Within the high-performance composite manufacturing industry, there exists a need to improve the reliability of LCM (Liquid Composite Molding) manufacturing processes in producing composite parts that better approach the quality and consistency of pre-impregnated composite tapes that are cured in autoclaves with cost being a driving factor of this need. One obstacle to that end is the phenomenon of void formation during the LCM infusion processes. The formation of these voids through different mechanisms leads to composite parts with lower mechanical properties and consistency than their pre-impregnated autoclave cure counterparts. The objective of this research is to investigate the different mechanisms through which voids form during LCM processes as well as potential actions that can be taken to reduce the total percent volume of voids that form during the infusion. This research also aims to investigate the correlation of the void content observed at the tool-ply interface compared to the void content through the thickness of composite laminates. Finally, this research investigates the effect that chemical modification of the wettability of carbon fiber fabrics has on void formation during infusions.

The purpose of this research is to empirically investigate flow front void formation rates and post-formation bubble mobility behavior for composites produced via resin transfer molding (RTM).For this study, in situ observation of bubble formation and migration was accomplished by photographing resin flow progression during infusion tests of carbon reinforcements. An analysis strategy for use in batch processing sequential image sets is presented. The use of MATLAB to process and analyze binary images of infusions for void content has garnered satisfactory results and has shown that analysis of progressive image sequences can greatly enrich the volume of in situ measurements for a given study without compromising the data quality.Semi-automated MATLAB software analysis employed the representative image area (RIA) method to evaluate v0. It was found that the shorter the RIA length, and the more it follows the true flow front shape, the more representative the measured v0 was of the void formation at the flow front.Experimental evidence of in situ bubble formation and mobility behavior is presented. Stitch architecture of NCF reinforcements is shown to influence bubble formation at the flow front. Bubble mobility mechanisms (such as escape and entrapment) are related to stitch orientation relative to the fluid flow direction. Different stitching orientations exhibited different effects on post-formation mobility.Void formation is presented as a function of flow front velocity. Despite differences in preform configurations (stitch orientation with respect to flow) and injection flowrates, bubbles seem to form in a similar fashion for the 3 infusions of carbon fiber NCF reinforcement analyzed in this study. It is observed that bubbles form at stitch lines, regardless of stitch orientation.Bubble migration is documented for infusion of NCF reinforcement with stitching at different orientations. Qualitative observations of bubble migration during infusions of a dense preform of STW, plain weave fabric are discussed. Recommendations are given for future studies involving image-based analysis of in situ bubble formation and migration.

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.

Liquid composite molding (LCM) is growing in importance alternative to traditional prepreg-autoclave methods for manufacture aerospace composites. The most significant roadblock to industry's implementation of LCM is the optimization of resin flow to ensure high quality parts. This study developed process optimization tools to foster the adaptation of LCM. The following dissertation characterized the permeability of reinforcement fabrics under various processing conditions, and investigated in-situ bubble flow with carbon fiber. The purpose of this research is to extend the understanding of LCM and push forward the state of the art via sub-studies captured in five chapters, or manuscripts. Research from these manuscripts is as follows. Chapter 3 sets the groundwork for LCM optimization by extending the current theory for assessing 3D permeability of reinforcement fabrics using an ellipsoidal point infusion experiment. The aim was to improve 3D permeability measurement accuracy for LCM processing models. This work is the first to compare solutions in the context of 75 experiments. Chapters 4 and 5 extend permeability analysis to curved and sheared geometries, typical to real-world aerostructures. Chapter 4 demonstrates a method for measurement of curvature effects on permeability with vacuum infusion. A correlation was shown between curvature (as evaluated over four radii) and effective permeability. Chapter 5 researches the shearing of reinforcement fabric (e.g. when reinforcements are draped over double curvature). The study shows that permeability actually increases for mid-range shear angles beyond the shear-locking angle, and develops a technique for obtaining the 3D permeability of sheared fabric.Chapter 6 investigates carbon fiber voids in situ. LCM optimization requires improved void monitoring for carbon fiber. It is challenging to monitor void flow in situ with carbon fiber reinforcements because of fiber opacity. The research builds upon a new automated fluorescent imaging method to monitor void flow in-situ. Results include high-resolution and high-contrast images and 230 data points for infusion velocity vs. void content data.Chapter 7 contributes to the growing interest in LCM processes for aerospace applications by providing a short cost summary of typical processes for manufacturing aerospace composite parts. Data shows that LCM is a financially wise alternative to automated fiber placement (prepreg-autoclave) manufacturing when a void content of 2-2.5% is acceptable. Work on LCM processes optimization indicates that these percentages will reduce in coming years.

Resin transfer molding (RTM) is an infusion-based closed-mold manufacturing process where resin is injected into a preform of dry reinforcement to create a net shape part. Often, when a preform is draped over a mold with complex geometry, such as the double curvature of a dome, a reorientation of the fibers takes place in the form of in-plane shear. This deformation of the reinforcement structure has the potential to adversely affect the resin flow and the filling of the mold during RTM if the manufacturer fails to properly account for the shear effects. Various process simulation tools are being developed and used to simulate infusions in a virtual environment and assist manufacturers in optimizing tooling features and process parameters before needing to invest in tooling or prototypes. Such simulation requires material characterization of the resin viscosity and reinforcement permeability. The latter is a function of the reinforcement architecture and is highly sensitive to perturbations such as shear. Permeability measurement is well represented in the literature, but for ideal fabric arrangements without the deformations caused by complex mold geometries typical to industrial parts. The purpose of this study is to develop the first method for measuring the three-dimensional (3D) permeability tensor of a sheared fiber reinforcement in a single test and empirical models to show the effect shear has on permeability. The method and models are intended to enhance the accuracy of infusion simulation and further advance the development of liquid composite molding processes. Building off the work of previous researchers who have used trellis tools to induce uniform shear on fabric samples and 3D point-infusion tools for radial flow tests, these two methods were combined to measure the sheared permeability of a carbon fiber non-crimp fabric (NCF) in the x, y, and z directions. To mitigate the amount of spring-back that occurs when transferring the sheared preform from the trellis tool to the permeability tool, a method of incorporating an adhesive binder into the preform is presented. Lastly, the permeability data obtained from testing samples sheared at 0, 10, 20, 30, and 40 degrees is documented. Mathematical models are provided based on the data gathered in this work that show the permeability of a NCF in the x, y, and z directions as a function of shear angle. The resulting models indicate an inverse correlation between permeability and shear due to the reorientation of the fibers and closure of preferential flow channels in the preform. These models can be used to predict the permeability for shear angles less than 40 degrees. To validate these results, theoretical shear permeability models are included for comparison. Recommendations for future studies involving the measurement of 3D sheared permeability are discussed.

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.

Global competition is pushing the composites industry to advance and become more cost effective. Liquid Composite Molding or LCM is a family of processes that has shown significant promise in its potential to reduce process times and cost while maintaining high levels of part quality. However, the majority of research and information on composite processes have been related to prepreg-autoclave processing which is significantly different than LCM. In order for LCM processes to gain large scale implementation, significant research is required in order to model and simulate the unique nature of the resin infusion process. The purpose of this research is to aid in the development of in situ void measurement and characterization during LCM processing, particularly for carbon fiber composites. This will allow for the gathering of important empirical data for the validation of models and simulations that aid in the understanding of void formation and movement during LCM. For such data to be useful, it needs to include details on the formation, mobility and evolution of the void over time during infusion. This was accomplished by creating a methodology that allowed for in situ images of voids to be captured during the infusion process. A clear mold was used to visually monitor infusions during RTM with UV dye and lighting to enhance contrast. Consecutive images were acquired through the use of macro lens photography. This method proved capable of yielding high quality images of a variety of in situ voids during infusions with carbon fiber composites. This is believed to be the first instance where this was accomplished. A second methodology was then developed for the analysis of the collected images. This was done by using ImageJ software to analyze and process the acquired images in order to identify and characterize the voids. Success was found in quantifying the size and circularity of a wide range of micro and macrovoids in both a satin weave and double bias NCF woven fabrics. To facilitate the burden of collecting large amounts of data, this process was made to be automated. A user generated macro script could be applied to large sets of images for rapid processing and analysis. This automated method was then evaluated against manually processed images to determine its overall effectiveness and accuracy as tool for validating void theory.

ALes travaux présentés sont une contribution à l’élaboration de composites à matrices thermoplastiques (TP) par un procédé de type Liquid Composite Molding non réactif pour l’industrie automobile. La thèse a été effectuée dans le cadre du projet ANR TAPAS (ThermoplAstic Process for Automotive composite Structure) et s’est focalisée sur la mise en œuvre de plaques composites en renfort continu injectées avec des matrices polyamides 6,6 (PA 6,6) de hautes fluidités par Resin Transfer Molding. Le premier objectif est porté sur l’optimisation des cadences d’injection à travers l’étude de la perméabilité de préformes unidirectionnelles (UD) en fibres de verre et à hauts modules mécaniques. L’architecture de ces UD a ainsi été modifiée de manière à faciliter les écoulements. La perméabilité des différents tissus a pu être évaluée par un couplage entre des mesures expérimentales et une modélisation analytique basée sur un raisonnement à deux échelles de pores : l’écoulement intra et inter-torons. Le deuxième objectif sur lequel les travaux de thèse se sont concentrés s’est reposé sur la maitrise de l’état d’imprégnation par le bais d’une étude complète sur les phénomènes qui se déroulent à l’interface entre la fibre et la matrice à haute température. Plusieurs viscosités et formulations du PA 6,6 ainsi qu’un traitement appliqué sur le verre ont pu être caractérisés et discutés en termes de mouillabilité et d’adhésion. Enfin, la dernière partie du manuscrit présente les résultats obtenus sur les plaques mises en œuvre par RTM-TP en injection in-plane. Les conditions optimales de fonctionnement ainsi que les aspects de saturation, de santé matière et des propriétés mécaniques sont ensuite présentés et discutés. / The present work is a contribution to the thermoplastic composites manufacturing by a non-reactive Liquid Composite Molding process for the automotive industry. The thesis was carried out by the « ANR TAPAS » project (Thermoplastic Process for Automotive Composite Structure) and was focused on the elaboration of continious-fiber reinforced composites plates injected with a high-fluidity polyamide 6,6 (PA 6,6) by the Resin Transfer Molding process. The first goal was focused on increasing injection rates through the study of the in-plane permeability of unidirectional (UD) glass fiber fabrics with high mechanical modulus (HM). Experiments and modelling results showed that the permeability of these UD has been enhanced by modifying specific structural parameters of their architecture. The analytical model developped and used is based on a flow distribution according two differents scales of porosity : in and inter-yarns. The second part of the work was focused on the understanding of phenomenas that take place at the interface created between glass fiber and the matrix during the impregnation step. The wettability and adhesion of molten PA 6,6 dropped on a glass substrate is studied at different processing temperature. The last part introduce the thermoplastic composite plates elaborated by RTM-TP process. The optimum operating conditions as well as preforms saturation and mechanical properties are also studied and discussed.

No description available.