholzl, The effects of heat input on adjacent paths during Automated Fibre Placement, Composites Part A: Applied Science and Manufacturing 68 (2015) 387–397.  J. P. Belnoue, O. J. Nixon-Pearson, D. Ivanov, S. R. Hallett, A novel hyper-viscoelastic model for consolidation of toughened prepregs under processing conditions, Mechanics of Materials (2016).
Infusion of integrated structures with semi-cured elements
Student: Michael O'Leary
Supervisors: James Kratz, Dmitry Ivanov, Ivana Partridge, Dr Turlough McMahon (Airbus), Dr Dominic Bloom (Airbus)
Fabricating large integrated structures with many different preforms greatly increases the complexity of manufacturing operations and also the likelihood of defects. A multistage cure process could have the potential to reduce the complexity of manufacturing such large infused structures with various preform elements. Semi-curing certain preforms that make up a larger integrated structure would allow for fewer post cure joining operations as the structure would already be its net geometry and has the potential to undergo non-destructive testing prior to final cure, it would also greatly reduce the risk of preform misalignment during cure.
The aim of this PhD is to produce semi-cured preforms to be integrated into a larger structure prior to infusion and cure, which will provide an interface between the semi-cured preform and the larger structure which retains the highest level of mechanical performance when compared to an interface produced without a multistage cure process. To do this, simple structures will first be produced before moving to more complex geometries. Initial work will centre around optimising the degree to which the semi-cured parts are cured to as well as optimising and understanding how best to form the interface between the semi-cured preforms and the wider structure. This work will then lead to exploration of how the semi-cured part is affected by handling and machining.
The results of this work should allow manufacture of large, complex, infused structures with a degree of ease not previously seen in resin infusion processes.
Within this project, research will be focused around:
- Development of interface properties between semi-cured and uncured preforms.
- Optimisation of the cure processing window.
- Handling and verification of semi-cured preforms.
- Novel tooling and infusion techniques.
This work is directly supported by EPSRC and Airbus.
Framework for life cycle engineering assessment of structural laminates for application to the marine environment
Student: Will Proud
Supervisors: Richard Trask, Ian Hamerton, Marco Longana
Minimising the impact we have on the environment by developing sustainable composites should be a key focus for composite designers. Composite production processes are often energy intensive and certain materials present significant challenges for recycling and end-of-life processes. Whilst companies could hold the viewpoint that sustainability is an opportunity to innovate, it appears that many see it as a challenge and by investing in improving sustainability, they are put at a disadvantage to their competitors. To make informed decisions and avoid assuming investing in sustainable composites is dis-advantageous, a full information set covering mechanical, economic and environmental factors in a design or re-design process should be created. A methodology of gathering this information is Life Cycle Engineering, presenting economic, mechanical and environmental factors in a semi-quantitative process utilising a ternary diagram. However, the at best semi-quantitative nature of this method limits its usefulness in a full industrial design process. This project will seek to develop an integrated LCE methodology which will bring sustainability alongside costing and mechanical performance in the design of composite components. Each aspect will be covered by a discrete, linked process. Iterative design will be included so that the process is not just applicable to the initial materials selection phase but also upstream design processes. Aim: The aim of this project is to investigate how an LCE framework can be implemented into an integrated environment and successfully applied to a composites design or re-design scenario.
- Understand the challenges of implementing a full LCE method to a composite scenario
- Develop an integrated LCE framework which is deployable to an industrial setting
- Test and validate the framework on a number of case scenarios
The current intended application is to the marine and wind turbine industries. However, a key focus of this work is to ensure that the method is fully adaptable to different scenarios. As such, it is hoped that the method will eventually be applicable to any user of composites. However, as marine and wind both have common challenges such as erosion and saline degradation, the validation and testing methods will relate to these considerations. As such, validation and test would be required for any other industries this framework may be applied to. The most significant benefit of this research would be improving the accessibility for the composites industries to a tool enabling them to make informed decisions on multiple criteria when attempting to integrate greater sustainability. The novelty of this work is derived from the development of a fully integrated LCE framework in a method with greater rigour than a simple ternary diagram. As such, the results will be quantitative as opposed to qualitative. Furthermore, during the case study it is possible that novel biocomposites will be developed. As such, novel research into challenges of bio-composite use such as interfacial weakness or moisture resistance will also be undertaken.
Advanced continuous tow shearing process for manufacturing defect-free complex composite parts
Student: Michelle Rautmann
Supervisors: Byung Chul Eric Kim, Dmitry Ivanov
Current state of the art material deposition technology used to manufacture large composite parts has a limited capability in laying up on double curved surfaces. Fibre steering processes, such as automated tape laying (ATL) or automated fibre placement (AFP) are not capable of producing composite parts with complex geometry without inducing defects like tow gaps or overlaps. The minimum steering radius is dependent on the tape/tow width; Therefore, the steering radius requires to be kept as large as possible in the design phase, which significantly constrains the design flexibility and manufacturability.
The novel continuous tow shearing (CTS) technology, developed at the University of Bristol, allows to eliminate defects by steering fibres utilising in-plane shear deformation of the tape or tow. Its greatest advantage is that there is no coupling between the tape width and the minimum steering radius. Hence, even wide ATL grade tapes may be used allowing high material deposition rates.
However, this process requires to be improved for complex 3D layup, as it still does not eliminate defects when laying up on double curved surfaces. To date, triangular shaped resin pockets are induced by cutting individual tows during a 3-dimensional complex layup, which have a highly negative impact on the mechanical properties of the composite.
The aim of this PhD project is to advance the current CTS technology to enable manufacturing of defect-free 3D complex composite parts. In order to eliminate the tow gaps and overlaps that are inevitably produced in the current AFP process, a novel mechanism that can control the geometry of the tow/tape will be developed, which will eliminate geometry induced defects in production of 3D complex shapes. The Advanced CTS process with this novel mechanism will become an innovative solution to the quality problems of the current AFP process and significantly expand the design space of composite structures, allowing for ultra-high structural efficiency.
Improving the Mode II delamination bridging behaviour of Z-Pins through materials solutions
Student: Eduardo Santana De Vega
Supervisors: Giuliano Allegri, Bing Zhang, Ian Hamerton, Stephen Hallett, James Finlayson (Rolls-Royce)
Z-pinning is the most effective method for embedding through-thickness reinforcement in prepeg laminates. Composite Z-pins have been traditionally manufactured employing carbon fibres combined with a bismaleimide (BMI) resin. These materials allow achieving a substantial enhancement of the mode I interlaminar fracture toughness of Z-pinned laminates. Nonetheless, the toughness improvements in mode II are much more modest, due to the inherent brittleness of fibrous carbon/BMI when transversely loaded.
Through-thickness reinforcement elements such as Z-pins act as micro-fasteners, restraining the opening and sliding displacements of delaminations and hence inhibiting interlaminar crack growth. As such, ensuring a balanced mode I and mode II performance will reduce the chance of catastrophic failure of damaged composite structures. Mode II delamination toughness is especially crucial in reducing the damage and crack propagation in impacted composite laminates. This includes applications such as gas turbine blades.
The aim of this PhD project is to address the poor performance of composite Z-pins under mode II delamination conditions. This will be carried out by means of materials solutions, with the objective of understanding the inherent properties that affect the pin’s performance. Ultimately, single-type Z-pins that provide a balanced mode I and mode II delamination toughness will be identified.
The key objectives to be completed throughout the length of the project are:
- Identify potential reinforcement/matrix materials to be used in Z-pins to achieve a balanced bridging performance in mode I and mode II regimes.
- Develop, as necessary, resin blends and fibre architectures that increase the mechanical performance of the pins under mode II loading.
- Successfully manufacture candidate Z-pins using the selected materials and architectures, optimising the process to obtain a high-quality output.
- Experimentally characterise the meso-mechanics of the candidate Z-pins for the full range of mode mixity and compare with the performance of commercially available Z-pins.
Most published work on Z-pinning has concentrated on understanding the performance of only the commercially available carbon/BMI Z-pins. These materials have remained almost unchanged for over a decade. Throughout the project, novel resin blends and fibre architectures will be developed to specifically satisfy the requirements of Z-pins and Z-pinned laminates. Additionally, the study will help to better understand the relationship between the mechanical properties of the pins and their ability to supress mode II delamination.