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Many large composite structures are manufactured using sandwich laminates to achieve high specific bending strength and stiffness. Examples include wind turbine blades, where self-weight becomes increasingly important as blade size increases. Resin infusion of three-dimensional sandwich laminates can result in complex resin flow paths, and subsequent defect formation, which are difficult to predict. The core material used for sandwich construction and its interaction with liquid resins may also influence the formation of defects, and in the case of balsa this effect can be used to reduce defect severity. In order to evaluate the effect of cored sandwich laminate construction on the formation of defects, this thesis concentrates on the characterisation of commonly used core materials and their interaction with liquid resin under high vacuum conditions. It also considers two numerical flow-modelling packages which are shown to be effective at the prediction of flow front convergence for monolithic laminate, but over-estimate defect severity when modelling air- permeable cored laminates. For balsa core, experiments indicate that the available pore space can act as sink for trapped air, which can aid the reduction of defects where multiple flow fronts converge due to the complexity of flow in sandwich laminates. Empirical data for air absorption and desorption rates in balsa core were obtained using a custom-designed experiment. Using these data a theoretical model was developed that can indicate available pore space, which can inform optimum processing conditions, such as time under vacuum. The diffusion coefficients obtained for air absorption and desorption in balsa are very similar, and lie in the middle of published ranges for hard woods at around 2 x 10 -7 m2/s. The methodology developed for this research project represents actual behaviour of air absorption/desorption during resin infusion, whilst other techniques do not, merely measuring diffusion of air through a sample not allowing for finite pore space. In consequence, infusion strategies can be planned more precisely because core/resin interaction is better understood. Knit line defect formation could be predicted with greater accuracy with suitably modified flow-modelling programs.

Cette thèse fait l’objet du développement d’un modèle numérique de la phase d’imprégnation d’une préforme lors du procédé de fabrication d’un matériau composite par infusion sous vide de type VARI (Vacuum Assisted Resin Infusion). La caractérisation in situ du comportement mécanique dans l’épaisseur d’une préforme (essais d’infusion réels) est confrontée à sa caractérisation ex situ par une machine de traction/compression. L’effet de différents paramètres est mis en valeur sur le comportement de la préforme (type de chargement appliqué, état de saturation, influence de la viscosité du fluide et de la vitesse de déformation). Les essais ont révélé le caractère viscoélastique d’un renfort de type tissé lors de sa décompression à l’état imprégné et ont permis la définition d’une loi de comportement viscoélastique non-linéaire du renfort lors de cette phase. En vue d’étendre l’usage de cette loi de comportement mécanique à une large gamme de renforts, la même démarche expérimentale est appliquée au cas d’un renfort de type mat, au comportement élastique non-linéaire. La forme générale de la loi de comportement proposée se veut être adaptée à la description des deux types de renforts testés, où les éventuels effets viscoélastiques sont pris en compte selon le renfort étudié. Suite à l’implantation de la nouvelle loi de comportement dans un code numérique dédié à la simulation de la phase de remplissage d’un moule, la comparaison entre les résultats numériques et ceux des essais d’infusion sous vide prouve la fiabilité de ce nouveau modèle pour ces deux renforts à l’architecture bien distincte. / This thesis focuses on the development of a numerical model for the preform impregnation during the VARI (Vacuum Assisted Resin Infusion) process for the manufacturing of a composite material. The in situ characterization of the mechanical behavior in the thickness direction of a preform (real infusion tests) was compared with an ex situ characterization by a universal testing machine. The preform behavior was characterized for different parameters such as loading type, saturation state, influence of fluid viscosity and strain rate. All the tests revealed the viscoelastic behavior of a woven fabric during its decompression in the impregnated state, leading to the definition of a non-linear viscoelastic constitutive law of the woven fabric during this phase. To extend the use of this mechanical constitutive law to a wide range of fabrics, the same experimental approach was applied to the case of a random mat fabric with non-linear elastic behavior. The generalized form of the constitutive law is adapted to the description of the two types of fabrics while the potential viscoelastic effects are taken into account according to the fabric type. After the implementation of the new constitutive law in a numerical code for the simulation of a mold filling process, the comparison between numerical and experimental results has proved the reliability of the new numerical model for these two reinforcements with distinct architectures.

As the aerospace industry continues to incorporate composites into its aircraft, there will be a need for alternative solutions to the current autoclaving process. Liquid composite molding (LCM) has proven to be a promising alternative, producing parts at faster rates and reduced costs while retaining aerospace grade quality. The most important factor of LCM is controlling the resin flow throughout the fiber reinforcement during infusion, as incomplete filling of fibers is a major quality issue as it results in dry spots or voids. Void formation occurs at the resin flow front due to competition between viscous forces and capillary pressure. The purpose of this work is to characterize capillary pressure in vacuum infusion, and develop a model that can be incorporated into flow simulation. In all tests performed capillary pressure was always higher for the carbon fiber versus fiberglass samples. This is due to the increased fiber packing associated with the carbon fabric. As the fabric samples were compressed to achieve specific fiber volumes an increase in capillary pressure was observed due to the decrease in porosity. Measured values for capillary pressure in the carbon fabric were ~2 kPa, thus the relative effects of Pcap may become significant in flow modeling under certain slow flow conditions in composite processing.

The purpose of the proposed research is to evaluate the mechanical response of stitched T-joints under tension, bending, and combined tensile-flexure loading. The use of fiber-reinforced polymer matrix composites has increased in primary load-bearing structures due to their many attributes, such as their high strength and stiffness-to-weight ratio, and tailorability. Composite T-joints are often used in aerospace, marine, and wind turbine structures to provide load connectivity between orthogonal components, such as stiffeners to skins. However, one of the main drawbacks of polymer matrix composites is their low interlaminar strength, which can lead to delamination when subjected to out-of-plane loads. Techniques such as braiding, knitting, stitching, tufting, and z-pinning have been used to reinforce T-joints in the through-thickness direction. Most research has been focused on the tensile or bending behavior of T-joints, although these joints are often subjected to a combination of tensile and bending loads in service. A few experimental and analytical studies have been conducted on the mechanical response under combined tensile-flexure loading conditions, but no studies have been conducted on stitched T-joints. In this study, mechanical tests of 3D stitched and unstitched T-joints under tension, bending, and combined tensile-flexure were conducted, and the ultimate load, displacement, and absorbed energy were obtained. The average displacement at total failure under tension, bending, and combined tensile-flexure loading conditions for the stitched specimens were found to be 34%, 51%, and 24% greater, respectively, when compared to their unstitched counterparts. Similarly, the average absorbed energy for stitched specimens is 58%, 82%, and 51% greater under tension, bending, and combined tensile-flexure loading conditions. The failure surfaces of stitched and unstitched T-joints were analyzed using an optical microscope, and areas of interest, such as resin-rich regions, stitches, and different damage types, were identified. Furthermore, the skin-flange interface fracture surface of the combined loading T-joint specimens were analyzed using a scanning electron microscope. Significant differences in the fracture surface indicated varying degrees of mixed-mode loading conditions within a specimen for all specimen types. A numerical analysis of a stitched double cantilever beam specimen was conducted to evaluate smeared cohesive laws to represent stitched regions. Overall, stitching results in improved damage tolerance in T-joints subjected to various loading conditions.

En aéronautique, l’élaboration via des pré-imprégnés n’est pas toujours adaptées àla fabrication de nouvelles pièces de formes complexes ou de grandes dimensions. Desprocédés directs existent, dénommés Liquid Composites Molding (LCM), tels que leResin Transfer Moulding (RTM) ou les procédés d’infusion de résine, comme le LiquidResin Infusion (LRI) et le Resin Film Infusion (RFI). Actuellement, environ 5 à 10%des pièces composites sont fabriqués par ces procédés directs. Avec le procédé RTM,les tolérances dimensionnelles et la porosité peuvent être maîtrisées et on peut atteindredes pièces haute qualité, mais son industrialisation est complexe et les modèlesmécaniques doivent être améliorés pour réaliser des simulations représentatives. Parcontre, les procédés d’infusion peuvent être utilisés dans des conditions plus flexibles,par exemple, dans des moules ouverts à sac vide en nylon ou silicone, à faible coût. Parconséquent, les procédés de LRI et RFI sont particulièrement adaptés pour les petites etmoyennes entreprises car les investissements sont plus faibles par rapport à d’autresprocédés de fabrication.Les procédés par infusion de résine LRI ou RFI sont basés sur l’écoulement d’unerésine liquide (pour RFI, après le cycle de température, la résine solide obtenir son étatliquide) à travers l’épaisseur d’un renfort fibreux sec dénommé préforme.L’optimisation du procédé est difficile à réaliser car le volume de la préforme changefortement pendant le procédé car elle est soumise à une pression extérieure et qu’il n’ya pas de contre-moule. Pour optimiser les paramètres de fabrication des matériauxcomposites par infusion de résine, il est nécessaire de mettre en oeuvre un modèlenumérique. Récemment, une modélisation de l'écoulement d’un fluide isotherme dansun milieu poreux compressible a été développée par P. Celle [1]. Avec ce modèlenumérique, nous avons simulé des cas test en 2D pour des géométries industriellesclassiques. Pour valider ce modèle numérique, des essais d’infusion d’une plaque par leprocédé LRI dans des conditions industrielles ont été réalisés. D’une part, la simulationnumérique permet de calculer le temps de remplissage, l’épaisseur de la préforme et lamasse de la résine durant l’infusion. D’autre part, nous avons suivi de procédéexpérimentalement par des micro-thermocouples, la fibre optique et la projection defranges. Un des points clefs de l’approche expérimentale est que l’écoulement de larésine et le comportement de la préforme dépendent intrinsèquement de paramètres quiévoluent pendant l’infusion de la résine, tels que la variation de l’épaisseur, le temps deremplissage et le taux volumique de fibres, via la perméabilité. Enfin, une comparaisonentre les résultats expérimentaux et la simulation numérique permet de valider lemodèle numérique. Cette confrontation des résultats permettra de mettre en lumière lesdifficultés et les limites de ce modèle numérique, afin d’améliorer les futurs modèles.De plus, ces deux approches constituent un bon moyen d’étudier et d’approfondir nosconnaissances sur les procédés d’infusion de résine, tout en développant un outil desimulation indispensable à la conception de pièces composites avancées. / Weight saving is still a key issue for aerospace industry. For instance 50% in weightof the B787 and A350 aircraft structures is made of CFRP, so it is necessary to makelighter thick and complex parts. Direct processes called Liquid Composite Molding(LCM), such as Resin Transfer Moulding (RTM) or Resin Infusion Process (LRI, RFI).At the present time, around 5 to 10% of the parts are manufactured by direct processesand the current trend is clearly to go ahead. In RTM process, the dimensional tolerancesand porosity fraction can be kept under control and high quality parts produced, but itsindustrialisation is complex and refined models are still needed to perform simulations.On the contrary, the resin infusion process can be utilized in flexible conditions, such asin low cost open moulds with vacuum bags in nylon or silicone. This type of processonly requires low resin pressure and the tooling is less expensive than RTM rigidmoulds. Therefore LRI and RFI processes are particularity suitable for small andmedium size companies because the investments are rather low compared to othermanufacturing process.Liquid Resin Infusion (LRI) processes are promising manufacturing routes toproduce large, thick or complex structural parts. They are based on the resin flowinduced across its thickness by pressure applied onto a preform / resin stacking.However, both thickness and fibre volume fraction of the final piece are not wellcontrolled since they result from complex mechanisms which drive the transientmechanical equilibria leading to the final geometrical configuration. In order tooptimize both design and manufacturing parameters, but also to monitor the LRIprocess, an isothermal numerical model has been developed by P. Celle [1], whichdescribes the mechanical interaction between the deformations of the porous mediumand the resin flow during infusion. With this numerical model, we have investigated theLRI process with classical industrial piece shapes. To validate the numerical model andto improve the knowledge of the LRI process, the researcher work details a comparisonbetween numerical simulations and an experimental study of a plate infusion testcarried out by LRI process under industrial conditions. From the numerical prediction,the filling time, the resin mass and the thickness of the preform can be determined. Onanother hand, the resin flow and the preform response can be monitored bymicro-thermocouples, optical fibre sensor and fringe projection during the filling stage.One key issue of this research work is to highlight the major process parameterschanges during the resin infusion stage, such as the preform and resin temperature, thevariations of both thickness and fiber volume fraction of the preform. Moreover, thesetwo approaches are both good ways to explore and improve our knowledge on the resininfusion processes, and finally, to develop simulation tools for the design of advancedcomposite parts.

Há um aumento da demanda mundial por materiais de alto desempenho, e os compósitos encaixam-se perfeitamente nesse nicho por serem resistentes e leves. Os compósitos poliméricos são feitos a partir da mistura de resina e reforço, no entanto, existem diversas metodologias para fazê-la. O presente trabalho compara em amplo aspecto, três desses métodos: laminação manual, infusão de resina e pré-impregnado. O primeiro é o método mais simples, o segundo é utilizado principalmente pela indústria náutica, e o terceiro pela indústria aeroespacial. Foram realizados diversos ensaios mecânicos como tração, flexão, cisalhamento, impacto, entre outros, e os resultados foram comparados entre si para demonstrar qualitativamente e quantitativamente as diferenças entre eles. Este trabalho demonstrou que o processo de pré-impregnado produz peças com propriedades cerca de 30% melhores que a infusão de resina, que por sua vez, possui um acréscimo de 25% sobre as propriedades da laminação manual. / There is an increasing worldwide demand for high performance materials, and therefore the composites reach a prominent position for being resistant and light weighted. Polymeric composites are made from mixing a given resin and the fibre; however, there are several different methodologies to do so. This work intends to compare, in a wide array, three of those methods: hand lay-up, resin infusion under flexible tooling and pre-pregs. The first is the simplest of all, the second is mainly used by the marine industry, and the third by the aerospace industry. Several mechanical tests such as tension, flexural, shear, impact, among others, were performed and their results were compared to infer their qualitatively and quantitatively differences. This study shows evidence that pre-preg properties are about 30% better than resin infusion, and the last has 25% better properties than hand lay-up.

Composite materials have traditionally been used in high-end aerospace parts and low-end consumer parts. The reason for this separation in markets is the wide gap in technology between pre-preg materials processed in an autoclave and chop strand fiberglass blown into an open mold. Liquid composite molding has emerged as a bridge between inexpensive tooling and large, technical parts. Processes such as vacuum infusion have made it possible to utilize complex layups of reinforcement materials in an open mold style set-up, creating optimal conditions for composites to penetrate many new markets with rapid innovation. Flow simulation for liquid composite molding is often performed to assist in process optimization, and requires the permeability of the reinforcement to be characterized. For infusion under a flexible membrane, such as vacuum infusion, or for simulation of a part with non-uniform thickness, one must test the permeability at various levels of compaction. This process is time consuming and often relies on interpolation or extrapolation around a few experimental permeability measurements. To accelerate the process of permeability characterization, a small number of methodologies have been previously presented in the literature, in which the permeability may be tested at multiple fiber volume contents in a single test. Some of the methods even measure the permeability over a continuous range of thicknesses, thus requiring no later interpolation of permeability values. A novel method is presented here for the rapid measurement of permeability over a continuous range of fiber volume content, in a single unidirectional vacuum infusion flow experiment. The thickness gradient across the vacuum bag, as well as the fluid pressure at several locations in the mold, were concurrently measured to calculate the fabric compressibility. An analytical flow model, which accounts for the compressibility, is then used by iterating the fitting constant in a permeability model until the predicted flow front progression matches empirical measurement. The method is demonstrated here for two reinforcement materials: 1) a fiberglass unbalanced weave and 2) a carbon bi-ax non-crimped fabric. The standard deviation of calculated permeabilities across the multiple infusion experiments for each material and flow orientation ranged from 12.8% to 29.7%. Validation of these results was performed by comparing the resulting permeability with multiple non-continuous permeability measurement methods.

Há um aumento da demanda mundial por materiais de alto desempenho, e os compósitos encaixam-se perfeitamente nesse nicho por serem resistentes e leves. Os compósitos poliméricos são feitos a partir da mistura de resina e reforço, no entanto, existem diversas metodologias para fazê-la. O presente trabalho compara em amplo aspecto, três desses métodos: laminação manual, infusão de resina e pré-impregnado. O primeiro é o método mais simples, o segundo é utilizado principalmente pela indústria náutica, e o terceiro pela indústria aeroespacial. Foram realizados diversos ensaios mecânicos como tração, flexão, cisalhamento, impacto, entre outros, e os resultados foram comparados entre si para demonstrar qualitativamente e quantitativamente as diferenças entre eles. Este trabalho demonstrou que o processo de pré-impregnado produz peças com propriedades cerca de 30% melhores que a infusão de resina, que por sua vez, possui um acréscimo de 25% sobre as propriedades da laminação manual. / There is an increasing worldwide demand for high performance materials, and therefore the composites reach a prominent position for being resistant and light weighted. Polymeric composites are made from mixing a given resin and the fibre; however, there are several different methodologies to do so. This work intends to compare, in a wide array, three of those methods: hand lay-up, resin infusion under flexible tooling and pre-pregs. The first is the simplest of all, the second is mainly used by the marine industry, and the third by the aerospace industry. Several mechanical tests such as tension, flexural, shear, impact, among others, were performed and their results were compared to infer their qualitatively and quantitatively differences. This study shows evidence that pre-preg properties are about 30% better than resin infusion, and the last has 25% better properties than hand lay-up.

Resin infusion of composite parts has continually been reaching to achieve laminate quality equal to, or exceeding, the quality produced with prepreg in an autoclave. In order for this to occur, developers must understand the key process variables that go in to producing a laminate with minimal void content. The purpose of this research is to continue efforts in understanding 1) the effect of process conditions on the resultant void content, with a focus on resin infusion flow rate, 2) applying statistical metrics to the formation, location and size of voids formed, and 3) correlate these metrics with the local mechanical properties of the composite laminate. The variation in dispersion and formation of micro-voids and macro-voids varied greatly between the rates of flow the infusion occurred, especially in the non-crimp carbon fiber samples. Higher flow rates led to lower volumes of micro-voids in the beginning section of the carbon fiber laminates with macro-voids being introduced approximately half-way through infusion. This was determined to have occurred decreasing pressure gradient as the flow front moved away from the inlet. This variation in void content per location on the laminate was more evident in the carbon fiber samples than the fiberglass samples. Micro-voids follow void formation modeling especially when coupled with a pressure threshold model. Macro-void formation was also demonstrated to correlate strongly to void formation models when united with void mobility theories and pressure thresholds. There is a quick decrease in mechanical properties after the first 1-2% of voids signaling strength is mostly sensitive to the first 0-2% void content. A slight decrease in SBS was noticed in fiberglass laminates, A-F as v0 increased but not as drastically as represented in the NCF laminates, G and H. The lower clarity in the exponential trend could be due to the lack of samples with v0 greater than 0% but less than 1%. Strength is not well correlated to void content above 2% and could possibly be related to void morphololgy.