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

Abstract The demand for temporary buildings has increased during the last few years. The main reason for this increase is the need for temporary housing.  This thesis contains the development of a construction solution that locks, joins and seals sandwich panels together in temporary buildings. This solution also explains how to mount, de-mount and re-mount the buildings and was developed in cooperation with the commissioning company Swetech Design AB.   The project has been limited to just the horizontal joints of Swetech Design AB´s sandwich panels. The panels which the construction solution is going to be applied on have the dimensions of 2703 mm in height, 2297 mm in width and a thickness of 206 mm. The core consists of cellular plastic that is laminated with a material that consists of fiberglass and polyester.   The pre-study consisted of literature studies of various types of joints, the important properties needed in order to make an airproof construction and the various building requirements for temporary residents. The pre-study also consisted of a visit to the module building company Mobile Composite Solutions (MCS) which helped in acquiring knowledge of mounting big units and the manufacturing technique RTM. Even inventories in different branches were made, to get knowledge around the different possibilities and variations of locking, sealing and joining parts.   After the completion of the pre-study a requirements specification and two function analysis where made that put demands and requirements on the concept solution and expressed the concept in terms of functions instead of solutions. The requirements specification and the function analysis became the base of the idea generating phase. This phase resulted in partial solutions and complete solutions which were then combined together to establish different concepts. Through elimination methods, screening tools and discussions together with the commissioning company a final concept was chosen. The concept is built on a tension joint in combination with a sealing profile and a locking device made of an angle steel, bolt and nut which can join and seal the sandwich panels.   The concept has been visualized in the form of sketches, CAD-model, a physical model on a cut-out between two sandwich panels in scale 1:1, basic drawings and a component list. A proposal for continued work has also been presented in order to assure that the construction meets all the requirements listed in the requirements specification and to make it possible for the commissioning company to implement the concept.

The sound transmission loss characteristics of plywood based sandwich panels were investigated. Measurements were made of the sound transmission loss of a range of materials and used as a baseline for comparison while a sound transmission loss optimisation method was developed. A unique test rig was built and calibrated to determine selected mechanical properties of materials of interest. The results of sound transmission loss and material properties measurements were used to select an appropriate prediction model, which was then used in conjunction with a mathematical optimisation model to determine combinations of materials and panel parameters which result in improved sound transmission loss. An effort was made to reproduce these predictions in experimental testing by constructing several prototype panels.

On s'intéresse à la modélisation numérique des structures sandwichs trois couches à âme viscoélastique utilisées pour l'amortissement passif et le contrôle des vibrations dans divers domaines d’application tels que l’aéronautique, l’aérospatial, l’automobile et l’électroménager. La complexité de la modélisation en dynamique de ces structures est liée d'une part aux non linéarités matérielles dues à la dépendance en fréquence et en température de la rigidité et d'autre part aux non linéarités géométriques dues aux grands déplacements. Nous proposons dans ce travail un cadre de modélisation des vibrations linéaires et non linéaires des poutres et des plaques sandwichs avec la dépendance en fréquence du comportement viscoélastique. En couplant la méthode asymptotique numérique et les techniques de différentiation automatique, nous avons développé un algorithme générique de résolution du problème numérique de valeur propre complexe gouvernant les vibrations libres linéaires des structures sandwichs viscoélastiques. L'algorithme est implémenté sous un solveur numérique en langage Matlab et permet pour une structure sandwich donnée, de déterminer de façon directe et exacte les propriétés amortissantes et les modes propres de vibration, quelle que soit la non linéarité en fréquence de la loi viscoélastique. L'efficacité de l'algorithme est illustrée sur trois modèles viscoélastiques différents: le modèle à module constant, le modèle de Maxwell généralisé et le modèle à dérivées fractionnaires. Puis, nous avons présenté une théorie basée sur la méthode des éléments finis pour la modélisation des vibrations non linéaires des poutres sandwichs. Cette théorie combine la technique d'équilibrage harmonique à la méthode de Galerkin à un mode et permet de réduire le problème de vibration non linéaire à une équation d'amplitude complexe. La résolution de l'équation d'amplitude permet de caractériser les propriétés modales et les réponses en amplitude. L'évaluation du choix de la base de Galerkin pour différentes approximations des modes a permis de montrer l'inadéquation des modes réels classiquement utilisés dans l'analyse des vibrations linéaires et non linéaires. Enfin, la théorie est appliquée pour modéliser les vibrations non linéaires des plaques sandwichs. Les coefficients de l'équation d'amplitude gouvernant les vibrations non linéaires sont formulés dans le cadre élément fini et nécessitent la résolution numérique de trois problèmes: un problème de valeur propre complexe non linéaire et deux problèmes linéaires. Pour l'efficacité de la méthode numérique ainsi proposée, la base de Galerkin est améliorée en utilisant des modes complexes afin de prendre en compte l'amortissement dans les modes de vibration. Les résultats obtenus montrent l'effet des non linéarités géométriques, des conditions aux limites et de la température sur les propriétés modales et les réponses en amplitude / The problem of interest is the numerical modeling of three layered viscoelastic sandwich structures used for passive damping and vibration control. The complexity in the dynamic modeling of these structures lies in the presence on the one hand of the material nonlinearities due to the frequency and temperature dependence of the stiffness and on the other hand of the geometrical nonlinearities due to large amplitude vibrations. We propose in this work a modeling framework of linear and nonlinear vibrations of viscoelastic sandwich beams and plates that takes into account the frequency dependent behaviour. Coupling the asymptotic numerical method to automatic differentiation techniques, we developed a generic algorithm for the solution of the complex eigenvalue problem governing the linear free vibrations of viscoelastic sandwich structures. The algorithm is implemented using Matlab language and a numerical solver has been designed for direct and exact computation of damping properties and vibration modes, whatever the dependence on frequency of the viscoelastic law. The efficiency of the algorithm is illustrated on three different viscoelastic models: the constant modulus model, the generalized Maxwell model and the fractional derivative model. Then, we presented a finite element based theory for nonlinear vibration analysis of viscoelastic sandwich beams. This theory combines the harmonic balance technique to one mode Galerkin's procedure and allows to reduce the nonlinear vibration problem in a complex amplitude equation. Solving the amplitude equation yields the modal properties and the amplitude responses. An assessment of the Galerkin's basis choice for various eigenmodes approaches shows the inaccuracy of the classical real eigenmodes used for linear and nonlinear vibration analysis. The theory is applied to model the nonlinear vibrations of viscoelastic sandwich plates. The amplitude equation coefficients are established in the finite element framework by numerically solving three problems: a nonlinear complex eigenvalue problem and two linear problems. For the efficiency of the proposed method, the Galerkin's basis has been improved using complex eigenmodes in order to take account the damping in the vibration modes. The obtained results show the effects of geometrical nonlinearities, boundary conditions and temperature on the modal properties and amplitude responses

Sandwich panels using fibre-reinforced composite skins and low-density cores are being increasingly used in the aerospace industry due to their superior specific strength and stiffness, and increased design flexibility over traditional metallic and composite structures. However, it is well-known that sandwich panels are highly vulnerable to the effects of impact damage, with even low-energy impacts potentially causing very severe reductions in the in-plane compressive strength of these structures. The objective of this project was to produce a faithful and reliable numerical model for the simulation of the compression-after-impact strength of composite sandwich panels. An in-depth literature review revealed that delamination within the skins of a sandwich panel is a damage mechanism that has gone almost entirely neglected in previous efforts at modelling this problem, despite the proven significance of this mechanism in the failure of impact damaged sandwich panels in compression. Consequently, the use of the cohesive zone model for delamination initiation and propagation is the key unique feature of this model, with Hashin s criteria being used for intra-laminar damage formation, and a simple plasticity response capturing core crushing. An experimental study is performed to produce a thorough dataset for model validation, featuring differing levels of damage induced via quasi-static indentation, and novel asymmetric panels with skins of unequal thickness (the thinner skin being on the unimpacted side). The experimental study revealed that the use of a thinner distal (undamaged) skin could improve the strength of mildly damaged sandwich panels over undamaged sandwich panels using the same asymmetric configuration. It is believed that this effect is due to the movement of the neutral plane of the sandwich panel caused by the reduction in the stability of the damaged skin through stiffness reduction and geometric imperfections. This removes the eccentricity of the compressive loading that exists in the undamaged asymmetric panels, which has mismatched axial stiffness between the indented skin and the thinner distal skin, and thus a noticeably lower ultimate strength than the undamaged symmetric panels. The sandwich model is developed using pre-existing experimental and material data, and trialled for a variety of different skin lay-ups, core thicknesses and indenter sizes. The numerical model generally agreed well with the ultimate stress found in the experiments for these different configurations, but is quite poor at estimating the magnitude of the damage induced by the indentation. When used to model the experimental study, the model gave generally good, conservative estimates for the residual compressive strength of both the symmetric and asymmetric panels. The tendency of the asymmetric panels to become stronger with mild damage was not captured by the model per se, with the numerical results instead showing an insensitivity to damage in the asymmetric panels, which was not shared by the symmetric panels. However, the numerical model did exhibit erroneous strain-stress responses for both panel configurations, particularly for the undamaged and mildly damaged cases. Investigations revealed that this erroneous behaviour was caused by inconsistency in the material data, which had been collected partially via experimentation and partly from literature sources. Overall, the model developed here represents a promising advancement over previous efforts, but further development is required to provide accurate damage states.

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.

No description available.

The goal of this study is to design and analyze a sandwich composite panel with lightweight graphite foam core and carbon epoxy face sheets that can function as a radiator for the given payload in a satellite. This arrangement provides a lightweight, structurally efficient structure to dissipate the heat from the electronics box to the surroundings. Three-dimensional finite element analysis with MSC Visual Nastran is undertaken for modal, dynamic and heat transfer analysis to design a radiator panel that can sustain fundamental frequency greater than 100 Hz and dissipate 100 W/m2 and withstand launch loads of 10G. The primary focus of this research is to evaluate newly introduced graphite foam by Poco Graphite Inc. as a core in a sandwich structure that can satisfy structural and thermal design requirements. The panel is a rectangular plate with a cutout that can hold the antenna. The panel is fixed on all the sides. The objective is not only to select an optimum design configuration for the face sheets and core but also to explore the potential of the Poco foam core in its heat transfer capacity. Furthermore the effects of various parameters such as face sheet lay-up, orientation, thickness and material properties are studied through analytical models to validate the predictions of finite element analysis. The optimum dimensions of the sandwich panel are determined and structural and thermal response of the Poco foam is compared with existing aluminum honeycomb core.

The focus of this research is characterizing a new material system composed of carbon and graphite foams, which has potential in a wide variety of applications encompassing aerospace, military, offshore, power production and other commercial industries. The benefits of this new material include low cost, light weight, fire-resistance, good energy absorption, and thermal insulation or conduction as desired. The objective of this research is to explore the bulk material properties and failure modes of the carbon foam through experimental and computational analysis in order to provide a better understanding and assessment of the material for successful design in future applications. Experiments are conducted according to ASTM standards to determine the mechanical properties and failure modes of the carbon foam. Sandwich beams composed of open cell carbon foam cores and carbon-epoxy laminate face sheets are tested in the flexure condition using a four point setup. The primary failure mode is shear cracks developing in the carbon foam core at a critical axial strain value of 2,262 με. In addition to flexure, the carbon foam is loaded under tensile and shear loads to determine the respective material moduli. Computational analysis is undertaken to further investigate the carbon foam's failure modes and material characteristics in the sandwich beam configuration. Initial estimates are found using classical laminated plate theory and a linear finite element model. Poor results were obtained due to violation of assumptions used in both cases. Thus, an additional computational analysis incorporating three dimensional strain-displacement relationships into the finite element analysis is used. Also, a failure behavior pattern for the carbon foam core is included to simulate the unique failure progression of the carbon foam on a microstructure level. Results indicate that displacements, strains and stresses from the flexure experiments are closely predicted by this two parameter progressive damage model. The final computational model consisted of a bond line (interface) study to determine the source of the damage initiation, and it is concluded that damage initiates in the carbon foam, not at the bond line.

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.