Thesis (Ph. D.)--West Virginia University, 2004. / Title from document title page. Document formatted into pages; contains xviii, 224 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 201-208).

Thesis--Auburn University, 2009. / Abstract. Vita. Includes bibliographic references (p.118-121).

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Honeycomb sandwich panels, formed by bonding a core of honeycomb between two thin face sheets, are in wide use in aerospace, automotive and marine applications due to their well-known excellent density-specific properties. There are many technological methods of damping vibrations, including the use of inherently lossy materials such as viscoelastic materials, viscous and friction damping and smart materials such as piezoelectrics. Some have been applied to damping of vibrations, in particular to sandwich panel and honeycomb structures, including viscoelastic inserts in the cell voids. Complete filling of the cell with foam, viscoelastic or particulate fillers have all been demonstrated to improve damping loss in honeycombs. However, the use of an additional damping material inside the core of a sandwich panel increases its mass, which is often deleterious and may also lead to a significant change in dynamic properties. The work presented in this thesis explores the competing demands of vibration damping and minimum additional mass in the case of secondary inserts in honeycomb-like structures. The problem was tackled by initially characterising the main local deformation mechanism of a unit cell within a sandwich panel subjected to vibration. Out-of-plane bending deformation of the honeycomb unit cell was shown to be the predominant mode of deformation for most of the honeycomb cells within a sandwich panel. The out-of-plane bending deformation of the honeycomb cells results in relatively high in-plane deformation of the cells close to the skins of the sandwich panels. It was also highlighted that the magnitude and loading of the honeycomb unit cell are dependent on its location within the honeycomb or sandwich panel and the mode shape of the panel. An optimisation study was carried out on diverse honeycomb unit cell geometries to find locations at which the relative displacement between the honeycomb cell walls of the void is maximal under in-plane loadings. These locations were shown to be dependant of the nature of the loading, i.e. in-plane tension/compression or in-plane shear loading of the honeycomb unit cell and the unit cell geometry. Analytical expressions and finite element analyses were used to investigate the partial filling of the honeycomb unit cell with a damping material, in this case a viscoelastic elastomer, in the target locations identified previously where the relative displacement between the honeycomb cell walls is maximal. Damping inserts in the form of ligaments partially filling the honeycomb cell void have shown to increase the density-specific loss modulus by 26% compared to cells completely filled with damping material for in-plane tension/compression loading. The form of the damping insert itself was then analysed for enhancement of the dissipation provided by the damping material. The shear lap joint (SLJ) damping insert placed in the location where the relative displacement between the honeycomb cell walls of the void is maximal under in-plane loadings was characterised with very significant damping improvements compared to honeycomb cells completely filled with viscoelastic material. A case study of a cantilever honeycomb sandwich panel with embedded SLJ damping inserts demonstrated their efficiency in enhancing the loss factor of the structure for minimum added mass and marginal variation of the first modal frequency of the structure. Partial filling of the cells of the honeycomb core was shown to be the most efficient at increasing damping on a density basis.

Diese Arbeit beschäftigt sich mit der Untersuchung der Wabenstruktur eines thermoplastischen Sandwichmaterials anhand von Daten, die durch das Verfahren der Computertomographie gewonnen wurden. Diese Untersuchung dient zur Modellierung eines repräsentativen Volumenelementes, welches die Zellgeometrie des Wabenkerns möglichst genau wiedergibt. Das Ziel dieses Vorgehens ist die Bestimmung effektiver Materialkennwerte, die zur mechanischen Simulation nach der Methode der finiten Elemente angewendet werden können.:1. Einleitung 2. Grundlagen 2.1 Sandwichbauweise 2.1.1 Biegeverhalten von Sandwichmaterialien 2.1.2 Spannungen in den Schichten 2.2 ThermHex-Waben als Kernmaterial 2.2.1 Material und Werkstoffkennwerte 2.2.2 Fertigung 2.2.3 Definition der Wabengeometrie 2.3 Mechanisches Verhalten von Wabenkernen im elastischen Bereich 2.4 Möglichkeiten zur Bestimmung von Elastizitätstensoren 2.4.1 Analytische Herangehensweise 2.4.2 Experimentelle Herangehensweise 2.4.3 Morphologische Herangehensweise mittels repräsentativen Volumenelement 3. Mikrostrukturelle Analyse 3.1 Untersuchungsobjekte 3.2 Untersuchungsmethodik 3.3 Untersuchungsergebnisse 3.3.1 Auswertungsproblematik 3.3.2 Zellweiten und Reihenabstände 3.3.3 Wandstärke 3.3.4 Zellwandlängen und -winkel 3.3.5 Zellwand- und Wölbungsradien 3.3.6 Kernhöhe und Schrägungswinkel des Wabenkerns mit Deckschichten 4. Modellierungsmethodik und -ergebnisse 4.1 Werkstoffabhängige und geometrische Parameter 4.2 Validierung der periodischen Randbedingungen 4.3 Validierung der Simulation mittels analytischer Herangehensweise 4.4 Geometrische Modellerstellung 4.5 Simulationsergebnisse der RVE-Grundtypen 5. Sensitivitätsanalyse 5.1 Komplexitätsvariation 5.2 Einfluss streuender Parameter 6. Simulierter 3-Punkt-Biegeversuch 7. Diskussion 7.1 Simulationsvalidierung 7.2 Verhalten der RVE-Grundtypen 7.3 Erkenntnisse der Sensitivitätsanalyse 7.3.1 Komplexitätsgrad 7.3.2 Modellsensitivität 7.4 Anwendungsfall der 3-Punkt-Biegung 8. Zusammenfassung und Ausblick 9. Literatur- und Quellenverzeichnis 10. Anhang

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Spacecraft structures rely on honeycomb panels to provide a light weight means to support the vehicle. Honeycomb panels can carry significant load but are most vulnerable to structural failure at their joints where panels connect. This research shows that predicting sandwich panel joint capability using finite element analysis (FEA) is possible. This allows for the potential elimination of coupon testing early in a spacecraft design program to determine joint capability. Linear finite element analysis (FEA) in NX Nastran was used to show that adhesive failure can be predicted with reasonable accuracy by including a fillet model on the edge of the fitting. Predicting the ultimate failure of a joint using linear FEA requires that engineering judgment be used to determine whether failure of certain bonds in a fitting will lead to ultimate joint failure or if other bonds will continue to carry the joint's load. The linear FEA model is also able to predict when the initiation of core failure will begin. This has the limitation that the joint will still be able to continue to carry significantly more load prior to joint ultimate failure even after the core has begun to buckle. A nonlinear analysis is performed using modified Riks' method in Abaqus FEA to show that this failure mode is predictable. The modified Riks' analysis showed that nonlinear post-buckling analysis of a honeycomb coupon can predict ultimate core failure with good accuracy. This solution requires a very high quality mesh in order to continue to run after buckling has begun and requires imperfections based on linear buckling mode shapes and thickness tolerance on the honeycomb core to be applied. / Master of Science

Cellular core structures are the state-of-the-art technology for light weight structures in the aerospace industry. In an aerospace product, sandwich panels with cellular core represent the primary structural component as a given aerospace product may contain a large number of sandwich panels. This reveals the necessity of understanding the mechanical behavior of the cellular core and the impact of that behavior on the overall structural behavior of the sandwich panel, and hence the final aerospace product. As the final aerospace product must go through multiple qualification tests to achieve a final structure that is capable of withstanding all environments possible, analyzing the structure prior to testing is very important to avoid any possible failures and to ensure that the final design is indeed capable of withstanding the loads. To date, due to the lack of full understanding of the mechanical behavior of cellular cores and hence the sandwich panels, there still remains a significant lack of analytical capability to predict the proper behavior of the final product and failures may still occur even with significant effort spent on pre-test analyses. Analyzing cellular core to calculate the equivalent material properties of this type of structure is the only way to properly design the core for sandwich enhanced stiffness to weight ratio of the sandwich panels. A detailed literature review is first conducted to access the current state of development of this research area based on experiment and analysis. Then, one of the recently developed homogenization schemes is chosen to investigate the mechanical behavior of heavy, non-corrugated square cellular core with a potential application in marine structures. The mechanical behavior of the square cellular core is then calculated by applying the displacement approach to a representative unit cell finite element model. The mechanical behavior is then incorporated into sandwich panel finite element model and in an in-house code to test the predicted mechanical properties by comparing the center-of-panel displacement from all analyses to that of a highly detailed model. The research is then expanded to cover three cellular core shapes, hexagonal cores made of corrugated sheets, square cores made of corrugated sheets, and triangular cores. The expansion covers five different cell sizes and twenty one different core densities for each of the core shapes considering light cellular cores for space applications, for a total of 315 detailed studies. The accuracy of the calculated properties for all three core shapes is checked against highly detailed finite element models of sandwich panels. Formulas are then developed to calculate the mechanical properties of the three shapes of cellular cores studied for any core density and any of the five cell sizes. An error analysis is then performed to understand the quality of the predicted equivalent properties considering the panel size to cell size ratio as well as the facesheet thickness to core thickness ratio. The research finally expanded to understand the effect of buckling of the unit cell on the equivalent mechanical property of the cellular core. This part of the research is meant to address the impact of the local buckling that may occur due to impact of any type during the manufacturing, handling or assembly of the sandwich panels. The variation of the equivalent mechanical properties with the increase in transverse compression load, until the first folding of the unit cell is complete, is calculated for each of the three core shapes under investigation. / Ph. D.