PhD projects 2011 cohort
- The nonlinear thermostructural behaviour of composite bimorphs at elevated temperatures
- Structural magnetic materials for use in electro-mechanical applications
- Integrated multi-scale modelling of 3D woven composites
- Self-healing agents for application in fibre-reinforced polymers
- Biologically inspired adaptive camouflage: polychromatic smart materials
- Non-classical effects in straight-fibre and tow-steered composite beams and plates
- Analysis of geometrically non-linear shells for morphing applications
- Self-healing for structural applications
- Novel through thickness reinforcement development
- Kirigami cellular structures: mechanical properties and shape morphing behaviour
- Development of 3D printed vascular networks for repeated self-healing
- Robust and reliability-based aeroelastic design of composite wings
- Multifunctional elastomers with tailored anisotropic response
The nonlinear thermostructural behaviour of composite bimorphs at elevated temperatures
Student: Eric Eckstein
Supervisors: Paul Weaver, Alberto Pirrera and Jacopo Ciambella (Sapienza Università di Roma)
Composite morphing structures often rely on thermal loading to achieve multistability, however new understanding of these structures may enable their use as temperature-driven actuators. In particular, the ability of multistable structures to undergo large displacements whilst remaining relatively stiff can give them a performance advantage over conventional thermal actuators such as bimetallic strips. To achieve this, the anisotropic elastic and thermal expansion properties of fibrous composite materials are exploited to create internal bending moments in laminated beams, plates, and shells. These bending moments drive displacements, which can be non-linear with respect to temperature and can even be tailored to produce snap-though behaviour at specific triggering temperatures. These techniques are becoming well-understood with conventional polymer-matrix composites, though perhaps their greatest potential for exploitation lies with high-temperature ceramic-matrix composites (CMCs).
The modern gas turbine engine presents itself as excellent candidate for high-temperature morphing technology. Conventional actuators are not only heavy and difficult to package, but also have temperature limitations that currently restrict their use to the relatively cold regions of the engine. This means that concepts such as variable-geometry turbine vanes remain out of reach with our current actuation technology, however achieving that goal seems quite possible using morphing CMCs.
Several key challenges are being overcome to make high-temperature morphing structures a reality. Models have recently been developed and validated which capture the geometrically nonlinear behaviour of multistable shallow composite shells subject to temperature change and through-thickness thermal gradients. In addition, equations have been developed which predict the optimum ratio of one material constituent to another in an anisotropic bi-material thermal actuator. These developments guide experimental research and may serve as useful “first order” analysis tools for future engineers. On-going research will focus on understanding and overcoming the challenges of building a working CMC thermal actuator.
Structural magnetic materials for use in electro-mechanical applications
Student: Laura Edwards
Supervisors: Ian Bond, Phil Mellor and Jason Yon
High specific output electrical machines and actuators are finding increased application in transportation and renewable power generation. Of the various electrical machine formats permanent magnet machines have the highest torque/power volumes, primarily due to the absence of rotor excitation losses, and recent advances in high strength rare earth permanent magnets offer new possibilities for novel electromagnetic topologies that extend the current capabilities.
Contactless high torque magnetic gearboxes have been proposed which require no lubrication or maintenance and are acoustically quiet. This research focuses on developing magnet composite materials for application in such devices.
Integrated multi-scale modelling of 3D woven composites
Student: Bassam El Said
Supervisors: Stephen Hallett, Dmitry Ivanov and Andrew Long (University of Nottingham)
This PhD project aims to develop a complete framework for the mechanical modelling of full scale 3D woven composite structure. 3D woven composites are a viable solution to the inherent low through thickness properties of conventional composites. Additionally, 3D woven preforms can reduce composites production costs which are yet another disadvantage of high performance composites. However, one of the main challenges facing the wide use of 3D woven composites is the lack of numerical tools capable of modelling fabric behaviour. Several modelling techniques have been proposed in literature which can model deformation and defects in 3D woven fabrics with reasonable accuracy. However, most these techniques are computationally expensive which limits the applicability of such models to the unit cell scale. In practice, deformations and defects occurring in a woven perform during production are a result of tool/fabric interactions. Consequently, these deformations and defects are dependent on the tool geometry as well as the fabric architecture. Unit cell models fall short of capturing the tool geometry effects on the compacted fabric which can be of paramount importance for complex components. Hence, the need arises for simple and computationally efficient modelling techniques which can predict fabric deformation and defects at the component or feature scales without compromising accuracy. In order to simultaneously achieve computational efficiency and accuracy, a mutli-scale approach combining high and low fidelity techniques is adopted in this project. For geometric modelling of fabric deformation and defects, a high fidelity digital element model is used to find a realistic woven geometry for a single unit cell. The as woven geometry is then tessellated to form a component or feature scale fabric. The full size fabric model is then meshed and combined with tool geometry. The overall model is solved to find the deformed fabric geometry. The detailed fabric architecture is then used as input to multi-scale mechanical models. Such models employ the hierarchal nature of the fabric to calculate the structures global mechanical behaviour as well as the strength characteristics on the level of the individual unit cells, thus providing a vivid image of all the aspects of the structure mechanical performance.
Bassam was awarded a Faculty of Engineering Commendation for his thesis.
Self-healing agents for application in fibre-reinforced polymers
Student: Daniel Everitt
Supervisors: Ian Bond, Duncan Wass and Richard Trask (University of Bath)
The design of new catalysts is a crucial step toward the realisation of self-healing materials. This project currently involves the synthesis, solution phase testing, and integration into composite materials of new catalytic systems. Eventual aims include achieving mechanical activation of such catalysts, allowing for low temperature autonomous healing.
Biologically inspired adaptive camouflage: polychromatic smart materials
Student: Ian Gent
Supervisors: Ian Hamerton, Richard Trask (University of Bath) and Annela Seddon
Camouflage technology has been an active area of research and development since the beginning of the 'industrialised' war. In this continually evolving field, the ability to mimic the surroundings not only greatly increases the safety of operational personnel, but can also provide a significant tactical advantage. However, theatres of war are rarely homogeneous. The British Army is now deploying Multi Terrain Pattern (MTP) in recognition that Helmand Province presents a range of operational terrains, with traditional woodland and desert variants of Disruptive Pattern Material (DPM) only being effective in different subsets. There is an alternative to the current compromises in camouflage systems: adaptive camouflage. The challenge for any current camouflage system is to defeat not only the human eye, but also the on-going technological advances in automated target recognition systems.
There is a growing need in engineering research and industry for structures and mechanisms that exhibit properties beyond conventional mechanical consideration, which this project seeks to address by considering the application of engineering materials to camouflage. Several examples found in nature of adaptive camouflage exist; these range from the slow, monochromic colour change of the//Pacific Tree Frog, to the fast and dramatic transitions of the cephalopods.
This project aims to assess the feasibility of a biologically inspired polyphenic-colour changing smart material, focussed at the single unit-cell level. The goal of this PhD study is to investigate the applicable natural camouflage systems, develop relevant colour shifting chemistry in soft materials and incorporate the resulting material into a lab scale demonstrator.
Non-classical effects in straight-fibre and tow-steered composite beams and plates
Student: Rainer Groh
Supervisors: Paul Weaver and Jacopo Ciambella (Sapienza Università di Roma)
Multilayered composites are widespread in load-bearing structures of the aeronautical and wind energy industries. Increasingly, advanced composites are spreading into the mass-market automotive sector, where the lightweight advantages of composites are improving structural efficiencies, and are thereby enabling a new generation of electric cars.
Composite laminates are mostly employed in thin-walled, semi-monocoque structures as the manufacturing processes, such as pre-preg curing and resin infusion, are amenable to this type of construction. However, their imminent diversification to new applications will benefit from extending the range of possible laminate configurations in terms of layer material properties, including both stacking sequences and laminate thicknesses, as well as the nature of service loading.
Such a diversification can add significant complexity, when for example, the layer material properties differ by multiple orders of magnitude, or when the composite comprises of relatively thick cross-sections. In case of the former, the structural response is non-intuitive and cannot be modelled adequately using classical lamination theory. The latter adds non-classical effects due to transverse shearing and transverse normal stresses, which are particularly pernicious due to the lack of reinforcing material in the stacking direction, and can lead to the delamination of layers.
Reliable design of these multilayered structures requires tools for accurate stress analysis that account for these non-classical or higher-order effects. Despite offering high fidelity, three-dimensional (3D) finite element models are prohibitive for iterative design studies due to their high computational expense. Consequently, a large number of approximate, higher-order two-dimensional (2D) theories have been formulated over the last decades, with the aim of predicting accurate 3D stress fields while maintaining superior computational efficiency. The majority of these formulations have focused on purely displacement-based approaches, which often require post-processing steps to recover accurate transverse stresses.
The present work uses the Hellinger-Reissner mixed-variational principle to derive a higher-order, 2D equivalent single-layer formulation that predicts variationally consistent 3D stress fields in laminated beams and plates with 3D heterogeneity, i.e. laminates comprised of layers with material properties that differ by multiple orders of magnitude and that also vary continuously in-plane. The formulation is shown to be accurate to within a few percent of 3D elasticity and 3D finite element solutions. A novelty of the present approach is that the computational expense is reduced by basing all stress fields on the same set of unknowns. Furthermore, by enforcing Cauchy's equilibrium equations in the variational statement via Lagrange multipliers, and then solving the ensuing governing equations in the strong form using spectral methods, boundary layers in the stress fields towards boundaries and surfaces are captured robustly.
The present formulation is also used to ascertain the relative effects of transverse shear, transverse normal and zig-zag deformation fields. By studying non-traditional materials and stacking sequences with pronounced transverse anisotropy, the model provides physical insight into the governing factors that drive non-classical effects, with the aim of aiding the intuition of structural engineers in preliminary design stages. In general, the formulation is well-suited for accurate and computationally efficient stress analysis in industrial applications.
Rainer was awarded a Faculty of Engineering Commendation for his thesis.
Analysis of geometrically non-linear shells for morphing applications
Student: Ettore Lamacchia
Supervisors: Paul Weaver, Alberto Pirrera and Isaac Chenchiah
Morphing structures are able to perform large scale geometric changes in order to better adapt to radically different environmental conditions. Multistable structures may be design to meet this requirement. Multistability is commonly obtained through anisotropy, stiffness tailoring or pre-stress. This research aims at investigating a novel class of multistabe structures. By exploiting the non-linear behaviour of structures with specific geometries, a completely new scenario opens up and new fields of investigation appear. For instance, it is possible to overcome the limit of few stable positions in conventional multistable structures devising concepts with neutrally stable positions. Objectives of this research are first to demonstrate the viability of this concept of multistable structures and then to investigate possible applications and design prototypes which could benefit from it.
Self-healing for structural applications
Student: Rafael Luterbacher
Supervisors: Ian Bond and Richard Trask (University of Bath)
The aim of the project is to develop and demonstrate, within industrially relevant composite assemblies, how the growth of rapid, unstable cracks under static, cyclic and impact loading can be suppressed and ameliorated. Approaches for damage arrest and redirection -e.g selective interleaving of a thermoplastic film- and damage healing -e.g. heat activated remendable thermoplastic interleaved film, or a self healing agent delivered via a local vascular network- will be incorporated into structural features such as stringer run-outs.
Novel through thickness reinforcement development
Student: Beene M'Membe
Supervisors: Stephen Hallett, Ivana Partridge and Mehdi Yasaee
Composite laminates are predisposed to failure through delamination due to their poor through-thickness properties. Several approaches, such as tufting, stitching and Z- anchoring, exist to improve their interlaminar strength. Discontinuous through-thickness pinning is a novel and alternative method of reinforcing prepreg laminates in the through-thickness. This PhD aims to develop novel through-thickness reinforced (TTR) pins, which will combine material and geometrical features to provide improved energy absorption mechanisms during service relative to traditional carbon fibre z-pins. The individual pins and arrays of pins will be experimentally tested to characterise their performance when embedded in the composite. The novel data that will be generated with this project will be suitable for publications in high quality journals as well as providing Rolls-Royce with useful inputs in both manufacturing and design process of TTR composite structures.
Kirigami cellular structures: mechanical properties and shape morphing behaviour
Student: Robin Neville
Supervisors: Fabrizio Scarpa, and Manuel Collet (CNRS Besancon, France)
This project concerns the application of Kirigami (Origami + cutting) to engineering sheet materials, in order to create multifunctional cellular structures. Kirigami is a powerful technique for forming 2D sheets into 3D structures, which grants the user considerable freedom of design. During this project a new family of cellular structures – called “open honeycombs” – was developed. These structures derive unique properties and behaviour as a direct result of the Kirigami manufacturing process. The key features of open honeycombs are tailorable geometry, directional stiffness, and the ability to form cylindrical curvature. These features solve some of the problems associated with traditional honeycombs, and open up areas of application where traditional honeycombs are not viable – e.g. morphing and deployable structures.
Development of 3D printed vascular networks for repeated self-healing
Student: Isabel Qamar
Supervisors: Ian Bond, Richard Trask (University of Bath) and Anna Scott (University of Southampton)
A key limitation in the vascular self-healing systems to date has been the inability to achieve repeated healing for an infinite number of cycles due to fracture of the fluid-carrying vessel, which ultimately restricts and then terminates the transportation of the healing agent throughout the structure. In order to overcome this limitation a novel concept employing a porous thermoplastic network integrated within a fibre-reinforced composite laminate has been utilised. This approach promotes adhesive failure between the network and the surrounding host matrix material, thus exposing a series of radial pores and permitting the secretion of the liquid healing agent into the damage crack plane. In this study, mechanical characterisation of the crack-vascule interaction through a fracture mechanics assessment of hollow thermoplastic tubes embedded within a fibre reinforced composite has been undertaken. The experimental characterisation of this system is on-going with self-healing trials imminent.
Robust and reliability-based aeroelastic design of composite wings
Student: Carl Scarth
Supervisors: Jonathan Cooper, Paul Weaver and Pia Sartor
There has been much recent interest in using the anisotropic characteristics of composite materials to enable "aeroelastic tailoring" of aircraft wings. Such an approach can be used to increase the aerodynamic performance, eliminate "divergence" and reduce gust and manoeuvre loads. However, it is not possible to manufacture structures that do not possess some variation from the ideal design due to variability in the composite material, manufacturing process, etc. There is a need to quantify the uncertainty in the structural, aerodynamic and aeroelastic performance due the variability in material and manufacturing process and then to determine robust designs that are resistant to these variations.
The aim of the PhD is to develop techniques for efficient quantification of uncertainty due too relevant sources of variability in composite materials, and to ultimately determine optimal, robust, aeroelastically tailored designs for composite wings. State-of-the-art techniques such as Polynomial Chaos Expansion and Bayesian methods will be used to model the uncertainty alongside robust design techniques such as FORM and SORM. Aeroelastic response will be modelled using a range of techniques, from simple analytical structural and aerodynamic models applied to simple yet representative wing idealisations, to complex finite element models of full scale aircraft structures.
Multifunctional elastomers with tailored anisotropic response
Student: David Stanier
Supervisors: Sameer Rahatekar and Jacopo Ciambella (Sapienza Università di Roma)
Traditional engineering design focusses on the load-bearing capabilities of materials, and develops functionality by adding the structure. However, hybridising with functional materials allows non-load bearing capabilities to be integrated, that simplify the design. Material anisotropy can effect this multi-functionality in complimentary ways, however its effects need to be understood.
The aims of this study were to produce tailored anisotropy in elastomers, and to investigate the resulting mechanical and magnetic performance. Hence, a combination of experimental testing, and numerical & constitutive modelling were used to analyse the mechanical behaviour.
The placement of fibres with a homogeneous magnetic field is demonstrated and described by a simple model. The resulting transversely isotropic elastomers, reinforced by short nickel-coated carbon fibres, are compared to a number of numerical and constitutive mechanical models. Current methods for describing the behaviour of aligned reinforcements assume an inherent anisotropy, analogous to continuous fibre reinforcement, rather than discontinuous reinforcements. However, it is demonstrated that a simple constitutive model can adequately describe the behaviour up to moderate strains (<30%). In addition, a simplified numerical model is shown to represent the behaviour, and could be adapted to investigate complex effects, such as failure and interfacial properties.
In addition, the specimens actuate in a magnetic field, due to the nickel-functionalised fibres. The actuation is dependent on the reinforcement angle and is described by a simple model; furthermore, the combination of the magnetic and mechanical models allows the complimentary behaviour of these properties to be described.
The multi-functional material could be envisaged in a number of high performance applications, such as the active surface of micro-swimmers & -controllers. However, there are a number of challenges in experimental testing of anisotropic materials that require further investigation. Never-the-less, the orientation of reinforcements could be used to produce bespoke fibre alignments or for through-thickness composite repair.