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Multifunctionality in composites is a developing area of research which shows much promise for the production of efficient and integrated structural designs. Incorporating further functionality criteria on top of the basic requirement for structural strength provides a range of possibilities for weight saving. Formability of functionalized preforms has not yet been fully explored and presents the main subject of this thesis. Most current development in multifunctional composites is focussed around functional materials. However, often functionalization, i.e. by incorporating micro/nano-additives, makes materials less compliant, handleable, or formable. An alternative approach explored in this thesis is to create multifunctional structures by separating the domains. The creation of multi-matrix structures, where two matrix materials occupy separate zones of a continuous fabric, is a relatively unexplored field with no established manufacturing process. The primary benefit is the ability to provide multifunctionality in carefully tailored regions, allowing the manufacturer to designate areas as either multifunctional or purely structural based on design requirements. This is especially relevant when the multifunctional component inhibits formability, requiring the addition of flexible 'hinges' between multifunctional regions. This thesis tackles many of the manufacturing hurdles and presents an effective technique for the creation of such structures. This work constitutes a part of the EPSRC Hub project on the manufacturing of multifunctional composites, where the contribution of this thesis focuses on the application of this technique towards enhancing the formability of structural supercapacitors This is developed in collaboration with Imperial College London, who have pioneered the development of structural supercapacitors. Multifunctional elements tend to inhibit formability, however their introduction can also be used to improve forming if applied wisely, the modification of textile properties in regions where a defect is likely to form can lead to formability enhancement. Forming of dry fibre textile pre-forms is a commonly used technique to achieve complex shapes for light weight composite structures. Most conventional engineering fabrics can only deform through shearing. Excessive shearing often results in the formation of detrimental features, such as wrinkles, folds and fabric distortions. Modification can be achieved by the integration of patches: additional materials, such as reactive thermally-conditioned resins, tufted or stitched yarns, thermoplastic films, locally activated binder, etc. This method is simple and effective for a certain class of forming problems. Once formed the primary structural resin is added and the patch material becomes a part of the finished component. The success of the forming operation depends on the balance of properties between dry and patched materials. This thesis shows the feasibility of improving formability by local modification of preforms and explores various numerical modelling tools for optimising this process. Modification is implemented by depositing localised resin patches onto textile preforms. The technique allows for continuity of fibres throughout the preform with the additional benefit that the deposited resin can be thermally staged to the desired level of cure to tune the viscosity of the region for forming and consolidation. The location and dimensions of patches are specified through numerical modelling to inform the subsequent manufacturing process. Manufacturing trials successfully demonstrate the possibility of defect mitigation using this technique.

In the aerospace and automotive industries, demand for lightweight, fuel-efficient designs have resulted in an increase in carbon fibre reinforced plastic (CFRP) structures. However, concerns regarding difficult-to-detect damage in CFRP and the risk of in-service failure restricted further application. Current methods for damage inspection require costly downtime, and in-service sensors incur a weight penalty and potentially introduce defects. The development of multi-functional composites is key to improving material performance beyond the purely structural, and further reduce structural mass and volume. The research aim was to manufacture and incorporate a reinforcing multi-material tufting thread and create a multifunctional composite by increasing mechanical performance and through-thickness conductivity (TTC). Two potential functionalities were investigated. The first, examined a fully integrated crack sensing capability by monitoring changes in through-thickness electrical resistance (TTER). The second, examined the susceptibility of a composite to inductive heating and investigated a new approach for composite thermoset curing and repairability. The study described the braiding of a hybrid multi-material tufting thread comprising carbon fibre tows and metal wires. The braided thread enabled multi-material hybridisation that was fully integrated into the structure. Crack progression was successfully monitored by assessing changes in TTER. The presence of the hybrid thread also aided the induction cure of thermosets by increasing thermal uniformity and reducing hot spots during curing. This research successfully demonstrated the manufacture of multi-material multi-functional reinforcement tufting threads. The hybrid thread provided an in-service fault detection method that addressed the parasitic nature of current probes without compromising on mechanical properties. It generated a sensing method that has been fully integrated within a composite structure with the potential to reduce servicing and inspection schedules. This study expanded material capability and has the potential to advance CFRP and composite use in industry.

An automotive structure must perform a number of functions including all of those required for crash, NVH (noise, vibration, and harshness) and durability attributes. It is in the industrial interest that these attributes are assessed using virtual methods such as Finite Element models. Due to their excellent mechanical and weight-efficient properties, carbon fibre composites are gaining popularity for structural applications. One of the most commonly used formats is textile reinforcement, due to its low manufacturing cost and ability to be formed into complex shapes. These composites are highly anisotropic, which means the material properties are dependent on the structural geometry of the fabric. Consequently, the properties might vary between components of different designs. This could be due to the carbon fibre alignment directions, the shape of the carbon fibre tows and the change in carbon fibre to resin ratio. All of this will have an effect on stiffness and strength locally in different directions. To optimise the use of this material, its properties must be understood. The traditional method, where physical experiments are conducted to characterise the material behaviour, is time-consuming, expensive and has poor repeatability as manufacture procedures may vary, and so the experimental work will need to be repeated if there are any changes to the material configuration. Thus it is crucial to develop an innovative workflow, where these complex factors and changing variables can be accounted for using Finite Element simulations. This consists of a multi-scale modelling approach. Where the material properties are extracted from a meso-scale model, to be used in macro-scale component level models. The effect of shear and unit cell rotation during forming are also taken into account throughout the meso-macro workflow. A forming simulation is first performed. A range of realistic textile unit cell geometries is then generated using an in-house digital element method for a discrete selection of shear angles, informed by the forming simulation. This realistic unit cell geometry is used to create a mechanical voxel mesh model, which is then subjected to displacement-controlled periodic boundary conditions. The homogenised material properties are obtained and mapped onto a macro-scale component shell model, where its material properties are varied based on the shear angle and unit cell orientation. Lastly, loading conditions are applied to obtain the final part performance. An extensive experimental testing campaign has been carried out in support of the modelling work and is divided into three parts: To validate the realistic unit cell geometry from the digital element method using CT scans, to validate the homogenised material property predictions from meso-scale unit cell models using tensile coupon tests with DIC, and to generate numerical inputs for the forming simulation with shear stiffness and bending stiffness experiments. This PhD primarily focuses on a 2D twill woven carbon fibre prepreg material with a 3-minute cure resin system. Composite plates with various shear angles were manufactured for the tensile experiments. Both the prepreg and its dry fabric counterpart were characterised to compare the differences. While the focus is on a single ply fabric under room temperature, the effect of multiple ply lay-up and forming temperature is also explored.

Conventional fibre polymer composite materials consist of a single matrix system for a whole part and are selected on the dominant requirements of the part. A multi-matrix composite would provide an expanded design space for a composite part. However, there are few processes available to realise such a composite and these rely on modified forms of resin transfer moulding. These processes require active venting using an expensive vacuum system and the quality is reliant on bag quality. This research is devoted to developing a foundational principle for a processing strategy that uses the localised introduction of resin combined with an out-of-bag consolidation process containing a preheating stage that can form a multi-matrix composite. The resin flow in local deposition and consolidation processes is determined by a complex interplay between the resin and preform. Hence the chosen resin and fabric were characterised extensively. A range of viscosity-tailored consolidation schemes was implemented using a setup to estimate the in-line viscosity using the rheo-kinetics of the fast-curing resin system. By initiating the consolidation pressure at a higher resin viscosity, the voids were found to be suppressed. A simplified model to capture the dominant mechanisms during consolidation was developed. The dominant flow transitioned from a 2D yarn-scale to macro-scale flow on increasing the activation viscosity. The increased resin pressure associated with high resin viscosity during consolidation was found to be a dominating factor in reducing the voids. The model was then used to develop a simple analytical design tool for tailoring the consolidation parameters to minimise the void content. A feasibility study was conducted on combining different matrices to improve the mechanical performance of composites in an open hole tension test. The use of high stiffness carbon nanotube modified resin to locally manipulate the region around the hole increased the strength of the composite. The enhancement of multi-matrix composite was then qualitatively explained using a simple finite element analysis combined with classical laminate theory.

Any composite manufacturing method requires an application of a carefully designed consolidation process to ensure the suppression of voids in the laminate, establish bonding in laminate layers and prevent dimensional or fibre-path defects. The optimisation of consolidation processes relies on the characterisation of the composite precursors' deformability. There are multiple mechanisms occurring in consolidation and various experimental programmes have been suggested in the literature to describe these mechanisms and deduce relevant material properties. The selection of a testing methodology often relies on an initial hypothesis or prior knowledge regarding the deformation modes. This may be a source of significant errors. This research poses questions on the testing rationales, on subjectivity in material testing and on how data-rich programmes should be designed. The proposed solution to the consolidation characterisation problem is a real-time adaptive testing strategy that enables a "conversation with the material" - flexible autonomous steering of a testing programme reacting on the obtained output. Such system is implemented within this research and is called adaptive consolidation sensor framework. The system's algorithm is set to analyse composite precursor's feedback to compression in real time and to track characteristic signatures relevant to existing consolidation models. Based on that feedback the framework formulates a load programme for the test in a reactive manner. The developed testing system focuses on the identification of the underlying physical mechanisms rather than material properties identification in a rightly or wrongly assumed resin flow mode. To validate the proposed testing approach, a series of characterisation experiments for different material systems was performed. The obtained results highlight a way forward in terms of rethinking experiments for materials used in manufacturing and beyond.

The generation of manufacture-induced defects, such as wrinkles formed during the tow steering process is one of the main drawbacks of automated fibre placement (AFP). It is a consequence of the mismatch between the inner and outer radius when a prepreg tape is deposited onto a curved surface or along a curved path. Some studies report that these defects can cause as much as 36% loss of the in-service product's performance. Hence, they must be minimised or fully avoided. Traditional physical trials are costly and time-consuming. Numerical process simulations could help to drastically reduce part development costs through optimisation in the virtual space. However, the complexity of the tape behaviour (which is anisotropic as well as temperature and rate-dependent) and the dynamic interactions between the different constituents (the prepreg tape, the machine roller and the substrate on which the material is deposited) involved in the process make any attempt at predicting of AFP steering defect extremely challenging. Good understanding of the evolution of the tape behaviour with processing condition, adequate models able to represent the observed behaviour and a robust numerical platform (to be practical the run time of such a platform should be relatively short) for the process are needed to reach this goal. Based on these considerations, the aim of this PhD thesis was to conduct a thorough study of thermosetting epoxy prepreg's behaviour under processing condition. Two of the main deformation mechanisms (in-plane shear and tack), which determine wrinkle initiation were comprehensively studied. Recognising there are no standard methods and only a few studies available in the open literature on the behaviour thermosetting prepregs under processing condition, two novel characterisation methods were proposed to investigate in-plane shear and tack behaviour of prepreg under different combination of temperature and deformation rates. Corresponding numerical models were proposed based on the experimental results and were then implemented into an AFP steering model as user material subroutines for the Finite Element (FE) package Abaqus/Explicit. The comparison with experimentally collected AFP data highlight the good predictive capability of the proposed models. The work presented here brings the composite manufacturing community one step closer to virtual manufacturing of the AFP process.

This thesis aims to reduce the costs associated with optimising a part, process, and tool design in order to avoid defects when using the thermoplastic stamp forming process. This was done by investigating the defects that can occur when using the process, the mechanisms by which they occur, and how they can be avoided. This knowledge can then be used to avoid defects in other designs, hence reducing design optimisation costs. This work focuses on an omega geometry that was stamp formed from a carbon fibre PEEK laminate in a series of manufacturing trials. Wrinkling, corner thinning, edge squeeze out, and fibre breakage defects were observed in the trials. Hypothetical mechanisms that caused these defects were proposed, and design feature modifications that prevented each of them were identified. However, the modification that prevented wrinkling caused fibre breakage, so an optimum design that prevented both wrinkling and fibre breakage was not tested. However, this optimum design was proposed based on the hypothetical defect mechanisms. The use of process simulation was also investigated as this has the potential to reduce design optimisation costs by predicting whether defects will occur in a given design. A forming simulation was conducted for three of the designs tested in the trials, and for the proposed optimum design. A verification and validation process was conducted in order to establish and improve the credibility of the simulations. It was initially found that there was uncertainty in the defect predictions due to the use of fibre stress to indicate defects. However, this uncertainty was reduced by using basic methods that accounted for the compaction and temperature reduction of the material. The fact that the defects were predicted with reasonably low error and uncertainty suggested that the models used in the simulations could be used to predict defects in similar designs.

The integration of vibroacoustics constraints in the design of sandwich panels is nowadays of utmost importance. Countless industrial sectors and applications now require multifunctionality in terms of mechanical, vibrational and acoustical performances. The main challenge dwells in the fact that good mechanical properties of a sandwich panel are normally related to poor acoustic performances. Numerous solutions have been proposed to enhance their vibroacoustic properties, ranging from porous and viscoelastic insertions or layers to Helmholtz resonators. However, the main drawback of such additions is the increase of weight which undermines the lightweight advantage of the panel. Thus, the complexity of designing structures which are mechanically and vibroacoustically efficient at the same time is further aggravated by the necessity of maintaining the additional mass at low levels. This thesis illustrates the development of a novel sandwich panel for vibroacoustics applications with emphasis on vibration damping, paramount indicator of vibroacoustics performances of sandwich structures. The design strategy adopted revolves around the utilisation of novel materials and viscoelastic inserts as passive damping mechanisms. Specifically, a flax-fibre reinforced composite laminate is chosen for the square-cell core and the skins. Natural-fibre composites generally provide higher vibration energy dissipation than commercial materials traditionally used for sandwich and composite structures. Carbon-nanotube reinforced viscoelastic inserts are embedded in critical locations within the lattice core - junction corners between interlocking walls - to further increase its vibration damping capabilities and, by consequence, to potentially increase its strength as well. Restrictions to the value of additional mass brought by the inserts are imposed to ensure the preservation of the lightweight advantage of the sandwich panel. Mechanical static analyses are carried out to guarantee good-quality structural performances. The quasi-static flatwise compression characterisation of bare and insert-reinforced core specimens shows that both configurations outperform other comparable benchmark cases present in open literature, in terms of normalized stiffness and strength. The reinforced configuration, in particular, has 24.5% higher normalized strength compared to the bare panel. Two different types of tests are performed to investigate vibroacoustic performances of the sandwich panels. Results of out-of-plane vibration transmissibility tests show that the bare configuration outperforms that with inserts due to Coulomb dry friction effects which are triggered during the test at the interlocking areas between walls. On the other hand, the reinforced panel offers the highest viscoelastic damping values under free vibrations. In this case, indeed, bending deformation shapes are excited and the dry friction mechanisms in the bare lattice are not initiated.

Laminated composite structures manufactured from pre-impregnated carbon fibre preforms have very high in-plane strength and stiffness. These properties make carbon-fibre composites ideal for withstanding the adverse loading conditions experienced by many aerospace primary load-carrying structures under normal operating conditions. However, such structures are very susceptible to delamination failure in the event of high-velocity impact by a foreign object. This is particularly important for aerospace components where foreign-object damage (FOD) can lead to catastrophic failure of a component due to the low composite material interlaminar fracture toughness. This property depends on the strength of the interfacial layers between plies, which for an unreinforced laminate is governed by the relatively low strength of the highly brittle matrix resin. Lack of fibrous material in the through-thickness direction means this strength is easily overcome by the large interlaminar shear stresses induced by a high-velocity impact event. Z-pin through-thickness reinforcement is an effective solution to this problem by adding fibrous material in the laminate Z-direction to enhance through-thickness impact performance. The ability of Z-pin reinforcement in enhancing the interlaminar fracture toughness of carbon-fibre composites has been well-documented for smaller scales and low strain-rates. However, Z-pins must encounter a delamination crack that is of sufficient scale to invoke large-scale bridging to have maximum efficacy in arresting crack propagation. Existing small-scale test methods do not produce this large-scale bridging effect, and full-scale Z-pinned component testing produces complex failure modes and is often prohibilitively expensive. In this thesis, a new sub-element scale test has been developed which produces large-scale Mode II crack-bridging at high strain-rate in a field of Z-pins. The test was designed using a design-by-analysis approach with the explicit finite element solver LS-DYNA R7.1.3. A cantilevered, tapered plate manufactured from IM7/8552 pre-impregnated carbon-fibre unidirectional material is subjected to impact by a gelatine projectile using a gas-gun. The specimens are produced using a combination of hand layup and autoclave curing with one-sided taper and soft tooling only for simple laminate manufacture. Undesirable additional failure modes such as fibre-failure near the fixture have been completely avoided. Test conditions, configuration and geometry have been well-scaled to produce a large, single delamination near the mid-plane which is driven in both initiation and propagation by structural bending. Experimental data shows that this large delamination crack may be effectively arrested by embedded, normally-aligned carbon-fibre Z-pins via large-scale bridging. Finite element modelling predictions are compared with experimental results and the modelling approach is developed further to establish some predictive ability in terms of predicting Z-pin large-scale bridging behaviour. Novel Z-pin configurations are then assessed using the predictive model.

This project focuses on investigating several methods, such as magnetic positioning and liquid resin printing, to introduce and position carbon nanotubes (CNTs) in a viscous medium. The combination of magnetic manipulation through static magnetic field and liquid resin printing represents an unprecedented approach that could be used in composite manufacture, specifically for patch repair applications of composite materials. The possibility of spatially positioning the CNTs in a tailored and accurately controlled fashion is a focal point of the presented project. The main advantages of the proposed approach are the local CNT positioning and heat dissipation tailoring by conductive networks within the part. More precisely, the generation of such conductive pathways through spatial additive positioning can be used to trigger inductive heating by alternated magnetic field. Homogeneous heat distribution within the inductively sensitive region could be aided by an inhomogeneous magnetic field. CNTs were metal functionalised through chemistry-based electroless plating. Metal-plated CNT positioning through static magnetic field was achieved both in epoxy resin and within glass fabric. The CNT positioning within fabric was assessed by introducing the CNTs within fabric through liquid resin printing. The CNT positioning was characterised within fabric by elemental analysis of selected fracture surfaces across the patch by assessing the metal particle distribution. Metal-plated CNTs affected the preform resin cure kinetics. The effect of grading the electrical conductivity within the patch by spatially locating the nanotubes was assessed through numerical inductive modelling. Homogeneous spatial distribution of CNTs provided uniform heat dissipation within the patch by means of eddy current generation. Enhancement of dielectric properties of the resin matrix by synergetic effect of dielectric particles and CNTs can be combined with eddy currents to aid and better control the cure process. Further experimental work on inductive heating for patch repair represents the next step of this investigation.

The development of cost effective and reliable bonded structures ideally requires an NDT method to detect the presence of poor quality, weak, or kissing bonds. If these bonds are more compliant in tension than in compression stress-strain nonlinearities provide a possible route to detection with the use of nonlinear ultrasonic techniques. This work focuses on the kissing bond case and the resulting contact acoustic nonlinearity of the interface. The technique developed in this research is a non-collinear ultrasonic method; involving the interaction of two beams, it removes all signal related to system nonlinearity except for that produced in the region of beam overlap. Mixing of two shear waves producing a sum frequency longitudinal wave is the selected mode of interaction. The frequencies and angle between the two beams are varied during the experiment. By measuring the nonlinear mixing response as these two parameters are swept through a 'fingerprint' of the nonlinear properties in the interaction region can be obtained. This fingerprint is shown to contain information about the bulk material and the interface status. To interpret the fingerprints an understanding of the bulk mixing response is first required. This was gained through modelling and experimental testing of solid metal samples. The characteristic pattern of bulk mixing could then be distinguished from that of kissing bonds which were also investigated with models and experimentally. Kissing bonds were created by compressive loading of two aluminium blocks. It was found that the bulk mixing and interface mixing occurred in overlapping regions of the parameter space and thus could not be measured independently in the compressively loaded samples. The experimental contacting interfaces are tested with varied interfacial loading, and surface roughness. Many changes in the fingerprint patterns are observed, some suggest that non-collinear mixing does not evenly sample the area of interaction. In the process of the above investigation a new mixing phenomenon is discovered which occurs at the interface between water and the solid samples. A finite element model is developed that can replicate this mixing behaviour.

Calculating mechanical properties of structures at the nanoscale is a problematic task. Different methods exist for performing the calculations but there is always a trade off between speed and accuracy. This work explores a number of different methods for simulating nanostructures. First, it describes a novel extension to an atomistic finite element method to enable it to perform calculations of the piezoelectric properties of nanostructures. The results are then compared with those from molecular mechanics methods. Next it benchmarks various reactive molecular mechanics potentials and electronic structure methods as tools for predicting the tensile strengths and strains at failure of carbon-carbon bonds. The third-order density-functional tight-binding method (DFTB3) and the adaptive intermolecular reactive empirical bond order potential (AIREBO) are shown to offer the best accuracy while still being computationally cheap enough to model nanostructures. Finally, the two most successful methods from the previous section, DFTB3 and AIREBO, are applied to a selection of nanoribbons undergoing uniaxial tensile strain to the point of failure. The importance of the electronic structure to the mechanical properties is examined and a previously unseen relationship between the tensile modulus of armchair nanoribbons and their nanoribbon index is revealed. Additionally, calculations of the tensile properties of experimentally derived nanoribbons are performed that show cove-types nanoribbons to have excellent mechanical properties.