Student Number : 0001361N - MSc(Eng) dissertation - School of Mechanical, Industrial and Aeronautical Engineering - Faculty of Engineering and the Built Environment / Thick composite cylinders are important structural elements which cannot be analysed by traditional techniques due to through-thickness effects. This work presents analyses for thick composite tubes of finite length including through-thickness shear. A numerical thermal analysis is implemented for the determination of the transient through-thickness behaviour of tubes. A me- chanical analysis, based on the Rayleigh-Ritz technique, is presented which analyses finite length, composite tubes under a variety of loadings. The anal- yses are shown to be accurate and efficient and are validated against existing results. Results are presented for two ring-stiffened tubes under pressure and thermal loading and also for the transient thermal behaviour of these tubes. It was found that both the through-thickness and transient effects are of im- portance as the stress variation through the thickness and with time was significant for both tubes.

Z-pinning is a through-thickness reinforcement technology for polymer composite materials that has been developed and commercialised over the past fifteen years. The through-thickness reinforcement of composites with thin metallic or fibrous pins aids in suppressing delamination, improving impact damage tolerance and increasing joint strength. Z-pins are applied to the composite part during its manufacture. Pins are embedded within sheets of foam and placed over the unconsolidated part. Subsequently, the foam is compacted and the pins transferred into the part, which is usually an uncured prepreg. In this manner, large numbers of pins can be inserted quickly and easily. The pinned composite is then cured using conventional processes. The use of z-pins is currently limited to several high performance composite structures, most notably Formula One racing cars and F/A-18 E/F (Superhornet) fighter aircraft, although the technology has potential applications in a d iverse variety of aerospace and non-aerospace composite structures. A limited understanding of the mechanical performance of z-pinned parts under high load and fatigue loading conditions currently hinders the application of z-pinned composites. The aim of this PhD project is to investigate the mechanical properties, strengthening mechanics and failure mechanisms of z-pinned carbon/epoxy laminates and joints. The effect of z-pin reinforcement on the tensile and flexural properties of laminates under monotonic and fatigue loading is studied. The sensitivity of these properties to the volume content and diameter of the z-pins is systematically studied by experimentation and analytical modelling. This PhD also evaluates the efficacy of z-pins in improving the load-bearing properties of carbon/epoxy lap joints. Improvements to the room temperature and elevated temperature properties of z-pinned lap joints under monotonic and fatigue tensile loading were determined. The effect of strain rate on the load-bearing properties of z-pinned lap joints was also evaluated. A further aim of the PhD project was to assess the z-pin manufacturing process and the microstructural damage caused by that process. The outcome of this study augments the analysis of the me chanical properties of z-pinned laminates and joints.

This thesis aims to establish a greater understanding of the effects of the single-sided robotic tufting process on component manufacturing costs and weight, and subsequently determine whether potential economic benefits can be realised through its implementation. To complete this assessment, the study uses the novel composite rib post application of the Next Generation Composite Wing (NGCW) project as an example case, investigating the component’s manufacturing cost, performance and weight relative to those of a non-tufted rib post with the same initial fabric layup, and a prepreg rib post with the same fibre orientation and fibre volume fraction. Detailed analyses of the tufted composite rib post manufacturing process have established the relationships between its manufacturing activities and the consumed resources, facilitating the creation of a novel cost model incorporating algorithms defining the robotic tufting process, which may be adapted for different geometries and tuft parameters. In addition, novel permeability data for the considered tufted preform have been obtained experimentally allowing the simulation of the liquid composite manufacturing process. The insertion of tufting in the studied arrangement was shown to decrease the in-plane permeability of the preform, reducing the characteristic by a factor of 3.5 in the direction perpendicular to the tuft seams, and 1.8 in the direction parallel. Completing structural analyses and further cost modelling, the study considers the manufacturing costs and weight of each of the studied components when performance is normalised. Implementing weight penalties including lifetime fuel consumption and lifetime revenue, appreciation is given to the potential lifetime costs generated by each of the structural solutions. Comparing the amalgamated costs of each component, relative economy has been evaluated both when considering equipment amortisation and also the use of pre-amortised equipment. Considering lifetime fuel cost/kg as the weight penalty, the novel composite rib post has greater economy within the pure pull-off load case without tufting due to its lower manufacturing costs. However, with the consideration of a more significant weight penalty, such as potential increased lifetime revenue, the use of the novel composite rib post results in superior economy, its lifetime cost/part is estimated to be 14% less than the component without tufting and 61% less than the lifetime cost/part of the prepreg component. Critical evaluations of the project’s methodologies have highlighted the various assumptions made due to the limitations of available information and means to perform certain validations. Under these circumstances the assumptions are believed to be the best representations of what is experienced in reality. As far as possible, errors believed to be the result of assumptions have been accounted for in rigorous error analyses to establish their propagation through the various calculations performed. These analyses have provided confidence in the results of the study, ensuring the significance of the determined economical differences between the investigated components.

This study investigates the through-thickness behaviour of carbon/epoxy laminates. A through-thickness compression test regime was conducted utilising three specimen designs, which are waisted, hollow cylindrical and cubic specimens. An assessment and comparison of each specimen is given regarding their advantages and disadvantages in characterising the through-thickness response of [+45/-45/90/0]s quasi-isotropic AS4/8552 carbon/epoxy laminates. A finite element (FE) study of the three specimens is presented which results in specimen geometries that provided a macroscopically uniform stress response throughout the gauge length whilst also minimising other features such as stress concentrations. Further to the final geometries being presented, the method of manufacture for the laminate and machining processes for each of the specimens is given. A mesoscopic FE study is presented relating to the free-edge effects induced by through-thickness loading in quasi-isotropic laminates. The results presented show that free-edge effects will be present in the test specimens and will have a larger overall impact on the hollow cylindrical specimen. The free-edge effects also increase the stress concentrations present in the corners of the waisted and cubic specimens. Characteristic stress strain curves are presented for each specimen with strain data taken from post yield strain gauges attached to the specimens. The extracted initial Young's modulus Ez and Poisson's ratios vzx and vzy show a small variation between specimens. The strength values for the three specimens vary greatly with the waisted specimen being the strongest and cylindrical specimen the weakest, indicating that the chosen specimen geometry dominates failure. The experimental data will be used for test case 12 in the Second World Wide Failure Exercise (WWFE-II). A study is presented to predict the effective elastic properties of Z-pinned laminates. The materials under consideration are UD and [0/90]s cross-ply AS4/3501-6 carbon/epoxy laminates. Estimates on the effective properties are provided by two FE approaches and two analytical bounding approaches; namely Voigt and Reuss bounds and Walpole's bounding theory. The two FE approaches are based on extreme assumptions about the in-plane fibre volume fraction in the presence of Z-pins and provide a tight range of values in which the real result should lie. Furthermore, whilst the bounding methods are simple and in the case of Young's moduli produce very wide bounds the selection of the suitable bound result can lead to a good estimate in comparison with the FE data. Typically the best bounding method result for each elastic property is within 10% of the FE predictions.

In most application to date reinforced carbon fiber composites have been used in relatively smaller thickness, less than 10mm thick and essentially for carrying in-plane loads. As a result, design and testing procedures were developed which reflected the need to understand the in-plane response of the material. recently, engineers and designers have begun to use reinforced carbon fiber composites in thicker sections, where an understanding of the through-thickness response is of para-mount importance in designing reliable structures, particularly where the through-thickness strength has a controlling influence on the overall structural strength of the component. In this thesis tests will be done on carbon fiber non-crimp fabric (NCF) which will be loaded in compression and shear and elastic moduli and strength will be evaluated. In characterizing the through-thickness mechanical properties of a composite, the objective is to produce a state of stress in the test specimen which is uniform and will repeatedly measure the true properties with accuracy. In this study, specimens were machined from two blocks of thick (~20 mm) laminates of glass/epoxy and NCF carbon fiber infused with vinylester and tested in compression, and shear.

No description available.

Cette étude portait sur l’évaluation des performances mécaniques et des mécanismes de défaillance des composites cousus dans différentes conditions de chargement. Des plaques stratifiées et des raidisseurs renforcés par tufting ont été fabriqués avec différents paramètres de couture afin d'évaluer leur effet sur les propriétés des composites. L'investigation a été assistée par une caractérisation multi-instrumentée pendant les tests. Les plaques cousues soumises à des tests de cisaillement à poutre courte sont utilisées dans l'analyse du comportement de la densité et de l'angle de couture dans des conditions de chargement en mode II, tandis que des tests d'impact et de compression après impact (CAI) sur la tolérance aux dommages. Des tests de fatigue en éprouvettes trouées ont également été réalisés afin d’évaluer la réponse des coutures, en particulier leur position par rapport au trou central, à la concentration de déformation générée par le trou. La suite de ce travail a consisté en des tests mécaniques sur panneau raidi oméga renforcé par tufting. La procédure a optimisé les paramètres de touffetage utilisés pour renforcer les structures du lot précédent d'échantillons jusqu'à atteindre un point optimal où les propriétés principales, principalement trouvées dans les tests d'arrachement, sont égales ou supérieures à celles des échantillons témoins. Cette amélioration tenait également compte des modifications de la forme des raidisseurs. En outre, une nouvelle approche basée sur l’effet piézorésistif des coutures en fibres de carbone lors du chargement des éprouvettes composites est réalisée. Cela peut faciliter la surveillance de l’état de santé des fils cousus et donc du composite en raison de la nature structurelle des coutures. Les résultats ont montré que les renforts par tufting sont capables d'augmenter considérablement la ténacité entre les composites et la tolérance aux endommagements des composites, principalement en raison de leurs phénomènes de pontage des fissures. Les paramètres de tufting sont des facteurs décisifs pour obtenir les meilleures propriétés mécaniques. Cependant, ces travaux ont montré que les fils de coutures sont également responsables de la création de fissures dues à la concentration de contrainte et aux défauts causés par leur insertion et, par conséquent, à la diminution de la résistance des composites. L'enquête conclut que l'insertion aléatoire des touffes n'est pas idéale pour la performance du matériau et doit donc être évitée. Le développement de l'insertion des coutures dans les raidisseurs oméga a été soutenu par la caractérisation multi-instrumentée qui a permis d'optimiser le renforcement de la structure. Bien que l’étude ait permis d’obtenir des propriétés mécaniques nettement supérieures à celles des panneaux oméga renforcés par touffetage, il est évident que la procédure employée n’est pas optimale. Le présent travail propose également un modèle préliminaire d'éléments finis pour surmonter le coût et la perte de temps des tests expérimentaux. Il vise principalement à optimiser les paramètres de tufting dans la structure. Le modèle développé était capable de prédire les mêmes endommagements que ceux observés expérimentalement, mais encore éloignés des prévisions quantitatives des résultats. Le contrôle de l’état structurel des stratifiés composites cousus par les fils de carbone semble prometteur et pourrait aider à l’avenir à fournir des informations sur l’état de santé des coutures sous chargement qui ne sont pas atteintes par les méthodes de caractérisation classiques utilisées dans ce travail. / This study focused mainly on the assessment of the mechanical performance and the failure mechanisms of tufted composites under divers loading conditions. Laminated plates and stiffened panels reinforced by tufting was manufactured with different tufting parameters to evaluate their effect in the properties of the composites. Multi-instrumented characterization carried out during the tests assisted the investigation. The tufted plates subjected to short-beam shear tests aided especially in the behavior analysis of tufting density and angle in mode Il loading condition, while impact and compression after impact (CAI) tests on the damage tolerance. Open-hole fatigue tests were also performed to evaluate the tufts response, especially regarding their position to the center hole, to the strain concentration factor generated by the hole. The following part of this work consisted of the mechanical tests on omega stiffened panel reinforced by tufting. The procedure optimized the tufting parameters employed for reinforcing the structures from the previous batch of specimens until reaching an optimal point that the main properties, primarily found in pull-off tests, are equal or superior to those of the control specimens. This improvement also considered the modifications in the shape of the stiffeners. Furthermore, a novel approach based on the piezoresistive effect of carbon tufts under loading of the composite specimens is performed. This may support the monitoring of the health status on the tufted threads and therefore of the composite because of the structural nature of the tufts. The results showed that tufting reinforcements are capable of increasing the interlaminar fracture toughness and damage tolerance of the composites considerably owing mainly to their crack bridging phenomena. The tufting parameters are decisive factors for achieving the best mechanical properties. However, this work reported that tuft threads are also responsible for generating cracks due to the strain concentration and defects caused by their insertion and consequently, can decrease the strength of the composites. The investigation concludes that the random insertion of the tufts is not ideal for the performance of the material and thus must be avoided. The development of the tufting insertion in the omega stiffeners was supported by the multi-instrumented characterization that led to optimizing reinforcement in the structure. Although the study achieved the goal of obtaining mechanical properties significantly superior to the omega panels reinforced by tufting, it is noticeable that the procedure employed is not optimal. The present work also proposes a preliminary finite element model to overcome the costly and time consuming of the experimental tests. It intends primarily optimizing the tufting parameters in the structure. The model developed was capable of predicting the same damage events as observed experimentally, but it still distant from the quantitative predictions of the results. The structural health monitoring of the tufted composite laminates by the carbon threads seems promising and could help in the future for supplying data about the tufts health status under loading that are not achieved by the conventional characterization methods employed in this work.

The level of residual stresses in welded components is known to have a significant influence on their failure behaviour. It is, therefore, necessary to understand the combined effect of mechanical loading and residual stresses on the ductile fracture behaviour of these structures in order to provide the accurate structural safety assessment. Recently, STYLE (Structural integrity for lifetime management-non-RPV component) performed a large scale bending test on a welded steel pipe containing a circumferential through-thickness crack (the MU2 test). The purpose of this test is to study the impact of high magnitude weld residual stresses on the initiation and growth of cracks in austenitic stainless steels. This research presents the simulation part of the STYLE project which aims to develop the finite element model of MU2 test in ABAQUS to enhance the understanding and ability to predict the combined influence of mechanical loading and residual stresses on the ductile fracture behaviour of nuclear pressure vessel steels. This research employs both fracture mechanics principles (global approach) and Rousselier damage model (local approach) to study this behaviour including crack initiation and growth. In this research, the Rousselier model was implemented into ABAQUS via the user defined subroutines for ABAQUS/Standard and ABAQUS/Explicit modules, i.e. UMAT and VUMAT. The subroutines were developed based on the integration algorithm proposed by Aravas and Zhang. The validation of these subroutines was checked by comparing the FE results obtained from the implementation of these subroutines with the analytical and other benchmark solutions. This process showed that UMAT and VUMAT provide accurate results. However, the UMAT developed in this work shows convergence problems when the elements start to fail. Hence, only VUMAT was used in the construction of the finite element model of the MU2 test. As mentioned above, the results obtained from both fracture mechanics approach and Rousselier model are compared with the experimental data to validate the accuracy of the model. The results shows that both fracture mechanics approach and the Rousselier model predict similar final crack shapes which correspond closely to the test results in south direction. The other conclusions about the influence of residual stress on ductile fracture obtained from this work are also summarized in this thesis.

Dans le cadre d'un programme de recherche visant à l'allègement de la structure des véhicules, l'origine de lignage dans des tôles en aluminium AA6016 a été étudiée. Ce phénomène, qui peut apparaître à la suite d'une déformation plastique, est apparenté à de la rugosité de surface alignée dans la direction de laminage (DL). Sa présence est néfaste à une bonne finition de surface, et son intensité est appréciée visuellement par les fabricants.Une méthode de quantification rationnelle a été développée. La caractérisation de la distribution morphologique des motifs de rugosité a été rendue possible par l'utilisation de fonctions fréquentielles telle la densité de puissance spectrale. La note globale, construite à partir de la quantification individuelle des composantes de lignage pur et de rugosité globulaire, s'est montrée en bon accord avec les estimations visuelles, et notamment avec le niveau de lignage intermédiaire regroupant plusieurs aspects de surface différents.La microstructure des matériaux à l'état T4 a été expérimentalement mesurée couche de grains par couche de grain à l'aide d'un couplage entre polissage contrôle et acquisition par EBSD. Les 4 à 5 premières couches sous la surface (-120μm) semblent jouer un rôle mécanique prépondérant dans la formation du lignage car elles offrent à la fois une grande taille de grains moyenne, une importante ségrégation d'orientations cristallines, et une forte similitude de longueurs d'onde entre la rugosité de surface et les motifs de la microtexture.Des simulations numériques ont permis de vérifier que les couples de texture identifiés (Cube/Goss, Cube/Aléatoire et Cube/CT18DN) possédaient des différences d'amincissements hors-plans suffisantes pour générer l'ondulation d'une couche d'éléments. En revanche, l'influence mécanique de cette même couche décroit très rapidement avec son enfouissement dans la profondeur et devient négligeable sous plus de 4 couches d'éléments. / As part of a project on aluminium alloys for vehicle weight reduction, the origins of roping in AA6016 aluminium sheets have been studied. This strain-induced phenomenon is related to surface roughness but involves narrow alignments along rolling direction (RD). Its lowers the surface quality, and its intensity is visually evaluated by vehicle manufacturers.An original quantification method is proposed. The morphological characterization of roughness features has been measured by using frequency functions such as the areal power spectral density. The overall roping quality mark, determined from quantifications of both the isotropic and unidirectional components, shows good agreement with the visual assessment, especially for the intermediate roping levels which exhibit several different surface appearances.The material microtexture has been experimentally measured through grain to grain layers by using serial sectioning and EBSD scans. The first 4 to 5 layers under the surface (-120μm) seem to play a leading role in the micromechanics of roping developpment since they simultaneously exhibit a high average grain size, significant segregation of crystallographic orientations, and a close similitude between surface roughness and microstructural feature wavelenghts.Numerical simulations verified that the identified texture pairs (Cube/Goss, Cube/Random and Cube/CT18DN) have sufficient out-of-plane strain difference to promote one element thick layer undulations. But, the mechanical influence of this layer decreases gradually with depth, and becomes negligible below 4 other layers.

This thesis explores new methods for achieving load-carrying joints between the dissimilar materials of continuous fibre reinforced polymer matrix composites and structural metals. The new composite-to-metal joining methods investigated in this work exploit the metal-to-metal joining techniques of arc micro-welding, resistance spot welding, and metal filler brazing, to form novel micro-architectured metal adherends that can be used for enhanced composite-to-metal joining. Through a combination of equipment instrumentation and metallographic inspection of fabricated prototype joints, understanding is gained of how materials respond when processed by manufacturing techniques that have not previously been exploited for dissimilar material joining. Mechanical testing of prototype joints; both to ultimate loading strength and partial failure states, with subsequent inspection of specimens and comparative performances evaluation enabled joining performance characterisation of the new joining methods. Key results include: the identification of micropin reinforced adhesive joints to exhibit pseudo-ductile failure characteristics, resistance spot weld reinforcement of adhesive joints to boost bonding performance, and the use of a polymer infused metal foam to overcome difficulties of thermoplastic to metal adhesion. Through this work knowledge of how novel micro-architectures reacted under mechanical loading enabled insights to be gained into how perceived manufacturing defects can benefit joining performance. Such examples include, localised material weakness that lead to global pseudo-ductile failure behaviour, and low-strength secondary joining mechanisms boosting primary load transfer systems. By comparison of the diverse joining methods investigated in this work, trends were identified that suggest joining performance between the two dissimilar materials is improved by increasing the direct interaction between the composite reinforcement fibres and the metal structure. It is demonstrated that joining improvements are gained by forming mechanical connections between metals and composite precursory material before the final manufacturing process of the composite.