My work reflects the change, growth, metamorphosis, duality, and balance that exists in my life. I focus on the process of change and necessity for adaptation. I’m interested in finding balance among the chaos of everyday living. I’m an observer, an introvert, and a thinker. I see and experience opposing forces all around me. Some are simple observations, such as good versus evil, chaos versus order, organic versus geometric, while some are more personal, such as anxiety versus depression, conflict between the inner versus outer self, disorganization versus obsessive compulsive. I’m fascinated by the delicate point at which opposing forces meet to create balance. I use a variety of papers, found objects, fiber, paint, drawing, and stitching to represent things that are picked up along my journey. Using a variety of materials and imagery represents the chaos of life. Editing, rearranging, dissecting, and reassembling these materials and imagery with deliberate choice are a way to balance the chaos I see and experience to take back control.

The Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) concept is characterized by through-thickness VectranTM stitching of warp-knit carbonabric prior to resin infusion. A series of novel lightning strike tests were performed on a PRSEUS panel. The panel’s lightning damage resistance was characterized as a function of peak current (50-200 kA) and strike location (mid-bay, stringer, frame). Both visual inspections and through-transmission-ultrasonic C-scans were used to investigate the damage resistance at the strike locations and to assess various damage mechanisms and morphologies. The size and severity of the damaged area depend strongly on the fiber orientation in the outermost ply, the amount of current injected into the panel, and the strike location relative to stitching lines. Increasing the current magnitude drastically increases the damaged area. Also, the presence of VectranTM structural stitches profoundly reduced the size and severity of lighting damage relative to similar strikes performed in panel regions without stitching.

In recent years, the prevalence and application of composite materials has exploded. Due to the demands of commercial transportation, the aviation industry has taken a leading role in the integration of composite structures. Among the leading concepts to develop lighter, more fuel-efficient commercial transport is the Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) concept. The highly integrated structure of PRSEUS allows pressurized, non-circular fuselage designs to be implemented, enabling the feasibility of Hybrid Wing Body (HWB) aircraft. In addition to its unique fabrication process, the through-thickness stitching utilized by PRSEUS overcomes the low post-damage strength present in typical composites. Although many proof-of-concept tests have been performed that demonstrate the potential for PRSEUS, efficient computational tools must be developed before the concept can be commercially certified and implemented. In an attempt to address this need, a comprehensive modeling approach is developed that investigates PRSEUS at multiple scales. The majority of available experiments for comparison have been conducted at the coupon level. Therefore, a computational methodology is progressively developed based on physically realistic concepts without the use of tuning parameters. A thorough verification study is performed to identify the most effective approach to model PRSEUS, including the effect of element type, boundary conditions, bonding properties, and model fidelity. Using the results of this baseline study, a high fidelity stringer model is created at the component scale and validated against the existing experiments. Finally, the validated model is extended to larger scales to compare PRSEUS to the current state-of-the-art. Throughout the current work, the developed methodology is demonstrated to make accurate predictions that are well beyond the capability of existing predictive models. While using commercially available predictive tools, the methodology developed herein can accurately predict local behavior up to and beyond failure for stitched structures such as PRSEUS for the first time. Additionally, by extending the methodology to a large scale fuselage section drop scenario, the dynamic behavior of PRSEUS was investigated for the first time. With the predictive capabilities and unique insight provided, the work herein may serve to benefit future iteration of PRSEUS as well as certification by analysis efforts for future airframe development. / PHD / In recent years, the prevalence and application of composite materials has exploded. Due to the demands of commercial transportation, the aviation industry has taken a leading role in the integration of composite structures. Among the leading concepts to develop lighter, more fuel-efficient commercial transport is the Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS) concept. The highly integrated structure of PRSEUS allows a new type of fuselage design to be implemented, known as Hybrid Wing Body (HWB) aircraft. PRSEUS unique fabrication process, the through-thickness stitching utilized by PRSEUS overcomes the low post-damage strength present in typical composites. Although many proof-of-concept tests have been performed that demonstrate the potential for PRSEUS, efficient computational tools must be developed before the concept can be commercially certified and implemented. In an attempt to address this need, a comprehensive modeling approach is developed that investigates PRSEUS at multiple scales. The majority of available experiments for comparison have been conducted for small specimens. Therefore, a computational predictive methodology is developed to accurately model the response of PRSEUS. A thorough analysis is performed to identify what needs to be considered in the model to predict an accurate result while remaining computationally efficient. From the baseline analysis, realistic models of the PRSEUS structure are created numerically and validated against the existing experiments. Finally, the validated approach is extended to panel and a fuselage section to compare PRSEUS to the current state-of-the-art. Throughout the current work, the developed methodology is shown to make accurate predictions that are well beyond the capability of existing predictive models. While using commercially available softwares, the methodology developed herein can accurately predict local behavior up to and beyond failure for structures such as PRSEUS for the first time. Additionally, by applying the methodology to a fuselage section drop scenario, the dynamic behavior of PRSEUS was investigated for the first time. With the predictive capabilities and unique insight provided, the work herein may serve to benefit future iteration of PRSEUS as well as certification by analysis efforts for future airframe development.

Lightweight sandwich composite structures exhibit high strength and bending stiffness and have been used in various load-bearing aerospace structures. A primary weakness of these structures is the low interfacial strength that can result in the disbonding between the facesheet and the core. Through-thickness reinforcement techniques, such as stitching, can be utilized to improve the interfacial strength of sandwich composites. This study focuses on fabrication and structural testing of stitched sandwich composite main landing gear (MLG) strut doors. The MLG strut doors are constructed from carbon fiber non-crimp fabric facesheets and closed-cell foam core that are stitched using a modified lock stitching technique. To assess the effects of stitching, two doors were fabricated, one with and one without through-the-foam stitching. A vacuum-assisted resin transfer molding process was used to infuse the doors. The strut doors were subjected to quasi-static loading, and the mechanical responses of the stitched and unstitched strut doors are presented.

L'objectif des travaux exposés dans cette thèse est d'identifier les différentes propriétés des constituants du sandwich cousu in-situ. Cette identification est indispensable pour simuler le comportement de ce type de matériau composite à différentes sollicitations statiques ou dynamiques. Nous proposons dans cette thèse de faire Une identification dynamique faite sur un échantillon représentatif (une plaque) qui prend en compte l'hétérogénéité et complexité de la structure. Cette identification est basée sur la corrélation ent.re un essai vibratoire et un calcul par éléments finis. La corrélation se fait en minimisant une fonction coût qui porte sur le décalage entre fréquences propres expérimentales et numériques. Cette minimisation est précédée par une identification du couple (fréquence propre/mode propre) numérique et expérimental en utilisant le MAC (Modal Assurance Criterion). Pour optimiser, nous passons, dans un premier temps; par une analyse de sensibilité qui permet de classifier les paramètres en fonction de• leur importance et ainsi ne lancer le processus d'identification que sur un nombre réduit de paramètres. Nous avons appliqué cette approche sur une poutre sandwich cousue et une plaque sandwich cousue. Dans une deuxième partie de ce travail de thèse, nous avons pu mettre au profil l'aspect périodique de ces structures composites sandwichs cousues, en utilisant le théorème de Floquet­-Bloch, et cela sur deux plans : l'aspect numérique pour la réduction du modèle et du temps de calcul et sur le plan physique par l'étude des bandes d'arrêt ('stopband') qui ont un intérêt applicatif assez intéressant. / The sandwich structures are well known for their high bending stiffness. This type of structure is also capable of including acoustic and thermal functionalities. However, they also have weaknesses such as the connection between the faceplate and the core and the weakness in the transverse direction due to the property of the core. The core is usually made of soft materials like foam for acoustic functionality. To overcome these weaknesses, it is possible to connect the different layers of the sandwich by transverse stitches. This is the concept of ‘stitched sandwich’. The stitch will deeply change the behavior of the structure and complicate the determination of its properties. The objective of this thesis is to identify the properties of the constituents of the stitched sandwich in situ. These properties are essential to simulate the behavior of this type of composite material under static or dynamic excitation. The identification of the properties of the constituents by mechanical tests is difficult for various reasons: the heterogeneity makes it complex to extract representative specimen; the behaviors of the constituents may change in non-in-situ tests; several different types of mechanical test, such as tensile-compression and torsion, are necessary to determine all the engineering constants in the case of orthotropic material which is common for composite materials. To overcome all these difficulties, we propose in this thesis a dynamic identification method conducted on the structure (the whole plate for example). Compared to mechanical test which is based on a sample, this method takes into account the heterogeneity and complexity of the structure. This method is based on the correlation between the vibration test and a finite element model of stitched sandwich. The parameters are identified by minimizing a cost function which can measure the gap between the experimental frequencies and the calculated frequencies. The correspondence of experimental mode and calculated mode is guaranteed by MAC (Modal Assurance Criterion). Before the optimization, we propose firstly a sensitivity analysis to classify the parameters according to their importance. Then the identification process is only conducted on a reduced number of parameters. We have applied this dynamic identification method to both a stitched sandwich beam and a stitched sandwich plate. 9 parameters of different constituents are identified in the case of plate. In the second part of this thesis, based on the theorem of Floquet-Bloch, we have profited from the periodic characteristic of the stitched sandwich structures in two aspects: in the numerical aspect, the periodicity has served to reduce the calculation of forced response of periodic structure; in the physical aspect, we have studied the stop band of stitched structures.

Producing composite parts using low-cost processes such as resin transfer moulding (RTM) has received much interest in the aerospace industry. RTM manufactured components require near net shape preforms which closely fit mould cavities. To reduce labour costs associated with composite production, automated preforming processes must be utilized. However, obtaining reproducible high quality preforms is required for manufacturing consistent high quality parts. Stitched multilayer 3D non crimp fabric preforms are well suited for automation and an investigation into quality and performance of components manufactured from these preforms is required. This thesis provides an initial evaluation of quality and mechanical properties of components made from stitched multilayer 3D non crimp fabric preforms using RTM. Similar sized flat plates of varying fibre volume fractions were manufactured to evaluate flexural modulus and strength, short beam shear strength and drop weight impact resistance of the material. Also, integral reinforced panels (IRPs) featuring a reinforcing section joined to a flat plate of varying laminating sequences were manufactured to evaluate debonding strength between sections. Optical microscopy was performed on component samples to determine quality based on void content and was found to be within acceptable limits for production composites. Flexural moduli were found to be comparable with theoretical expected values, however flexural strength was limited by the presence of transverse stitches. Short beam shear strength results showed high consistency between specimens, however were lower than comparable values found in literature. Impact specimens showed consistency among specimens, with greater damage resistance than comparable values found in literature. Determining debonding strength proved difficult as different failure modes were observed between IRPs, however, initial baseline values were acquired.

This research aims to investigate the influence of 3D through-thickness stitching on the gas permeability and transverse microcracking of cryogenically cycled carbon/epoxy composites. 3D through-thickness stitching can be used to improve the interlaminar properties of polymer matrix composites (PMCs) and produce lightweight, unitized structures for cryogenic storage tanks. To fully utilize stitched composite structures for these applications, their inherent gas permeability challenges must be understood. Therefore, in this study, the stitched composites' damage evolution and gas permeability was experimentally characterized under a) pure thermal stress, b) thermal and uniaxial mechanical stress, and c) thermal and biaxial mechanical stress. Helium gas permeability was measured for each specimen at room or cryogenic temperatures under a mechanically strained state following the thermo-mechanical cycles. Optical microscopy was used to measure microcrack densities and monitor their evolution through the thickness of the composite specimens. Thin plies, graphene nanoplatelets (GNP) modified resin, and a hybrid barrier layer comprising of both were incorporated in the stitched specimens as barrier layers to reduce their gas permeability. The dependence of gas permeability of stitched composites on the mechanical strain, test temperature, and load history was evaluated and correlated to microcrack density. A significant reduction in permeability and damage evolution (transverse microcracks and delaminations) was obtained for all thermo-mechanical cases using the hybrid barrier layer laminate. Additionally, the permeability was several orders of magnitude lower than the allowable. Overall, the hybrid barrier layer shows tremendous promise as a viable barrier layer for stitched/unstitched composites undergoing thermo-mechanical fatigue involving a cryogenic environment.

The purpose of this research is to evaluate the influence of through-the-thickness reinforcements on the fracture behavior of stitched sandwich composites and to develop predictive methodologies to aid in simulating their damage-tolerant capability. Sandwich composites are widely used for their high stiffness-to-weight ratio due to their unique material architecture, which is composed of two rigid, outer facesheets that are bonded to a light-weight internal core. However, sandwich composites are limited by their low interlaminar strengths and can develop core-to-facesheet separation when subjected to low out-of-plane loads. In this study, sandwich composites were manufactured with through-the-thickness reinforcements, or stitches, to act as crack-growth inhibitors and to improve interlaminar properties. Stitch processing parameters, such as the number of stitches per unit area (stitch density) and stitch diameter (linear thread density), have considerable influence on the in-plane and out-of-plane behavior of composite structures. A design of experiments (DoE) approach is used to investigate stitch processing parameters and their interaction on the fracture behavior of stitched sandwich composites. Single cantilevered beam (SCB) tests are performed to estimate the required energy to propagate crack growth, or Mode I fracture energy, during the separation of the facesheet from the core. Additionally, embedded optical fibers within the SCB test articles are used to determine the internal crack front variation. During testing, unique fracture morphologies are obtained and show dependency on stitch processing parameters. Furthermore, embedded optical fibers indicate that the internal crack front is approximately 10% greater than visual edge measurements, which is primarily attributed to Poisson’s effect. The DoE approach is then used to develop a statistically informed response surface model (RSM) to optimize stitch processing parameters based on a maximum predicted fracture energy. Novel analytical formulations are developed for estimating the mode I fracture energy using the J-integral approach. The DoE approach is then used to inform and validate finite element models that simulate the facesheet-to-core separation using a discrete cohesive zone modeling approach. The predicted load and crack growth response show good agreement to experimental measurements and highlights the capability of stitching to arrest delamination in stitched sandwich composites.

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.

Les nouveaux renforts NCF (Non Crimp Fabrics) sont adaptés aux procédés RTM (Resin Transfer Moulding) ou RIM (Resin Infusion Moulding) et permettent d’élaborer des structures aéronautiques complexes et de grande taille. Cependant, la présence de la couture peut conduire à une morphologie spécifique hétérogène du matériau avec un réseau 3D de zones riches en résine. Ces dernières, sous cycles de vieillissement hygrothermiques, sont à l’origine d’un état spécifique de fissuration. Ainsi, le présent travail se concentre sur la caractérisation morphologique et la fissuration d’une famille particulière des NCF - NC2 (Non Crimp New Concept), soumis au vieillissement hygrothermique cyclique. Pour cela, des cycles accélérés de vieillissement sont définis, diverses méthodes de caractérisation sont utilisées et différentes variables représentatives sont introduites. Au sujet de la morphologie du matériau, une hétérogénéité multi-échelles a été visualisée en surface et dans l’épaisseur en effectuant des coupes sous microscope 2D et de la reconstruction volumique sous tomographie 3D à RX. En ce qui concerne la fissuration hygrothermique, son initiation et son développement ainsi que sa morphologie ont été étudiés. L’influence de la morphologie et des paramètres de chargement au cours des cycles a été identifiée. De plus, afin de maîtriser le comportement des zones riches en résine, un couplage thermique/hygrothermique-mécanique à différents états de vieillissement du matériau a été décrit finement par des mesures de champs. Enfin, la tenue mécanique du matériau vieilli a été étudiée. / Stitched multiaxial laminates NCF (Non-Crimp Fabric) are potential candidate materials as new high performance preforms for manufacturing complex and large aeronautical composite structures by RTM (Resin Transfer Moulding) or infusion processes. Stitching within the preform leads to a particular morphology including 3D resin-rich regions and to a specific crack network developed in the bulk of the laminate when this is subjected to hygrothermal ageing cycles. The present work focuses on the characterization of the morphology and the crack development in a particular family of NCF - NC2 (Non Crimp New Concept) subjected to hygrothermal cycling. For this purpose, different accelerated thermal/hygrothermal ageing cycles were defined, various characterisation methods were adopted and representative variables were introduced. Regarding the structural morphology, a multi-scale heterogeneity of the NC2 could be visualized on the surface and through the thickness by optical microscopy as well as by the non-destructive volumetric analysis of X-Ray tomography. Regarding hygrothermal cracking, its initiation, its development and its morphology were studied. The influence of the morphology and the role of loading parameters on crack development were identified. Furthermore, for a better control of resin-rich region behaviour, the thermal/hygrothermal-mechanical coupling at different ageing states was investigated by full-field image correlation. Finally, the mechanical strength of the aged material was determined.