• Investigating geometrical and manufacturing effects on the impact performance of UHMWPE composites
• Development of improved fibre reinforced feedstocks for high performance 3D printing
• Improving atomic oxygen resistance of composite materials for flexible deployable structures in space applications
• An adaptive shell model with variable-kinematics for the analysis of laminated structures
• Analysis and tailoring of stiffened panels with asymmetries via extended modal nudging
• On the sensitivity and validation of bending, twisting, and bend-twist coupling behaviour of wind turbine blade cross-sections
• Towards creating multi-matrix continuous fibre polymer composites using an out-of-vacuum bag process
• Exploiting thin-ply materials to establish controlled failure in carbon composites
• Structural and aerodynamic performance of a three-dimensional compliance-based composite
• Developing capabilities in materials and manufacturing for wind turbine blades by the application of an anhydride-cured epoxy resin system
• Suppressing delamination through Verticality Aligned Carbon Nanotube (VACNT) interleaves

Investigating geometrical and manufacturing effects on the impact performance of UHMWPE composites

Student: Behjat Ansari
Supervisors: Luiz Kawashita, Stephen Hallett, Ulrich Heisserer (DSM Dyneema) and Harm van der Werff (DSM Dyneema)

The need to continually enhance the ballistic performance of UHMWPE composite body armour has prompted numerous investigations into the failure mechanisms of these material systems, and the effects of dimensional and manufacturing parameters on their ballistic per- formance. Past studies have identified the contribution of the fibres to the laminate impact performance, while relatively little attention has been paid to the role of the matrix and its contribution to the overall energy dissipation. Likewise, while flat laminate panels have been studied extensively, in reality, panels used in impact protection are not necessarily flat, with many possessing single or double curvature. Furthermore, modern processing methods such as drape-forming, used in the fabrication of UHMWPE composite shells such as ballistic-grade helmets, induce the geometrical and manufacturing deformations of curvature and in-plane shear. The effects of these deformations on the ballistic impact performance of UHMWPE composites have, however, not previously been investigated. The two features must therefore be studied in isolation, in order to gain an understanding of their effects on impact performance.

In this thesis, cohesive elements are implemented into existing numerical tools to model interlaminar contact in flat laminates. The cohesive elements are used to investigate the in-plane and through-thickness dissipation of energy at sub-laminate interfaces under ballistic impact loading, as well as highlighting the contribution of the matrix to overall energy absorption by the laminate. Curved panels are tested under ballistic impact, demonstrating the geometrical effects of curvature on laminate response. In addition, existing numerical tools are shown to require modifications not previously necessary for flat configurations, to capture the impact response of curved laminates. A process is then developed for manufacturing sheared plates that are tested under ballistic impact, demonstrating the effects of in-plane shear deformation on the ballistic performance of UHMWPE composite plates. Finally, it is shown that current manufacturing standards are unsuitable for promoting uniform impact performance across the surface of doubly-curved components.

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Development of improved fibre reinforced feedstocks for high performance 3D printing

Student: Lourens Blok
Supervisors: Ben Woods, Kevin Potter, HaNa Yu and Marco Longana

Composite materials made from carbon fibres and polymer matrices can provide excellent mechanical properties and allow for significant design tailorability. A fundamental challenge with fibre reinforced plastics, however, is the combination of the reinforcements into the polymer matrix with good consolidation, maximum control of fibre orientation and low cost. While a wide range of processing methods for composites are available, most methods rely on applying a high pressure on the final part geometry to force continuous fibres into the required shape with a high fibre volume fraction and minimal amount of manufacturing defects. Small defects such as voids and wrinkles can have a large effect on the performance of the composite material and large presses or autoclaves are required to obtain high quality parts which increases the total cost of the part.

In this work, an alternative approach to composite manufacture is proposed for fibre reinforced plastic materials. Instead of consolidation of the composite material through high pressure on the final part geometry, an additive manufacture (AM) approach was used where the fibres are first embedded in a thermoplastic matrix in a pre-processing stage. The novel aspect of the project is that the continuous fibres were replaced by highly aligned short fibres to allow for improved printing performance and increased design freedom. Careful selection of the fibre length to be longer than the critical length of the fibre/matrix system allows for retention of the majority of the mechanical performance of a continuous fibre solution, without the need for cumbersome and restrictive fibre cutting and initial laydown procedures within the additive manufacturing process.

The aim of this project is to perform the analysis on creating and printing the short fibre reinforced filament, as well as creating a prototype for manufacturing the reinforced filament. The analysis part will focus on flow induced alignment of short fibres in viscous thermoplastic matrices during printing. The prototype is built around the High Performance Discontinuous Fibres (HiPerDiF) module, which is a fibre alignment method under development at the University of Bristol. The output of the HiPerDiF module is a continuous preform of aligned dry fibres with a width of 3 mm. To be turned into a printable filament, the preforms of dry fibres first need to be consolidated with a thermoplastic matrix to improve handling. The second step is to shape the resulting short fibre plastic strip into a circular filament with uniform properties. The third step is to print the short fibre filament, investigating the fibre alignment and flow dynamics.

The two main advantages of this process are: (i) enabling 3D printing of short fibre reinforced composites parts with high mechanical properties and (ii) high suitability for recycling by using short fibres and a thermoplastic resin. These advantages may lead to a broader range of applications for 3D printed parts, where higher mechanical properties are required whilst keeping the advantage of rapid prototyping and customisability. The local alignment of short fibres directly influences the mechanical properties and control of the short fibre placement would enable advanced topology optimisation.

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Improving atomic oxygen resistance of composite materials for flexible deployable structures in space applications

Student: Yanjun He
Supervisors: Ian Hamerton, Mark Schenk and Alex Brinkmeyer (Oxford Space Systems)

Launching satellites into space is expensive, costing around £20,000 per kilogramme of payload placed into orbit. However, ‘new space’ companies such as SpaceX, Blue Origin, and start-ups such as Oxford Space Systems, are developing radical innovations that attempt to make space access more affordable. Deployable structures, commonly used to deploy RF antennas, solar panels, and booms are conventionally complex, heavy, and expensive. Oxford Space Systems is pioneering the use of flexible composite structures, which offer the space sector a simpler, lighter, and more cost-effective solution.

However, one of the challenges of using these structures is to demonstrate their ability to resist the extremely harsh environment of space – the combination of deep vacuum, UV, proton and electron radiation and atomic oxygen interaction can strongly affect the mechanical performance of the material. In addition, the extreme temperature fluctuations encountered in space can lead to thermal distortion, degradation and/or failure.

This project aims to enhance the material properties of flexible composite structures used in a space environment, by developing a new inorganic compound that can be introduced to the matrix material. The objective is to impart a high creep resistance and the ability to withstand a long duration in the space environment, while maintaining good flexural properties.

The objectives of the project are as follows:

• Identify suitable nanomaterials as additives in the compound.
• Investigate whether the chosen nanomaterial(s) can promote the mechanical performance of the material.
• Investigate whether the developed material can meet the requirements of the flexible deployable structures for space applications.

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An adaptive shell model with variable-kinematics for the analysis of laminated structures

Student: Aewis Hii
Supervisors: Luiz Kawashita, Alberto Pirrera, Stephen Hallett and Adam Bishop (Rolls-Royce)

For the past five decades, the finite element (FE) method has been a versatile virtual testing and optimisation tool of engineering structures. Fibrous composite structures are increasingly used in the aerospace industry as they offer lightweight solution to stiffer, stronger structures with excellent fatigue properties. However, fibrous composites display highly nonlinear response under dynamic loading, especially when irreversible damage mechanisms are involved. In the case of large and thick composite structures with complex geometry and thickness tapering, an accurate analysis using currently available modelling tools will require infeasible computational costs. Currently, the industry has to utilise low fidelity modelling techniques to model large composites, which compromises the analysis accuracy.

The aim of the project is to develop efficient numerical tools to model accurately the dynamics of large and thick composite structures with complex geometry accounting for various damage mechanisms. Different analysis methodologies are studied to better understand limitations of the current analysis tools. The most strategic solution to the research challenge will be identified and further developed, with evidence in terms of literature data and numerical experiments. Lastly, the developed numerical tool(s) will be benchmarked against existing experimental/numerical results in terms of accuracy and numerical efficiency.

Currently available high fidelity modelling approaches for composites allow only for the analysis of structures with modest size and geometrical complexity. The envisioned numerical tool will improve upon the current modelling capability for fibrous composites, by allowing for high fidelity analysis of larger, thicker and geometrically more complex structures. With the computationally efficient numerical tool(s) or technique(s) in hand, structural analysts can perform more rigorous optimisation in terms of geometry, layups and tapering scheme in early design phases.

Over the past two decades, many advances have been made on structural theories, some of which could resolve the research challenge addressed in this project. However, only a small proportion of these technologies have been transferred onto commercially off-the-shelf analysis tools. The novelty of this project is in the application of advanced theories in contexts that are more industrially relevant, with scope to integrate a numerical ‘toolbox’ for design optimisation of composite structures in aerospace applications.

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Analysis and tailoring of stiffened panels with asymmetries via extended modal nudging

Student: Olivia Leão Carvalho
Supervisors: Alberto Pirrera, Paul Weaver and Rainer Groh

Nonlinearities have long been treated as undesired aberrations in structural design due to the complexities they add to the problem. As a result, the majority of structures are designed to be stiff and exhibit small deformations when subjected to a certain load condition, guaranteeing that they only work in the linear regime. However, with the capabilities offered by composite materials and with more advanced numerical methods and computational power now available for solving nonlinear problems, new possibilities have emerged. This unprecedented scenario makes it easier to understand nonlinearities and, therefore, conceivable to use them, rather than avoid them, in structural design.

This research project proposes to embrace nonlinearities and exploit the unexplored capabilities they can provide as a means of achieving superior structural efficiency. The underlying idea is that restricting the structure to behave linearly may correspond to overdesigning it. The principal objectives are to investigate and demonstrate the concept of structural efficiency via stiffness adaptation. The project involves extensive design studies using numerical/analytical techniques, optimisation procedures and physical prototyping to assess the manufacturability of the proposed structural concepts.

This unexplored design philosophy, i.e. efficiency via stiffness adaptation, can be beneficial to applications in which structures have to operate under different conditions, while also meeting strict mass requirements. It would not only further improve high-performance designs such as adaptive structures, but also provide a new mentality and understanding to re-assess well established design systems. For example, a change in stiffness can be useful to prevent the collapse of buildings and infrastructure subject to occasional extreme wind and earthquake loads.

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On the sensitivity and validation of bending, twisting, and bend-twist coupling behaviour of wind turbine blade cross-sections

Student: Vincent Maes
Supervisors: Paul Weaver, Alberto Pirrera, Terence Macquart and Tomas Vronsky (Vestas)

Scaling up of rotor technology is a key challenge faced by industry for future turbines. Larger rotors provide greater total energy capture, but also suffer increased loads on the blades and, subsequently, accelerated wear of all components of the turbine. To reduce the wear, load alleviation techniques have received significant interest from both academia and industry. Various authors have proposed both active and passive solutions, and one prominent technique is the implementation of Bend-Twist Coupling (BTC). When used to induce twist towards feather under flap-wise bending, BTC has been shown to reduce the maximum loads on the turbine drastically. While the advantages of BTC are well documented, the underlying modelling approaches are still mostly unvalidated. Code-to-code comparisons and validation exercises between techniques have shown significant discrepancies. It has also been demonstrated that at least some of the difference can be traced to small discrepancies in inputs and cross-sectional representation.

This thesis contributes to the body of research on cross-sectional modelling of wind turbine blades through both numerical and experimental work. Using two cross-sectional modellers (i.e. BECAS and VABS) and 3D Finite Element Models in ABAQUS, a numerical sensitivity study has been carried out. Comparison of the compliance terms for three typical turbine blade cross-sections highlights the innate performance variability caused by manufacturing tolerances and modelling simplifications. The results show the importance of the Trailing Edge (TE) bondline, previously unstudied, to the accurate prediction of the stiffness of compliant and coupled beams. The TE bondline is also shown to contribute meaningfully to the disagreement observed between 3D shell and solid model predictions.

The numerical insights are supported by experimental work through testing of several demonstrators, designed to contain BTC, under cantilever loading. Each of demonstrator has a different cross-sectional design, capturing the influence of various common features of wind-turbine blade cross sections. Together, these demonstrators help validate the modelling techniques investigated and provide important practical insights for cross-sectional properties of compliant and coupled beams. As such, this work helps establish best practices for modelling and testing of bend-twist coupled beams. In doing so, it also provides reliable experimental data for validation and benchmarking purposes of current and future numerical and/or analytical modelling techniques.

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Towards creating multi-matrix continuous fibre polymer composites using an out-of-vacuum bag process

Student: Arjun Radhakrishnan
Supervisors: Dmitry Ivanov, Ian Hamerton, Fabrizio Scarpa and Milo Shaffer (Imperial College London)

This project aims to bring together concepts of 3D printing and nano-engineering, which have a potential to generate exceptional composite properties. The primary focus is to create multi-matrix patterned functionalised composite systems, containing additives in an optimum pattern and enhancing composites structurally and functionally. These composites would have wide range of potential applications in the aerospace and renewable energy sectors.

Liquid Resin Printing (LRP) is a novel method to 3D print liquid reactive resins into continuous textile preforms. The technique allows the local introduction of resin, using high precision injections, to form potentially enhanced patches of matrix. Various additives, such as carbon nanotubes (CNTs), can be used to enhance the properties of the printed patches. The technique forms a novel composite with multi-level features ranging from structural to nanoscale. Hence, this project would formulate the processing map in order to achieve control of the resin printing for the development of multi-matrix patterned functionalised composite systems. The major objectives are:

Validate the concept of additive matrix manufacturing in application to nano-engineered polymers and to draw the map of method limitations, challenges and solutions.
Development of process simulation to understand the flow of additives within the solution during injection and consolidation.
Create a range of patterned composites and determine their mechanical and functional properties.
Investigate the damage accumulation mechanism of the patterned composites.

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Exploiting thin-ply materials to establish controlled failure in carbon composites

Student: Tamas Rev
Supervisors: Michael Wisnom, Ian Bond and Gergely Czel

Determining accurate values for strength of composites under multi-axial loading is crucial for the successful and safe exploitation of these materials. However there are fundamental problems with even determining uniaxial tensile strength since specimens tend to fail at the grips where stress concentrations arise [1]. Whilst such tests are acceptable for conservative design, they are problematic for establishing the real behaviour of the materials. Premature failure also masks underlying size effects, where the strength of composites tends to decrease with increasing volume of material due to the statistical effects of defects [2], which is often ignored due to lack of accurate experimental data. A recent breakthrough in Bristol as part of High Performance Ductile Composites Technology programme (HiPerDuCT) has shown that stress concentrations at the point of load introduction can be eliminated in hybrid specimens, for example with carbon plies sandwiched between layers of glass.

In a unidirectional hybrid, the outer layers act as in-situ tabs, and although stress concentrations occur in the glass layer, stresses are actually lower in the carbon layer. Failure therefore occurs in the gauge section, and substantially higher failure strains and stresses are obtained than with conventional tests. Combining the method mentioned above and using hybrid specimens with thin plies creates an exciting opportunity to conduct tests on carbon/epoxy composite laminates under combined stress states eliminating the issues of tab and edge failure which have plagued previous experimental programmes. The main research challenge of the project is to look at the real effect of transverse tension, compression and shear on the ultimate fibre direction strain to failure of carbon fibre / epoxy composites. Thin-ply hybrid tests are proposed, offering potential for a breakthrough in establishing the interactions between these stress components by avoiding premature failure. This is important from a fundamental point of view in understanding the behaviour of these materials, and is also critical for applications in order to take maximum advantage of the real properties, reduce the requirement for structural testing, and avoid premature failures.

The main objectives of this project are:

• Design hybrid composite laminates which give appropriate combinations of stresses in the carbon plies.
• Carry out tensile tests to determine the failure strains and mechanisms for the different layups.
• Construct failure envelopes, establish failure criteria, and compare with existing approaches in the literature.
• Carry out a similar investigation of the effect of interaction of other stress components on compressive failure.

[1] Wisnom MR MM. Tensile strength of unidirectional carbon fibre-epoxy from tapered specimens. 2nd Eur. Conf. Compos. Test. Stand., 1994, p. 239–47.

[2] Wisnom MR, Jones MI. Size effects in interlaminar tensile and shear strength of unidirectional glass fibre/epoxy. J Reinf Plast Compos 1996;15:2–15.

[3] Czél G, Jalalvand M, Wisnom MR. Hybrid specimens eliminating stress concentrations in tensile and compressive testing of unidirectional composites. Compos Part A Appl Sci Manuf 2016. doi:10.1016/j.compositesa.2016.07.021.

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Structural and aerodynamic performance of a three-dimensional compliance-based composite

Student: Andres Rivero Bracho
Supervisors: Ben Woods, Paul Weaver and Jonathan Cooper

Traditional fixed wing aircraft (i.e. commercial airliners) are usually designed, aerodynamically, for optimal performance at one specific cruise condition. As consequence, the performance of aircraft at flight stages and scenarios that do not occur at cruise condition (e.g. takeoff, climb, landing, unexpected gust, etc.) is far from optimal. This results in lower profit margins for airlines due to fuel consumption and noise, as well as higher emissions, per distance travelled, of carbon dioxide (CO2), oxides of nitrogen (NOx), among others greenhouse gases.

The Fish Bone Active Camber (FishBAC) concept is an active morphing trailing edge device that creates large magnitude, continuous, and smooth changes in aerofoil camber, and therefore, changes in lift distribution with a ‘low’ drag penalty. The aim of this project is to design a three dimensional aeroelastically optimised composite wing structure that combines active camber morphing for lift control and load alleviation, using the FishBAC morphing trailing edge device. Specific objectives of this project are listed as follows:

• Design, manufacture and wind tunnel test of the first composite FishBAC active camber device
• Development of analytical fluid-structure interaction routine for design and optimisation
• Design, manufacture and wind tunnel test of a static aeroelastically tailored three-dimensional camber morphing wing manufactured using composite laminates
• Quantitative study of system level benefits of a three-dimensional camber morphing fixed wing.

Potential benefits include reductions in drag, fuel consumption, noise and weight due to optimisation of the spanwise lift distribution and a decrease in induced and control surface drag throughout the entire flight envelope.

The novelty of this research project lies in the fact that it combines active and passive structural morphing techniques for optimising aircraft performance, as well as a simultaneous optimisation of the chordwise (i.e. 2D aerofoil aerodynamics) and spanwise (i.e. 3D wing aerodynamics) lift distributions. The outcome of this project will not only deliver a novel wing design, but will also develop modelling techniques that combine three-dimensional aerodynamics with structural mechanics, at multiple levels of fidelity.

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Developing capabilities in materials and manufacturing for wind turbine blades by the application of an anhydride-cured epoxy resin system

Student: Bethany Russell
Supervisors: Ian Hamerton, Carwyn Ward and Shinji Takeda (Hitachi Chemical)

Glass fibre reinforced composites are widely exploited in the manufacture of wind turbine blades. Epoxy resin matrices have been widely used in this application, due to their inherent high specific strength. Current trends in wind turbine technology shows that the sizes of the rotor blades for both on- and offshore systems are increasing. As turbine blades increase in size the turbine efficiency increases, and there is a significant drive to replace the E-glass reinforcement with carbon analogues that are both stiffer and lighter.

A new, infusible epoxy resin system has been developed which offers enhanced toughness, lower weight, high strength and particularly well developed interphase region between the resin and fibre. As part of a smaller six-month project the physical cure of the resin and some mechanical properties (interlaminar strength and impact resistance) of the composite part were examined. The PhD project will look to extend this chemistry into higher performance carbon reinforced composite analogues and assess their suitability for use in larger turbine blades.

To achieve this several objectives must be achieved:

• To blend commercially available monomers to achieve suitable rheological properties to infiltrate carbon fibre reinforcement. This will also examine the infusion process over more complex geometries which are more representative of the finished structure.
• To produce CFRP samples suitable for mechanical testing, both static and dynamic. To demonstrate the fitness for purpose for use in wind turbine blades.
• Improve the impact resistance, which has been identified as a key performance parameter, over existing epoxy CFRP structures. In addition to macroscopic tests, the influence of wind and water erosion on the laminate will also be examined. The modification of surface plies with the aim to increase abrasion resistance and surface hardness through the incorporation of e.g. nanosilica species will be explored.
• The potential to increase the multifunctionality, such as increased interfacial toughness and electrical/thermal conductivity, through the formation of nanocomposites can also be examined.
• A series of CFRP demonstrators will be manufactured to various scales, which integrate all the features examined throughout the project to fully demonstrate their utility.

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Suppressing delamination through Verticality Aligned Carbon Nanotube (VACNT) interleaves

Student: Robert Worboys
Supervisors: Luiz Kawashita, Ian Hamerton, Stephen Hallett and Rob Backhouse (Rolls-Royce)

Background

Nano-composites are materials made up of two or more constituents where at least one is dispersed (or exhibits discrete features) at the nanometre length scale. This may produce an extremely high interfacial area density, which can result in the macroscopic material properties being radically different from those of the individual constituents, without a dramatic increase in structural mass. This applies to all physical properties including mechanical, optical, electromagnetic, etc.

In the area of mechanical reinforcement of engineering materials, some technologies have recently become available for large scale industrial applications. Examples include:

• The N12™ NanoStitch™ inter-layer toughening film with vertically aligned carbon nanotubes, which can, it is claimed, improve the delamination toughness of the composite by up to four times.
• The 3M™ Matrix Resin 4831 with “highly uniform dispersions of silica nanoparticles”, which can, it is claimed, increase the impact-burst strength of filament wound pressure vessels by up to 30%.
• Other novel techniques developed within the University via academic collaboration. Examples include the alignment of graphene flakes, the growth of CNTs on to carbon fibres (Fuzzy Fibres) and the use of graphene / graphene oxide as toughening agents for commercial material systems.

Aims and objectives

The aim of this PhD project is to compare nano-reinforcement improvements in fracture toughness and impact resistance relative to other through thickness reinforcement techniques such as z-pinning, stitching, 3D-weaving, etc. Particular attention will be paid to reinforcement techniques that are usable at an industrial scale.

In order to do this, the following objectives will be set:

• Manufacture simple coupons with and without different types of nano-reinforcements.
• Test for basic strength / fracture properties under a variety of loading and environmental conditions.
• Conduct fractography and material characterisation analysis to help interpret the results of the mechanical tests.
• Develop computational models that enable the equivalent elastic properties of the nanomaterials to be estimated.
• Using these elastic properties, develop additional computational models with increased geometric complexity to predict failure.
• Manufacture and test geometrically more complex specimens and compare failure response to model predictions.

The development of modelling tools for nanocomposites will enable them to be used in composite design, for example as a method to delay or guide failure in a controlled manner.

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