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PhD projects: 2021 cohort

Mycellium composites for sustainable construction materials in developing countries

CDT21 Student: Stefania Akromah
Supervisors: Steve Eichhorn and Neha Chandarana

Mycelium-based composites (or mycelium composites) are a class of novel sustainable materials based on organic agricultural and agro-industrial wastes bound by a fungal mycelial network. They are gaining increasing attention due to their biocompatible and biodegradable nature, and tuneable properties which are dependent on the fungal strain and growth medium characteristics. Mycelium composites are produced by fungal decomposition of organic materials and, thus, are cost- and energy-efficient, and can add new value to organic waste streams. Therefore, they have become an attractive alternative material with potential for use in the construction industry. Additionally, the use of mycelium composites could substantially reverse carbon emissions and reduce the global carbon footprint of this industry.

Currently, mycelium composites are only used for packaging and in non-structural applications due to the typically low strength, stiffness, and foam-like characteristics [1]. This has been attributed to the high content of weak non-structural compounds (e.g. glucans and proteins) and the random orientation of fungal hyphae in the mycelium network which minimises the mycelium-substrate bonding interfaces (Figure 1).

Fig. 1 (a) Mycelium composite; (b) SEM of mycelium network of hyphae; (c) Schematic diagram of the structure of the fungal cell wall [2].

 

 

The aim of this project is to investigate the potential use of mycelium composites in load-bearing applications. The project will particularly focus on optimising the structure of the mycelium network using some of the structure modification strategies used in engineered woods such as the “SuperWood”. SuperWood, a high-performance structural engineered wood, is made by partial-delignification of natural wood followed by densification [3], [4]. The enhanced properties of engineered woods are favoured by the structural anisotropy of natural wood as a result of the high directionality of the cellulose fibrils at different scales. However, mycelium networks are characterised by randomly oriented filaments (hyphae).

The project will investigate orientation and densification approaches to enhance crystallinity, inter-fibrillar bonding, and the structural performance index of mycelium composites. Such composites could be used in Africa and other developing countries to support their efforts towards sustainable development. Africa, for example, is richly endowed with natural reserves; however, high processing costs poses a limitation to local production. Meanwhile, the exponential population growth has resulted in mounting food demands and increasing crop output, leading to the high generation of agricultural by-products and wastes. Thus, a huge potential for sustainable materials based on organic wastes. Mycelium-based composites as a cost-efficient and sustainable alternative could improve the SDG index of these countries, supporting the SDG 2030 Agenda for Sustainable Development (i.e., “Leave No One Behind (LNOB)”) [5].

References

[1] M. Jones, A. Mautner, S. Luenco, A. Bismarck, and S. John, “Engineered Mycelium Composite Construction Materials from Fungal Biorefineries: A Critical Review,” Mater Des, vol. 187, p. 108397, 2020, doi: https://doi.org/10.1016/j.matdes.2019.108397.

[2] M. R. Islam, G. Tudryn, R. Bucinell, L. Schadler, and R. C. Picu, “Morphology and Mechanics of Fungal Mycelium,” Sci Rep, vol. 7, no. 1, pp. 1–12, 2017, doi: 10.1038/s41598-017-13295-2.

[3] J. Song et al., “Processing Bulk Natural Wood Into a High-Performance Structural Material,” Nature, vol. 554, pp. 224–228, 2018, doi: 10.1038/nature25476.

[4] C. Chen et al., “Structure–Property–Function Relationships of Natural and Engineered Wood,” Nature Reviews Materials, vol. 5, no. 9. Nature Research, pp. 642–666, Sep. 01, 2020. doi: 10.1038/s41578-020-0195-z.

[5] UNSDG, “Principle Two: Leave No One Behind,” https://unsdg.un.org/2030-agenda/universal-values/leave-no-one-behind, 2022.

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Wind blade design for disassembly

CDT21 Student: Tom Brereton
Supervisors: Paul Weaver, Alberto Pirrera, Terence Macquart

The wind energy industry is currently the most prominent renewable energy sector in the UK, accounting for 24% of its total energy production in 2020. In order to increase the feasibility and sustainability of this vital technology, significant steps need to be taken to reduce the material waste generated by wind turbine blades at end of life.

Primarily, blades are manufactured from glass or carbon fibre epoxy composites. While this enables them to withstand the extreme weather and cyclic loads they are exposed to, recycling or repurposing these materials upon the blade’s decommission is a currently unsolved problem. The most common practice is simply to bury the blades in a landfill, thus completely disregarding the possibility of potential reuse and recirculation of these high value composites. Alternative measures such as burning or mechanical grinding are becoming more popular. However burning the blades for fuel is far from ideal due to the low energy content of the materials and the hazardous pollutants emitted when burning glass fibre. While mechanical grinding of the blades for use in the construction industry is more environmentally conscious, with the amount of energy used to manufacture and process the blades initially, is this really the best we can do in terms of reuse?

In order to maximise the material value of the blades at end of life, the most efficient reuse strategies would aim to repurpose whole, as-manufactured composite components. This would allow for minimal loss of mechanical performance as well as maximal energy efficiency in terms of manufacturing and processing replacement parts. To implement such a strategy in the wind energy industry, a “Design for Disassembly” philosophy needs to be implemented at the point of initial blade design.

This project aims to embody this design philosophy by conceiving a new blade design focusing entirely on disassembly. In this work, ‘disassembly’ is defined by two main criteria; modularity and structural separation of internal blade components. A blade design combining both these aims will allow for the most efficient reuse of blade materials while preserving as much of their intrinsic value as possible. To achieve this, research into the most effective joining mechanisms in terms of load-transfer and ease of disassembly will be conducted. A thorough investigation into the manufacturability and cost of the proposed designs will be completed, by which structural performance and industrial practicality can be correctly balanced. Accurate turbine simulation modelling will also be undertaken to ensure the dynamic performance of the redesigned blade is sufficient for operation, and can be verifiably considered as a promising next stage of wind turbine blade design.

This project is supported by Vestas.

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Hybrid composites for improved impact performance

CDT21 Student: An Chen
Supervisors: Michael Wisnom, Luiz Kawashita, Stephen Hallett, Xun Wu

Carbon fibre reinforced polymer (CFRP) materials have excellent in-plane properties and are widely used in primary structures of aircraft because significant weight savings can be achieved without compromising safety. However, CFRP is vulnerable to out-of-plane loading due to lack of through-thickness reinforcement and low strain to failure. Impact is a typical loading on the transverse direction and is very important in many aerospace applications for safety and certification, e.g., engine casings and wing leading edges. Recent experimental work in Bristol has shown that, compared to CFRP, hybrid composites composed of carbon, glass and PBO fibre composites have better impact performance. The underlying factors controlling the response and detailed mechanisms for the improvements are not well understood. Numerical models for predicting behaviour can give good insight and understanding of the damage development in composites under impact, where this cannot be determined from experiments alone. However, models for understanding of hybrid behaviour are lacking.

The PhD project aims to develop a numerical model for simulating impact events of hybrid laminates and to use the model for designing hybrids with different types of carbon, glass and PBO fibres. It is expected that a material models library of different type of composites will be developed, and new modelling capabilities will be implemented to simulate the behaviour which are not available in the standard code. The simulation result will be used for understanding the driving damage mechanisms and the effect of different hybridisation strategies during the penetration process. More specifically, how fibre failure and delamination interact to control the behaviour of composites under impact, how this is affected by combing different materials, and how the behaviour can be improved.

To achieve the aim, several objectives of increasing complexity have been planned out as shown below:

  • Initially, a simplified axisymmetric finite element model will be developed to simulate the impact response of hybrid laminates. Post-processing will be used to understand the driving mechanisms and compare the existing experimental data. The analysis will then be used to undertake parametric studies to characterise important material properties, and to propose novel hybrid configurations with potential to give improved impact performance.
  • Secondly, the work will be extending the modelling to high fidelity 3D simulations incorporating state-of-the-art approaches and carrying out further experimental investigations. These analyses will be used to investigate the behaviour in full detail and validate the assumptions of simpler models. Correlation can be undertaken with existing experimental data for static indentation and impact tests, and new tests carried out where needed. The results can then be applied to designing new optimised configurations which can be manufactured, tested, and evaluated.
  • Finally, the final objective will be a validated design methodology that can be applied to real applications in collaboration with industrial partners.

It is believed that if the damage mechanisms of hybrid composites can be characterised using the novel model developed in this project, the proposed design methodology could lead to breakthroughs in applications of composites requiring high resistance to impact.

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Bioinspired composites for the future: sustainable hybrid composites for compression

CDT21 Student: Eleni Georgiou
Supervisors: Richard Trask, Ian Hamerton, Gustavo Quino Quispe (Imperial)

In recent decades, continuous synthetic fibre-reinforced polymers have become increasingly popular for advanced structural applications. This is largely due to their superior specific properties, corrosion resistance and potential for enhanced fatigue performance when compared to their metallic counterparts. However, with the sustainability of engineering materials becoming an increasingly important consideration amid growing environmental concerns, interest in natural fibre composites (NFCs) is growing due to their potential to replace synthetic fibre-reinforced polymers with improved sustainability and at a lower cost. NFCs can also offer specific properties comparable to E-glass fibre, which is one of the most common reinforcing fibres for polymer composites. Despite this, the uptake of NFCs for structural and load-bearing applications is limited.

One key reason for this is the current gap in knowledge regarding their mechanical behaviour. In particular, very few reports in open literature discuss the compressive performance of NFCs, and the resulting scarcity of data severely degrades confidence in the use of NFCs for load-bearing applications. The longitudinal compressive performance is design-limiting for continuous fibre-reinforced polymers. The compressive loads that arise in structures are typically accounted for at the design stage with higher safety factors. This often results in overdesigned and inefficient structures, further solidifying the need for investigating the compressive performance of NFCs.

The aim of the project is to characterise the compressive failure of NFCs and develop an understanding of principles that govern the failure of NFCs in compression. Hybridisation will be investigated as a means to improve mechanical performance. The application of hierarchical and functional grading will also be considered since there is undoubted potential for bioinspired hierarchical architectures to help resolve the challenges that arise from intrinsically conflicting property demands. The end goal of the PhD is to produce and conduct macro-scale testing to assess the structurally representative performance of an NFC component. The design of this component will utilise the new understanding of sustainable hybrid composites developed throughout the project. Given the importance of future-proofing composites, the findings of this work could help industries working towards sustainable material solutions. The development of novel material systems with improved compressive performance may encourage new industries, which currently do not routinely use composites, to use higher-performance materials. They may be enabled to do so by the resulting less stringent design limits. Industries of interest include but are not limited to, the marine, wind energy and automotive sectors.

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Developing crack resistant polymer composite matrices for liquid hydrogen storage

CDT21 Student: James Griffith
Supervisors: Ian Hamerton, Valeska Ting, Karthik Ram Ramakrishnan, Sebastien Rochat

With the international effort to reach Net Zero by 2050, the aviation industry is in a race to adopt zero-carbon emission fuel sources within the coming decades. It is widely accepted that liquid hydrogen (LH2), a cryogenic liquid stored at 20 K (-253 °C), will serve as this fuel source for the majority of the aircraft market, as it offers a better payload and range to alternative solutions such as batteries or gaseous hydrogen. Nevertheless, there are many challenges associated with the transition to liquid hydrogen that the aviation industry will need to overcome. One significant challenge is the storage of LH2 aboard the aircraft. While metallic tanks are currently used in the space industry for single-use LH2 launch vehicles and are likely to be first to market in civil aviation; carbon fibre reinforced polymer (CFRP) tanks offer significant gravimetric efficiency benefits which, over the multi-cycle 20–25-year lifetime of a tank, translates to considerable cost savings. While CFRP tanks are considered promising long-term storage vessels, the susceptibility of the polymer matrix to microcracking at these extremely low temperatures is currently a primary barrier to adoption. There are a number of issues with microcracking in this application, not least the safety and thermal issues associated with increased hydrogen permeation, but also the integrity of the tank being compromised and the chance of liquid hydrogen boil off within crack networks causing delamination or tank rupture.

Matrix microcracking at cryogenic temperatures is understood to be caused by the build-up of thermally induced residual stresses through several possible mechanisms. On the microscopic level, the mismatch of co-efficient of thermal expansion (CTE) between the fibre and matrix leads to residual stresses in both constituents during thermal cycling. On the next level of structural hierarchy, the mismatch of effective CTE between adjacent plies with varying fibre orientation is a possible cause. In addition to this, when cooled down to cryogenic temperatures a material can experience thermal shock via inhomogeneous temperature distributions, where neighbouring domains encounter different temperatures, creating a steep temperature gradient across the material. This can also result in the development of transient thermally induced stresses and in turn cause microcracking. The overarching aim of this project is therefore to develop a polymer composite matrix which can withstand repeated exposure to a 20 K cryogenic environment without microcracking and be suitable for use in LH2 storage tanks.

To address this, the key objectives of this project include:

  • Determine, through a design of experiments testing approach, which polymeric molecular properties or toughening methods enable matrices with the desired thermomechanical and physical properties to supress microcracking.
  • Design and synthesise new matrices or adapt existing materials to incorporate these material properties or toughening methods.
  • Characterisation of these matrix materials to validate the design process and subsequent manufacture of composite panels using the best candidates.
  • Design and conduct a rigorous testing campaign to characterise the composite materials with respect to the key performance indicators for LH2 tanks, such as microcrack fracture toughness, hydrogen permeability and resistance to cracking under repeated cryogenic cycling.

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Novel, bioinspired, aligned-discontinuous reclaimed fibre composites for enhanced compressive performance

Student: Ian Lee
Supervisors: Giuliano Allegri, Laura Pickard, Ian Hamerton, Marco Longana

Despite over 60 years of active research and considerable improvements in properties relating to tensile strength and impact resistance, compressive strength levels in composites are still some 40% lower than measured longitudinal tensile strengths. This relative weakness, and the difficulty in systematically modelling the resultant anisotropic failure mechanisms, represents a significant barrier to the wider industrial adoption of these materials.

This research project forms part of the EPSRC supported Next Generation Fibre-Reinforced Composites (NextCOMP) programme, concentrated on developing novel composite materials to meet this challenge. A principal focus is investigating hierarchically structured materials, inspired by natural composites such as bone and wood. These biomaterials feature dissimilar but complementary reinforcement systems across length scales and exhibit higher compressive load carrying capacities than traditional manufactured composites. It is anticipated that a new generation of manufactured composite materials able to mimic such architectures will find numerous innovative industrial applications including in the aerospace, energy, and automotive sectors.

Discontinuous fibrous materials have historically been used as bulk reinforcement in manufactured composite structures due to lower overall mechanical properties. A key determinant of their performance being the degree of fibre alignment. The High-Performance Discontinuous Fibre (HiPerDif) process developed at the University of Bristol is a proven method for producing composite tapes of highly aligned discontinuous fibres of between 1 and 12mm in length, utilising water as a transfer medium. The process offers the potential to produce materials with mechanical properties comparable to those of continuous fibre composites, given a fibre aspect ratio high enough to allow load transfer and fibre pull out. Furthermore, highly aligned fibre composites have shown strong promise in overcoming current limitations of continuous fibre materials such as: lack of ductility, and the resultant restrictions in available forming methods; difficulties in high volume, defect-free automated production of complex geometries; and integration of the truly sustainable production methods required in a circular economy.

This project will investigate the behaviour of a range of manufactured highly aligned fibrous materials in compression and assess their potential for use in the hierarchically structured composite materials being researched within the NextCOMP programme.

AIMS:

  • Investigate the behaviour of a range of highly-aligned discontinuous carbon-fibre composites in compression.
  • Assess the potential for utilising these materials within larger composite structures, inspired by natural composites, and featuring hierarchical architectures.
  • Investigate potential processing methods for such composites that improve beneficial material property and production rate characteristics. To include mechanised processes such as automated tape laying, prepreg filament winding and human-robot collaboration.
  • Undertake trials of discontinuous fibre composites to determine compressive performance and manufacturing efficiency.
  • Manufacture, test and assess demonstration structures, featuring discontinuous fibre composites as one element within a more complex hierarchical architecture. Structural geometries and material composition to be industrially relevant.

OBJECTIVES:

  • Identify and assess processing methodologies and sources of discontinuous carbon-fibre composite materials.
  • Determine appropriate geometries and manufacturing methods for useful sample and demonstrator testing and data acquisition.
  • Acquire suitable experience in using automated composite processing methods.
  • Identify suitable mechanical testing regimes that generate useful and repeatable comparative data.
  • Continually assess results in the broader context of the NextCOMP research program.

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Fatigue and damage tolerance of 3D-woven composites

CDT21Student: Christian Stewart
Supervisors: Stephen Hallett and Bassam Elsaied

Recently, 3D woven composite materials are seeing wide use in engineering applications such as aerospace, marine, automotive and renewable energy sectors. This category of composites provides several economic and performance advantages over their laminated counterparts and metal alloys. 3D woven composites are usually prepared in near net-shape preforms before being infused by resin and cured to form the final structure. This production method eliminates the costs associated with layup of laminated composites or with use of expensive prepreg materials. Additionally, 3D woven composites provide some key mechanical performance advantages such as higher damage tolerance, as well as better impact and delamination resistance.

Since the main area of application for 3D woven composites involves impact, it is necessary to develop modelling capabilities that can capture behaviour such as fatigue life after impact events. The fatigue behaviour of textile composites is less well understood than for laminated structures. To this end, this project aims to develop a deep understanding of the fatigue and damage tolerance of 3D woven composites. This will be achieved via a combination of experimental characterisation and high-fidelity modelling. Research has shown that small scale damage (e.g. matrix cracking and debonding) initiates in 3D woven composites at low loads. Due to these materials’ high tolerance to damage and the ability to redistribute stresses, this meso-scale damage doesn’t affect the 3D woven structure’s mechanical performance under static loading conditions and is only detectable using advanced Non-Destructive Testing (NDT) techniques. However, when subjected to increased load levels and cyclic fatigue loading, the meso-scale damage progresses, reduces the stiffness, and will ultimately lead to the material’s final failure.

The main interest of the project is the progression of meso-scale damage in these materials under cyclic fatigue loading post-impact. Low-velocity impact tests add a certain level of uncertainty and variability in the level of damage introduced into the specimen prior to cyclic fatigue loading. This makes it difficult for several post-impact fatigue tests to start with the same level of damage. Conversely, the use of a notched specimen ensures a level of damage which is more measurable and repeatable. Therefore, the first part of the experimental testing will focus on notched 3D woven composites.

Fatigue testing of 3D woven composites containing pre-initiated meso-scale damage will then be carried out. Their purpose is to gain a deep understanding of how damage progresses in 3D woven composites under cyclic fatigue loading. To this end, advanced NDT and imaging techniques (e.g., CT scanning, acoustic emission, etc.) will be employed. This understanding will help the development of a high-fidelity model capable of predicting meso-scale damage progression in 3D woven composites under fatigue loading. This model will then be validated against the experimental data.

In this project, the meso-scale level is considered because several researchers have shown that the development and progression of meso-scale damage in textile composites can be modelled with reasonable accuracy. However, the associated high computational cost prohibits the modelling of damage at structural scales. Therefore, this project ultimately aims to combine the developed fatigue model with larger, macro-scale analysis tools, to be able to predict the fatigue life of 3D woven composite components at the structural scale.

In view of the above, the key aims and objectives of this project can be summarised as follows:

  • Fatigue testing of 3D woven composites containing pre-initiated meso-scale damage.
  • Development of high-fidelity fatigue model at the meso-scale.
  • Extension of model capability to predict fatigue life of 3D woven composites at the structural scale.

This project is supported by Rolls-Royce.

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Composite overmoulding for complex, multifunctional loaded structures

CDT21 Student: Maria Veyrat Cruz-Guzman
Supervisors: Dmitry Ivanov, Jonathan Belnoue, Steve Eichhorn

Thermoplastic semi-crystalline composites have experienced a resurgence in interest and demand due to their attractive properties such as their ability to reshape and fuse at elevated temperatures. This makes them ideal candidates for emerging applications in new structural components, additive manufacturing materials, adhesives, and coatings as well as applications that require reusable, economical, and functional products. The quality of the heat-fused interface in the thermal welding process for thermoplastics is important as the regenerated interface plays a critical role in the bulk performance of the composite as well as plays a part in toughening the composite for aerospace applications. The process where the two thermoplastics are fused, known as interface healing, describes the multi-scale physical process that occurs. The interface healing process controls the subsequent interfacial properties, affecting the bulk mechanical properties.

Currently, no design tool nor definitive method is available to predict these thermoplastic interfaces' interface strength, especially in the context of composite over-moulding, a manufacturing process used to create 3D structural components. This is because most current models looking at interface healing are based on theories initially developed for amorphous polymers and then adapted for semi-crystalline polymers. In addition, these models do not account for crystallisation which is the most important phase transition that needs to be considered in semi-crystalline polymer processing. The crystallisation distribution will affect the interfacial properties and in turn, affect the bulk properties. Additionally, the manufacturing process employed when forming the thermoplastic interface will also affect the crystallisation distribution adding another level of complexity to modelling this process.

This PhD study will aim to:

  • Develop a tool to model the crystallisation through-thickness in thermoplastic composites.
  • Perform optimisation trials on the cooling cycle in the thermal welding process to improve the crystallinity at the interface.
  • Manufacture a sample thermoplastic composite with a potential gradient crystallinity structure.

This model tool would be able to provide information for the optimisation of the cooling cycle in the manufacturing of thermoplastic composites. It will also provide more information on non-isothermal crystallisation modelling for the development of multi-scale interface healing models as well as allow for novel methods of manufacturing 3D-shaped thermoplastic composites with higher interfacial bond strengths.

This project is supported by Victrex.

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Composite structural housing with integrated thermal management

CDT21 Student: Toby Wilcox
Supervisors: Richard Trask, Ian Bond, Julian Booker, Ian Farrow 

As modern rotorcraft design shifts away from conventional power and towards more electrical systems, the need for efficient thermal regulation has never been higher. Systems are currently in place to combat this in rotorcraft but they would benefit from higher integration and optimisation. The key to achieving this may lie in further utilisation of materials that are already commonplace in the aerospace industry; composites. Composite materials, namely carbon and glass fibre reinforced composites (CFRPs/GFRPs) have widespread applications in modern aircraft and can comprise as much as 40-50% of structural components. The prevalence of composite materials is mainly due to their high strength-weight ratio and stiffness tailoring ability. They are however limited in temperature critical areas due to their poor thermal performance. This means they are generally unsuitable for structural applications around components that require a large amount of heat removal. However, if the thermal performance of these composite materials could be improved without compromising the mechanical properties of the material itself, the benefits would be numerous.

This project aims to investigate ways to improve the thermal characteristics of composite materials in ways that would aid the removal of heat from temperature critical components. There are currently a few novel concepts that can do this on a small scale, but current literature and research into the area is scarce. This likely means a new technique, or a combination of techniques would have to be used to achieve this. There are two types of techniques that could be used; passive and active cooling. A passively cooled system would employ microstructural or geometric features and take advantage of the surrounding environment to promote heat dissipation without the need for energy consumption. Microstructurally, this may include thermally conductive additives into the composite matrix or improved crystallinity within the matrix. Geometrically, this may involve ventilation features that take advantage of the surrounding conditions and the airspeed produced by the rotors. Possibly the most promising concept however would be to improve thermal conductivity in the through-thickness direction of the composite using z-pinning for tufting (stitching). This would create thermally conductive pathways within the structure with more conductive materials such as carbon or metals. These two techniques already have uses from a mechanical performance perspective, but their thermal effects have not been investigated in research. Preliminary experiments have already been carried out to investigate z-pinning as part of this project, with promising initial results.

An actively cooled system would require some means of energy consumption in order to remove heat from the system. This would most easily be done by pumping a cooling fluid around the surface of the structure. Some similar systems exist in the modern rotorcraft but integration into composite structures is very complex. Channels can however be embedded within the composite to create a ‘vascular network’ through which coolant can be pumped. Based on the limited literature, this technique offers the most potential to achieve the cooling effect required, and will form the bulk of the experimental work of this project. The size, configuration, and fabrication method of the channels are all factors that need to be investigated further, as well as choice of coolant and flow velocity. These variables will create a strong starting point for research.

The project will use a two pronged approach to evaluate both passive and active systems experimentally, before identifying the concept with the highest potential. This concept will then be evaluated in more detail and with a specific application in mind, in the hopes of raising the TRL level and furthering the research for future projects.

This project is supported by Leonardo.

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Modal nudging and elastic tailoring for blade-stiffened wing structures

CDT21 Student: Lichang Zhu
Supervisors: Alberto Pirrera, Rainer Groh, Mark Schenk, Jiajia Shen

Aerospace requirements put an emphasis on developing lightweight structures to help reduce fuel consumption and related costs. Typically, designers in aerospace engineering use thin-walled structures which are periodically stiffened with ribs, frames and longerons, i.e. semi-monocoque structures, as an efficient solution. However, slender and thin-walled structures often exhibit undesirable elastic nonlinearities and instabilities that need to be remedied at the cost of mass efficiency. However, it has been demonstrated that incorporating well-behaved elastic nonlinearities offers the means to recuperate the baseline’s efficiency or even improve upon it.

Modal nudging is a recently introduced tailoring technique, whereby mode shapes from the post buckling regime are seeded as initial perturbations to the geometry of the perfect structure. The small alterations to the geometry of a structure can be used to connect stable pre-buckling responses to stable post-bucking ones. This characteristic increases the load carrying capability of the structure by removing critical bifurcations and stabilising the post-buckling response removing any of the undesirable instabilities which may typically be encountered post-bucking. As an additional benefit, in stabilising the post-buckling response, modal nudging also ameliorates imperfection sensitivity. Ultimately, the stabilisation of post-buckling responses, the increase in load-carrying capacity, and the reduction in imperfection sensitivity, are conducive to further lightweighting of aerospace, semi-monocoque structures.

Preliminary work has shown that modal nudging via geometric alterations can successfully be used to increase the load carrying capacity and the compliance of blade-stiffened wing structures. With a judicious selection of the post-buckling modes seeded onto the original geometry, the nonlinear load-displacement trajectory of a structure can be closely controlled and optimised for compliance, load-carrying capacity, or additional functionality. The drawback of the geometric approach is that small perturbations to the initial geometry are difficult and costly to manufacture. Moreover, certain applications do not permit geometric changes. That is the case, for instance, in aerodynamic structures where any geometric alteration would disrupt flow and performance. A more suitable approach to nudging could then be controlling the nonlinear behaviour by elastic tailoring. This can, for example, be achieved by localised shifting of the neutral axis, by laminate design, or by smoothly varying the material properties using composite tow-steering. This project will investigate the efficacy of elastic tailoring through composite materials to replace geometric imperfection seeding for the modal nudging technique.

The first objective is to demonstrate, by design and analysis of numerical prototypes, that semi-monocoque structures can be nudged through stiffness tailoring.

The second objective of the project is to verify the numerical findings in experimental tests, by designing, building, and testing a prototype blade-stiffened aircraft panel. The challenge is to design and build a physical prototype that exhibits the desired structural behaviour, and which is robust to manufacturing imperfections. To achieve this objective, an understanding of the effect of manufacturing imperfections on the mechanical behaviour of the nonlinear structure is necessary. Accurate experimentation on the nudged prototype structures will enable the validation of the numerical analyses and will enable practical applications of well-behaved nonlinear structural responses.

This project is supported by Embraer.

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