The first part of the master’s thesis deals with sandwich materials. Starting with composition of parts which the sandwich panel is made of. The thesis continues with loading distribution and testing of entire sandwich panel. The impact of environment is considered as well. The second part is its goal – determine degradation of the properties of sandwich panels with metal sheets and core after 15 years aging in the common atmosphere and aging in moistheat and salt environment by experiment.

In Luftfahrzeugen werden häufig Sandwich-Strukturen verwendet, da somit vergleichsweise hohe gewichtsspezifische Steifigkeiten und Festigkeiten erreicht werden können. Hierbei werden für Deckschichten überwiegend Faserverbund-Kunststoffe angewendet. Die Kerne bestehen zumeist aus Honigwaben, welche aus phenolharzbeschichtetem Aramid-Papier gefertigt sind. Somit können Anforderungen an die Feuer- und Korrosionsresistenz erfüllt werden. Sandwich-Strukturen im Allgemeinen sind dabei anfällig für lokale Belastungen, sowie Lasten senkrecht zur Struktur. Dies können beispielsweise Schlagbelastungen, Lasteinleitungen durch Verbindungselemente oder Druckunterschiede sein. Folglich bedarf die Zertifizierung von Luftfahrtstrukturen zumeist umfangreiche experimentelle Untersuchungen zum Nachweis des Tragverhaltens und der Schadenstoleranz. Dieses Vorgehen ist äußerst zeitaufwendig und somit kostenintensiv. Virtuelle Tests, welche durch einzelne reale Versuche validiert werden, können den experimentellen Aufwand erheblich reduzieren. Dazu bedarf es fundierter Kenntnisse der mechanischen Eigenschaften der einzelnen Komponenten der Sandwich-Struktur. Während diese für Faserverbund-Kunststoffe als gegeben angenommen werden kann, trifft dies für Honigwabenkerne bestehend aus Aramid-Papier nicht zu. Demzufolge wird in dieser Arbeit ein Vorgehen vorgestellt, welches eine mechanische Charakterisierung und numerische Simulation von papierartigen Materialien ermöglicht. Dabei werden zunächst anwendbare Prüfmethoden für Aramid-Papier evaluiert. Darauf aufbauend werden ein verbessertes Schubprüfverfahren und ein neuartiges Druckprüfverfahren für Papier erarbeitet. Anschließend werden verschiedene luftfahrttaugliche Papiere mechanisch charakterisiert und Anforderungen an ein Materialmodell für die numerische Simulation abgeleitet. Daran anknüpfend wird ein spezielles Materialmodell entwickelt, welches das elastisch-plastische orthotrope Materialverhalten mit unterschiedlicher Druckplastifizierung und regressivem Versagen abbilden kann. Dieses Modell wird in LS-DYNA implementiert und validiert. Darauf aufbauend werden Validierungsrechnungen am Aramid-Papier sowie an Honigwaben- und Faltkern-Strukturen durchgeführt. Abschließende exemplarische Simulationen von Deckschichtablöseversuchen demonstrieren die mit dem Vorgehen erreichbare Qualität der Ergebnisse sowie Möglichkeiten zum virtuellen Testen und virtuelle Parameterstudien. / A variety of components in aircraft are made out of sandwich structures because of its high weight-specific stiffness and strength. In many cases, fiber composite plastics are used for face-layers and cores consist of honeycombs, which are made of phenolic resin coated aramid paper. Thus, requirements for fire and corrosion resistance can be met. Sandwich structures in general are prone to local loads as well as loads perpendicular to the structure. This can be, for example, impact loads, load applications by connecting elements or pressure differences. Consequently, certification of aerospace structures usually requires extensive experimental tests to demonstrate structural behavior and damage tolerance. This procedure is extremely time-consuming and therefore cost-intensive. Virtual tests, which are validated by individual experiments, can significantly reduce the experimental effort. This requires a knowledge of the mechanical properties of the individual components of the sandwich structure. While this is given for fiber composite plastics, this is not true for honeycomb cores consisting of aramid paper. Consequently, this work presents a procedure that allows mechanical characterization and numerical simulation of paper-like materials. First, applicable test methods for aramid paper are evaluated. Based on this, an improved shear test method and a novel compression test method for paper are developed. Subsequently, various paper-like materials are mechanically characterized. The requirements for a material model for numerical simulation are derived. Following on from this, a special material model is developed that can reproduce the elastic-plastic, orthotropic material behavior with different plastification for compressive loads and a regressive failure model. This material model is implemented and validated in LS-DYNA. Based on this, validation calculations are carried out on aramid paper, honeycomb and foldcore structures. Final exemplary simulations of single-cantilever-beam tests demonstrate the achievable quality of the results as well as possibilities for virtual testing and virtual parameter studies.

Due to high specific strength and stiffness as well as outstanding energy-absorption characteristics, sandwich structures are extensively used in aircraft, aerospace, automobile, and marine industries. With the objective of finding lightweight blast-resistant sandwich structures for protecting infrastructure, we have found, for a fixed areal mass density, one- or two-core doubly-curved sandwich shell's (plate's) geometries and materials and fiber angles of unidirectional fiber-reinforced face sheets for it to have the maximum first failure load under quasistatic (blast) loads. The analyses employ a third-order shear and normal deformable plate/shell theory (TSNDT), the finite element method (FEM), a stress recovery scheme (SRS), the Tsai-Wu failure criterion and the Nest-Site selection (NeSS) optimization algorithm, and assume the materials to be linearly elastic. For a sandwich shell under the spatially varying static pressure on the top surface, the optimal non-symmetric one-core (two-core) design improves the first failure load by approximately 33% (27%) and 50% (36%) from the corresponding optimal symmetric design with clamped and simply-supported edges, respectively. For a sandwich plate under blast loads, it is found that the optimal one-core design is symmetric about the mid-surface with thick face sheets, and the optimal two-core design has a thin middle face sheet and thick top and bottom face sheets. Furthermore, the transverse shear stresses (in-plane transverse axial stresses) primarily cause the first failure in a core (face sheet). For the computed optimal design under a blast load, we also determined the collapse load by using the progressive failure analysis that degrades all elasticities of the failed material point to very small values. The collapse load of the clamped (simply-supported) sandwich structure is approximately 15%–30% (0%–17%) higher than its first failure load. Incompressible materials such as rubbers, polymers, and soft tissues that can only undergo volume preserving deformations have numerous applications in engineering and biomedical fields. Their vibration characteristics are important for using them as wave reflectors at interfaces with a fiber-reinforced sheet. In this work we have numerically analyzed free vibrations of plates made of a linearly elastic incompressible rubber-like material (Poison's ratio = 0.5) by using a TSNDT for incompressible materials and the mixed FEM. The displacements at nodes of a 9-noded quadrilateral element and the hydrostatic pressure at four interior nodes are taken as unknowns. Computed results are found to match well with the corresponding either analytical or numerical ones obtained with the commercial FE software Abaqus and the 3-dimensional linear elasticity theory. The analysis discerns plate's in-plane vibration modes. It is found that a simply supported plate admits more in-plane modes than the corresponding clamped and clamped-free plates. / Doctor of Philosophy / A simple example of a sandwich structure is a chocolate ice cream bar with the chocolate layer replaced by a stiff plate. Another example is the packaging material used to protect electronics during shipping and handling. The intent is to find the composition and the thickness of the "chocolate layer" so that the ice cream bar will not shatter when dropped on the floor. The objective is met by enforcing the chocolate layer with carbon fibers and then finding fiber materials, their alignment, ice cream or core material, and its thickness to resist anticipated loads with a prescribed level of certainty. Thus, a sandwich structure is usually composed of a soft thick core (e.g., foam) bonded to two relatively stiff thin skins (e.g., made of steel, fiber-reinforced composite) called face sheets. They are lightweight, stiff, and effective in absorbing mechanical energy. Consequently, they are often used in aircraft, aerospace, automobile, and marine industries. The load that causes a point in a structure to fail is called its first failure load, and the load that causes it to either crush or crumble is called the ultimate load. Here, for a fixed areal mass density (mass per unit surface area), we maximize the first failure load of a sandwich shell (plate) under static (dynamic) loads by determining its geometric dimensions, materials and fiber angles in the face sheets, and the number (one or two) of cores. It is found that, for a non-uniformly distributed static pressure applied on the central region of a sandwich shell's top surface, an optimal design that has different materials for the top and the bottom face sheets improves the first failure load by nearly 30%-50% from that of the optimally designed structure with identical face sheets. For the structure optimally designed for the first failure blast load, the ultimate failure load with all of its edges clamped (simply supported) is about 15%-30% (0%-17%) higher than its first failure load. This work should help engineers reduce weight of sandwich structures without sacrificing their integrity and save on materials and cost. Rubberlike materials, polymers, and soft tissues are incompressible since their volume remains constant when they are deformed. Plates made of incompressible materials have a wide range of applications in everyday life, e.g., we hear because of vibrations of the ear drum. Thus, accurately predicting their dynamic behavior is important. A first step usually is determining natural frequencies, i.e., the number of cycles of oscillations per second (e.g., a human heart beats at about 1 cycle/sec) completed by the structure in the absence of any externally applied force. Here, we numerically find natural frequencies and mode shapes of rubber-like material rectangular plates with different supporting conditions at the edges. We employ a plate theory that reduces a 3-dimensional (3-D) problem to a 2-D one and the finite element method. The problem is challenging because the incompressibility constraint requires finding the hydrostatic pressure as a part of the problem solution. We show that the methodology developed here provides results that match well with the corresponding either analytical or numerical solutions of the 3-D linear elasticity equations. The methodology is applicable to analyzing the dynamic response of composite structures with layers of incompressible materials embedded in it.

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

La mise en oeuvre de matériaux acoustiques est une méthode efficace et très utilisée pour réduire le bruit le long de sa propagation. Les propriétés acoustiques de nouvelles structures sandwich multifonctionnelles et de matériaux absorbants poreux sont étudiées dans la thèse. Les principales contributions de la thèse sont les suivantes: Les panneaux sandwich ont généralement d'excellentes propriétés mécaniques et un bon indice de perte en transmission sonore (STL), mais aucune capacité d'absorption acoustique. De nouvelles structures sandwich multifonctionnelles sont développées en intégrant des microperforations et des matériaux absorbants poreux aux panneaux sandwich ondulés et en nid d’abeilles conventionnels, structurellement efficaces pour obtenir de bons STL et de bonnes absorptions en basses fréquences. Le coefficient d'absorption acoustique (SAC) et la perte en transmission (STL) des panneaux sandwich ondulés sont évalués numériquement et expérimentalement en basse fréquence pour différentes configurations de perforations. Les modèles éléments finis (EF) sont construits en tenant compte des interactions vibro-acoustiques sur les structures et des dissipations d'énergie, visqueuse et thermique, à l'intérieur des perforations. La validité des calculs FE est vérifiée par des mesures expérimentales avec les échantillons testés obtenus par fabrication additive. Par rapport aux panneaux sandwich ondulés classiques sans perforation, les panneaux sandwich perforés (PCSPs) avec des perforations dans leur plaque avant présentent non seulement un SAC plus élevé aux basses fréquences, mais aussi un meilleur STL, qui en est la conséquence directe. L'élargissement des courbes des indices d’absorption et de transmission doit être attribué à la résonance acoustique induite par les micro-perforations. Il est également constaté que les PCSPs avec des perforations dans les plaques avant et les parois internes onduleés ont les fréquences de résonance les plus basses de tous les PCSPs. En outre, les performances acoustiques des panneaux sandwich en nid d'abeilles avec une plaque avant microperforée sont également examinées. Un modèle analytique est présenté avec l'hypothèse que les déplacements des deux plaques sont identiques aux fréquences inférieures à la fréquence de résonance des plaques. Le modèle analytique est ensuite validé par des modèles d'éléments finis et des résultats expérimentaux existants. Contrairement aux panneaux sandwich en nid d'abeilles classiques qui sont de piètres absorbeurs de bruit, les sandwichs en nid d'abeilles perforés (PHSPs) conduisent à un SAC élevé aux basses fréquences, ce qui entraîne en conséquence un incrément dans le STL basse fréquence. Les influences de la configuration du noyau sont étudiées en comparant les PHSPs avec différentes configurations de noyaux en nids d'abeilles. […] / Implementation of acoustic materials is an effective and popular noise reduction method during propagation. Acoustic properties of novel multifunctional sandwich structures and porous absorbing materials are studied in the dissertation. The main contributions of the dissertation are given as, Sandwich panels generally have excellent mechanical properties and good sound transmission loss (STL), but no sound absorption ability. Novel multifunctional sandwich structures are developed by integrating micro perforations and porous absorbing materials to the conventional structurally-efficient corrugated and honeycomb sandwich panels to achieve good SAC and STL at low frequencies. Low frequency sound absorption and sound transmission loss (STL) of corrugated sandwich panels with different perforation configurations are evaluated both numerically and experimentally. Finite element (FE) models are constructed with considerations of acousticstructure interactions and viscous and thermal energy dissipations inside the perforations. The validity of FE calculations is checked against experimental measurements with the tested samples provided by additive manufacturing. Compared with the classical corrugated sandwich panels without perforation, the perforated corrugated sandwich panels (PCSPs) with perforations in its face plate not only exhibits a higher SAC at low frequencies but also a better STL as a consequence of the enlarged SAC. The enlargement of SAC and STL should be attributed to the acoustical resonance induced by the micro perforations. It is also found that the PCSPs with perforations in both the face plates and corrugated cores have the lowest resonance frequencies of all the PCSPs. Besides, the acoustic properties of honeycomb sandwich panels with microperforated faceplate are also explored. An analytical model is presented with the assumption that displacements of the two faceplates are identical at frequencies below the faceplate resonance frequency. The analytical model is subsequently verified by finite element models and existing experimental results. Unlike classical honeycomb sandwich panels which are poor sound absorbers, perforated honeycomb sandwiches (PHSPs) lead to high SAC at low frequencies, which in turn brings about increment in the low frequency STL. Influences of core configuration are investigated by comparing PHSPs with different honeycomb core configurations. In order to enlarge the SAC bandwidth of perforated sandwich panels, porous absorbing materials are added to the cores of novel perforated sandwich panels. FE models are set up to estimate the SAC and STL of perforated sandwich panels with porous materials. Results show that perforated sandwich panels with porous material can provide SAC with broader bandwidth and lower resonance frequency than that without porous materials. Whereas the peak values in the SAC and STL curves are reduced due to the weakened acoustical resonance by the porous materials. […]

Un nouveau matériau d’âme à base de fibres enchevêtrées et réticulées a été précédemment développé dans le but d’améliorer certaines propriétés des structures sandwichs dont l’amortissement vibratoire. Cependant, son comportement mécanique et vibratoire doit être optimisé afin de l’utiliser dans le domaine aérospatial. Plusieurs paramètres morphologiques entrent en jeu lors de sa fabrication. L’objectif de cette thèse est de développer un modèle numérique permettant de mieux comprendre le comportement de ce matériau enchevêtré réticulé. Le comportement d’un volume élémentaire représentatif de fibres de carbone enchevêtrées et non-réticulées est d’abord étudié en compression par éléments finis. La géométrie numérique du réseau de fibres s’appuie sur les données morphologiques du matériau réel. Les simulations numériques permettent de suivre, au cours de la compression confinée, l’évolution des différents paramètres, tels que la distribution des orientations des fibres, la distance entre contacts ou la fraction volumique. Ces résultats constituent une base robuste pour le développement du modèle numérique du matériau enchevêtré et réticulé qui est ensuite utilisé pour modéliser le comportement mécanique en cisaillement, et en particulier pour simuler et expliquer les boucles d’hystérésis observées expérimentalement. A la fin de ce travail, une étude numérique est proposée afin de décrire l’influence des différents paramètres morphologiques sur la rigidité en compression et en cisaillement du matériau enchevêtré réticulé. / A new core material based on entangled and cross-linked fibers has been previously developed in order to improve certain properties of sandwich structures including vibration damping. However, its behavior must be optimized for use in the aerospace field. Several morphological parameters can be modified during the manufacturing process. The aim of this thesis was to develop a numerical model to better understand the behavior of this entangled cross-linked material. The behavior of a representative volume element of entangled carbon fibers without cross-links is first studied in compression using finite element. The numerical geometry of the fiber network relies on the morphological parameters of a real sample. Numerical simulations make it possible to follow, during the confined compression, the evolution of the different parameters, such as the distribution of fiber orientations, the distance between contacts or the volume fraction. These results provide a robust basis for developing the numerical model of the entangled cross-linked material which is then used to model the mechanical behavior in shear, and in particular to simulate and explain the hysteresis loops observed experimentally. At the end of this work, a numerical study is proposed to study the influence of different morphological parameters on the compressive stiffness and shear stiffness of the entangled cross-linked material.

La surveillance de santé structurale (SHM) a attiré beaucoup d'attention dans de nombreux domaines tels que l'industrie civile, aéronautique, mécanique, etc., car il est important de surveiller l'état de la structure afin d'éviter des défaillances structurelles imprévues. Le processus d'identification des endommagements à quatre niveaux: existence, localisation, sévérité et prédiction de l'évolution des endommagements peut être partiellement réalisé si un propre indicateur est bien choisi. Il existe différents indicateurs d'endommagements dont la gamme d'application de la fréquence s'étend de la réponse vibratoire à basses fréquences aux régimes ultrasoniques dans la gamme méga hertz.Les structures sandwich sont largement utilisées dans diverses applications d'ingénierie en raison de son rapport rigidité / poids exceptionnellement élevé par rapport aux structures monocoques. Dans ce travail, une structure sandwich a été étudiée et des indicateurs basés sur la réponse vibratoire ont été conçus en utilisant ses caractéristiques de directivité de propagation et d'amortissement relativement élevé de la structure. Des investigations numériques sur différents scénarios d'endommagement (càd, différents types d'endommagement et leurs combinaisons) et une discussion associée sur la plage d'application ont d'abord été effectuées. La configuration expérimentale a été facilement réalisée à l'aide d'un vibromètre laser à balayage Doppler (SLDV). L'endommagement a été détecté avec succès par les indicateurs proposés. / Structural health monitoring (SHM) has attracted much attention in many engineering fields like civil, aeronautic, mechanical industry, etc. since it is important to monitor the healthy condition of the operational structure in order to avoid unpredicted structural failure which may have severe consequences. The four-level damage identification process: existence, localization, severity and prediction of damage evolution, can be partly realized if a suitable indicator is chosen. It exists different damage indicators whose application range of frequency spans from vibrational response at low frequencies to the ultrasonic regimes in the mega hertz range.The sandwich structures are widely used in various engineering applications due to its exceptionally high flexural stiffness-to-weight ratio compared to monocoque structures. In this thesis a specified twin-wall sandwich structure in polypropylene was studied and vibration-based indicators were designed by taking use of its relative high damping and propagation directivity characteristics. Numerical investigations on different damage scenarios (i.e., different types of defect and their combinations) and an associated discussion on the range of application were first carried out. Experimental configuration was easily realized with the help of a scanning laser doppler vibrometer (SLDV). Defect was successfully detected by the proposed indicators.

No description available.

Pour être commercialisés et installés à bord des navires, les panneaux structuraux tels que les cloisons et les pontsdoivent passer avec succès un essai normalisé de résistance au feu de type ISO 834. De tels essais sont longs et coûteux,les constructeurs de panneaux souhaitent par conséquent maximiser les chances de succès lors du passage de leur produitau test en particulier pour les designs alternatifs que constituent les sandwichs composite.La simulation par éléments finis est un outil pour modéliser le comportement thermomécanique de telles structures.Les codes industriels comme SAMCEF qui a été utilisé dans ce travail sont capables de réaliser des analyses thermiquesavec dégradation et des analyses mécaniques mettant en jeu la gestion du contact, la dégradation des propriétésmécaniques et la perte des structures par ruine.L’enjeu de cette étude est d’abord identifier les spécificités de telles structures particulières par leur taille, leurconception, de déterminer quelles sont les données thermiques et mécaniques nécessaires pour alimenter le modèlenumérique et le cas échéant de les construire à partir des résultats de la réaction au feu des matériaux. Des modèlesnumériques que l’on souhaite robustes et utilisables dans un contexte industriel, sont développés pour déterminer lecomportement thermomécanique de tels panneaux. Ils prennent en compte l’évolution des propriétés thermiques etmécaniques des matériaux en cours de dégradation. Ces modèles doivent permettre par la suite l’estimation desperformances de nouveaux designs lors d’un essai de certification ISO 834. / Structural panels used in naval industry such as bulkheads and decks must succeed in standard certification testssuch as ISO 834 to be commercialized and settled on board. As these tests are long and expensive, panel manufacturerswish to maximize chances of success for their panels when submitted to certification tests especially when it comes toalternative designs such as composite sandwich panels.Finite elements analyses are used to model thermo-mechanical behavior. Industrial software such as SAMCEF,which was used to conduct this work, are able to solve thermal analyses with degradation and mechanical analyses involvingcontact conditions, degradation of mechanical properties and loss of structures due to failure.The objective in this study is to first identify characteristics of these structures. They are special due to their size andtheir manufacturing. This study aims also to determine thermal and mechanical data required for numerical modeling.When necessary some data can be computed from results coming from the results of the materials’ reaction to fire.Numerical models are developed to determine thermo-mechanical behavior and are designed to be robust and used inindustrial context. They include the evolution of thermal and mechanical properties during the degradation process. Thesemodels must enable to estimate the performances of innovative designs during an ISO 834 certification test.

Favorable properties of Balsa wood make it an interesting alternative in a number of applications including thermal insulation or as a lightweight core material in sandwich composites. Increasing use in construction necessitates a better understanding of its mechanical and failure properties. In the present work, mode I and mode II fracture toughness for different types of balsa wood and a sandwich structure (balsa as core and fiber glass as skin layer) are studied experimentally by using load-displacement diagrams and visually acquired crack growth data. / Graduation date: 2013