Дисертації з теми "Composite and sandwich plate"

Three series of tests were conducted on a sandwich plate bridge deck, which consisted of two steel plates and an elastomer core. The first series of testing was conducted by applying a static load on a full scale sandwich plate bridge deck panel. Local strains and deflections were measured to determine the panelâ s behavior under two loading conditions. Next, fatigue tests were performed on the longitudinal weld between two sandwich plate panels. Two connections were tested to 10 million cycles, one connection was tested to 5 million cycles, and one connection was tested to 100,000 cycles. The fatigue class of the weld was determined and an S-N curve was created for the longitudinal weld group. Finally, a series of experiments was performed on a half scale continuous bridge deck specimen. The maximum positive and negative flexural bending moments were calculated and the torsional properties were examined. Finite element models were created for every load case in a given test series to predict local strains and deflections. All finite element analyses were preformed by Intelligent Engineering, Ltd. A comparison of measured values and analytical values was preformed for each test series. Most measured values were within five to ten percent of the predicted values. Shear lag in the half scale bridge was studied, and an effective width to be used for design purposes was determined. The effective width of the half scale simple span sandwich plate bridge deck was determined to be the physical width. Finally, supplemental research is recommended and conclusions are drawn.
Master of Science

Shear links are used as fuse elements in lateral load resisting systems to provide ductility and dissipate seismic energy. These links have traditionally been employed in eccentrically braced frames, but have more recently been suggested for use in the innovative linked column frame system (LCF). Current design specifications for shear links require intermediate web stiffeners to provide out-of-plane web stability so ductility requirements can be achieved. This research focused on moving from discrete transverse web stiffening to continuously stiffened webs in built up shear links. Built up links were designed to yield in shear when subjected to severe cyclic loading, however the webs of the links were designed using two metal sheets joined by an elastic core. These composite "sandwich" webs allowed for an increase in web thickness (and inherent flexural rigidity) without increasing the shear strength of the links. Numerical and experimental investigations were conducted to assess the performance of composite sandwich links subjected to severe loading. Numerical results showed improved web behavior in sandwich links in which the core material was assigned an elastic modulus greater than 5000psi. Due to fabrication limitations, experimental specimens were fabricated with a core material elastic modulus of 1000psi. These specimens did not perform as well as unstiffened base case links in terms global hysteretic behavior or ductility.

The random wetlay process is used to make fiber-reinforced thermoplastic sheets that can be compression molded into composite panels at little cost. By utilizing these composite panels as the facesheets of honeycomb sandwich structures, it is possible to greatly increase the bending stiffness of the composite without adding significant weight. The random wetlay composite facesheets used in this research consisted of 25% E-glass fibers and 75% PET by weight. The thickness uniformity of the facesheets was difficult to control. The core of the sandwich structure was HexWeb&174; EM. Three low-cost adhesives were examined for secondarily bonding the facesheets to the core: polyurethane glue; epoxy paste; and 3M Scotch-Grip&174; plastic adhesive. The polyurethane glue mixed with Cab-O-Sil filler was easiest to apply and provided the largest flatwise tensile strength. Mathematical models were developed to predict the static behavior of sandwich beams and plates in bending. Three-point bend tests were performed on a sandwich beam in accordance with ASTM C 393. A sandwich plate simply supported along two opposite edges and free along the other two edges was subjected to a line-load using weights and a wiffle tree arrangement. An effective facesheet modulus and Poissonâ s ratio were found by comparing the measured displacements to the sandwich plate theory. The shadow moiré technique was used to visualize the displacement of the line-loaded sandwich plate. The overall shape of the displacement was very similar to the shape predicted by the sandwich plate theory.
Master of Science

In this study, Ritz method has been employed to analyze the following problems: free vibrations of plates with curvilinear stiffeners, the lowest 100 frequencies of thick isotropic plates, free vibrations of thick quadrilateral laminates and free vibrations and static deformations of rectangular laminates, and sandwich structures. Admissible functions in the Ritz method are chosen as a product of the classical Jacobi orthogonal polynomials and weight functions that exactly satisfy the prescribed essential boundary conditions while maintaining orthogonality of the admissible functions. For free vibrations of plates with curvilinear stiffeners, made possible by additive manufacturing, both plate and stiffeners are modeled using a first-order shear deformation theory. For the thick isotropic plates and laminates, a third-order shear and normal deformation theory is used. The accuracy and computational efficiency of formulations are shown through a range of numerical examples involving different boundary conditions and plate thicknesses. The above formulations assume the whole plate as an equivalent single layer. When the material properties of individual layers are close to each other or thickness of the plate is small compared to other dimensions, the equivalent single layer plate (ESL) theories provide accurate solutions for vibrations and static deformations of multilayered structures. If, however, sufficiently large differences in material properties of individual layers such as those in sandwich structure that consists of stiff outer face sheets (e.g., carbon fiber-reinforced epoxy composite) and soft core (e.g., foam) exist, multilayered structures may exhibit complex kinematic behaviors. Hence, in such case, Cz0 conditions, namely, piecewise continuity of displacements and the interlaminar continuity of transverse stresses must be taken into account. Here, Ritz formulations are extended for ESL and layerwise (LW) Nth-order shear and normal deformation theories to model sandwich structures with various face-to-core stiffness ratios. In the LW theory, the C0 continuity of displacements is satisfied. However, the continuity of transverse stresses is not satisfied in both ESL and LW theories leading to inaccurate transverse stresses. This shortcoming is remedied by using a one-step well-known stress recovery scheme (SRS). Furthermore, analytical solutions of three-dimensional linear elasticity theory for vibrations and static deformations of simply supported sandwich plates are developed and used to investigate the limitations and applicability of ESL and LW plate theories for various face-to-core stiffness ratios. In addition to natural frequency results obtained from ESL and LW theories, the solutions of the corresponding 3-dimensional linearly elastic problems obtained with the commercial finite element method (FEM) software, ABAQUS, are provided. It is found that LW and ESL (even though its higher-order) theories can produce accurate natural frequency results compared to FEM with a considerably lesser number of degrees of freedom.
Doctor of Philosophy
In everyday life, plate-like structures find applications such as boards displaying advertisements, signs on shops and panels on automobiles. These structures are typically nailed, welded, or glued to supports at one or more edges. When subjected to disturbances such as wind gusts, plate-like structures vibrate. The frequency (number of cycles per second) of a structure in the absence of an applied external load is called its natural frequency that depends upon plate's geometric dimensions, its material and how it is supported at the edges. If the frequency of an applied disturbance matches one of the natural frequencies of the plate, then it will vibrate violently. To avoid such situations in structural designs, it is important to know the natural frequencies of a plate under different support conditions. One would also expect the plate to be able to support the designed structural load without breaking; hence knowledge of plate's deformations and stresses developed in it is equally important. These require mathematical models that adequately characterize their static and dynamic behavior. Most mathematical models are based on plate theories. Although plates are three-dimensional (3D) objects, their thickness is small as compared to the in-plane dimensions. Thus, they are analyzed as 2D objects using assumptions on the displacement fields and using quantities averaged over the plate thickness. These provide many plate theories, each with its own computational efficiency and fidelity (the degree to which it reproduces behavior of the 3-D object). Hence, a plate theory can be developed to provide accurately a quantity of interest. Some issues are more challenging for low-fidelity plate theories than others. For example, the greater the plate thickness, the higher the fidelity of plate theories required for obtaining accurate natural frequencies and deformations. Another challenging issue arises when a sandwich structure consists of strong face-sheets (e.g., made of carbon fiber-reinforced epoxy composite) and a soft core (e.g., made of foam) embedded between them. Sandwich structures exhibit more complex behavior than monolithic plates. Thus, many widely used plate theories may not provide accurate results for them. Here, we have used different plate theories to solve problems including those for sandwich structures. The governing equations of the plate theories are solved numerically (i.e., they are approximately satisfied) using the Ritz method named after Walter Ritz and weighted Jacobi polynomials. It is shown that these provide accurate solutions and the corresponding numerical algorithms are computationally more economical than the commonly used finite element method. To evaluate the accuracy of a plate theory, we have analytically solved (i.e., the governing equations are satisfied at every point in the problem domain) equations of the 3D theory of linear elasticity. The results presented in this research should help structural designers.

The Sandwich Plate System (SPS) is created by bonding two steel plates together with an elastomer core that is injected into a cavity formed by the steel plates and perimeter bars. The result is a stiffer and lighter panel that can be used for plate-like structures such as bridge decks, stadium risers or ship decks. For more versatility, the effects of welding post-injection to the SPS panels were investigated. Three post-injection welded joints were tested to determine fatigue resistance and the effects of cyclic loading on the localized debonding of the heat affected zone at the post-injection welded joint of a SPS bridge deck. Seven panels containing one of three post-injection weld configurations were investigated. Each panel was fatigue tested to ten million cycles or until failure, by applying remote bending to the post-injection welded joint. Experimental deflections and strains were compared to finite element analyses. Fatigue-life predictions were made using code based S-N curves, and a relatively new mesh-insensitive structural stress method with a master S-N curve approach. The post-injection welded joint demonstrated good fatigue resistance to recommended AASHTO loading when shims were used under the middle support to offset the camber in the SPS panels. It was also found that stresses caused by draw down of the camber had an adverse affect on the post-injection welded joint and greatly reduced its fatigue resistance.
Master of Science

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

Composite sandwich plates are widely used in aerospace, automobile and shipbuilding industries. Composite sandwich plates have many different types of failure modes. A comparative study of composite sandwich plates with different finite element modeling approaches for predicting buckling and wrinkling failure response is described in this thesis. The research considers composite sandwich plates with isotropic and anisotropic face-sheets with a thick core. Finite element solutions are obtained using Abaqus/CAE 2016 software by conventional shell element models and conventional shell/solid element models. This study investigates results obtained using finite element methods and compares them to experimental and analytical solutions for overall buckling and face-sheet wrinkling. Results of the study indicate that finite element methods provide an accurate and effective modeling approach for predicting both overall buckling and wrinkling response.

Furthermore, the study also explored buckling response of composite sandwich panels with different core thickness and face-sheet fiber angle orientation. The study found that the shell/solid element model provides an appropriate and effective modeling method to predict both overall buckling and local wrinkling behavior in composite sandwich plates.

An innovative approach to possible construction or rehabilitation of bridge decks can be found in a bridge construction system called the Sandwich Plate System (SPS). The technology developed and patented by Intelligent Engineering Canada Limited in conjunction with an industry partner, Elastogran GmbH, a member of BASF, may be an effective alternative to traditional bridge rehabilitation techniques.

Although the systemâ s behavior has been studied the connection of the SPS deck to the supporting girders has not been investigated. Two types of connection are presented in this research. The use of a bent plate welded to the SPS deck and subsequently bolted to the supporting girder utilizing slip-critical connections has been utilized in the construction of a SPS bridge. A proposed SPS bridge system utilizes the top flange of the supporting girder welded directly to the SPS deck as the deck-to-girder connection.

The fatigue performance of a deck-to-girder connection utilizing a bent plate welded to the deck and bolted to the supporting girder using slip-critical connections was tested in the Virginia Tech Materials and Structures Laboratory. The testing concluded that the fatigue performance of the welded and bolted bent plate connection was limited by the weld details and no slip occurred in the slip-critical connections. Finite element modeling of the two types of deck-to-girder connections was also used to determine influence of the connections on the local and global behavior of a SPS bridge system. A comparison of the different connection details showed that the connection utilizing the flange welded directly to the SPS deck significantly reduces the stresses at location of the welds in the connections, but the connection type has a limited influence on the global behavior of a SPS bridge.
Master of Science

Two new multi-layered plate bending elements (DKT/CST and DKT/LST) are developed based on a combination of the three model Discrete Kirchhoff theory (DKT) triangular plate bending element, the three model constant strain triangle (CST) and the six noded linear strain triangle (LST). Both frequency independent and frequency dependent damping of viscoelastic materials are considered. An iterative complex eigensolver is used to compute the natural frequencies and model loss factors. Several bench mark problems are solved using these new multi-layer plate elements. As the plate bending elements previously developed on the basis of Kirchhoff's theory are inadequate for thick plate analysis, several quadrilateral Mindlin plate bending elements are developed to study the behaviour of Mindlin plates. The plate bending elements based on Mindlin theory require shear correction factors in their formulations. Hence two new Co assumed strain finite element formulations of a refined third order theory which does not require shear correction factors, are developed and used to analyse isotropic, orthotropic, and layered anisotropic composite and sandwich plates under free vibration, damping and transient loading conditions. Parametric effects of plate aspect ratio, length to thickness ratio, degree of orthotropy, number of layers and lamination scheme on the natural frequencies (free vibration), model loss factors (damping) and dynamic (transient) responses have been shown. The results presented in this investigation could be useful in better understanding the behaviour of sandwich laminates under dynamic conditions and potentially beneficial for designers of sandwich structures.

This thesis presents the work of an experimental investigation into the impact damage resistance and damage tolerance for symmetrical and unsymmetrical composite honeycomb sandwich panels through in-plane compression. The primary aim of this research is to examine the impact damage resistance of various types of primarily carbon/epoxy skinned sandwich panels with varying skin thickness, skin lay-up, skin material, sandwich asymmetry and core density and investigate the residual in-plane compressive strengths of these panels with a specific focus on how the core of the sandwich contributes to the in-plane compressive behaviour. This aim is supported by four specifically constructed preconditions introduced into panels to provide an additional physical insight into the loading-bearing compression mechanisms. Impact damage was introduced into the panels over a range of IKEs via an instrumented drop-weight impact test rig with a hemi-spherical nosed impactor. The damage resistance in terms of the onset and propagation of various dominant damage mechanisms was characterised using damage extent in both impacted skin and core, absorbed energy and dent depth. Primary damage mechanisms were found to be impacted skin delamination and core crushing, regardless of skin and core combinations and at high energies, the impacted skin was fractured. In rare cases, interfacial skin/core debonding was found to occur. Significant increases in damage resistance were observed when skin thickness and core density were increased. The reduction trends of the residual in-plane compressive strengths of all the panels were evaluated using IKE, delamination and crushed core extents and dent depth. The majority of impact damaged panels were found to fail in the mid-section and suffered an initial decline in their residual compressive strengths. Thicker skinned and higher density core panels maintained their residual strength over a larger impact energy range. Final CAI strength reductions were observed in all panels when fibre fracture in the impacted skin was present after impact. Thinner skinned panels had a greater compressive strength over the thicker skinned panels, and panel asymmetry in thin symmetrical panels appeared to result in an improving damage tolerance trend as IKE was increased due to that the impact damage balanced the in-plane compressive resistance in the skins with respect to the pre-existing neutral plane shift due to the uneven skin thickness.

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.

This thesis documents the findings of a three year experimental investigation into the impact damage resistance and damage tolerance of composite honeycomb sandwich panels. The primary area of work focuses on the performance of sandwich panels under quasi-static and low-velocity impact loading with hemispherical and flat-ended indenters. The damage resistance is characterised in terms of damage mechanisms and energy absorption. The effects of varying the skin and core materials, skin thickness, core density, panel boundary conditions and indenter shape on the transverse strength and energy absorption of a sandwich panel have been examined. Damage mechanisms are found to include delamination of the impacted skin, core crushing, limited skin-core de bonding and top skin fibre fracture at high loads. In terms of panel construction the skin thickness is found to dominate the panel strength and energy absorption with core density having a lesser influence. Of the external factors considered the indenter noseshape has the largest effect on both failure load and associated damage area. An overview of existing analytical prediction methods is also included and the most significant theories applied and compared with the experimental results from this study. The secondary area of work expands the understanding obtained from the damage resistance study and assesses the ability of a sandwich panel to withstand in-plane compressive loading after sustaining low-velocity impact damage. The importance of the core material is investigated by comparing the compression-after-impact strength of both monolithic carbon-fibre laminates and sandwich panels with either an aluminium or nomex honeycomb core. The in-plane compressive strength of an 8 ply skinned honeycomb sandwich panel is found to be double that of a 16 ply monolithic laminate, with the type of honeycomb also influencing the compressive failure mechanisms and residual compressive strength. It is concluded that under in-plane loading the stabilising effect of the core opposes the de-stabilising effect of any impact damage.

Steel-concrete-steel (SCS) sandwich panels are an efficient means of achieving a strong and stable composite wall. Development in the 70's and 80's focussed on tunnelling, with other applications, particularly in the defence and offshore sectors, appearing later. Renewed focus has been placed on the system in recent years due to a proliferation of proposals for new nuclear power stations in Europe. Many new nuclear projects that have been completed in recent years have been significantly delayed by problems with reinforcement congestion. SCS construction offers a potential solution to this, since reinforcement is either significantly reduced or eliminated entirely in most designs. As a result of this renewed interest, industry has sought to develop improved design rules, both for economy and easier regulatory approval. As with any composite system, the strength of the system is derived from the ability of the materials to interface efficiently with each other where they are connected. Review of existing design guides and research showed a gap in understanding of the effects of shear connection on the overall behaviour of the system, particularly when resisting out-of-plane loads. This thesis aims to improve this understanding, leading to improved design provision and a wider range of applications for SCS panels in industry. An extensive literature search found a large body of test results. However, the majority of these tests are for designs where shear connection is over-provisioned, meaning shear connection is not critical. The tests that were conducted with lower degrees of shear connection were found to be insufficient to draw definitive conclusions about changes in behaviour. For this reason, numerical modelling using finite element analysis was used to supplement the test data. A validation and verification exercise was performed, which showed that the model accurately predicted the behaviour seen in testing, for all of the relevant failure modes. This thesis focusses on the three design checks that are required for panels subject to out-of-plane loads; bending resistance, shear resistance and deflection. The effect of reduced shear connection on each of these design checks is explored in turn. For bending resistance, design rules based on first principles cross-section equilibrium are found to accurately predict the point of failure for the majority of cases. However, the existing assumption of a smooth profile of shear connector force is found to be incorrect on the tension plate, with tensile cracking leading to discontinuities in the stud force profile. Further interpretation of this result shows that this can lead to an unconservative prediction of the failure load when a panel with a low degree of shear connection is subject to a uniformly-distributed load (UDL). A new design rule is presented for this situation. Design equations for shear resistance are found to vary considerably between design codes and countries. As with the bending check, the test database is found to be lacking in tests with low enough degrees of shear connection to draw definitive conclusions about any changes in behaviour. A parametric FE study is presented to investigate these effects. The study focusses on varying the degree of shear connection for groups of beams loaded at different shear-span to depth ratios. Different behaviour is observed in each group, with the influence of shear connection varying, depending on which shear transfer action is dominant. The study shows that unconservative predictions are made for a number of the design models, particularly for slender beams with low degrees of shear connection. A new adjustment is presented for the Eurocode shear resistance model that removes the unconservative predictions. The models from the fib Model Code are suggested as a better alternative, again with some adjustment to account for reduced degree of shear connection. Deflection of SCS panels is usually predicted using linear-elastic models. Debate has occurred about whether to base the stiffness used on the contribution of the steel plates only, or whether the concrete stiffness should be included. This work finds that a partial concrete contribution should be assumed. It is also found that simple bending prediction models, based on Euler-Bernoulli principles, tend to overestimate stiffness for beams with low shear-span to depth ratios. In these cases, models that include shear deformation (such as the model by Timoshenko) are found to produce more accurate predictions. Reduced shear connection is found to lead to non-linear load deflection response curves, which cannot be easily approximated with linear-elastic models. A new load-stiffness curve is proposed for simplified non-linear modelling, which could be easily implemented in most current software packages with non-linear solvers. Finally, partial resistance factors for the bending and shear design checks are calculated, using the procedure presented in Annex D of Eurocode 0. This method takes into account the precision and conservativeness of a particular design equation through a systematic comparison with available test data, and penalises studies that are based on limited test data. The procedure is found to be deficient when the design model includes contributions from multiple materials and large numbers of parameters. To overcome this, a novel extension to the existing procedure is proposed, termed the 'matrix method'. In general, it is concluded that lower degrees of shear connection are not immediately detrimental to the performance of the system. This thesis highlights the changes in behaviour that can occur, which designers should account for when calculating the resistance of panels. This thesis also presents new adjustments and design rules to allow resistance to be accurately calculated in such cases.

Cette etude s'interesse au comportement mecanique des structures en materiaux composites epais, utilisees en construction navale (monolithiques epais et sandwichs). Une attention particuliere est apportee au comportement en flambement. La premiere etape consiste a identifier les caracteristiques materiaux. Des essais permettant de determiner la contrainte de compression, les modules de cisaillement transverse ainsi que le comportement non-lineaires sont examines. Une procedure de caracterisation en vue du calcul est proposee. La seconde etape aboutit au developpement de deux elements coques, non lineaires, grands deplacements et grandes deformations. Le champs de cisaillement transverse est decrit soit par une formulation variationnelle, soit par une evolution quadratique dans l'epaisseur. Une ecriture de la loi de comportement en reperage materiel est proposee. Lors de la troisieme etape deux essais sont proposes : plaque sous pression hydrostatique pour la validation du logiciel, flambement de plaques raidies par cisaillement afin de mettre au point une procedure de calcul.

L’objet de ce travail est de produire et de caractériser des composites structuraux à base de polypropylène et de mousse. La première partie est consacrée au renforcement du polypropylène avec des fibres de chanvre en étudiant l’effet de la concentration de la fibre, de la taille des fibres et de la concentration en agent de couplage sur les propriétés mécaniques. Une étude morphologique par photomicrographies a permis d’expliquer les résultats mécaniques en tension et flexion. On montre que 2% d’agent couplant est suffisant pour optimiser les modules. Dans la deuxième partie, des mousses de polypropylène sont produites par compression avec différentes concentrations d'agent gonflant afin de déterminer l’effet de la réduction de densité et du profil de densité sur les propriétés en tension et flexion. Une caractérisation complète de la morphologie des mousses en termes de taille de cellules, de densité de cellules et d’épaisseur de la peau est faite. L'utilisation du profil de densité est nécessaire afin d’obtenir une bonne prédiction des propriétés mécaniques. Finalement, des structures sandwich avec différents pourcentages de peau et de densité de cœur sont produites. Une analyse morphologique du cœur est rapportée avec les propriétés mécaniques en tension et flexion. On montre qu’une très bonne prédiction peut être faite en utilisant simplement la loi des mélanges et le modèle quadratique avec le profil de densité pour l’effet de la peau et du cœur, respectivement.
The aim of this work is to produce and characterize polypropylene structural composite foams. To do so, the work is divided in three parts. The first part is devoted to study the reinforcement of polypropylene with hemp fibres by changing the fibre content, fibre size and coupling agent concentration. Micrographs are used to explain the results of the mechanical properties measured under tensile and flexural stress. It is found that 2% of coupling agent gives the optimum modulus values. In the second part, polypropylene foams are produced by compression moulding with different concentrations of blowing agent to determine the effect of density reduction and density profile on the tensile and flexural properties. The morphological characteristics (cell size, cell density and skin thickness) of the foams are also examined. It is found that the use of the complete density profile is necessary to predict with high precision the mechanical results. Finally, sandwich structures are produced with different skin ratio and core densities. A complete morphological analysis is reported with mechanical properties (tensile and flexural). It is shown that the simple law of mixture and the square power-law combined with the density profile are enough to predict the effect of the skins and core, respectively.

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 thesis examines the sound transmission loss (STL) through composite sandwich panel systems commonly used in the marine industry. Experimental, predictive and optimisation methods are used to evaluate the acoustic performance of these systems and to improve their acoustic performance with noise treatment. The complex nature of the material properties of composite sandwich panels was found to be dependent not only on the physical properties but also the frequency of incident noise. Young’s modulus was found to reduce with increasing frequency as has been predicted in the literature which is due to the shear stiffness dominating over the bending stiffness. Two methods for measuring these properties were investigated; ‘fixed-free’ and ‘free-free’ beam boundary condition modal analyses. The disagreement between these methods was identified as the clamping fixed nature that increased flexibility of the beam. Composite sandwich panels can be modelled as homogeneous isotopic materials when predicting their acoustic performance provided the dilatational resonance is above the frequency range of interest. Two such panels were modelled using this simple sound insulation prediction method, but the agreement between theory and experimental results was poor. A variable Young’s modulus was included in the model but agreement remained relatively poor especially in the coincidence frequency region due to variation of Young’s modulus with frequency. A statistical method of optimisation of the parameter settings by fractional factorial design proved successful at identifying the important parameters that affect the sound transmission class (STC) of a noise treatment material applied to a panel. The decouple foam layer and attachment method were the most significant factors. The same method, with higher resolution was then used to identify the important parameters that affected the noise reduction class (NRC) finding that the outer foam thickness without a face sheet were the most significant factors. The independent optimisation studies performed for each of the STC and NRC produced conflicting results meaning that both could not be achieved simultaneously.

Transcendental stiffness matrices for vibration (or buckling) analysis have long been available for a range of structural members. Such stiffness matrices are exact in the sense that they are obtained from an analytical solution of the governing differential equations of the member. Hence, assembly of the member stiffnesses to obtain the overall stiffness matrix of the structure results in a transcendental eigenproblem that yields exact solutions and which can be solved with certainty using the Wittrick-Williams algorithm. Convergence is commonly achieved by bisection, despite the fact that the method is known to be relatively slow. Quicker methods are available, but their implementation is hampered by the highly volatile nature of the determinant of the structure's transcendental stiffness matrix, particularly in the vicinity of the poles, which may or may not correspond to eigenvalues. However, when the exact solution exists, the member has a recently discovered property that can also be expressed analytically and is called its member stiffness determinant. The member stiffness determinant is a property of the member when fully clamped boundary conditions are imposed upon it. It is then defined as the determinant of the member stiffness matrix when the member is sub-divided into an infinite number of identical sub-members. Each sub-member is therefore of infinitely small length so that its clamped-ended natural frequencies are infinitely large. Hence the contribution from the member stiffness matrix to the Jq count of the W-W algorithm will be zero. In general, the member stiffness determinant is normalised by dividing by its value when the eigenparameter (i.e. the frequency or buckling load factor) is zero, as otherwise it would become infinite. Part A of this thesis develops the first two applications of member stiffness determinants to the calculation of natural frequencies or elastic buckling loads of prismatic assemblies of isotropic and orthotopic plates subject to in-plane axial and transverse loads. A major advantage of the member stiffness determinant is that, when its values for all members of a structure are multiplied together and are also multiplied by the determinant of the transcendental overall stiffness matrix of the structure, the result is a determinant which has no poles and is substantially less volatile when plotted against the eigenparameter. Such plots provide a significantly better platform for the development of efficient, computer-based routines for convergence on eigenvalues by curve prediction techniques. On the other hand, Part B presents the development of exact dynamic stiffness matrices for three models of sandwich beams. The simplest one is only able to model the flexural vibration of asymmetric sandwich beams. Extending the first model to include axial and rotary inertia makes it possible to predict the axial and shear thickness modes of vibration in addition to those corresponding to flexure. This process culminates in a unique model for a three layer Timoshenko beam. The crucial difference of including axial inertia in the second model, enables the resulting member dynamic stiffness matrix (exact finite element) to be included in a general model of two dimensional structures for the first time. Although the developed element is straight, it can also be used to model curved structures by using an appropriate number of straight elements to model the geometry of the curve. Finally, it has been shown that considering a homogeneous deep beam as an equivalent three-layer beam allows the beam to have additional shear modes, besides the flexural, axial and fundamental shear thickness modes. Also for every combination of layer thickness, the frequencies of the three-layer beam are less than the corresponding frequencies calculated for the equivalent beam model with only one layer, since it is equivalent to providing additional flexibility to the system. However, a suitable combination of layer thicknesses for any mode may be found that yields the minimum frequency. It is anticipated that these frequencies would probably be generated by a single layer model of the homogeneous beam if at least a third order shear deformation theory was incorporated. Numerous examples have been given to validate the theories and to indicate their range of application. The results presented in these examples are identical to those that are available from alternative exact theories and otherwise show good correlation with a selection of comparable approximate results that are available in the literature. In the latter case, the differences in the results are attributable to many factors that vary widely from different solution techniques to differences in basic assumptions.

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.

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1999.
Includes bibliographical references (leaves 237-250).
Experimental and numerical work was conducted to understand better the compressive response of notched composite sandwich panels. The quasi-static uniaxial compressive response of notched (circular through hole) E-glass/epoxy- NomexTM sandwich panels were studied experimentally. Two different woven fabric architectures were examined. The key failure mechanism was observed to be linear damage zones (LDZs) emanating from the notch tip (in both materials). LDZ's behaved in a macroscopically similar manner to a bridged crack under tensile loading, and were characterized by semi-stable propagation. Crosssectioning studies revealed the key damage mechanisms operating within the LDZ. Progressive cross-sections indicated that individual fiber microbuckling led to out-of-plane warp tow kinking. The LDZ wake was characterized by kinking in all warp tows and transverse tow splitting. Strain gages were used to measure the in situ damage zone tractions as the LDZ propagated across the width of the specimen; a softening trend was observed. Consistent with observations, a two parameter linear strain softening traction law was used to model the LDZ constitutive behavior. The traction law was treated as a material property. The damage zone modeling (DZM) framework was investigated to determine its validity, specifically its ability to predict three experimentally observed phenomena: the notched strength, local strain distribution, and LDZ growth characteristics. A self-consistent physically-based model should be able to predict all three phenomena. Two models were created in order to interrogate the DZM. The damage growth model was used to determine the ability of the DZM to predict the LDZ growth behavior and notched strength. A finite element model that used discrete nonlinear springs in the wake of the LDZ to model the LDZ as a continuous spring, was implemented to determine if the DZM could predict the local strain distribution. Results showed that the current traction law provided excellent agreement with the phenomenon used to calibrate the traction law, for all specimen sizes. Extension of predictive power to other phenomena resulted in weaker correlations. The modeling framework and methodology established provide a robust tool for investigating the potential of adding physical bases to the DZM.
by Michael Garcia-Lopez Toribio.
S.M.

In recent years advances in technology have allowed the transition of composite structures from secondary to primary structural components. Consequently, a lot of applications demand development of thicker composite structures to sustain heavier loads. Typical sandwich panels consist of two thin metallic or composite face sheets separated by a honeycomb or foam core. This configuration gives the sandwich panel high stiffness and strength and enables excellent energy absorption capabilities with little resultant weight penalty. This makes sandwich structures a preferred design for a lot of applications including aerospace, naval, wind turbines and civil industries. Most aerospace structures can be analyzed using shell and plate models and many such structures are modeled as composite sandwich plates and shells. Accurate theoretical formulations that minimize the CPU time without penalties on the quality of the results are thus of fundamental importance. The classical plate theory (CPT) and the first order shear deformation theory (FSDT) are the simplest equivalent single-layer models, and they adequately describe the kinematic behavior of most laminates where the difference between the stiffnesses of the respective phases is not huge. However, in the case of sandwich structures where the core is a much more compliant and softer material as compared to the face sheets the results from CPT and FSDT becomes highly inaccurate. Higher order theories in such cases can represent the kinematics better, may not require shear correction factors, and can yield much more accurate results. An advanced Extended Higher-order Sandwich Panel Theory (EHSAPT) which is a two-dimensional extension of the EHSAPT beam model that Phan presented is developed. Phan had extended the HSAPT theory for beams that allows for the transverse shear distribution in the core to acquire the proper distribution as the core stiffness increases as a result of non-negligible in-plane stresses. The HSAPT model is incapable of capturing the in-plane stresses and assumes negligible in-plane rigidity. The current research extends that concept and applies it to two-dimensional plate structures with variable aspect ratios. The theory assumes a transverse displacement in the core that varies as a second order equation in z and the in-plane displacements that are of third order in z, the transverse coordinate. This approach allows for five generalized coordinates in the core (the in-plane and transverse displacements and two rotations about the x and y-axes respectively). The major assumptions of the theory are as follows: 1) The face sheets satisfy the Euler-Bernoulli assumptions, and their thicknesses are small compared to the overall thickness of the sandwich section; they undergo small strains with moderate rotations. 2) The core is compressible in the transverse and axial directions; it has in-plane, transverse and shear rigidities. 3) The bonding between the face sheets and the core is assumed to be perfect. The kinematic model is developed by assuming a displacement field for the soft core and then enforcing continuity of the displacement field across the interface between the core and facesheets. The constitutive relations are then defined, and variational and energy techniques are employed to develop the governing equations and associated boundary conditions. A static loading case for a simply supported sandwich plate is first considered, and the results are compared to existing solutions from Elasticity theory, Classical Plate Theory (CPT) and First-Order Shear Deformation Plate Theory (FSDT). Subsequently, the governing equations for a dynamic analysis are developed for a laminated sandwich plate. A free vibration problem is analyzed for a simply supported laminated sandwich plate, and the results for the fundamental natural frequency are compared to benchmark elasticity solutions provided by Noor. After validation of the new Extended Higher Order Sandwich Panel Theory (EHSAPT), a parametric study is carried out to analyze the effect of variation of various geometric and material properties on the fundamental natural frequency of the structure. After the necessary verification and validation of the theory by comparing static and free vibration results to elasticity solutions, a nonlinear static analysis for square and rectangular plates is carried out under various sets of boundary conditions. The analysis was carried out using variational techniques, and the Ritz method was used to find an approximate solution. The kinematics were developed for a sandwich plate undergoing small strain and moderate rotations and nonlinear strain displacement relations were evaluated. Approximate and assumed solutions satisfying the geometric boundary conditions were developed and substituted in the total potential energy relations. After carrying out the spatial integrations, the total potential energy was then minimized with respect to the unknown coefficients in the assumed solution resulting in nonlinear simultaneous algebraic equations for the unknown coefficients. The simultaneous nonlinear equations were then solved using the Newton-Raphson method. A convergence study was carried out to study the effect of varying the number of terms in the approximate solution on the overall result and rapid convergence was observed. The rapid convergence can be attributed to the fact that the assumed approximate solution not only satisfies the geometric boundary conditions of the problem but also the natural boundary conditions. During calculations four cases of boundary conditions were considered 1) Simply Supported with moveable edges. 2) Simply Supported with fixed edges. 3) Clamped with moveable edges. 4) Clamped with fixed edges. For movable boundary conditions, in-plane displacements along the normal direction to the supported edges are allowed whereas the out-of-plane displacement is fixed. For the immovable boundary condition cases, the plate is prevented from both in-plane and out-of-plane displacements along the edges. For the simply supported cases rotations about the tangential direction are allowed, and for the clamped cases no rotations are allowed.In recent years advances in technology have allowed the transition of composite structures from secondary to primary structural components. Consequently, a lot of applications demand development of thicker composite structures to sustain heavier loads. Typical sandwich panels consist of two thin metallic or composite face sheets separated by a honeycomb or foam core. This configuration gives the sandwich panel high stiffness and strength and enables excellent energy absorption capabilities with little resultant weight penalty. This makes sandwich structures a preferred design for a lot of applications including aerospace, naval, wind turbines and civil industries. Most aerospace structures can be analyzed using shell and plate models and many such structures are modeled as composite sandwich plates and shells. Accurate theoretical formulations that minimize the CPU time without penalties on the quality of the results are thus of fundamental importance. The classical plate theory (CPT) and the first order shear deformation theory (FSDT) are the simplest equivalent single-layer models, and they adequately describe the kinematic behavior of most laminates where the difference between the stiffnesses of the respective phases is not huge. However, in the case of sandwich structures where the core is a much more compliant and softer material as compared to the face sheets the results from CPT and FSDT becomes highly inaccurate. Higher order theories in such cases can represent the kinematics better, may not require shear correction factors, and can yield much more accurate results. An advanced Extended Higher-order Sandwich Panel Theory (EHSAPT) which is a two-dimensional extension of the EHSAPT beam model that Phan presented is developed. Phan had extended the HSAPT theory for beams that allows for the transverse shear distribution in the core to acquire the proper distribution as the core stiffness increases as a result of non-negligible in-plane stresses. The HSAPT model is incapable of capturing the in-plane stresses and assumes negligible in-plane rigidity. The current research extends that concept and applies it to two-dimensional plate structures with variable aspect ratios. The theory assumes a transverse displacement in the core that varies as a second order equation in z and the in-plane displacements that are of third order in z, the transverse coordinate. This approach allows for five generalized coordinates in the core (the in-plane and transverse displacements and two rotations about the x and y-axes respectively). The major assumptions of the theory are as follows: 1) The face sheets satisfy the Euler-Bernoulli assumptions, and their thicknesses are small compared to the overall thickness of the sandwich section; they undergo small strains with moderate rotations. 2) The core is compressible in the transverse and axial directions; it has in-plane, transverse and shear rigidities. 3) The bonding between the face sheets and the core is assumed to be perfect. The kinematic model is developed by assuming a displacement field for the soft core and then enforcing continuity of the displacement field across the interface between the core and facesheets. The constitutive relations are then defined, and variational and energy techniques are employed to develop the governing equations and associated boundary conditions. A static loading case for a simply supported sandwich plate is first considered, and the results are compared to existing solutions from Elasticity theory, Classical Plate Theory (CPT) and First-Order Shear Deformation Plate Theory (FSDT). Subsequently, the governing equations for a dynamic analysis are developed for a laminated sandwich plate. A free vibration problem is analyzed for a simply supported laminated sandwich plate, and the results for the fundamental natural frequency are compared to benchmark elasticity solutions provided by Noor. After validation of the new Extended Higher Order Sandwich Panel Theory (EHSAPT), a parametric study is carried out to analyze the effect of variation of various geometric and material properties on the fundamental natural frequency of the structure. After the necessary verification and validation of the theory by comparing static and free vibration results to elasticity solutions, a nonlinear static analysis for square and rectangular plates is carried out under various sets of boundary conditions. The analysis was carried out using variational techniques, and the Ritz method was used to find an approximate solution. The kinematics were developed for a sandwich plate undergoing small strain and moderate rotations and nonlinear strain displacement relations were evaluated. Approximate and assumed solutions satisfying the geometric boundary conditions were developed and substituted in the total potential energy relations. After carrying out the spatial integrations, the total potential energy was then minimized with respect to the unknown coefficients in the assumed solution resulting in nonlinear simultaneous algebraic equations for the unknown coefficients. The simultaneous nonlinear equations were then solved using the Newton-Raphson method. A convergence study was carried out to study the effect of varying the number of terms in the approximate solution on the overall result and rapid convergence was observed. The rapid convergence can be attributed to the fact that the assumed approximate solution not only satisfies the geometric boundary conditions of the problem but also the natural boundary conditions. During calculations four cases of boundary conditions were considered 1) Simply Supported with moveable edges. 2) Simply Supported with fixed edges. 3) Clamped with moveable edges. 4) Clamped with fixed edges. For movable boundary conditions, in-plane displacements along the normal direction to the supported edges are allowed whereas the out-of-plane displacement is fixed. For the immovable boundary condition cases, the plate is prevented from both in-plane and out-of-plane displacements along the edges. For the simply supported cases rotations about the tangential direction are allowed, and for the clamped cases no rotations are allowed.

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.

Thesis (M.S.)--West Virginia University, 2007.
Title from document title page. Document formatted into pages; contains xviii, 141 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 140-141).

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