PhD projects: 2019 cohort

Transtibial prosthetic socket design: Understanding the requirements for a healthy residual limb

Student: Kevin Alarcon
Supervisors: Eric Kim, Ole Thomsen, Elena Seminati (University of Bath), HaNa Yu (University of Bath), Alex Dickinson (University of Southampton)

A prosthesis serves to restore the functions of an amputated limb partially. A prosthesis is, therefore, a gateway for a restored social and economic life for many. With over 30 million lower leg amputees worldwide, the prosthesis which are accessible to most must sufficiently cater to the user’s needs. However, with current transtibial prosthesis (for below the knee amputations) designs, around 25 – 57% of users choose to abandon their prosthesis. This is primarily attributed to the interface between the amputated limb and the prosthesis, which is comprised of the prosthetic socket and liner. The high proportion of abandonment indicates there is much room for research and development. The lack of effective design is due to the difficulty in holistically understanding and solving the design challenges because this requires multidisciplinary knowledge across biology, biomechanics, mechanical and manufacturing engineering, clinical knowledge, and experience.

In the literature, there is clear interconnectivity and interplay between the socket design parameters and the amputated limb. This systematic nature is largely not considered in the development of prosthetic socket design. The literature in which systematic nature is considered a clear improvement in design outcomes is recorded. This project, therefore, aims to expand in that trajectory by considering a holistic approach to design. And thus, answer the research question: how to design and manufacture an effective and accessible prosthetic socket?

The method to achieve this is outlined below in the objectives of this PhD project:

  • To holistically investigate the residual limb system and how to maintain it healthy in everyday use.
  • To investigate the limitations to current socket design and modelling techniques.
  • To investigate the design using Axiomatic design principles, consideration for the design for manufacture and biomimicry to find suitable design parameters for the socket.
  • Using the previous investigations to derive a novel manufacturing and design process for prosthetic sockets
  • To test and validate the design through the development of computer and physical models of an amputated limb and socket.

The potential application includes the production of a low-cost prosthetic socket which can be modified to include other types of amputation. Additionally, this research could improve the human-machine interaction, for example, in wearable devices and other wearable equipment.

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Development of multiscale modelling methodology for the discovery and design of composite materials

Student: Matthew Bone
Supervisors: Terence Macquart, Ian Hamerton, Brendan Howlin (University of Surrey)

Computational modelling is of increasing importance to materials science and engineering. Modelling has clear advantages over laboratory work: it is infinitely repeatable, generates a wealth of useable data, doesn’t produce waste material, and is considerably cheaper in terms of both cost, and time investment. However, current modelling simulations are limited to the length scale (e.g. macroscale or atomistic) for which they were designed, which ultimately limits their application scope. This work aims to develop a methodology that enables bridging between scales to produce a true multiscale modelling experiment. This novel advancement would enable the production of high accuracy models for complex structures. These models would facilitate rapid prototyping and reduce reliance on physical testing. Furthermore, an additional objective of this work will be to expand the modelling potential of composite structures, particularly in low length scale simulations, and incorporate this into the multiscale model methodology.

This work will model atomistic chemical structures of polymers, nanomaterials and carbon fibre by using molecular dynamics (MD). MD is well suited to modelling the curing process of polymer structures and how they interact with other materials such as at the surface of carbon fibre. The final structure and material properties determined by MD will then be fed into mesoscale models, raising the scale from nanometre to micrometre. These models will reveal how a polymer matrix and carbon fibre act in a single composite ply and can be used to explain the mechanical properties of a material. Finally, the results of the mesoscale model will be used to produce a high-quality finite element (FE) model. FE analysis is able to demonstrate how large, tangible structures, such as beams, panels or trusses, respond to external loads. Using material originally modelled in MD allows for detailed material properties in the FE model, enhancing the accuracy of the model and identifying materials that have potential to cater to a given task. This whole process can be repeated with different starting materials to refine the final properties of structure being modelled.

Application of this work is broad as any development of modelling technology not only enhances inorganic materials modelling (e.g. metals, batteries, superconductors) but also biological modelling for the world of drug design. In composites, this multiscale modelling would be of significant interest to the aerospace, automotive and renewables industries to aid in the design of complex structures such as wind turbines. Of equal importance, the technology could be of interest to materials manufacturers as it has the potential to accelerate materials discovery and further the development of bespoke materials.

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Design, fabrication and testing of porous material-metal hydride composites for hydrogen storage

Student: Charlie Brewster
Supervisors: Valeska Ting, Sebastien Rochat, Lui Terry, Ashkan Salamat (UNLV)

Hydrogen is widely acknowledged to be a promising renewable fuel for replacing petroleum. Current methods of storage focus on compression or cooling to increase the density of hydrogen. However, these often require cryogenic or high-pressure conditions which are costly to achieve and maintain.

Alternatively, hydrogen can be stored via adsorption onto a solid nanoporous scaffold, or reversibly forming a metallic hydride. Both techniques have their own sets of advantages and disadvantages with neither method meeting all criteria for practical hydrogen storage simultaneously. Achieving solid-state hydrogen storage is an important goal within engineering and chemistry, and is the key to realising a safe, cost-effective, environmentally friendly fuel centred around hydrogen.

Producing metal-hydride particles at the nanometre scale has previously been used to improve the hydrogen storage capabilities of various metal hydrides through maximising the surface area, increasing surface energies, and reducing internal diffusion paths. Typically, these nanosized materials are synthesised through mechanical milling, which produces inconsistent materials that are prone to contamination. Recently, incorporating the metal hydride within a porous network has proven to be a practical pathway to control the synthesis of nanosized metal hydrides. Nanoporous materials with pore diameters of only several nanometres demonstrate good potential for synthesising porous material-metal hydride composites, and show several beneficial properties, including reduced exposure to moisture, reduced formation enthalpies, and improved stability. Confinement effects from the porous scaffolds can further alter the phase diagram of the guest material, stabilising phases which may otherwise be unstable under the same pressure and temperature conditions.

Nanoporous material-metal hydride composites provide an exciting avenue to overcoming the current challenges in hydrogen storage and producing confined phases with unique properties. In this project, carbonaceous micro-mesoporous host material properties will be explored to identify the effect the scaffold has on the behaviour of the encapsulated guest metallic hydride.

Several steps are required to be able to design nanoporous material-metal hydride composites, including:

  • Systematically explore host properties such as pore size and pore geometry to determine the effect on the confined material arrangement.
  • Identify the extent to which the porous scaffold affects the formation/decomposition and physical properties of the confined metal hydride lattice.
  • Understand phase nucleation within the nanoporous material-metal hydride system and how the guest materials can vary throughout the composite.
  • Investigate the effect of different manufacturing techniques and conditions on the final composite system.
  • Perform computational simulations to understand underlying mechanisms within the material to predict the guest structure and composite properties.

The overarching goal of this project is to rationally design and fabricate novel nanoporous material-metal hydride composites to exploit desirable properties for hydrogen storage at commercially achievable temperature and pressure conditions. Furthermore, understanding and manufacturing of nanoconfinement composites may lead to developments in catalysts, electronics, and energy storage materials.

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Advanced high fidelity modelling of woven composites

Student: Ruggero Filippone
Supervisors: Bassam Elsaid, Stephen Hallett, Adam Thompson, Peter Foster, Adam Bishop (Rolls-Royce)

In the last decades, advanced composite laminates have represented one of the most used material technologies able to ensure unparalleled performance in terms of strength to weight ratio and range of applicability. However, the newest aerospace engineering challenges has led to ever increasing demand for better mechanical properties and higher reliability. Traditionally, the composites laminate exhibit poor through-thickness strength and impact toughness. Therefore, the need arises to use more reliable materials in some specific applications such as fan blades or impact resistance structural parts, which in turn led to the development of more complex fibre interweaving architectures such as the 3D woven composites. The introduction of the binder yarns aim to interlace multiple fabric planes, providing a three dimensional reinforcement. However, associated with the outstanding properties exhibited by the 3D Woven composites the exponential growth in architecture complexity poses new challenges, in terms of manufacturing and virtual modelling.

This research aims to develop state-of-the-art modelling capabilities for meso-scale damage modelling in woven textile composites. In particular, 3D woven composites debonding is one of the key damage mechanisms that have been extensively observed via experimental test studies. In the absence of debonding models, Matrix cracks can progress directly from matrix to yarn materials, resulting in a premature prediction of failure. Consequently, it is essential to include this damage mode in simulation for accurate predictions of the ultimate failure strength. Here, a novel modelling technique is proposed to include reliable debonding failure detection in the meso-scale model of textile composites. As a first step, the more traditional Voxel meshes needs to be improved due to the lack of a distinct representation of the yarn surface. So, the research starts by developing a dedicated mesh technique i.e. an original Hybrid Mesh that aims to represent the contact surfaces between matrix and yarns with a high fidelity. Finite Element Analysis models will be compared, in terms of mechanical characterisation, with experimental results. Finally, a dedicated mechanical damage models will be investigated, including the non-linear matrix behaviour under shear loading. The validation of the results will be carried out against X-ray CT scans experimental tests to capture damage morphology as well as failure strengths.

This project’s outcome will provide an important virtual modelling technique to enable the better design of structural components using woven composites. The proposed meso-scale model will be integrated in a multi-scale analysis framework, standing as cutting-edge software able to investigate the mechanical behaviour from the fibre/matrix constituents up to the components level made of woven composite materials.

Key aspects that will be covered to achieve a state-of-the-art 3D woven modelling capability include:

  • Dedicated matrix modelling including shearing non-linearity.
  • Yarn matrix debonding damage.
  • Enhanced damage progression algorithms for Voxel meshes.
  • 3D Woven dedicated conformal meshing methods.
  • Computational enhancements to achieve a stable implementation of damage models in an implicit integration framework.
  • Enhanced damage morphology experimental verification against CT-scans.

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Manufacture, characterisation, and optimisation of WrapToR stiffened skin panels for aerospace applications

Student: Chris Grace
Supervisors: Ben Woods, Mark Schenk, Terence Macquart, Michael Wisnom

This PhD will pioneer the use of highly efficient composite truss structures as reinforcement members for composite panels, with a particular focus on applications in the aerospace industries.

The patented wrapped tow reinforced (WrapToR) truss concept currently being researched at the University of Bristol has already been proven capable of producing low cost, consistent truss beam structures with a quick and simple fabrication process. An adapted filament winding technique allows continuous carbon fibre tow to be laid down as the shear elements, reducing the need for many individual members and automating much of the manufacturing. Previous work has already shown that these beams exhibit far superior stiffness and strength properties when compared to alternative structures of similar mass under a variety of loading conditions.

To date, research on this novel composite truss concept has focused on its use as beams and space frames, with the human powered helicopter Gamera II utilising the low mass and high stiffness properties of WrapToR trusses to break several FIA world records in human powered flight. The next step in developing this exciting technology is to integrate these naked beams and frames into closed panel structures to explore their use as reinforcement members. WrapToR reinforcement of skin panels has the potential to offer lower mass and higher stiffness and strength than alternative monolithic stiffening methods through exploitation of the excellent structural efficiency of trusses.

This project aims to use a multi-disciplinary approach to characterise and optimise the application of the WrapToR truss concept as a reinforcement member for structural panels, to demonstrate that WrapToR stiffened skin panels can improve the mechanical performance of aerospace vehicles such as rockets and reusable space planes, for a low mass budget. To achieve this aim, several objectives of increasing complexity have been planned out as shown below.

  • Initially, a stiffened flat panel unit cell will be fabricated and tested to characterise its mechanical performance and validate models currently being developed.
  • Secondly, validated models and optimisation algorithms will be used to design a larger, optimised flat panel structure to allow for comparative analysis of mechanical performance to alternative structures such as sandwich panels.
  • Thirdly, the work will move from flat panel geometry to more complex curved geometry, with a curved stiffened panel demonstrator unit built and tested for stiffness and strength. Further development of models will be used for validation.
  • Finally, a scaled down, curved stiffened structure will be designed, built, and tested for stiffness and strength to demonstrate the true potential of this novel technique. This scaled structure will replicate a relevant space or aerospace application to provide meaningful performance data that can be compared to current industry standards of panel stiffening.

Although this work will focus on aerospace vehicle structures, this concept would be suitable for a much wider range of industries and applications. From trains, planes, and rockets, to fuel tanks, ships, and wind turbine blades, where there is a need for mass efficiency in reinforcement, WrapToR stiffened skin panels can lead the way for the next generation of structural reinforcement.

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Lattice cores for high performance sandwich composite structures

Student: Athina Kontopoulou
Supervisors: Giuliano Allegri, Fabrizio Scarpa, Bing Zhang

Sandwich panels are employed in a wide range of engineering applications, especially in the aerospace and automotive sectors, to replace traditional bulk materials whenever structural mass is a critical performance metric. Notably, cellular cores provide a unique combination of properties to sandwich panels, including high specific stiffness and high specific strength, as well as low thermal conductivity, which makes them ideal for load-bearing, thermal insulation and energy-absorbing functions.

The mechanical performance of the core strongly depends on the deformation "mode", i.e. bending versus stretching, of the cell walls. Cores with cell walls undergoing axial stretching can be ten times stiffer than those deforming primarily through wall bending or buckling for the same relative density.

The aim of this PhD project is to investigate the feasibility of inducing stretch-dominated behaviour in lattices via the introduction of multi-material joints and flexural hinges in the core unit cell. This project will provide a novel and flexible method for tailoring specific stiffness, strength and thermal properties beyond what can be achieved by acting upon the topology of the unit cell alone. The properties of these novel cores will be explored both numerically, via high-fidelity finite element analysis, and experimentally, employing additive manufacturing methods to build prototypes.

The key objectives of this project are:

  • Development of high-fidelity finite element models to understand the elastic, inelastic and thermal response of lattice cores at the unit cell and assembly levels.
  • Optimisation of topology and multi-material hinged configurations to maximise the performance through parametric modelling.
  • Additive manufacturing initial prototypes of the optimised lattice cores.
  • Assessing the manufacturability in an efficient and robust fashion.
  • Testing the prototype structures and evaluate the results against the finite element analysis simulations.

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An investigation into the performance of aligned, discontinuous carbon fibre produced with the scaled-up HiPerDiF process

Student: Chantal Lewis
Supervisors: Ian Hamerton, Carwyn Ward, Marco Longana, Claude Billaud (Solvay)

Fibre reinforced polymer composites provide the opportunity to produce lighter, cheaper and, durable products with increased mechanical performance compared with metallics. However, this creates challenges related to the complex manufacturing processes, and composite waste. Waste fibres are in a discontinuous and randomly oriented form and to create a valuable product it is necessary to realign them to achieve high volume fractions. The HiPerDiF technology, invented at the University of Bristol, can process a variety of fibre types to produce a highly aligned discontinuous fibre preforms. The advantages of using discontinuous fibres are that they provide better handling and forming capabilities with the potential to reduce defects. The cured composite material can also achieve mechanical properties comparable to that of its continuous counterparts.

While significant progress has been made to optimize the mechanical performance of aligned discontinuous fibre prepregs produced using HiPerDiF technology, it has mostly been lab-based with a low throughput. As a result, this has allowed a limited characterisation of the produced material. A new generation machine, HiPerDiF 3G, capable of producing kg/hr quantities of aligned discontinuous fibre prepregs has been recently installed. This will industrialise the HiPerDiF technology, making it suitable to a wide range of composite applications. With the increased production rate, enough material can be produced to allow a full characterisation of the produced material both in terms of quality control and its mechanical performance under various loading conditions. The project will concentrate on virgin discontinuous fibres sourced from production waste.

The scale-up of any new technology brings with it new challenges. Therefore, this project will aim to identify risks associated with transforming the process into a larger scale and investigating the effects on the performance and quality of the produced material. To achieve this, the project will:

  • Produce panels using material from HiPerDiF 3G to compare with panels produced with continuous fibre prepreg of the same constituent materials.
  • Design and conduct a testing campaign that will analyse mechanical performance, failure properties, and fibre physical properties to determine the material's limits.
  • Experimentally determine the feasibility of optimising the process to maximise performance by varying the key process parameters.
  • Explore suitable measurement techniques which can be used to implement quality control into the HiPerDiF 3G manufacturing process.

Based on the results of this project, further research can be conducted to establish an efficient control strategy with a better understanding of the process parameters interactions and their effects.

This project will be supported by Solvay.

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Tow steering for the structural dynamics of launch vehicles

Student: Calum McInnes
Supervisors: Rainer Groh, Alberto Pirrera, Eric Kim

Typically, structural elements account for 60% of a launch vehicle’s dry mass, and hence significant effort is being undertaken by both academia and industry to develop highly mass-efficient structures. Such structures will allow for larger payloads to be delivered to orbit by next-generation launch vehicles. Consequently, NASA has identified lightweight materials and structures amongst the highest priorities for next-generation space vehicles to enable future manned exploratory missions beyond Low Earth Orbit. Tow-steered composites, those in which the reinforcement fibres follow curvilinear reference paths, represent structures which can be tuned by the designer to satisfy desirable criteria. Tow-steered composites have shown proven benefits to the axial compression load case of cylindrical launch vehicle structures.

During ascent, the loads experienced by launch vehicle structures are not solely static, significant dynamic loading arises from sources such as staging, engine noise and aerodynamic buffeting. Hence, the investigation of the benefits of tow-steered composites to the dynamic response of thin-walled cylinders is pertinent. However, very little research exists into the potential benefits of this concept to the dynamic loading regime. Hence, this project aims to address this scarcity.

The typical means of manufacturing tow-steered composites within the literature is by Automated Fibre Placement (AFP), which is prone to process-induced defects. Instead, this project will investigate tow steering using the Continuous Tow Shearing (CTS) process. CTS mitigates the process-induced defects of AFP by shearing instead of bending material tows. The in-plane shearing of material tows gives rise to an orientation-thickness coupling which can be exploited as integrated stiffening features on a CTS cylinder.

Aims & Objectives
This project aims to both numerically and experimentally develop tow-steered composites to optimise the dynamic response of launch vehicle structures. Furthermore, a link between the two loading cases, both axial compression and vibration, shall be developed as to produce fibre paths which are beneficial for structures under combined loading.

The project aims shall be fulfilled by following a staged work plan to meet the following objectives:

  1. Explore the potential design space of tow-steered composites through development of numerical models. Numerical tools shall be developed to quantify and explore these novel performance benefits.
  2. Conduct rigorous optimisation studies to identify tow-steered designs which exhibit both single and multiple load case performance benefits in addition to revealing the potential for significant mass efficiencies.
  3. Manufacture the optimised structure utilising the CTS process and evaluate the quality of this structure.
  4. Design and conduct experimental tests to validate the predicted dynamic performance benefits.

Applications & Benefits
The primary benefits to be found in this PhD are those afforded to launch vehicle structures. By improving the dynamic performance of thin-walled launch vehicle structures the opportunity to avoid instabilities will be revealed. Such instabilities may cause damage to sensitive payloads or the loss of the entire vehicle, and hence the opportunity to avoid these will prove to be invaluable when designing new structures.

Research Novelty
The novelty in this project is in the determination of potential dynamic performance benefits of thin-walled CTS cylinders. Additionally, the multi-loading case optimisation will propose a link between the two primary loading cases of launch vehicle structures and develop methodologies to satisfy requirements in both regimes.

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Topological optimization of large, additively manufactured composite structures with a graded lattice core

Student: Alex Moss
Supervisors: Alberto Pirrera, Terence Macquart, Ajit Panesar (Imperial College London)

Topology optimisation is a powerful tool in developing (often) non-intuitive structural solutions. There are many distinctive methods to achieve optimal topologies, as well as different levels of detail used in the underlying analyses. The most common approach is density-based topology optimisation, which varies density of each elemental volume of the structure to find the optimal layout of material. Generally, penalisation is used to drive the intermediate densities to reveal a solid-void solution, as these are more readily manufacturable. Recent developments in 3D printing of short fibre reinforced polymers will lead to increased use of additive manufacturing for structural components, and design of such components can be achieved through topology optimisation.

With the advent of 3D printing, cellular architectures allow intermediate density regions from topology optimisations to be readily manufactured and could therefore revolutionise current design paradigms and archetypes. In addition, short fibre reinforced polymer 3D printing material could further improve the performance of graded lattice structures by increasing both the stiffness and strength of the materials making up the cellular architectures.

Large structures require very strong and stiff components to resist the primary loads imposed upon them. For instance, unidirectional carbon fibre composites are used in many large structural applications. Additively manufactured short fibre composites cannot be used to replace these components, however their value lies in providing the opportunity to combine traditional composite laminates with high-performance graded lattice architectures, which, in synergy, will provide improved structural performance and manufacturability. In other words, the high strength backbone of unidirectional carbon fibre composites can be connected by topologically optimised graded lattices for overall structural and manufacturing benefits. Procedurally, topology optimisation can be used to find where primary reinforcement is required. Then, further optimisation can provide the detailed topology of the graded lattices, resulting in the best configuration for connectivity of the design domain.

The aim of this research is to devise a design method by which novel structural architectures comprising of graded lattices in combination with layered composite can be modelled and analysed accurately within a topology optimisation framework. The overall objective is to enable the design of these novel structural concepts, whilst ensuring manufacturability through 3D printing and conventional composite layup techniques and scalability all the way through large structural length scales (>100 m). To achieve accurate modelling and analysis, density-based topology optimisation research will be conducted, which will involve studying the pseudo material properties of cellular architectures. This multi-scale modelling approach will allow for faster computation of solutions, while still providing optimised designs which, once manufactured, perform as expected.

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Design for 4D printing: Modelling of smart porous networks for in-vivo deployment

Student: Joe Surmon
Supervisors: Richard Trask, Kate Robson Brown

This project has two key objectives: firstly, it aims to contribute to the 4D printing design space; with use of Abaqus and python smart composite materials can be modelled, and their subsequent shape changes simulated. This allows a user to model 4D composite materials and follow their triggered deformation over a period of time. The second objective will be to manufacture a 4D printable bio-inductive hip implant, this will be tested in various campaigns in order to optimise for use in-vivo. The aim is to use 4D materials to provide value to humans.

Osteoarthritis affects upwards of 250 million people worldwide; currently there is not a simple effective treatment that addresses the cause of the problem. Hence, one asks, can a 4D material be used to support and regenerate damaged joint interfaces? Both key objectives for the project will be carried out in unison, designed to support and optimise each other; modelling experiments should supplement physical experiments and vice versa.

Modelling in the 4D design space will be used to carry out exploratory experiments, allowing time in the laboratory to be focused and succinct. 4D material modelling will investigate the effect of hierarchical structure, i.e., can nano/micro-structures direct macroscopic shape change? This involves composite material modelling that explores micro/nano structures within a larger framework, enabling direct and intelligent shape change to provide support and growth where necessary. 4D material modelling will allow for a holistic material testing approach, many materials can be tested in a relatively short period of time. Thus, generating the most desirable material properties for supporting and stimulating a human hip joint. Following this, it will be possible to pick physical materials that closely match the most desirable material properties. In addition, multiple different material blends will be tested, producing tuneable material properties.

A stereolithographic 3D printer will be used to manufacture materials for testing, the printer utilises ultra-violet light to cure a resin instantaneously to produce a solid structure. Various printing methods will be investigated to produce the best material properties, the aim is to mimic material properties found within the body to provide the best support. Additive manufacturing produces anisotropic material properties, paired with complex loading patterns in the hip joint. It is essential that material properties are maximised in the appropriate plane. Thus, experiments will investigate how the angle of printing affects anisotropic material properties giving the optimal printing angle for implant manufacture. Furthermore, greyscale lighting techniques will be used during the cure process to produce functionally graded materials, this will mimic the joint interface that is found in a healthy human to provide further support. Modelling campaigns will supplement multi-material manufacture, in order to provide information on the most effective material blends for implant manufacture. Material testing will initially involve simple compression and shear testing outlined by standard ASTM manuals. As the project progresses, the aim will be to build a pseudo hip-joint in order to mimic the complex loading environment found within a human hip joint. In order for use in-vivo, biocompatibility must be investigated, the structure must interact in a complementary sense with the human body. Thus, experiments will be carried out to determine biocompatibility to avoid complications with implant insertion. Steps will be taken to produce a more advanced implant that stimulates joint regrowth. By incorporating bio-inks within the manufacturing process it is possible to stimulate positive regrowth, the aim will be to manufacture an implant that is bio-inductive. A bio-inductive implant will regenerate the joint interface and subsequently degrade as it is no longer needed.

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