Tesi sul tema "Structures composite sandwich architecturée"

La sécurité constitue un aspect essentiel au développement des pièces et assemblages aéronautiques. Pour un avion à hélices, les pales assurent la propulsion de l'appareil, et garantissent sa portance. Elles sont essentielles. C'est pourquoi des essais de certification à l'impact oiseau sont nécessaires afin de garantir la capacité de l'avion à se poser en mode dégradé. Ces essais ont lieu à la fin de la phase de développement. Une caractérisation du comportement à l'impact en amont est nécessaire afin d'éviter d'éventuels coûts supplémentaires liés aux mauvais résultats de ces tests Les pales d'avion étudiées sont des structures composites sandwiches complexes, composées d'une âme en mousse polymère et de plusieurs couches parmi lesquelles on trouve des tresses composites, des plis composites unidirectionnels, de la mousse polymère, des renforts métalliques, des interfaces collées et autres constituants spécifiques (protection foudre, peinture...). De nombreux matériaux sont donc à considérer pour l'étape de caractérisation. A cela s'ajoute la conciliation de phénomènes à des échelles microscopiques et macroscopiques. Le travail de recherche s'attachera dans un premier temps à comprendre et à caractériser les phénomènes d'endommagement dans les conditions de certification d'impact oiseau. Cette première étape doit permettre de hiérarchiser les phénomènes physiques afin de guider le développement d'un modèle de pré-dimensionnement. Dans un second temps, une campagne d'essais de caractérisation sera menée sur les éléments constitutifs de la pale pour alimenter le modèle. Enfin, dans un troisième temps, des essais sur une éprouvette représentative d'un tronçon de pale seront menés ainsi qu'une étude de sensibilité pour mettre en évidence la capacité du modèle à représenter le phénomène étudié
Safety is a crucial aspect in the development of aeronautical parts and assemblies. For a propeller aircraft, the blades ensure the propulsion of the aircraft and guarantee its lift. They are essential. This is why bird strike certification tests are necessary to ensure the aircraft's ability to land in a degraded mode. These tests take place at the end of the development phase. An upstream characterization of impact behavior is necessary to avoid potential additional costs associated with poor test results. The aircraft blades studied are complex sandwich composite structures, consisting of a polymer foam core and several layers, including composite braids, unidirectional composite plies, polymer foam, metallic reinforcements, bonded interfaces, and other specific components (lightning protection, paint, etc.). Therefore, numerous materials need to be considered during the characterization phase. Additionally, it is necessary to reconcile phenomena at both microscopic and macroscopic scales. The research work will initially focus on understanding and characterizing damage phenomena under bird strike certification conditions. This first step should allow the prioritization of physical phenomena to guide the development of a preliminary sizing model. In the second phase, a characterization test campaign will be conducted on the constituent elements of the blade to feed the model. Finally, in the third phase, tests on a representative specimen of a blade section will be conducted, as well as a sensitivity study to highlight the model's ability to represent the studied phenomenon

Low weight is one of the most important factors in the design process of high speed naval ships, road vehicles and aircrafts. Lower structural weight enables the possibility of down-sizing the propulsion system and thus decrease manufacturing and operating costs as well as reducing the environmental impact. Two efficient ways of reducing the structural weight of a structure is by using high performance composite materials and by using geometrically efficient structures such as the sandwich concept. In addition to good quasi-static performance different structures have dynamic impact requirements. For a road vehicle this might be crash worthiness, an aircraft has to be able to sustain bird strikes or debris impact and a naval ship needs to be protected against blast or ballistic loading. In this thesis important aspects of dynamic loading of composite and sandwich structures are addressed and presented in the appended papers as follows. In paper A the notch sensitivity of non-crimp fabric glass bre composites is investigated. The notch sensitivity is investigated for several different laminate con gurations at varying tensile loading rate. It is shown that the non-crimp fabrics have very low notch sensitivity, especially for laminate con gurations with a large amount of bres in the load direction. Further, the notch sensitivity is shown to be fairly constant with increasing loading rates (up to 100/s). In paper B a heuristic approach is made in order to create an analytical model to predict the residual strength of composite laminates with multiple randomly distributed holes. The basis for this model is a comprehensive experimental programme. It is found that unidirectional laminates with holes predominantly fail through three failure modes: global net-section failure, local net-section failure and local shear failure. Each failure mode can be described by a physical geometric constant which is used to create the analytical model. The analytical model can predict the residual strength of unidirectional laminates with multiple, randomly distributed holes with good accuracy. In paper C and paper D, novel prismatic high performance all-composite sandwich cores are proposed. In paper C an analytical model is developed that predicts the strength and sti ness properties of the suggested cores. In paper D the prismatic cores are manufactured and tested in shear loading and out-of-plane compression loading. Further, the analytical model is used to create failure mechanism maps to map out the overall behaviour of the different core con gurations. The novel cores show very high speci c strength and sti ness and are potential candidates as cores in high performance naval ship hulls. In paper E the dynamic properties of prismatic composite cores are investigated. The dynamic out-of-plane strength of an unit cell is tested experimentally in a gas gun - Kolsky bar set-up. Especially, different failure mechanisms and their e ect on the structural strength are investigated. It is found that cores with low relative density (slender core members) show very large inertial stabilisation e ects and have a dynamic strength that can be more than seven times higher than the quasi-static strength. Cores with higher relative density show less increase in dynamic strength. The main reason for the dynamic strengthening is due to the strain rate sensitivity of the parent material rather than inertial stabilisation of the core members.
QC 20101014

This study investigated the application of fibre reinforced composite materials to spacecraft sandwich structures. In particular, aspects of the manufacture, analysis and design optimisation of components fabricated using the co-cure process were studied. The manufacturing process was developed to ultimately enable a full size thrust tube structure to be built using a single step cure, the design of which was verified by a modal survey test. Techniques for the analysis of stiffness, strength., vibration frequencies and local instability were established and found to correlate well with tests on co-cured sandwich specimens. The current wrinkling theory for composite faced sandwich was extended to the more general case to allow facesheet constitutive matrix coupling and multiaxial loding to be accomodated. The analytical methods were incorporated within simple optimisation schemes, amenable to employment at the preliminary design stage, to allow alternative feasible designs for panel and thrust tube structures to be generated. These illustrated the benefits of the use of composite materials and the co-cure manufacturing technique for spacecraft sandwich components.

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The objective of this research is to examine the fluid structure interaction (FSI) effect on composite sandwich structures under a low velocity impact. The primary sandwich composite used in this study was a 6.35-mm balsa core and a multi-ply symmetrical plain weave 6 oz E-glass skin. The specific geometry of the composite was a 305 by 305 mm square with clamped boundary conditions. Using a uniquely designed vertical drop-weight testing machine, there were three fluid conditions in which these experiments focused. The first of these conditions was completely dry (or air) surrounded testing. The second condition was completely water submerged. The final condition was a wet top/air-backed surrounded test. The tests were conducted progressively from a low to high drop height to best conclude the onset and spread of damage to the sandwich composite when impacted with the test machine. The measured output of these tests was force levels and multi-axis strain performance. The collection and analysis of this data will help to increase the understanding of the study of sandwich composites, particularly in a marine environment.

Composite sandwich structures are widely regarded as a cost/weight-effective alternative to conventional composite stiffened panels and are extensively utilized for lightweight applications in various sectors, including the aeronautical, marine and transport industries. Nevertheless, their damage tolerance remains a critical issue. This work aims to develop reliable analytical and numerical tools for the design of damage-tolerant advanced foam-cored composite sandwich structures for aerospace applications. It comprises of original experimental observations together with novel numerical and analytical developments, as detailed below. A novel analytical model for predicting the post-crushing response of crushable sandwich foam cores is presented. The calibration of the model is performed using experimental data obtained exclusively from standard monotonic compressive tests. Hence, the need for performing time-consuming compressive tests including multiple unloading-reloading cycles is avoided. Subsequently, the translaminar initiation fracture toughness of a carbon-epoxy Non-Crimp Fabric (NCF) composite laminate is measured. The translaminar fracture toughness of the UD fibre tows is related to that of the NCF laminate and the concept of an homogenised blanket-level translaminar fracture toughness was introduced. A multiple length/time-scale framework for the virtual testing of large composite structures is presented. Such framework hinges upon a novel Mesh Superposition Technique (MST) and a novel set of Periodic Boundary Conditions named Multiscale Periodic Boundary Conditions (MPBCs). The MST is used for coupling different areas of the composite structure modelled at different length-scales and whose discretizations consist of different element types. Unlike using a sudden discretization-transition approach, the use of the MST eliminates the undesirable stress disturbances at the interface between differently-discretized subdomains and, as a result, it for instance correctly captures impact-induced damage pattern at a lower computational cost. The MPBCs apply to reduced Unit Cells (rUCs) and enable the two-scale (solid-to- shell) numerical homogenization of periodic structures, including their bending and twisting response. The MPBCs allow to correctly simulate the mechanical response of periodic structures using rUCs (same results as if conventional UCs were used), thus enabling a significant reduction of both modelling/meshing and analysis CPU times. The developments detailed above are finally brought together in a realistic engineering application.

The exploitation of sandwich structures as a means toachieve high specific strength and stiffness is relatively new.Therefore, the knowledge of its damage tolerance is limitedcompared to other structural concepts such as truss bars andmonocoque plate solutions.

Several aspects of the damage tolerance of sandwichstructures are investigated. The influence of impact velocityonresidual strength is investigated. Sandwich panels withfaces of glass fiber reinforced vinylester are impacted bothwith very high velocity and quasi static. The residual strengthafter impact is found to be similar for both cases of impactvelocity.

Curved sandwich beams subjected to opening bending momentare studied. Faceñcore debonds of varying size areintroduced between the compressively loaded face sheet and thecore. Finite element analysis in combination with a pointstress criterion is utilized to predict the residual strengthof the beams. It is shown that it is possible to predict thefailure load of the beams with face-core debond.

Using fractography the governing mode of failure ofcompressively NCF-carbon is characterized. Sandwich panelssubjected to compression after impact are shown to fail byplastic micro buckling.

The residual compressive strength after impact of sandwichpanels is investigated. Sandwich panels with face sheets ofnon-crimp fabric (NCF) carbon are subjected to different typesof impact damages. Predictions of residual strength are madeusing the Budiansky, Soutis, Fleck (BSF) model. The residualstrength is tested, and the results are compared topredictions. Predictions and tests correlate well, and indicatethat the residual strength is dependent on damage size and notthe size of the damaged panel.

A study of the properties of a selection of fiberreinforcements commonly used in sandwich panels is conducted.The reinforcements are combined with two types of core materialand three types of matrix. Also the influence of laminatethickness is tested. Each combination materials is tested inuni-axial compression, compressive strength after impact andenergy absorption during quasi static indentation. Thespecimens which are tested for residual strength are eithersubjected to quasi-static or dynamic impact of comparableenergy level. Prediction of the residual strength is made andcorrelates reasonably whith the test results. The tests showthat if weight is taken into account the preferred choice offiber reinforcement is carbon.

Currently, out-of-autoclave (OOA) technology is being used to design and manufacture composite structural components at lower costs. OOA technology enables composites to be produced using only vacuum pressure, eliminating the cost of purchasing and operating an autoclave. The key to OOA prepreg is that they are specially designed to remove air that is entrapped during the lay-up process. The in-plane and through thickness permeability of the prepreg were characterized to determine which bagging configuration would produce the best honeycomb sandwich structure. The bagging configuration that produced the lowest skin porosity was determined to be one ply of non-perforated release film with edge breathing around the perimeter of the panel. The resin content of the prepreg is such that any resin loss from the skin will create dry spots, pinholes, and porosity. The edge breathing allows the air inside the prepreg and core to be removed by the vacuum, and t he non-perforated release film presents resin starvation and subsequently reduces porosity. Caution should be used when debulking out-of-autoclave sandwich panels, since removing all the air from the core reduces the amount of skin compaction available during cure. The skin compaction is needed to suppress void growth, and the optimal internal core pressure was experimentally determined to be between 35-55 kPa. The resin was fully characterized such that when the internal core pressure has reached the optimal range, the resin can flow and close off the air passages. Five representative panels were manufactured using two low temperature cure film adhesives. The optimal curing temperature to minimize porosity of the composite skin was determined to be 100°C. Climbing drum peel tests were used to evaluate the mechanical performance of the panels.
Présentement, la technologie hors autoclave (OOA) est utilisée pour la conception et la fabrication à moindre coût des composants structuraux en composite. La technologie OOA permet de produire des composites en utilisant uniquement la pression générés lors des procédés d'ensachage sous vide, en éliminant par le fait même les coûts relatifs à l'achat et à l'exploitation d'un autoclave. L'avantage des matériaux pré-imprégnés destinés à des cuissons hors autoclave est qu'ils sont spécialement conçus pour éliminer les bulles d'air emprisonnées au cours de la préparation. Afin de déterminer la configuration d'ensachage optimale pour la production de structure en sandwich avec noyau en nid d'abeille, l'épaisseur du matériel pré-imprégné ainsi que la perméabilité à travers le plan ont été caractérisées. Les conditions d'ensachage ayant menées à la plus faible porosité correspondent à l'utilisation combinée d'une couche non perforée de pellicule antiadhésive et d'une bordure permettant l'extraction des produits gazeux. La pellicule antiadhésive non perforée permet de prévenir les pertes de résine, tandis que la bordure permettant l'extraction des produits gazeux permet d'évacuer l'air contenu dans le noyau et les couches pré-imprégnés. L'utilisation d'une pellicule non perforée s'est avérée nécessaire puisque la teneur en résine du matériel pré-imprégné est telle que toute perte de résine crée des régions dites sèches (non imprégnée), produit des trous ou défauts à la surface du laminé et augmente la porosité. Il convient d'être prudent lors du dégazage des panneaux en sandwich hors autoclave puisque la suppression de tout l'air contenue dans le noyau réduit le niveau de compaction disponible durant la cuisson. L'application d'une force de compaction adéquate doit être effectuée lors de la cuisson afin de réduire la formation de vide

The present document covers the studies over Structural Health Monitoring systems via vibration based methods. The topic is organized in two parallel studies. The first one analyzes the integrity of metal-composite single lap bonded joints. The second one approaches similar analyses for sandwich structures. The monitoring was made by investigating the dynamic response both computationally and experimentally to verify the reliability of applying vibration based SHM procedures, specifically with the objective of identifying the presence of debonding damage. The dynamic responses were obtained via accelerometers and piezoelectric sensors placed on top of the investigated structures (on the outward surface). The purpose for the accelerometers is to provide reference data for the analyses involving the piezoelectric sensors. Different metrics of damage identification were investigated, all working over a determined frequency range. They quantify the damage by analyzing either the magnitudes or phase angles of the dynamic responses among the undamaged and damage structures. This present work proposed modifications to some methodologies of damage quantification found in the literature and compared the results. The new metrics offered more reliable values for the damage quantification on several of the analyses. It was verified that the metrics are valid for the scenarios observed in the present study. The experimental analyses showed also the influence on the dynamic response due to the position of small elastomeric elements. In regards to the finite element analyses, the computational models showed similar results to the experimental data, the more accurate ones being the models for the bonded joints. For the computational models, improvements can be applied into the piezoelectric sensor (e.g. by using new finite element formulations), as well as the region of debonding (e.g. by using contact algorithms). It is important to highlight that the elastic properties of the skins for the sandwich structure were obtained by the literature, so the model can be improved in the future by applying properties obtained experimentally.
Esta dissertação aborda os estudos realizados no campo de Sistemas de Monitoramento da Integridade Estrutural por meio de métodos baseados em vibrações. O tópico abordado é organizado em dois estudos paralelos. O primeiro é relativo ao monitoramento da integridade de juntas coladas metal-compósito. O segundo versa sobre análises semelhantes em estruturas sanduíche. O monitoramento foi executado através das análises das assinaturas dinâmicas das estruturas, tanto computacionalmente quanto experimentalmente, visando avaliar a capacidade de metodologias vibracionais de SHM em detectar dano de descolamento. As respostas dinâmicas foram obtidas por meio de acelerômetros e sensores piezelétricos dispostos sobre a superfície das estruturas avaliadas. Os acelerômetros fornecem dados de referência para as análises realizadas com base nas respostas do sensor piezelétrico. Diferentes métricas de identificação de dano são abordadas, sendo que todas estão baseadas em análise no domínio da frequência, utilizando parâmetros de magnitude ou ângulo de fase das estruturas danificadas e intactas. O presente trabalho propôs alterações em algumas das metodologias encontradas na literatura e comparou os resultados das métricas originais com as modificadas. As métricas modificadas apresentaram resultados mais consistentes em vários cenários de análise. Constatou-se também que as métricas abordadas mostram-se válidas para os casos observados no presente estudo. As análises experimentais também evidenciaram a influência na assinatura dinâmica da estrutura sanduíche causada pelo posicionamento de pequenos elementos elastoméricos. Com relação às análises via elementos finitos, os modelos computacionais apresentaram resultados similares aos obtidos experimentalmente, sendo os da junta colada os mais precisos. Tais modelos computacionais podem ser melhorados no futuro por meio de uma modelagem mais detalhada dos elementos piezelétricos (por exemplo: por meio de novas formulações), como também da região de descolamento (por exemplo: por meio da implementação de algoritmos de contato). Deve-se ressaltar também que as propriedades elásticas das lâminas externas da estrutura sanduíche foram obtidas da literatura, assim sendo, o modelo poderá ser melhorado em estudos futuros por meio do emprego de propriedades obtidas experimentalmente.

Failure modes for sandwich beams of GFRP laminate skins and Nomex honeycomb core are investigated. Theoretical models using honeycomb mechanics and classical beam theory are described. A failure mode map for loading under 3-point bending, is constructed, showing the dependence of failure mode and load on the ratio of skin thickness to span length and honeycomb relative density. Beam specimens are tested in 3-point bending. The effect of honeycomb direction is also examined. The experimental data agree satisfactorily with the theoretical predictions. The results reveal the important role of core shear in a sandwich beam's bending behaviour and the need for a better understanding of indentation failure mechanism. High order sandwich beam theory (HOSBT) is implemented to extract useful information about the way that sandwich beams respond to localised loads under 3-point bending. 'High-order' or localised effects relate to the non-linear patterns of the in-plane and vertical displacements fields of the core through its height resulting from the unequal deformations in the loaded and unloaded skins. The localised effects are examined experimentally by Surface Displacement Analysis of video images recorded during 3-point bending tests. A new parameter based on the intrinsic material and geometric properties of a sandwich beam is introduced to characterise its susceptibility to localised effects. Skin flexural rigidity is shown to play a key role in determining the way that the top skin allows the external load to pass over the core. Furthermore, the contact stress distribution in the interface between the central roller and the top skin, and its importance to an indentation stress analysis, are investigated. To better model the failure in the core under the vicinity of localised loads, an Arcan- type test rig is used to test honeycomb cores under simultaneous compression and shear loading. The experimental measurements show a linear relationship between the out-of-plane compression and shear in honeycomb cores. This is used to derive a failure criterion for applied shear and compression, which is combined with the high order sandwich beam theory to predict failure caused by localised loads in sandwich beams made of GFRP laminate skins and Nomex honeycomb under 3-point bending loading. Short beam tests with three different indenter's size are performed on appropriately prepared specimens. Experiments validate the theoretical approach and reveal the nature of pre- and post-failure behaviour of these sandwich beams. HOSBT is used as a compact computational tool to reconstruct failure mode maps for sandwich panels. Superposition of weight and stiffness contours on these failure maps provide carpet plots for design optimisation procedures.

The next generation of European scientific satellites will carry extremely sensitive measurement devices that require platform stability orders of magnitude higher than current missions. It is considered that the meteoroid and space debris (M/SD) environment poses a risk to the success of these missions as disturbances induced by the impact of these particles at hypervelocity may degrade the platform stability below operational requirements. In this thesis, disturbances induced by the impact of M/SD particles at hypervelocity on a representative scientific satellite platform have been investigated. An extensive experimental impact test program has been performed, from which an empirical ballistic limit equation (BLE) which defines the conditions of structural perforation for composite sandwich panel structures with CFRP facesheets and aluminium honeycomb cores (CFRP/Al HC SP) has been defined. The BLE is used to predict impact conditions capable of inducing the different excitation modes relevant for a SP sandwich panel structure, enabling a significant reduction in the time and expense usually required for calibrating the protective capability of a new structural configuration. As experimental acceleration facilities are unable to cover the complete range of possible in-orbit impact conditions relevant for M/SD impact risk assessment, a Hydrocode model of the representative CFRP/Al HC SP has been constructed. A series of impact simulations have been performed during which the local impact-induced disturbance has been measured. The numerical disturbance signals have been validated via comparison with experimental disturbance measurements, and subsequently subject to a characterisation campaign to define the local elastic excitation of the SP structure equivalent to that induced by impact of a M/SD particle at hypervelocity. The disturbance characterisation is made such that it is applicable as an excitation force on a global satellite Finite Element (FE) model, allowing propagation of impact-induced disturbances throughout the complete satellite body to regions of critical stability (i.e. measurement devices). The disturbance induced upon measurement devices by M/SD impacts at both near- and far-body locations can then be made, allowing the threat to mission objectives to be assessed.

Les structures sandwich sont des structures légères composées de deux peaux superficielles minces, relativement denses, à haute résistance qui sont collées de part et d'autre d'une âme, épaisse, de faible densité telle que les mousses ou nids d'abeilles. Les matériaux sandwich à peau composite renforcée de fibres plastiques et à cœur en matériaux polymère représentent aujourd'hui une classe importante des matériaux structuraux légers dans de nombreux domaines de l'ingénierie tels que l'aéronautique et l'aérospatiale, l'automobile et les structures marines. Toutefois, certaines de ces structures sandwich ont des capacités d'absorption d'énergie très limitées. Cette limitation devient critique lorsque ces structures sont susceptibles d'être soumises à un impact. L'endommagement par impact dans le cas de structures sandwich peut être dû, notamment, à des chutes d'outils, des vols de débris sur piste d'atterrissage, des chocs à oiseaux, des averses de grêles ou des impacts balistiques.Les résines utilisées en tant que matrice dans le cas des sandwiches à peaux en composites stratifiés sont généralement des résines thermodurcissables comme les résines époxy. En raison de la nature fragile de la matrice, même la présence d'un léger délaminage interne se propage essentiellement à angle droit par rapport à la contrainte de compression appliquée ayant alors des résultats désastreux pour la structure sandwich. Une des solutions proposées est alors la modification des résines thermodurcissables avec l'ajout de particules organiques et inorganiques de taille nanométrique. Une nouvelle méthode de synthèse de copolymères par blocs qui s'auto-assemblent à l'échelle nanométrique permettrait de réduire sensiblement les problèmes liés à la dispersion des nanoparticules.L'objectif de ce travail est d'étudier et de mieux comprendre l'amélioration de la résistance à l'impact des panneaux sandwich à peau en stratifiés composites grâce à l'ajout de copolymère tribloc (Nanostrength®) dans la matrice Epoxy du composite Fibres/Epoxy. L'effet des nanoparticules sur les performances mécaniques des panneaux sandwich à peau Kevlar/Epoxy ou Verre/Epoxy et âme en mousse Rohacell® sera investigué : pour cela une comparaison des résultats entre résine pure et résine modifiée par l'ajout de 10% de Nanostrength sera effectuée en utilisant des essais expérimentaux et une modélisation numérique. Ce travail portera sur deux types de chargement d'impact différents ; des impacts à faible vitesse dont l'angle d'incidence est normal à la surface de l'échantillon et des impacts à faible vitesse et à trajectoire parabolique. Un dispositif d'impact tridimensionnel adossé à un hexapode de mouvement a été développé pour étudier la réponse mécanique d'un panneau sandwich soumis à une trajectoire parabolique.La méthode des éléments finis est un moyen largement usité pour étudier l'impact sur les structures et notamment les structures sandwich. Un modèle LS-Dyna a été développé pour la simulation de l'impact normal sur plaque de composites stratifiés et sur plaques sandwich Kevlar/Epoxy – mousse Rohacell®. Une loi de comportement basée sur la mécanique de l'endommagement, disponible dans la bibliothèque de modèles matériaux proposés par LS-Dyna et dénommé « Laminated Composite Fabric » (MAT58) a été utilisée pour représenter le comportement des plaques composites. Les paramètres d'entrée du modèle MAT58 ont été obtenus par combinaison d'essais et études paramétriques. Le modèle « CRUSHABLE FOAM » (MAT63) a été utilisé pour le cœur. Le modèle macroscopique avec une loi de comportement phénoménologique est capable de simuler la réponse macroscopique de composites stratifiés et plaques sandwich soumis à des impacts de faible vitesse.On peut souligner que le développement de panneaux sandwich à matrice renforcée de copolymère tribloc est un domaine prometteur de l'étude
Sandwich structures are lightweight structures composed of two thin, relatively dense, high strength facesheets that are glued on either side of a thick, low density core, such as foams or honeycombs. Sandwich panels with fibre reinforced plastic skins and core of polymer foam represent an important class of lightweight structural materials in many areas of such as aeronautics and aerospace, automotive and marine structures. However, some of these sandwich structures have very limited energy absorption capacity. This limitation becomes critical because these structures are susceptible to be subjected to impact. The impact damage in the case of sandwich structures may be due, in particular, to dropped tools, flights debris, bird strike, hailstorms or ballistic impacts.The resins used as the matrix in the case of sandwich panels with laminated composite facesheets are usually thermosetting resins such as epoxy resins. Due to the fragile nature of the matrix, the presence of even a slight internal delamination spreads at right angles to the applied compressive stress with disastrous results for the sandwich structure. One of the proposed solutions is the modification of the thermosetting resins with the addition of organic and inorganic particles of nanometric size. A new method of synthesis of block copolymers that self-assemble at the nanoscale would substantially reduce the problems associated with the dispersion of nanoparticles.The objective of this work is to study and better understand the improvement of impact resistance of sandwich panels with skin laminates with the addition of tri-block copolymer (Nanostrength®) in the epoxy matrix of fibre / epoxy composite. The effect of nanoparticles on the mechanical performance of the sandwich Kevlar / epoxy or glass / epoxy facesheets and Rohacell® foam core panels will be investigated by comparing the results between pure resin and resin modified by the addition of 10% Nanostrength performed using experimental testing and numerical modelling. This work will focus on two different types of impact loading; low velocity impacts with normal angle of incidence to the sample surface and low velocity impacts with parabolic trajectory. A device for three-dimensional impact has been developed to study the mechanical response of sandwich panels subjected to a parabolic trajectory impact.The finite element method is a widely used method to study the impact on structures including sandwich structures. An LS-Dyna model was developed to simulate the normal impact of composite laminates and Kevlar / Epoxy - Rohacell® foam sandwich plates. A constitutive law based on damage mechanics, available in the material library of LS-Dyna called "Composite Laminated Fabric" (MAT58) was used to represent the behaviour of composite facesheets. The input parameters of the model MAT58 were obtained by combination of tests and parametric studies. The model "Crushable foam" (MAT63) was used for the core. The macroscopic model with a phenomenological law is able to simulate the mechanical response of composite laminates and sandwich plates subjected to low velocity impacts. It may be noted that the development of sandwich panels reinforced with triblock copolymer in the matrix is a promising field of study

Fiber reinforced plastics offer advantageous specific strength and stiffness compared to metals and has been identified as candidates for the reusable space transportation systems primary structures including cryogenic tanks. A number of carbon and aramid fiber reinforced plastics have been considered for the liquid hydrogen tanks. Materials selection is based upon mechanical properties and containment performance (long and short term) and upon manufacturing considerations. The liquid hydrogen tank carries shear, torque, end load, and bending moment due to gusts, maneuver, take-off, landing, lift, drag, and fuel sloshing. The tank is pressurized to about 1.5 atmosphere (14.6psi or 0.1MPa) differential pressure and on ascent maintains the liquid hydrogen at a temperature of 20K. The objective of the research effort is to lay the foundation for developing the technology required for reliable prediction of the effects of various design, manufacturing, and service parameters on the susceptibility of composite tanks to develop excessive permeability to cryogenic fuels. Efforts will be expended on developing the materials and structural concepts for the cryogenic tanks that can meet the functional requirements. This will include consideration for double wall composite sandwich structures, with inner wall to meet the cryogenic requirements. The structure will incorporate nanoparticles for properties modifications and developing barriers. The main effort will be extended to tank wall’s internal skin design. The main requirements for internal composite stack are: • introduction of barrier film (e.g. honeycomb material paper sheet) to reduce the wall permeability to hydrogen, • introduction of nanoparticles into laminate resin to prevent micro-cracking or crack propagation. There is a need to characterize and analyze composite sandwich structural damage due to burning and explosion. Better understanding of the flammability and blast resistance of the composite structures needs to be evaluated.

En service, les pièces aéronautiques en matériaux composites et structures sandwiches subissent des dommages qui nécessitent des réparations. Les réparations par patch interne en biseau, en escalier ou par combinaison des deux offrent une excellente restauration de la résistance mécanique pour ces structures composites. Cependant, l’environnement de réparation peut se révéler être un défi de taille quant à leur mise en œuvre, au choix des paramètres géométriques (angle de biseau, nombre de plis extra), à leur comportement mécanique sous différents chargements ainsi qu’à leur processus d’endommagement. Cette thèse présente une étude expérimentale et numérique (éléments finis) du comportement mécanique de réparations par patch interne effectuées sur des structures avec des peaux en composites à renforts tissés fabriquées hors autoclave et âme en Nomex en nid d’abeille. Afin de déterminer l’effet de différents paramètres géométriques sur la résistance de la réparation et de comprendre son comportement mécaniqueet son processus d’endommagement, une série de tests de caractérisation sous différents chargements (traction, compression, flexion) a été effectuée sur des structures sandwiches faite avec deux matériaux composites tissés pour la peau : soit du composite tissé taffetas (PW) ou satin de 8 (8HS) Des simulations numériques ont été effectuées afin de prédire le comportement mécanique de la réparation. Cette étude numérique a été effectuée en plusieurs étapes. Un premier modèle 2D qui suppose que la colle ait un comportement linéaire élastique a été développé et permet d’étudier la distribution des contraintes dans le joint de colle pour différentes configurations de réparation rectangulaire. Ensuite, le modèle 2D est modifié pour tenir compte du comportement élastoplastique de la colle et ceci permet de prédire le comportement mécanique d’une réparation rectangulaire jusqu’à la rupture. Par la suite, un modèle 3D est développé pour prédire le comportement de réparations circulaires sous des chargements de compression. Ce modèle tient compte de l’endommagement progressif des peaux en composite. Les résultats de ces simulations numériques sont comparés par la suite aux mesures expérimentales. Les modèles par éléments finis, avec une loi de comportement élastoplastique pour le joint de colle, permettent une estimation adéquate de la résistance ainsi que de l’endommagement des structures sandwiches réparées. Une étude paramétrique a eu lieu afin d’étudier l’effet de différents paramètres géométriques sur la résistance de la réparation. La mise en œuvre et la détermination de la performance mécanique des réparations par patch interne des structures sandwiches est une tâche complexe avec de multiples paramètres de matériaux et de procédés. D’une manière générale, cette thèse contribue à une meilleure compréhension du comportement mécanique des structures sandwiches réparées et de leur processus d’endommagement. Les modèles par éléments finis développés dans ces travaux ont été validés expérimentalement et des simulations paramétriques ont contribué à une meilleure compréhension de l’influence des différents paramètres géométriques sur la résistance de la réparation par patch interne.
En service, les pièces aéronautiques en matériaux composites et structures sandwiches subissent des dommages qui nécessitent des réparations. Les réparations par patch interne en biseau, en escalier ou par combinaison des deux offrent une excellente restauration de la résistance mécanique pour ces structures composites. Cependant, l’environnement de réparation peut se révéler être un défi de taille quant à leur mise en œuvre, au choix des paramètres géométriques (angle de biseau, nombre de plis extra), à leur comportement mécanique sous différents chargements ainsi qu’à leur processus d’endommagement. Cette thèse présente une étude expérimentale et numérique (éléments finis) du comportement mécanique de réparations par patch interne effectuées sur des structures avec des peaux en composites à renforts tissés fabriquées hors autoclave et âme en Nomex en nid d’abeille. Afin de déterminer l’effet de différents paramètres géométriques sur la résistance de la réparation et de comprendre son comportement mécaniqueet son processus d’endommagement, une série de tests de caractérisation sous différents chargements (traction, compression, flexion) a été effectuée sur des structures sandwiches faite avec deux matériaux composites tissés pour la peau : soit du composite tissé taffetas (PW) ou satin de 8 (8HS) Des simulations numériques ont été effectuées afin de prédire le comportement mécanique de la réparation. Cette étude numérique a été effectuée en plusieurs étapes. Un premier modèle 2D qui suppose que la colle ait un comportement linéaire élastique a été développé et permet d’étudier la distribution des contraintes dans le joint de colle pour différentes configurations de réparation rectangulaire. Ensuite, le modèle 2D est modifié pour tenir compte du comportement élastoplastique de la colle et ceci permet de prédire le comportement mécanique d’une réparation rectangulaire jusqu’à la rupture. Par la suite, un modèle 3D est développé pour prédire le comportement de réparations circulaires sous des chargements de compression. Ce modèle tient compte de l’endommagement progressif des peaux en composite. Les résultats de ces simulations numériques sont comparés par la suite aux mesures expérimentales. Les modèles par éléments finis, avec une loi de comportement élastoplastique pour le joint de colle, permettent une estimation adéquate de la résistance ainsi que de l’endommagement des structures sandwiches réparées. Une étude paramétrique a eu lieu afin d’étudier l’effet de différents paramètres géométriques sur la résistance de la réparation. La mise en œuvre et la détermination de la performance mécanique des réparations par patch interne des structures sandwiches est une tâche complexe avec de multiples paramètres de matériaux et de procédés. D’une manière générale, cette thèse contribue à une meilleure compréhension du comportement mécanique des structures sandwiches réparées et de leur processus d’endommagement. Les modèles par éléments finis développés dans ces travaux ont été validés expérimentalement et des simulations paramétriques ont contribué à une meilleure compréhension de l’influence des différents paramètres géométriques sur la résistance de la réparation par patch interne.
In service, aeronautical components made of composite materials and sandwich structures are subject to type of damages that require repairs. Adhesively bonded repairs (scarf-scarf, step-step or scarf-step) offer an excellent mechanical strength recovery for these composite structures. However, the repair environment can be a significant challenge in terms of the choice of geometrical parameters (scarf angle, addition of an overply), damage process parameters and mechanical behavior under different loads.This thesis presents both experimental and numerical investigations of the mechanical behavior of internal patch repairs carried-out on Nomex honeycomb composite sandwich structures. The skins use an out-of-autoclave woven fabric made of carbon-epoxy composite materials. In order to determine the effect of different geometric parameters on the resistance of the internal patch repair and to better understand its mechanical behavior and damage processes, a series of mechanical tests under different loads (tensile, compression, bending) is conducted on the repaired sandwich panels made with either plain weave or 8 harness satin textile composites. Numerical simulations were carried out, in several stages, in order to determine the mechanical behavior of the repair. First, a 2D model that assumes a linear elastic behavior of the adhesive film was developed. This simple model allows to study the distribution of the stresses in the adhesive joint for different configurations of rectangular patch repair. Then, the 2D model is modified in order to account for the elastoplastic behavior of the adhesive film. The latter allows to predict the mechanical behavior of a rectangular internal patch repair until rupture. Subsequently, a 3D model is developed to predict the mechanical behavior of circular internal patch repairs under compressive loadings. This model takes into account the progressive damage and failure of the woven fabric skins. The results of these numerical simulations are validated by comparing them to experimental measurements. The finite element models that account for the elastoplastic behavior law for the adhesive joint allow predictions of the strength as well as the damage morphology of the repaired sandwich structures. A parametric study has also been conducted in order to determine the influence of the geometrical design parameters in the repair strength. Processing and assessment of the mechanical performance of internal patch repairs on sandwich structures is a complex task with multiple material and process parameters. In general, this thesis contributes to a better understanding of the mechanical behavior of adhesively bonded repaired sandwich structures and their damage process. The finite element models developed in this work and validated experimentally have contributed through parametric numerical simulations to an economical better understanding of the influence of different geometric parameters on the strength and failure of internal patch repaired sandwich panels.
In service, aeronautical components made of composite materials and sandwich structures are subject to type of damages that require repairs. Adhesively bonded repairs (scarf-scarf, step-step or scarf-step) offer an excellent mechanical strength recovery for these composite structures. However, the repair environment can be a significant challenge in terms of the choice of geometrical parameters (scarf angle, addition of an overply), damage process parameters and mechanical behavior under different loads.This thesis presents both experimental and numerical investigations of the mechanical behavior of internal patch repairs carried-out on Nomex honeycomb composite sandwich structures. The skins use an out-of-autoclave woven fabric made of carbon-epoxy composite materials. In order to determine the effect of different geometric parameters on the resistance of the internal patch repair and to better understand its mechanical behavior and damage processes, a series of mechanical tests under different loads (tensile, compression, bending) is conducted on the repaired sandwich panels made with either plain weave or 8 harness satin textile composites. Numerical simulations were carried out, in several stages, in order to determine the mechanical behavior of the repair. First, a 2D model that assumes a linear elastic behavior of the adhesive film was developed. This simple model allows to study the distribution of the stresses in the adhesive joint for different configurations of rectangular patch repair. Then, the 2D model is modified in order to account for the elastoplastic behavior of the adhesive film. The latter allows to predict the mechanical behavior of a rectangular internal patch repair until rupture. Subsequently, a 3D model is developed to predict the mechanical behavior of circular internal patch repairs under compressive loadings. This model takes into account the progressive damage and failure of the woven fabric skins. The results of these numerical simulations are validated by comparing them to experimental measurements. The finite element models that account for the elastoplastic behavior law for the adhesive joint allow predictions of the strength as well as the damage morphology of the repaired sandwich structures. A parametric study has also been conducted in order to determine the influence of the geometrical design parameters in the repair strength. Processing and assessment of the mechanical performance of internal patch repairs on sandwich structures is a complex task with multiple material and process parameters. In general, this thesis contributes to a better understanding of the mechanical behavior of adhesively bonded repaired sandwich structures and their damage process. The finite element models developed in this work and validated experimentally have contributed through parametric numerical simulations to an economical better understanding of the influence of different geometric parameters on the strength and failure of internal patch repaired sandwich panels.

Current insulation solutions across multiple industries, especially the commercial sector, can be bulky and ineffective when considering their volume. Aerogels are excellent insulators, exhibiting low thermal conductivities and low densities with a porosity of around 95%. Such characteristics make aerogels effective in decreasing conductive heat transfer within a solid. These requirements are crucial for aerospace and spaceflight applications, where sensitive components exist among extreme temperature environments. When implemented into insulation applications, aerogel can perform better than existing technology while using less material, which limits the amount of volume allocated for insulation. The application of these materials into composites can result in enhancing a material's thermal and mechanical properties when exposed to mechanical testing. The main objective of this study was to perform theoretical and experimental analysis on a corrugated composite sandwich structure integrated with aerogel insulation by studying its effective thermal conductivity. The aerogel material used was Pyrogel XT-E, a silica aerogel-based fiberglass insulation manufactured by Aspen Aerogels. Theoretical models of the corrugated composite sandwich structure were constructed in ANSYS Workbench based on geometry from a previous study. The main goal of the theoretical models was to analytically and computationally study the effective thermal conductivity of this sample; the conditions of these simulations were modeled after the experimental setup. Additionally, two insulation studies were performed using the thermal models. The first study was performed on a flat plate structure to determine the optimal thickness of Pyrogel XT-E in a flat plate orientation. The second study compared multiple types of common insulation materials to Pyrogel XT-E when integrated into the corrugated composite sandwich structure model. As expected, aerogel particles and Pyrogel XT-E outperformed all insulation materials and had the lowest effective thermal conductivity. Experimental data was obtained using a test enclosure and a heating element source with an integrated temperature control circuit that was designed and built for this study. This experimental data was compared to the theoretical data obtained from the thermal model simulations. The corrugated composite sandwich structure did not perform as well as expected due to thermal bridging along the composite corrugation. Its effective thermal conductivity was much higher than that of the flat plate structure, even though the effective Pyrogel XT-E layer in the corrugated composite sandwich structure was more than twice as thick as the layer in the flat plate structure. Despite thermal bridging, the corrugated composite sandwich structure exhibits superb thermal resistance, which adds to its impressive strength. Thermal conductivity results from this study can be used to design efficient materials for high structural and thermal stress applications.

This thesis will present the experimental and numerical analysis of composite sandwich structures under monotonic and fatigue loading. The sandwich skins were made of fiberglass and the core used was a closed cell PVC foam. Initial delaminations were introduced into the sandwich structures during manufacturing to see the effect of delamination size on the ultimate strength and monotonic fracture. Fiberglass rods, called shear keys, added to the foam core to determine whether or not they increased the strength of the test specimens. Furthermore, shear key locations were also varied and their effects noted. The fixed rate static behavior for all of the above cases listed were determined. The fatigue life and behavior were determined for sandwich structures with no initial delamination, 0.5 inch initial delamination, and 0.5 inch initial delamination with a shear key 0 inch from the delamination depth. The fatigue specimens were tested at various percentages of the ultimate monotonic failure loads to determine the fatigue life. A static numerical analysis was performed using Abaqus/CAE 6.7.1 to observe at the monotonic behavior of the test specimens with no initial delamination and with 0.5 inch initial delamination. The sandwich structures with an initial delamination and/or a shear key in the foam core experienced over a 70% reduction in the ultimate monotonic failure load. The two delamination lengths had no significant effect on the ultimate monotonic failure load, but the presence of an initial delamination corresponded to a material response dominated by plastic behavior. The experimental testing also showed that the location of the shear key in the sandwich structure had little effect on the monotonic strength, but moving the shear keys further away from the back edge of the delamination caused a reduction in strength. The monotonic testing determined that composite sandwich structures containing shear keys had approximately a 7% reduction in the monotonic failure load of test specimens with an initial delamination. Numerical analysis results matched the ultimate failure loads within 5% for the test specimens with a 0.5 inch an initial delamination and within 15% for the test specimens with no initial delamination. The fatigue testing showed that sandwich structures containing shear keys had life reduction of approximately 33%. Preliminary experiments involved with rotating the shear keys 90° showed increased ultimate monotonic failure loads of the composite sandwich structures by as much as 30%. Future funding and research would be necessary to verify the increased structural performance of the newly oriented shear keys.

The present study emerges from a present industry need for accurate and fast numerical modeling approaches to estimate the vibro-acoustic behaviours of multilayered composite and viscoelastic treatments configurations.The structure is modeled using a wave approach applied to various multilayer configurations such as: symmetrical laminate composite, symmetrical sandwich composite and general symmetrical or unsymmetrical laminate or sandwich composite as well as viscoelastic treatments. Three behavioural modeling approaches are investigated: smeared laminate, discrete layer sandwich and general discrete layer laminate. Smeared laminate approach is devoted to symmetrical laminate composite panels and uses equivalent elastic properties computed by smearing out the layers' properties through the panel's thickness. Discrete layer sandwich approach is devoted to symmetrical sandwich composite panels and uses individual displacement fields for each layer. Classical assumptions of thick skins sandwich panels are adopted. General discrete laminate approach accommodates both laminate and sandwich composite panels of symmetrical or unsymmetrical layout. Individual displacement fields are used for each layer. These three behavioural modeling approaches are applied in the present work to flat and curved panel configurations as well as laminated beams. Dispersion relations are developed for each configuration and solved in a generalized polynomial eigenvalue problem context. These solutions are used in a SEA framework to compute the group velocity, the modal density, the radiation efficiency as well as the resonant and non-resonant contributions to the transmission coefficient. Moreover, the dispersion relations are used to develop general expressions to compute the ring frequency and the critical frequencies. In the context of viscoelastic treatments modeling the mechanical impedance, the input mobility, the deformation energy as well as the equivalent loss factor are computed for several boundary conditions.The presented approaches are successfully validated with experimental results and previously published theories. In addition to their proven accuracy, the proposed approaches are quick and general.

The use of composite sandwich structures is rapidly increasing in the aerospace industry because of their increased strength-to-weight and stiffness-to-weight characteristics. The effects of low velocity impacts on these structures, however, are the main weakness that hinders further use of them in the industry because the damages from these loadings can often be catastrophic. Impact behavior of composite materials in general is a crucial consideration for a designer but can be difficult to describe theoretically. Because of this, experimental analysis is typically used to attempt to describe the behavior of composite sandwiches under impact loads. Experimental testing can still be unpredictable, however, because low velocity impacts can cause undetectable damage within the composites that weaken their structural integrity. This is an important issue with composite sandwich structures because interlaminar damage within the composite facesheets is typical with composites but the addition of a core material results in added failure modes. Because the core is typically a weaker material than the surrounding facesheet material, the core is easily damaged by the impact loads. The adhesion between the composite facesheets and the core material can also be a major region of concern for sandwich structures. Delamination of the facesheet from the core is a major issue when these structures are subjected to impact loads. This study investigated, through experimental and numerical analysis, how varying the core and facesheet material combination affected the flexural strength of a composite sandwich subjected to low velocity impact. Carbon, hemp, aramid, and glass fiber materials as facesheets combined with honeycomb and foam as core materials were considered. Three layers of the same composite material were laid on the top and bottom of the core material to form each sandwich structure. This resulted in eight different sandwich designs. The carbon fiber/honeycomb sandwiches were then combined with the aramid fiber facesheets, keeping the same three layer facesheet design, to form two hybrid sandwich designs. This was done to attempt to improve the impact resistance and post-impact strength characteristics of the carbon fiber sandwiches. The two and one layer aramid fiber laminates on these hybrid sandwiches were always laid up on the outside of the structure. The sandwiches were cured using a composite press set to the recommended curing cycle for the composite facesheet material. The hybrid sandwiches were cured twice for the two different facesheet materials. The cured specimens were then cut into 3 inch by 10 inch sandwiches and 2/3 of them were subjected to an impact from a 7.56 lbf crosshead which was dropped from a height of 38.15 inches above the bottom of the specimen using a Dynatup 8250 drop weight machine. The impacted specimen and the control specimen (1/3 of the specimens not subjected to an impact) were loaded in a four-point bend test per ASTM D7250 to determine the non-impacted and post-impact flexural strengths of these structures. Each sandwich was tested under two four-point bend loading conditions which resulted in two different extension values at the same 100 lbf loading value. The span between the two supports on the bottom of the sandwich was always 8 inches but the span between the two loading pins on the top of the sandwich changed between the two loading conditions. The 2/3 of the sandwiches that were tested after being impacted were subjected to bending loads in two different ways. Half of the specimens were subjected to four-point bending loads with the impact damage on the top facesheet (compressive surface) in between the loading pins; the other half were subjected to bending loads with the damage on the bottom facesheet (tensile surface). Theoretical failure mode analysis was done for each sandwich to understand the comparisons between predicted and experimental failures. A numerical investigation was, also, completed using Abaqus to verify the results of the experimental tests. Non-impacted and impacted four-point bending models were constructed and mid-span deflection values were collected for comparison with the experimental testing results. Experimental and numerical results showed that carbon fiber sandwiches were the best sandwich design for overall composite sandwich bending strength; however, post-impact strengths could greatly improve. The hybrid sandwich designs improved post-impact behavior but more than three facesheet layers are necessary for significant improvement. Hemp facesheet sandwiches showed the best post-impact bending characteristics of any sandwich despite having the largest impact damage sizes. Glass and aramid fiber facesheet sandwiches resisted impact the best but this resulted in premature delamination failures that limited the potential of these structures. Honeycomb core materials outperformed foam in terms of ultimate bending loads but post-impact strengths were better for foam cores. Decent agreement between numerical and experimental results was found but poor material quality and high error in material properties testing results brought about larger disagreements for some sandwich designs.

Abstract Salt water environments are very harsh on materials that are used within them. Many issues are caused by either corrosion and/or internal degradation to the materials themselves. Composites are better suited for this environment due to their high strength to weight ratios and their corrosion resistance, but very little is known about the fracture mechanics of composites. The goal of this study is to gain a better understanding for the behavior of a composite boat hull under a shear loading, similar to the force water applies on the hull as the boat moves through the water; then attempt to strengthen the composite sandwich panel against the shear loading. A parametric study was conducted to investigate monotonic in-plane shear loading for composite sandwich panels used in commercial naval vessels. In order to model a conventional composite boat hull, test specimens were composite sandwich panels made of a Divinycell H100 foam core with four layers of fiberglass on both sides of the core. Specimens were tested under a monotonic loading with a rate of 0.2 in/min, and tested until complete failure using the standard test. Seawater specimens were manufactured in the same manner as the original test specimens, but then were submersed in either filtered seawater or the ocean. The differences between the filtered pieces and the ocean allowed us to determine if any changes found in the composite sandwich panels were related to environment conditions or if the changes were related to the saltwater interaction itself. To create these different environments the seawater specimens were taken to the Avila pier where 36 specimens were placed in a tub that was fed filtered saltwater, while 30 specimens were placed in a plastic mesh with weights and lowered to a depth of approximately 30 ft. in the ocean. Three specimens were then removed at monthly intervals from both filtered and ocean environments. Shear Keys were created as a method to strengthen the composite sandwich panels against the shear force that the previous specimens had been tested to. Eight Shear Keys were then placed into groves cut into the foam core (four on each side) and the four fiberglass layers were laid on top. Testing showed that the seawater did have an initial effect on the composite sandwich panels. The filtered pieces showed a decrease in yield strength and stiffness the longer they were subjected to the seawater. The raw unfiltered pieces placed in the ocean saw an even higher decrease in their yield strength and decrease in stiffness. However, for both the unfiltered and raw specimens there was an increase in the ultimate strength and fracture point of the specimens. The effects of the sea water seemed to taper off after the 3rd month however. The Shear Key specimens were tested with a 4mm and an 8mm Shear Key. The 8mm Shear Keys showed a decrease in shear strength, which was primarily due to removing too much material from the core and weakening the specimen. It was concluded that the decrease in area created a force concentration at the deepest part of the Shear Key causing the premature failure. The 4mm Shear Key showed an increase in the yield strength, ultimate strength, and fracture point. A finite model was built to simulate the original test specimen along with the 4mm and 8mm Shear Key cases, and the results were compared to the experimental results. The numerical results showed that it was possible to relate the experimental results to the linear or elastic portion of the plots. There was a difference between the maximum displacement of the model and the actual specimens, but this was attributed to potential inaccurate comparison of the loading on the model compared to the actual specimens. The correlation between the model itself and the experimental data was close enough to conclude that it could be used for predicting baseline trends. Further investigation of the specimens should include looking into the effects of a cyclic shear loading on the specimens. This combined with the seawater element used in this thesis would provide further insight to the initial degradation seen in the seawater specimens, and could potentially provide a closer relation to current hull failures. In addition to including a cyclic loading another numerical model should be created. A model that could be constrained both locally and globally would provide more accurate results. The FEM should also include the ability to run a crushable foam core model within the solver which would also increase the accuracy of the numerical solution.

The overall objective of this study was to develop and validate a predictive modelling methodology for simulating the static and dynamic impact failure response of thermoplastic composite sandwich structures. The work has primarily focused on sandwich constructions with commingled woven fabric glass/polypropylene composite skins and a crushable polypropylene foam core. The static and high strain rate mechanical properties of the thermoplastic composite skin material have been experimentally characterised. This investigation showed that the tensile and compressive modulus and strength increased with strain rate while the shear modulus and strength decreased as strain rate increased. A modelling methodology was developed for predicting damage in the thermoplastic composite using the advanced MAT 162 material model that is implemented in the LS-DYNA explicit finite element code. An inverse modelling technique for calibrating and validating the MAT 162 damage parameters was developed. The material model was validated for predictive simulation of the static and crash response of a large scale complex shaped demonstrator thermoplastic composite automotive component. The static and dynamic mechanical properties of the thermoplastic foam core have been experimentally investigated and presented. This was followed by an experimental investigation and finite element modelling of the failure modes of the thermoplastic composite sandwich under static and dynamic localised indentation and bending loads. A fracture criteria was implemented in the model to simulate core shear fracture. The main contributions to knowledge from this doctoral study are: the static and dynamic characterisation of the mechanical properties and failure modes of the thermoplastic composite and the crushable thermoplastic foam material; development of a validated modelling methodology for predicting damage in thermoplastic composites; and development of a finite element modelling procedure for simulating the static and dynamic impact failure behaviour of thermoplastic composite sandwich structures.

Over the last three decades, the aerospace industry has gradually shifted from metals to composites in many different applications due to the lightweight properties of composite materials. Some benefits of composites include higher strength-to-weight ratio and corrosion resistance. At this point in time, the composite industry researchers are focusing on renewable and sustainable materials (bio-composites). By understanding the structural capabilities of bio-composites that have been used for centuries, new developments of sustainable materials will spark more interest throughout the industry. Bio-composites include fibers such as hemp, bamboo, flax, etc. The high demand for bio-composites in composite structures can also reduce raw material costs. This study investigated, through experimental and numerical analysis, the mechanical behavior of sandwich panels under edgewise compressive loading. The first task of the study was to use four different facesheet materials and the same Nomex honeycomb core. The number of facesheet layers consecutively increased from one layer to four layers on each side of the core for each material. The facesheet materials used were Hexply AGP280-5H Carbon Fiber Pre-Preg, B601 Plain Weave Hemp, D118DKBR Split Herringbone Weave Hemp, and NB308T 7725 Texalium Fiberglass Pre-Preg. The sandwich panels were cured using a composite heat press and followed the recommended cure cycle for the material’s resin matrix. The variation of the facesheet materials while keeping the core consistent showed how the edgewise strength and displacement of the composite sandwiches were affected under compressive loading. The second task of the study was to try and create a multifunctional hybrid composite sandwich with two different facesheet materials; using one hemp material and one pre-preg material. The goal of this task was to try and minimize the damage occured upon failure. Being that the pre-preg materials are more brittle than the hemp material, the hybrid composite sandwiches can potentially create a superior composite structure. The sequence of stacking of the facesheet materials was manipulated to study how changing the outer and inner layers affected the results. All the specimen were loaded at a rate of 0.05 in/min in a steel jig specifically made per ASTM C364 standard using an Instron 8801 to determine the mechanical behavior. These experimental results combined with results from theoretical and finite element analysis using Matlab and Abaqus, respectively, were used to compare composite sandwich designs under compressive loadings. Failure mode comparison between the individual material composites and the hybrid composites were also discussed.

L'objectif de cette thèse est de développer des modèles d'homogénéisation analytiques de panneaux sandwichs en nid d'abeilles. A la différence des méthodes classiques, l'effet des peaux est pris en compte, conduisant à des propriétés mécaniques très différentes. Dans les cas des tractions, flexions, cisaillement dans le plan, cisaillements transversaux et torsion, différentes séries de fonctions analytiques sont proposées pour prendre en compte la redistribution des contraintes entre les parois du nid d'abeilles. Nous avons étudié l'influence de la hauteur du nid d'abeilles sur les propriétés élastiques. Les courbes des modules obtenues avec le modèle proposé sont bien bornées par les valeurs obtenues avec la théorie des poutres. Les contraintes d'interface sont également étudiées afin de comparer avec les modèles existant pour le problème de traction. De nombreux calculs numériques ont été réalisés avec nos H-modèles pour les problèmes de tractions, de flexions, de traction-flexion couplés, de cisaillement dans le plan, de cisaillement transversal et de torsion. De très bon accords ont été obtenus entre les résultats issus des H-modèles et ceux issus des calculs en éléments finis de coques en maillant complètement les panneaux sandwichs. Nos H-modèles ont été appliquées aux calculs de grandes plaques sandwichs industrielles en nid d'abeilles. La comparaison desrésultats entre les H-modèles et les calculs en éléments finis de coques du logiciel Abaqus sont en très bon accord
The aim of this thesis is to develop an analytical homogenization model for the honeycomb core sandwich panels. Unlike conventional methods, the skin effects are taken into account, leading to a very different mechanical properties. In the cases of extensions, bendings, in-plane shear, transverse shears andtorsion, different analytical function series are proposed to consider the stress redistribution between the honeycomb walls. We have studied the influence of the height of the core on its homogenized properties. The moduli curves obtained by the present H-models are well bounded by the moduli values obtained by the beam theory. The interface stresses are also studied to compare with existing models for stretching problem. Many numerical computations with our H-models have been done for the problems of stretching, bending, in-plan and transverse shearing, as well as torsion. Very good agreement has been achieved between the results of the H-models and the results obtained by finite element simulations by completely meshing thesandwich panel with shell elements. Our H-models have been applied to the computations of industrial large sandwich panels with honeycomb core. The comparison of the results between the H-models and the simulations with Abaqus shell elements are in very good agreement

El texto completo de este trabajo no está disponible en el Repositorio Académico UPC por restricciones de la casa editorial donde ha sido publicado.
In this paper, a three-dimensional numerical solution for the bending study of laminated composite doubly-curved shells is presented. The partial differential equations are solved analytically by the Navier summation for the midsurface variables; this method is only valid for shells with constant curvature where boundary conditions are considered simply supported. The partial differential equations present different coefficients, which depend on the thickness coordinates. A semi-analytical solution and the so-called Differential Quadrature Method are used to calculate an approximated derivative of a certain function by a weighted summation of the function evaluated in a certain grin domain. Each layer is discretized by a grid point distribution such as: Chebyshev-Gauss-Lobatto, Legendre, Ding and Uniform. As part of the formulation, the inter-laminar continuity conditions of displacements and transverse shear stresses between the interfaces of two layers are imposed. The proper traction conditions at the top and bottom of the shell due to applied transverse loadings are also considered. The present results are compared with other 3D solutions available in the literature, classical 2D models, Layer-wise models, etc. Comparison of the results show that the present formulation correctly predicts through-the-thickness distributions for stresses and displacements while maintaining a low computational cost.
Consejo Nacional de Ciencia, Tecnología e Innovación Tecnológica

En aéronautique, la sécurité des passagers et la fiabilité des structures sont des aspects essentiels. Dans le cas particulier des hélicoptères, les pales peuvent être sujettes à des sollicitations d'impact. La modélisation de ces phénomènes reste difficile et les essais remplacent souvent la prédiction. Ce travail concerne l'étude expérimentale et numérique d'un impact oblique sur la peau d'une pale. Ceci est équivalent dans une première approche à un impact sur une structure sandwich composée d'une âme en mousse polymère et d'une fine peau en tissu composite. Tout d'abord, les mécanismes d'endommagement de la peau pour ce type de sollicitations ont été identifiés expérimentalement et une étude d'influence des paramètres matériaux sur la réponse à l'impact a été menée. Ensuite un modèle représentatif de la cinétique du dommage adaptée à une modélisation à l'échelle de la structure a été développé. Ainsi un modèle E. F. Explicite a été développé. Il repose sur le développement d'un élément spécifique à l'échelle de la mèche. Enfin, les résultats numériques obtenus ont été comparés aux résultats expérimentaux. Le modèle permet d'identifier les mécanismes du dommage observés expérimentalement
In aeronautics, passenger safety and reliability of structures are essential aspects. For the specific case of helicopters, blades are subjected to impact solicitations. Modelling these phenomena is still difficult and experimental tests often replace the prediction. This work will be focused on the experimental and numerical study of an oblique impact on the skin of the blade. It is equivalent in a first approach to an impact on a sandwich panel made up with a foam core and a thin woven composite skin. First the mechanisms of damage in the skin for this kind of solicitation have been identified experimentally and a study of the influence of the materials on the impact response has been performed. Then a representative model of the damage kinetics adapted to the modelling of the complete structure has been developed. Thus, an F. E. Explicit model has been developed. It relies on the development of a specific damageable element at the bundles scale. Finally, the numerical results obtained have been compared to the experimental results. The modelling allows the identification of the damage mechanism of the woven skin

Sandwich structure is one type of composite materials that has been widely used in automotive, marine and aerospace structures. It offers high specific modulus and specific strength. Sandwich structures consist of two thin face plates that enclose a thick core. The face plate is generally stiff enough to resist bending load, whilst the foam core is of lower modulus, lightweight yet it can transfer the shear and compressive loadings. Sandwich structures with composite faces have been applied to fabricate aerospace structures (e.g., engine cowling, floor panels, etc.) because of their excellent bending stiffness and their resistance to impact (good energy absorption). However, sandwich composites commonly suffer face-core interface failure that could lead to catastrophic failure of the whole structure. This failure would also give a significant increase on its maintenance and repair cost. To alleviate the interface failure between face materials and core, there are several techniques that have been proposed: (a) through-thickness reinforcement by inserting z-pins to connect the faces and the core; (b) insertion of an interleaf (thin layer) which acts as an adhesive between the faces and the core; and (c) matrix modification utilizing polymer nanocomposites (or nanofillers) on the facing materials (which the modified matrix is able to penetrate into the top surface of the core material) to improve face/core interface bonding. The aim of this research is to investigate the improvement level of the face/core interface strength by z-pinning and interleaving. Here, the role of z-pins and interleaves with nanofillers in changing the delamination resistance under mode I (opening mode) was investigated by the double cantilever beam (DCB) test. The results showed improvement in both reinforcement methods on mode I debonding fracture toughness. Cracked sandwich beam (CSB) test was carried out to study the enhancement of fracture resistance of the face-core interface of the structure under mode II (shear loading mode). Almost similar to mode I test, mode II tests also showed a significant improvement in mode II fracture toughness up to 9.5 times in z-pinning. Unlike z-pinning, interleaving did not show a significant result with only 11% improvement at most. Particularly on z-pinning, the increased mode I fracture toughness was achieved by the role of z-pin bridging on crack growth whose effect is controlled by the crack-wake bridging law. Pullout tests were performed to study the bridging law due to the z-pins. The results showed that pullout was dominantly from the face laminates and that the interface bonding between the z-pin and the foam-core was very small and neglected in the analysis. Computer simulation of mode I DCB tests with z-pins was done to compare the model predictions with obtained experimental data.

Lightweight sandwich structures that are composed of high–performance core and face sheets, have been attracting attention in both civilian and military applications due to their outstanding mechanical properties. Honeycomb cores and fibre reinforced composite face sheets have specific advantages for resisting dynamic impact. For example, honeycomb cores possess higher specific-strength (ratio of strength to relative density) than the other sandwich cores under compression, and carbon fibre composites possess high tensile strength and low density. This thesis focuses on the understanding of the dynamic compressive response of high-performance honeycombs and the ballistic impact resistance of stiff/soft hybrid fibre composite laminate beams. For honeycomb cores, the out-of-plane compressive behaviour of the AlSi10Mg alloy hierarchical honeycombs and commercially available Nomex honeycombs have been experimentally and numerically investigated. Owing to the complex in-plane topology, hierarchical honeycombs were fabricated using the Selective Laser Melting (SLM) technique. The experimental measurement and finite element (FE) calculation indicate that the two hierarchical honeycombs, specifically two-scale and three-scale honeycombs, both offer higher wall compressive strengths than the single-scale honeycombs. With an increase in relative density, the single-scale honeycomb experiences a transition in terms of failure mechanism from local plastic buckling of walls to local damage of the parent material. Alternately, the two-scale and three-scale hierarchical honeycombs all fail with solely parent material damage. The dynamic compressive strength enhancement of the hierarchical honeycombs is dominated by the strain rate sensitivity of the parent material. For Nomex honeycombs, the dynamic failure mode under out-of-plane compression is different from the quasi-static failure mode, i.e. the honeycombs fail due to stubbing of cell walls at the end of specimens under dynamic compression, whereas fail due to local phenolic resin fracture after elastic buckling of the honeycomb wall under quasi-static compression. The dynamic compressive strength of Nomex honeycombs increases linearly, and the strength enhancement is governed by two mechanisms: the strain rate effect of the phenolic resin and inertial stabilization of honeycomb unit cell walls. The inertial stabilization of unit cell walls plays a more significant role in strength enhancement than the strain rate effect of the phenolic resin. In addition, the effect of key parameters such as impact method and initial geometrical imperfections on the compressive responses of honeycombs has also been numerically investigated. For face sheets, the ballistic resistance of the beams hybridizing stiff and soft carbon fibre composites has also been experimentally studied, and these results were compared with those of stiff and soft composite beams with identical areal mass. The failure modes of composite beams under different velocity impacts have been identified to be different. For monolithic beams, the hybrid and soft monolithic beams exhibited similar energy absorption capacity. As for the sandwich beams, the hybrid sandwich beams behaved better in terms of energy absorption than soft sandwich beams at high projectile velocities. Both the hybrid and soft composite beams absorbed more kinetic energy from projectiles than stiff composite beams. The advantages of the stiff/soft hybrid composites can be summarized as follows: (i) the soft composite part survives at low velocity impact; (ii) the stiff composite part of the hybrid monolithic/sandwich beams has a more uniform stress distribution than the stiff monolithic/sandwich beams owing to the buffer effect of the soft composite part. This work identifies the advantages of high performance honeycomb cores as well as fibre composite face sheets. These findings can be used to develop high strength, low weight and multi-functional sandwich structures, thereby widening their applicability to a wider array of fields.

Over the last three decades, composite materials have been increasingly used in different engineering field due to their high stiffness-to-weight and strength-to-weight ratios. Nowadays, relatively thick laminated composite and sandwich materials with one hundred or more layers find their applications in primary load-bearing structural components of the modern aircraft. To ensure a reliable design and failure prediction, accurate evaluation of the strain/stress state is mandatory. A high-fidelity analysis of multilayered composite and sandwich structures can be achieved by adopting detailed 3D finite element models that turn into a cumbersome modeling at high computational cost. Thus, most of the researchers efforts are devoted to the development of approximated models wherein assumptions on the distribution of displacements and/or stresses are made. In the ‘80s, thanks to the original works by Prof. Di Sciuva, a new modeling strategy of multilayered composite and sandwich structures arose: the so-called Zigzag theories, wherein accuracy comparable with that proper of the Layer-wise models is achieved but saving the computational cost. For accuracy, computational cost and efficient finite element implementation, the most remarkable Zigzag model, inspired by the Prof. Di Sciuva’s work, is the Refined Zigzag Theory. From its first appearance, the Refined Zigzag Theory has experienced several developments in terms of beam and plate finite element implementations and has been extensively assessed on static problems. The present research activity supports the great accuracy of the Refined Zigzag Theory and for this reason deals with some overlooked aspects, as the application to the functionally graded materials (Chapter 2), the mixed-field formulation (Chapter 3), the implementation of a beam finite element employing exact static shape functions (Chapter 5) and the correlation with experimental results (Chapter 8). By enriching the Refined Zigzag Theory and using the Reissner Mixed Variational Theorem, a novel higher-order mixed zigzag model is developed (Chapter 4). The higher-order zigzag model constitutes the underlying theory for a beam finite element, suitable for a thermo-mechanical analysis, and a plate element, formulated taking into account only mechanical loads. The results presented (Chapters 6-8), along with those already published in the open literature by other authors, still encourage the use of the Refined Zigzag Theory in the analysis of relatively thick multilayered composite and sandwich structures. Moreover, when the transverse normal stress and the transverse normal deformability effects are not negligible, the novel higher-order mixed zigzag model appears proficient to solve these cases in virtue also of its efficient finite element implementations. The author’s auspice is that the models belonging to the Refined Zigzag Theory class becomes to attract attention of the companies involved in the design and analysis of multilayered composite and sandwich structures and of the finite element commercial codes that still implemented models not suitable for the analysis of composite and sandwich structures, as extensively demonstrated.

Vacuum assisted resin transfer molding (VARTM) is a low cost resin infusion process being developed for the manufacture of composite structures. VARTM is being evaluated for the manufacture of primary aircraft structures, including foam sandwich composite materials. One of the benefits of VARTM is the ability to resin infiltrate large or complex shaped components. However, trial and error process development of these types of composite structures can prove costly and ineffective. Therefore, process modeling of the associated flow details and infiltration times can aide in manufacturing design and optimization. The purpose of this research was to develop a process using VARTM to resin infiltrate stitched and unstitched dry carbon fiber preforms with polymethacrylimide foam cores to produce composite sandwich structures. The infiltration process was then used to experimentally verify a three-dimensional finite element model for VARTM injection of stitched sandwich structures. Using the processes developed for the resin infiltration of stitched foam core preforms, visualization experiments were performed to verify the finite element model. The flow front progression as a function of time and the total infiltration time were recorded and compared with model predictions. Four preform configurations were examined in which foam thickness and stitch row spacing were varied. For the preform with 12.7 mm thick foam core and 12.7 mm stitch row spacing, model prediction and experimental data agreed within 5%. The 12.7 mm thick foam core preform with 6.35 mm row spacing experimental and model predicted data agreed within 8%. However, for the 12.7 mm thick foam core preform with 25.4 mm row spacing, the model overpredicted infiltration times by more 20%. The final case was the 25.4 mm thick foam core preform with 12.7 mm row spacing. In this case, the model overpredicted infiltration times by more than 50%. This indicates that the model did not accurately describe flow through the needle perforations in the foam core and could be addressed by changing the mesh elements connecting the two face sheets.
Master of Science