Dissertations / Theses on the topic 'Honeycomb structures'

Thesis (M. S.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2007.
Committee Chair: Mulalo Doyoyo; Committee Co-Chair: Reginald Desroches; Committee Member: Laurence J. Jacobs.

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.

In this dissertation, polyethylene oxide (PEO) and ethyl cellulose (EC) have been chosen as model polymers to investigate different aspects of electrohydrodynamic processing and forming. In the first part of the work, electrospraying of PEO was attempted choosing a wide range of single solvents and mixed solvents. The selection of solvents affects the solubility and spinnability of PEO and the morphology of electrospun fibres. In the second part of the research the creation of 3D nanofibrous structures using electrospinning of PEO was investigated. The results demonstrate how the process is influenced by physical and processing parameters. It is reported that electrospun polymer nanofibres self-assemble into three dimensional honeycomb-like structures. The underlying mechanism was studied by varying the polymer solution concentration, collecting substrates and collection distance. The polymer solution concentration was found to have a significant effect on the size of the electrospun nanofibres. The nature of the collection substrate and the magnitude of the collection distance affect the electric field strength, the evaporation of solvent and the discharging of nanofibres. Consequently both the collection substrate and the collection distance had a significant influence on the self-assembly of nanofibres. In the third part of the work, the ways in which relative humidity (RH) plays a key role in the formation of porous structures was investigated using the hydrophilic polymer (PEO) and the hydrophobic polymer (EC). The generation of a 3D honeycomb-like structure was achieved using PEO polymer when RH was increased to between 53% and 93%. The optimum RH was found to be 73%. But efforts to generate 3D honeycomb-like structures using EC were unsuccessful throughout the range of RH investigated (53% - 93%). High speed camera imaging has been an important feature of the work carried out in this thesis.

In recent times, it was divulged that highly ordered honeycomb structured porous films from a variety of polymers could be fabricated by breath figures (water droplets) templating technique. In contrast to existing macroporous fabrication techniques, this technique is simple, more versatile and very cost effective. Amphiphilic block copolymers composed of a hydrophobic and a hydrophilic block were employed in this research to examine the process of porous film formation and the outcome of films generated using breath figure technique. A customized film casting system, established according to the casting parameters affecting the outcome of films was used to generate honeycomb structured porous films for the studies. The casting method best suited to generate highly ordered honeycomb structured porous films and the procedures to manipulate the size of the pores in films generated from amphiphilic block copolymers were also investigated and identified. Analyses into the formation process of the honeycomb structured porous films revealed that the airflow casting method where the cast of polymer solution was supplied with a flow of moist air was the most suitable method to generate highly ordered honeycomb structured porous films from amphiphilic block copolymers. Variations to the casting conditions of the airflow casting method such as the rate of moist airflow could only provide limited alterations to the size of pores on films generated. However, changes to the chemical system of the casting solution such as the concentration and the molecular weight of polymers in the polymer solvent was more prominent in manipulating the size of pores in the generated films. On the other hand, any extreme variations to either the physical conditions or the chemical system could devastate the hexagonal arrangement of pores in these films. In the synthesis of amphiphilic block copolymers in this research, RAFT polymerization technique was used to generate the hydrophobic polymer block followed by the subsequent chain extension polymerization of the hydrophilic polymer block. The polymerization 'process, especially the hydrophilic chain extension polymerization, was investigated in details. It was established that there were significant dependence on the composition of the initial polymer block used, particularly the molecular weight and the type of chain transfer (RAFT) end group in the hydrophobic polymer chain. Incompatible RAFT end group and high polymer molecular weights of the initial block usually lead to slower rate of subsequent chain extension coupled with increased terminations. These copolymers generated were usually bimodal in molecular weight distributions and broad in polydispersity indexes. Honeycomb structured porous films generated from one of these amphiphilic block copolymers were assessed as scaffoldings for cell culture to regenerate cells. In particular, the effects of cellular attachments and proliferations on the honeycomb porous structures were investigated. The assessment of these honeycomb structured porous films indicated that not only were these films not cytotoxic but they also enhanced the quantity of cellular proliferation (2.7x) when used as cell culture substrate compared to standard non-porous polystyrene cell culture surfaces. Finally, this research had shown a simple way to generate a new class of highly ordered porous material that could be customized individually for a wide range of applications. The synthesis of amphiphilic block copolymers to generate these films could be achieved by RAFT polymerization with a board selection of polymers choices according to applications. A porous cell substrate such as honeycomb structured porous films could enhance cellular growth when used as a cell culture substrate.

In this thesis, design and analyses of honeycomb structures are investigated. Primary goal is to develop an equivalent orthotropic material model that is a good substitute for the actual honeycomb core. By replacing the actual honeycomb structure with the orthotropic model, during the finite element analyses, substantial advantages can be obtained with regard to ease of modeling and model modification, solution time and hardware resources . To figure out the best equivalent model among the approximate analytical models that can be found in the literature, a comparison is made. First sandwich beams with four different honeycomb cores are modeled in detail and these are accepted as reference models. Then a set of equivalent models with the same dimensions is generated. The material properties of the equivalent models are taken from different studies performed in the literature. Both models are analyzed under the same loading and the boundary conditions. In finite element analyses, ANSYS finite element program is used. The results are compared to find out the best performing equivalent model. After three major analyses loops, decision on the equivalent model is made. The differences between the total reaction forces calculated by the equivalent model and the actual honeycomb model are all found to be within 10%. The equivalent model gives stress results at the macro-scale, and the local stresses and the strains can not be determined. Therefore it is deemed that for stress analysis, equivalent model can be used during the preliminary design phase. However, the equivalent model can be used reliably for deflection analysis, modal analysis, stiffness determination and aero-elastic analysis.

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Dissertacao (Mestrado)
IPEN/D
Escola Politecnica, Universidade de Sao Paulo - POLI/USP

Sandwich panels with aluminium face sheets and honeycomb core material have certain advantages over panels made of wood. Some of the advantages of these constructions are low weight, good moisture properties, fire resistance and high stiffness to-weight ratio etc. As product development is carried out in a fast pace today, there is a strong need for validated prediction tools to assist during early design stages. In this thesis, tools are developed for predicting the sound transmission through honeycomb panels, typical for inner floors in trains and later through double-walled structures typical for rail-vehicles, aircrafts and ships. The sandwich theory for wave propagation and standard orthotropic plate theory is used to predict the sound transmission loss of honeycomb panels. Honeycomb is an anisotropic material which when used as a core in a sandwich panel, results in a panel with anisotropic properties. In this thesis, honeycomb panels are treated as being orthotropic and the wavenumbers are calculated for the two principal directions. The wavenumbers are then used to calculate the sound transmission using standard orthotropic theory. These predictions are validated with results from sound transmission measurements. The influence of constrained layer damping treatments on the sound transmission loss of these panels is investigated. Results show that, after the damping treatment, the sound transmission loss of an acoustically bad panel and a normal pane lare very similar. Further, sound transmission through a double-leaf partition based on a honeycomb panel with periodic stiffeners is investigated. The structural response of the periodic structure due to a harmonic excitation is expressed in terms of a series of space harmonics and virtual work theory is applied to calculate the sound transmission. The original model is refined to include sound absorption in the cavity and to account for the orthotropic property of the honeycomb panels. Since the solution of the space harmonic analysis is obtained in a series form, a sufficient number of terms has to be included in the calculation to ensure small errors. Computational accuracy needs to be balanced with computational cost as calculation times increases with the number of terms. A new criterion is introduced which reduces the computational time by up to a factor ten for the panels studied. For all the double-leaf systems analysed, the sound transmission loss predictions from the periodic model with the space harmonic expansion method are shown to compare well with laboratory measurements.

QC 20120607

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.

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

Thesis (M.S.)--West Virginia University, 2001.
Title from document title page. Document formatted into pages; contains xi, 127 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 127).

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.
Includes bibliographical references.
Small dimension engineering tubular structures subjected to a complex load system are designed like hollow circular shells. For minimum weight design, the ratio between the shell radius and the thickness has to be as large as possible, but its maximum value is limited by the onset of local buckling. Tubular natural structures subjected to a complex load system have often an outer shell of solid material supported by a low density, compliant core, which makes them more resistant to local buckling. Biomimicking of natural constructions offer the potential to improve the design of small diameter tubular engineering structures. Here, the fabrication technology of biomimicked engineering tubular structures integrating aluminum foam or honeycomb as core material is discussed. A viability analysis is presented including technical performance, cost, utility, and risk assessments. Aluminum compliant core shells have potential for substituting CFRP and aluminum tubular structures in aerospace and high-level sport applications. The case of sailboat masts was considered in detail. Results of our analysis proved that use of honeycomb as core material can lead to a significant reduction of the mast weight. Business opportunities based on this application are discussed.
by Stefano Bartolucci.
M.Eng.

Honeycomb-cored sandwich structures are widely used in transport for their high strength-to-mass ratio. Their inherent high stiffness and lightweight properties make them prone to high vibration cycles which can incur deleterious damage to transport vehicles. This PhD thesis investigates the performance of a novel passive damping treatment for honeycomb-cored sandwich structures, namely the Double Shear Lap-Joint (DSLJ) damper. It consists of a passive damping construct which constrains a viscoelastic polymer in shear, thus dissipating vibrational energy. A finite element model of such DSLJ damper inserted in the void of a hexagonal honeycomb cell is proposed and compared against a simplified analytical model. The damping efficiency of the DSLJ damper in sandwich beams and plates is benchmarked against that of the Constrained Layer Damper (CLD), a commonly used passive damping treatment. The DSLJ damper is capable of achieving a higher damping for a smaller additional mass in the host structure compared to the optimised CLD solutions found in the literature. The location and orientation of DSLJ inserts in honeycomb sandwich plates are then optimised with the objective of damping the first two modes using a simple parametric approach. This method is simple and quick but is not robust enough to account for mode veering occurring during the optimisation process. A more complex and computationally demanding evolutionary algorithm is subsequently adopted to identify optimal configurations of DSLJ in honeycomb sandwich plates. Some alterations to the original algorithm are successfully implemented for this optimisation problem in an effort to increase the convergence rate of the optimisation process. The optimised designs identified are manufactured and the modal tests carried out show an acceptable correlation in the trends identified by the numerical simulations, both in terms of damping per added mass and natural frequencies.

Thesis (Ph.D.)--University of Cincinnati, 2007.
Advisor: Dr. Jandro L. Abot. Title from electronic thesis title page (viewed June 29, 2010). Keywords: Honeycomb Structures; Mechanical Properties; Cellular Structures. Includes abstract. Includes bibliographical references.

Honeycomb constructions are the most widely used materials in contemporary aviation and space technology. They are the basis for the housings of practically all products of this sector, where reliability of all parts should meet the in-creased requirements. Special attention is paid to the quality of composite materials and to the absence of defects such as the places of adhesion failure (exfoliation) between the skin and the honeycomb filler. Therefore, increase in the efficiency and reliability of thermal flaw detection, based on in-depth analysis of the processes of detecting defects and development of the principles of optimization of both the procedure of control and subsequent processing of the obtained information, is an important and relevant task.

Thesis (M.S.)--West Virginia University, 2004.
Title from document title page. Document formatted into pages; contains xi, 107 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 87-89).

In this thesis I present the synthesis and characterization of materials exhibiting frustrated magnetism. In Chapter 1 I describe magnetic frustration and some of the magnetic states that can arise from it followed by the background on iridates and platinates with honeycomb structures and rare earth pyrochlores.

In Chapter 3 I discuss my work on the synthesis and properties of ternary sodium iridates with formulas Na_xM_2/3Ir_1/3O ₂ and Na_xM_1/3Ir_2/3O₂ (M = Mn, Fe, Co, Ni, Cu, Zn). The ternary iridates are based on the honeycomb compound Na₂IrO₃ but show more disorder in the honeycomb layer than the parent. The six new compounds are all spin glasses but show distinct magnetic properties from one another.

In Chapter 4 I continue my work on honeycombs by exploring new ternary sodium platinates. These three new compounds with formulas Na₃MPt ₂O_6+x (M = Mg, Cu, Zn) are structurally very similar to the iridates discussed in Chapter 2 but have non-magnetic Pt⁴⁺ in place of magnetic Ir⁴⁺. The Mg and Zn variants are non-magnetic while the Cu variant is paramagnetic at 2 K.

Chapter 5 is a synchrotron X-ray diffraction study of the magnetically frustrated rare earth pyrochlores Ho₂Ti₂O₇, Er₂Ti₂O₇ and Yb₂Ti₂O ₇. Previous neutron scattering studies have shown reflections that are forbidden by the assigned space group Fd-3m, therefore high intensity, high resolution X-ray diffraction data was collected to determine if the reflections are present. Slight variations in sample stoichiometry were studied to account for possible sample variation. The forbidden reflections are absent from the X-ray diffraction patterns, providing strong evidence that the extra reflections in neutron scattering experiments are not structural in origin.

There is currently an interest in optimizing the structural design to improve materials' strength to weight ratio or improve stiffness for energy absorption. As such, cellular structures are continuously studied and improved. However, it is a well-known fact in the literature that one primary mechanism of failure of a honeycomb is the formation of shear bands. The impacts of these shear bands bring many questions and unknowns, especially when the cellular structures are created with the increasingly popular manufacturing technique of 3D printing. Therefore, understanding the deformations in 3D printed honeycomb structures is necessary to explain the behavior of materials generated through new additive manufacturing techniques and further the knowledge of the deformation localization and, consequently, formations of shear bands in the deformation process of cellular structures.In the first phase of this work, samples with a unit cell regular hexagonal honeycomb format were designed and manufactured using masked-stereolithography (M-SLA). After the curing process, the samples were prepared with a paint application in the format of speckle, and DIC was realized in a compression experiment to identify and analyze the presence of high strain regions indicating the presence of shear bands. A second phase was then conducted, aiming to consider the control and manipulation of the shear band through the utilization of relief holes. The results demonstrated that adding incisions in specific parts of the polymeric honeycomb makes it possible to change its strain spread through the shear band and change its toughness.

Thesis (M.S.)--University of Missouri-Columbia, 2007.
The entire dissertation/thesis text is included in the research.pdf file; the official abstract appears in the short.pdf file (which also appears in the research.pdf); a non-technical general description, or public abstract, appears in the public.pdf file. Title from title screen of research.pdf file (viewed on April 9, 2009) Includes bibliographical references.

Thesis (M.S.)--University of Cincinnati, 2008.
Advisor: Yijun Liu. Title from electronic thesis title page (viewed Feb.25, 2009). Includes abstract. Keywords: Honeycomb; FGM; BEM; FEM. Includes bibliographical references.

This paper applies to the design and simulation of a shipping container made of sandwich panels. The amount of stresses acting on the body of the container is calculated and is optimized to reduce stresses for the better design output of the structure. The design aims to produce an application to reduce the tare weight of the container in order to increase the payload. Finite Element Analysis (FEA) is performed to evaluate the strength of structures of both old and new models helps us to compare which model is better and more efficient. Complete design and analysis is performed using Autodesk Inventor.
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Honeycomb core sandwich panels are widely used in the aeronautic industry due to their excellent flexural stiffness to weight ratio. Generally, classical homogenized model is used to model honeycomb core sandwiches in order to have an efficient but not expensive numerical modeling. However, previous works have shown that, while the homogenized models could correctly represent the membrane waves' behavior of sandwiches in a large frequency range, they could not give satisfying simulation results for the flexural waves' behavior in the high frequency range (HF). In fact, the honeycomb core layer plays an important role in the propagation of the flexural waves, so that when the involved wavelengths become close to the characteristic lengths of honeycomb cells, the cellular microstructure starts interacting strongly with the waves and its effect should no longer be neglected, which is unfortunately not the case of the homogenized models. In the present work, we are interested in improving the theoretical and numerical analysis of HF elastic waves' propagation in honeycomb core sandwich panels by a numerical approach based on the Bloch wave theorem, which allows taking into account the periodic characteristics of the honeycomb core. In fact, by decomposing non-periodic wave solutions into their periodic Bloch wave basis modes, numerical models are defined on a basic cell and solved in a efficient way, and provide a better description and so a better understanding of the interaction between HF wave propagation phenomena and the periodic structures. Our numerical approach is developed and validated in the cases of one-dimensional periodic beam structures, of two-dimensional periodic hexagonal and rectangular beam structures and of honeycomb core sandwich plates. By solving the eigenvalue problem of the Bloch wave modes in one primitive cell of the periodic structure for all the wave vectors located in the corresponding first Brillouin zone in the phase space, the dispersion relation between the wave vector and the eigenvalue is calculated. The analysis of the dispersion relation provides important results such as: the frequency bandgaps and the anisotropic and dispersive characteristics of periodic structures, the comparison between the first Bloch wave modes to those of the classical equivalent homogenized models and the existence of the retro-propagating Bloch wave modes with a negative group velocity.

This thesis has investigated composite-honeycomb sandwich materials commonly used in Formula 1 nosecone structures. Experimental work has investigated their failure behaviour under static and dynamic crash loading, from which new constitutive failure laws for implementation in the explicit Finite Element code PAM-CRASHTM have been proposed. An investigation using an improved Arcan apparatus has been conducted to establish the mixed shear-compression properties of the honeycomb. An investigation has also been performed to establish relationships between in-plane deformation and out-ofplane compression properties. These relationships have been identified and successfully implemented into a honeycomb solid element material model available in PAMCRASHTM. A further investigation to represent honeycomb using geometrically accurate shell representation of the honeycomb has also been presented. This model was shown to reproduce trends observed during testing. The composite skin material has also been experimentally investigated and presented. This investigation made use of digital image correlation to examine the onset of intralaminar shear failure mechanisms, from which a non-linear damage progression law was identified. This law was successfully implemented into the Ladevéze damage model in PAM-CRASHTM for composite material modelling and has been shown to improve the representation of in-plane shear damage progression and failure. A series of experimental investigations to examine the energy absorbing properties of the sandwich have been conducted and presented. These investigations include three point bend flexural testing and edgewise impact loading. Failure mechanisms in the skin and core have been identified for each loading case. Experimental findings were used to assess the capability of PAM-CRASHTM for sandwich material modelling. This investigation has highlighted deficiencies in the material models when representing the sandwich, specifically with the existing composite skin and honeycomb models. Improvements introduced to the core and skin material models have shown some improvement when representing sandwich structures.

In the industry, all passenger vehicles are treated with damping materials to reduce structure-borne sound. Though these damping materials are effective to attenuate structure-borne sound, they have little or no effect on the air-borne sound transmission.The lack of effective predictive methods for assessing the acoustic effects due to added damping on complex industrial structures leads to excessive use of damping materials.Examples are found in the railway industry where sometimes the damping material applied per carriage is more than one ton. The objective of this thesis is to provide a better understanding of the application of these damping materials in particular when applied to lightweight sandwich panels.

As product development is carried out in a fast pace today, there is a strong need for validated prediction tools to assist in the design process. Sound transmission loss of sandwich plates with isotropic core materials can be accurately predicted by calculating the wave propagation in the structure. A modified wave propagation approach is used to predict the sound transmission loss of sandwich panels with honeycomb cores. The honeycomb panels are treated as being orthotropic and the wave numbers are calculated for the two principle directions. The orthotropic panel theory is used to predict the sound transmission loss of panels. Visco-elastic damping with a constraining layer is applied to these structures and the effect of these damping treatment on the sound transmission loss is studied. Measurements are performed to validate these predictions.

Sound radiated from vibrating structures is of great practical importance.The radiation loss factor represents damping associated with the radiation of sound as a result of the vibrating structure and can be a significant contribution for structures around the critical frequency and for composite structures that are very lightly damped. The influence of the radiation loss factor on the sound reduction index of such structures is also studied.

QC 20100519
ECO2-Multifunctional body Panels

Edgewise crushing responses of composite aluminum honeycomb sandwich structures were predicted using finite element analysis (FEA) software LS-DYNA by modeling the honeycomb as a material with anisotropic properties. The goal of the project was to develop a process for modeling the sandwich structure to rapidly iterate possible solutions for a safer workstation train table. Current workstation tables are too rigid and may cause injury or death in a head-on collision. Experimental compression tests were used to calibrate the aluminum honeycomb core with material type 26 (MAT 26, honeycomb). A published composite tensile test was used to validate the use of material type 22 (MAT 22, composite damage) for laminates. Finally, a model was made to recreate the results of a published compression test of an aluminum honeycomb sandwich structure with aluminum sheet metal face sheets to confirm contact types. With each component of the model verified separately, three plain weave composite aluminum honeycomb sandwich structures were modeled, one with [0/90] composite sheets completely bonded to the core, one with [0/90] composite sheets partially bonded to the core, and one with [±45] composite sheets partially bonded to the core. The failure modes for each sandwich structure were previously shown through research and the elastic region of the response was checked for accuracy using a simple beam theory. The analysis suggests that incorporating unbonded zones into the sandwich structure will change the failure mode from general buckling to face wrinkling, which effectively lowers the failure strength while not sacrificing energy absorption throughout loading. The analysis also indicates that using an angled ply orientation will lower the initial stiffness and the failure load. Future work is recommended such as performing compression tests with composite aluminum honeycomb sandwich structures and integrating delamination failure modes into the model using cohesive elements.

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

This study will present the Experimental, numerical and analytical characterizations of composite sandwich structures needed to optimize structure design. In this study, the effects of varying honeycomb core ribbon orientation and varying face sheet thickness’s have on the flexural behavior of honeycomb sandwich structures was investigated. Honeycomb sandwich panels were constructed using Hexcel 6367 A250-5H carbon fiber face sheets and Hexcel Nomex HRH-10-1/8-5 honeycomb cores. The mechanical properties of the constituent materials were discovered experimentally using ASTM standards and theoretical models using honeycomb mechanics and classical beam and plate theory are described. A failure mode map for loading under three point bending is developed from previous works by Triantafillou and Gibson26, showing the dependence of failure mode on face sheet to core thickness and honeycomb core ribbon orientation. Beam specimens are tested with the effects of Honeycomb core ribbon orientation and unequal face sheet thickness’s examined. Experimental data sufficiently agrees with theoretical predictions. A finite element model was developed in ABAQUS/CAE to validate experimental and analytical analysis and produced agreeable results. Optimal bending stiffness and strength with respect to minimum weight was analyzed. The results reveal an important role core ribbon orientation has in a sandwich beam’s bending behavior, and design of unequal ply count face sheets can produce higher stiffness to weight ratios than conventional symmetric sandwich structures of similar weight when subjected to a single static load.