Dissertations / Theses on the topic 'Composite Honeycomb Sandwich Panels'

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

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

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

The master thesis describes application of a sandwich construction for Formula Student car. It will replace the current tubular space frame according to FSAE rules. The introduction is focused on the information of international Formula Student competition, including TU Brno Racing Team. Then there are important rules for the construction of frames and composite monocoques. For the selected production technology are found strength properties of face sheets made of carbon and hybrid fibres. Another part of thesis deals with the design of the sandwich panel using analytical calculation to determine the bending stiffness. On the basis of these proposals are made three-point bending and shear tests. Using of FEM simulation is detected torsional stiffness of the tubular space frame and the monocoque concept from the proposed sandwich panels. The conclusion is devoted to a summary of the important information and possible monocoque manufacturing process.

Numerous studies have been performed in search of structures providing better blast protection and to understand the various effects influencing the structural performance. This thesis reports on an investigation into the behaviour of circular sandwich panels with aluminium honeycomb cores subjected to air blast loading. It focuses on the effect of varying core thickness, plate thickness and panel configuration. In this study, aluminium honeycomb core is sandwiched between mild steel face plates. Quasi-static tensile and compression experiments are performed to characterise the face plates and the honeycombs. Four sandwich panel configurations are proposed and subjected to blast loading. The impulse is generated by detonating plastic explosives at a constant stand-off distance and measured using a ballistic pendulum. The impulse is varied by using different charge masses. It is observed that the panels experience front plate deflection and tearing; honeycomb core crushing and densification; and back plate deflection and tearing. The deformations of the face plates and the cores increased with increasing impulse. Increasing the core thickness delayed the onset of core densification and decreased back plate deflection; and increasing the plate thickness also decreased back plate deflection. The use of an extra sandwich layer helps to provide better structural support but has the penalty of extra mass.

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

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics, 1991.
Aero hard copy bound in 1 v.
Includes bibliographical references.
by James Edward Williamson.
M.S.

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 sandwich panels offer an extremely lightweight solution for aerospace structures. As efficiency demands increase, low-cost non-autoclave manufacturing solutions are sought for honeycomb and other composite structures. Vacuum-bag-only (VBO) manufacturing is one possible solution that relies on vacuum to remove all entrapped volatiles prior to cure, and then the differential pressure between the inside and outside of the vacuum bag consolidates the layers during cure. This technique can be very effective for monolithic laminates made with out-of-autoclave (OOA) prepregs, but honeycomb structures introduce two additional manufacturing nuisances. First, the core entraps up to 98 % of its volume during lay-up, and second, non-metallic cores readily absorb ambient moisture. Entrapped air and moisture can increase the honeycomb core pressure during processing, reducing part quality. Given that the honeycomb core pressure is crucial to achieving success in VBO manufacturing of honeycomb panels, a threefold approach was used in this thesis to study the transport phenomena that influence this behaviour. First, the transport phenomena of the constituent materials were characterized. Applying an impermeable boundary condition to the tool-side skin allowed for simple air permeability characterization of honeycomb skins by considering only the bag-side skin. An instrumented test fixture was used to measure the honeycomb core pressure during the pre-processing vacuum hold. The results revealed that a transverse interconnected pore space was required in OOA prepreg skins for gas evacuation to proceed in honeycomb panels. The same test fixture was used to characterize the honeycomb skin air permeability and honeycomb core moisture diffusivity during elevated temperature processing. The evolving skin air permeability and core diffusivity were observed to cause the honeycomb core pressure to increase during the temperature ramp and decrease during the temperature hold. Second, a process model was developed to predict honeycomb core pressure throughout the manufacturing process. The process model identified that the honeycomb core pressure can exceed the vacuum bag consolidation pressure due to the high core moisture adsorption and elevated temperature diffusivity. Choosing, or creating, a honeycomb skin with high air permeability was identified as a key process parameter to avoid exceeding the consolidation pressure. Finally, the material characterization and process modelling were successfully scaled to reproduce the honeycomb core pressure behaviour in holistic honeycomb panels. The in-situ honeycomb core pressure was measured throughout the manufacturing process in dual-skin honeycomb panels using embedded pressure sensors. The embedded pressure sensor response validated the material characterization assumptions and model simplifications used to predict the honeycomb core pressure during the VBO manufacturing process. Manufacturing honeycomb panels is a complex activity with many material and processing variables. A suitable skin material and bagging configuration was selected for VBO manufacturing of honeycomb panels by coupling transport phenomena modelling and tailored material characterization. This approach could be used to reduce manufacturing trial and error before scaling these materials to larger applications.
Les panneaux sandwich en nid d'abeille offrent une solution extrêmement légère pour les structures aérospatiales. Avec l'augmentation de la demande pour les structures en matériaux composites, les solutions de fabrication de ces structures hors de l'autoclave sont recherchées afin de réduire les coûts. La méthode de fabrication avec sac sous vide requiert une pompe à vide pour enlever tous les gaz piégés après le drapage des matériaux préimprégnés et créer le différentiel de pression entre l'intérieur et l'extérieur du sac à vide afin de consolider les couches de composite. Cette technique peut être très efficace pour les laminés monolithiques, mais les structures en nid d'abeille présentent deux difficultés supplémentaires lorsque des nids d'abeilles non métalliques sont utilisés. D'abord, le nid d'abeille contient 98% du volume d'air piégé pendant le drapage, et deuxièmement, les nids d'abeilles non métalliques absorbent l'humidité pendant leur manipulation. L'air emprisonné dans le nid d'abeilles et l'humidité va augmenter la pression pendant la mise en forme, et peuvent créer des défauts. Cette thèse est divisée en trois thèmes pour étudier et pour optimiser le processus de fabrication des panneaux de composite sandwich avec nid d'abeilles. Tout d'abord, une condition imperméable a été appliquée sur le côté de l'outil, ce qui permet une caractérisation simple des matériaux utilisés pour la mise en forme combinés avec les matériaux préimprégnés de côté de sac à vide. La perméabilité à l'air pour les matériaux préimprégnés a été mesurée durant l'évacuation de l'air avant la cuisson, révélant un degré significatif de l'anisotropie de perméabilité à l'air. Pendant la cuisson à température élevée, la perméabilité à l'air a évolué avec le cycle de cuisson. En outre, le coefficient de diffusion de l'humidité du nid d'abeille non métallique a été caractérisé par une fonction de la concentration d'humidité et de la température. Deuxièmement, un modèle a été développé pour prédire la pression dans le nid d'abeille pendant le processus de fabrication. Des cartes de processus ont été créées afin d'identifier les combinaisons de conditions de traitement pouvant augmenter la pression dans le nid d'abeille au-dessus de la pression de consolidation. Finalement, des panneaux ont été fabriqués avec un laminé sur le côté de l'outil ainsi que sur le côté du sac à vide. Des capteurs de pression ont été incorporés pour mesurer la pression dans le nid d'abeilles pendant le processus de fabrication. La caractérisation des matériaux et la modélisation des processus développées à partir d'expériences simples à petite échelle ont permis de reproduire avec succès le comportement complexe de la pression dans le nid d'abeilles des pièces de grandes dimensions.

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

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.

Sandwich Panels with cellular cores are widely used in the aerospace industry for their higher stiffness to mass, strength to mass ratio, and excellent energy absorption capability. Even though, sandwich panels are considered state of the art for lightweight aerospace structures, the requirement to further reduce the mass exists due to the direct impact of mass on mission costs. Traditional manufacturing techniques have limited the shape of the cores to be either hexagonal or rectangular, but, with rapid advancements in additive manufacturing, other core shapes can now be explored. This research aims to identify and evaluate the mechanical performance of two-dimensional cores having standard wall geometry, which provide higher specific stiffness than honeycomb cores. Triangular cores were identified to have higher specific in-plane moduli and equivalent specific out-of-plane and transverse shear moduli. To consider practical use of the triangular cores, elastic and elastic-plastic structural analysis was performed to evaluate the stiffness, strength, failure, and energy absorption characteristics of both the core and sandwich panels. The comparison made between triangular cores and hexagonal cores having the same cell size and relative density showed that triangular cores outperform hexagonal cores in elastic range and for applications where in-plane loading is dominant. Triangular cores also have excellent in-plane energy absorption capabilities at higher densities.
Master of Science

A better understanding of the effect of impact damage on composite structures is necessary to give the engineer an ability to design safe, efficient structures. Current composite structures suffer severe strength reduction under compressive loading conditions, due to even light damage, such as from low velocity impact. A review is undertaken to access the current state-of-development in the areas of experimental testing, and analysis methods. A set of experiments on Nomex honeycomb core sandwich panels, with thin woven fiberglass cloth facesheets, is described, which includes detailed instrumentation and unique observation techniques. These techniques include high speed video photography of compression after impact (CAI) failure, as well as, digital image correlation (DIC) for full-field deformation measurements. The effect of nominal core density on the observed failure mode is described. A finite element model (FEM) is developed to simulate the experiments performed in the current study. The purpose of this simulation is to predict the experimental test results, and to conrm the experimental test conclusions. A newly-developed, commercial implementation of the Multicontinuum Failure Theory (MCT) for progressive failure analysis (PFA) in composite laminates, Helius:MCT, is included in this model. The inclusion of PFA in the present model gives it the new, unique ability to account for multiple failure modes. In addition, significant impact damage detail is included in the model as a result of a large amount of easily available experimental test data. A sensitivity study is used to assess the effect of each damage detail on overall analysis results. Mesh convergence of the new FEM is also discussed. Analysis results are compared to the experimental results for each of the 32 CAI sandwich panel specimens tested to failure. The failure of each specimen is accurately predicted in a high-fidelity, physics-based simulation and the results highlight key improvements in the understanding of honeycomb core sandwich panel CAI failure. Finally, a parametric study highlights the strength benefits compared to mass penalty for various core densities.
Ph. D.

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

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.

Tableau d'honneur de la Faculté des études supérieures et postdorales, 2015-2016
Le but de ce travail est de produire et de caractériser des structures sandwich à trois couches asymétriques avec ou sans cœur moussé. Pour ce faire, le travail est divisé en deux sections. Dans la première partie, l'effet de la variation des quantités d'agent de couplage et de fibres sont étudiés. La microscopie et la caractérisation mécanique sont utilisées pour évaluer l'effet du polyéthylène greffé d’anhydride maléique (MAPE) sur l'amélioration de la compatibilité entre les fibres de chanvre et le polyéthylène de haute densité (HDPE). Les résultats montrent que les propriétés mécaniques optimales (tension, flexion, torsion et impact) sont obtenues à 9% en poids de MAPE. Dans la deuxième partie, des structures sandwich asymétriques à trois couches, avec ou sans cœur moussé, sont produites par extrusion suivi par un moulage en compression. Les effets de paramètres tels que la densité du cœur, la concentration en chanvre dans les peaux, les épaisseurs des couches et la séquence d'empilage sur leurs comportements en flexion et en impact sont étudiés. Les effets combinés de tous les paramètres mènent à contrôler les propriétés mécaniques (traction, torsion, flexion et impact) des structures sandwich asymétriques.
The aim of this work is to produce and characterize asymmetric three-layer sandwich structures with and without foam core. In order to do so, the work is divided in two sections. In the first part, the effect of coupling agent and fiber content is investigated. Micrographs and mechanical characterizations are used to show that the addition of maleic anhydride polyethylene (MAPE) improved the compatibility between hemp and high density polyethylene (HDPE). It is found that the optimum mechanical properties (tension, flexion, torsion and impact) are obtained with 9% wt. of MAPE in the composite. In the second part, asymmetric three-layer sandwich structures with and without foam core were produced using extrusion followed by compression molding. The effect of different parameters such as core density, skin hemp content, layer thickness, and stacking sequence on their flexural and impact behaviors are studied. The combined effect of all the parameters was found to control the mechanical properties (tension, torsion, flexion and impact) of asymmetric sandwich structures.

Precast concrete sandwich panels are a structural system consisting of concrete layers with insulation layers in between. The concrete layers are connected through the insulation with specially designed connectors. For engineers to properly design and analyze the strength characteristics of sandwich panels and their connectors, the engineers need to obtain recommendations from the individual connector manufacturers, which can be a very rigorous process. This project tested eight full scale precast concrete sandwich panels with two concrete layers on either side of an insulation layer with connectors concentrated at either end of each panel. The objectives of this project were to evaluate the interaction between the two concrete layers and how well the connectors transferred forces between the layers (percent of composite action) and to validate simplified methods of predicting properties of the panels by comparing the predicted panel properties to the results of the testing series. Additionally, this study evaluated the panel’s different thicknesses and lengths and compared their results.

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

Present day composite sandwich panels provide incredible strength. Their largest problem, however, is early bonding failure between the core and the skin. This is due to the low bonding surface area of present cores like honeycomb. Corrugated structures could provide a remedy for this with their much larger bonding surface area. Corrugated structures have extreme mechanical properties deeming them particularly useful in aerospace and automotive applications. However, previous research has shown that the stiffness of carbon fiber causes debonding and drastic failure when used as both a core and a skin. Bio-composites have properties that could strengthen the corrugated sandwich panel against such debonding and increase the strength of the structure while making it cheaper and more environmentally friendly. This thesis presents the optimum design, manufacturing, and testing of corrugated sandwich panel structures with integrated bio-composites under edgewise compression loading. To do this, optimum corrugation geometry was identified using theoretical analysis of the moment and bonding area of the shape. Control tests with carbon fiber and hemp were conducted. The bio-composite was integrated in both the core and the skin individually in corrugated sandwich panels. The cases tested were all-carbon fiber, hemp skin with carbon fiber core, carbon fiber skin with hemp core, and all-hemp. These corrugated structures were analyzed by conducting compression loading tests on varying lengths of single-ligament panels utilizing trapezoidal corrugation as the core and a flat plate as the skin. The lengths tested were 1, 2, 3, and 4 inches. As many samples as possible were manufactured out of limited material with heavier focus on creating the shorter samples. The goal of this testing was, first, to determine if hemp fibers were viable as a substitute for certain sections of the traditional composite structure, and second, to see if integrating hemp fibers would solve the problems of debonding seen in the all-carbon fiber samples seen in previous research. To determine mechanical property viability, the ultimate load and stiffness were investigated for each sample, as well as investigation of the failure modes seen in the test. Secondary goals were to see at what length buckling behavior became an issue and to see if this corrugated structure and all its failure modes could be simulated in finite element analysis. At the 1-inch and 2-inch lengths where minimal buckling was encountered, the hemp core-carbon skin samples showed better results than both the all-carbon fiber and the all-hemp samples with a 4% and 6% increase in average ultimate load and a 11% and 47% increase in stiffness, respectively. From these results, it was concluded that hybrid bio-composite structures can have comparable mechanical properties to traditional composites and can solve bonding failure.

Thesis (Ph. D.)--University of California, San Diego, 2006.
Title from first page of PDF file (viewed December 1, 2006). Available via ProQuest Digital Dissertations. Vita. Includes bibliographical references (p. 155-161).

Blast risk associated with terrorist threats and accidental explosions has become an international concern over the past decade and has provoked structural engineers to implement protective design measures. Recent advances in this area of research has seen tremendous improvements in mitigating this risk through the installation of retrofits, advanced structural design, or pre-emptive protective measures. Tilt-up and precast panel walls are constructed using a unique approach in which the walls are cast horizontally and lifted, or tilted, into their final vertical position. These unique structures are cost effective, energy efficient, and can be rapidly constructed. This approach is commonly applied to the construction of large industrial facilities and the construction of schools which are categorized as high importance structures in the National Building Code of Canada. These panels are inherently flexible and have a surplus of mass making them desirable for protective design applications, however their behaviour under blast induced loads is not well defined. This experimental research project investigates the behaviour of non-composite tilt-up sandwich (NCTS) panels and solid reinforced concrete (SRC) panels with realistic support conditions subjected to blast-induced shockwaves. Previous research shows that NCTS panels, identifiable by their large structural wythe, exhibit some degree of composite behaviour and require between 5% to 10% composite action for successful erection. Five scaled specimens were constructed following common procedures used in practice, equipped with identical data acquisition instruments, and tested at the University of Ottawa shock tube testing facility under similar blast pressure-impulse combinations. Test results for the NCTS and SRC panels are compared graphically in terms of displacement–time histories and sectional strain distributions. The data is evaluated to approximate the composite behaviour at mid-span of the NCTS panel. Analytical results generated, using “RC Blast,” single-degree-of-freedom analysis software developed at the University of Ottawa, were validated with empirical data and are presented graphically. Each specimen was equipped with connections similar to those commonly used in the construction of NCTS panels. These connections were experimentally studied under simulated blast pressures and analysed using CSA A23.3-04 guidelines for punching shear capacity. Modified support iii | P a g e reinforcement layouts and surface bonded FRP laminates were evaluated as strengthening and retrofit techniques to prevent support failure. Dynamic support reactions and predicted support resistances are tabulated for each shot of every panel. The results indicate that it is possible to accurately predict the flexural behaviour and support resistance of a NCTS panel using RC Blast and CSA A23.3-04 guidelines. Several factors considered in this analysis include boundary conditions, dynamic material properties, and shear tie degradation. This analysis of flexural behaviour is highly dependent on shear stiffness, which is directly related to the composite action within NCTS panels. Support resistance was increased significantly through application of the strengthening techniques outlined in this thesis.

Thesis (M.S.)--West Virginia University, 2006.
Title from document title page. Document formatted into pages; contains ix, 125 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 124-125).

Doctor of Philosophy
Department of Mechanical and Nuclear Engineering
Liang-Wu Cai
X. J. Xin
Thin walled structures such as plates and shells have application in many fields of engineering because these structures are light weight and can support large loads when designed suitably. In real world, loads may cause these structures to vibrate which can be undesirable causing fatigue and failure of the structure. Such undesirable vibrations need to be reduced or eliminated. In this work, analytical studies of flexural wave propagation for idealized geometries are conducted and finite element method (FEM) is used to explore the effects of composite panel designs of finite size for the reduction of vibration transmission. In the analytical studies, the influence of the material properties on the reflection and transmission characteristics are explored for an infinite bi-material plate, and infinite plate with a strip inhomogeneity. In the analytical study of an infinite thin plate with a solid circular inclusion, the far and near field scattering characteristics are explored for different frequencies and material properties. All the analytical studies presented here and reported in the literature consider infinite plates to characterize the flexural wave propagation. Obtaining closed form solutions to characterize the flexural wave propagation in a finite plate with inclusions is mathematically difficult process. So, FEM is used to explore the composite panel designs. The understanding gained about the material properties influence on the flexural wave propagation from analytical studies helped with the choice of materials for FEM simulations. The concept of phononic crystals is applied to define the design variations that are effective in suppressing vibration transmission. Various design configurations are explored to study the effects of various parameters like scatterer’s material properties, geometry and spatial pattern. Based on the knowledge gained through a systematic parametric study, a final design of the composite sandwich panel is proposed with an optimum set of parameters to achieve the best vibration reduction. This is the first study focused on reducing vibration and wave transmission in composite rotorcraft fuselage panels incorporating the concept of phononic crystals. The optimum sandwich panel design achieved 98% vibration transmission reduction at the frequency of interest of 3000 Hz.

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

Thesis (M.S. in civil engineering)--Washington State University, December 2009.
Title from PDF title page (viewed on Jan. 26, 2010). "Department of Civil and Environmental Engineering." Includes bibliographical references.

Master of Science
Department of Architectural Engineering and Construction Science
Kimberly Waggle Kramer
Insulated precast concrete sandwich panels are commonly used for exterior cladding on a building. In recent years, insulated tilt-up concrete sandwich panels are being used for the exterior load-bearing walls on a building. The insulation is sandwiched between exterior and interior concrete layers to reduce the heating and cooling costs for the structure. The panels can be designed as composite, partially composite, or non-composite. The shear ties are used to achieve these varying degrees of composite action between the concrete layers. A parametric study analyzing the standard, solid single-wythe tilt-up concrete wall panel and solid sandwich (double-wythe separated by rigid insulation) tilt-up concrete wall panels subjected to eccentric axial loads and out-of-plane seismic loads is presented. The sandwich tilt-up panel is divided into two categories – non-composite and composite wall panels. The height and width of the different types of tilt-up wall panel is 23 feet (21 feet plus 2-foot parapet) and 16 feet, respectively. The solid standard panel (non-sandwich) is 5.5 inches in thickness; the non-composite sandwich panel is composed of 3.5-inch architectural wythe, 2.5-inch rigid insulation, and 5.5-inch interior load bearing concrete wythe; and the composite sandwich panel is composed of 3.5-inch exterior, load bearing concrete wythe, 2.5-inch insulation, and 5.5-inch interior, load bearing concrete wythe. The procedure used to design the tilt-up wall panels is the Alternative Method for Out-of-Plane Slender Wall Analysis per Section 11.8 of ACI 318-14 Building Code Requirements for Structural Concrete and Commentary. The results indicated that for the given panels, the applied ultimate moment and design moment strength is the greatest for the composite sandwich tilt-up concrete panel. The standard tilt-up concrete panel exhibits the greatest service load deflection. The non-composite sandwich tilt-up concrete panel induced the greatest vertical stress. Additionally, the additional requirements regarding forming materials, casting, and crane capacity is covered in this report. Lastly, the energy efficiency due to the heat loss and heat gain of sandwich panels is briefly discussed in this report. The sandwich tilt-up panels exhibit greater energy efficiency than standard tilt-up panels with or without insulation.

La modélisation du comportement vibroacoustique en flexion des structures sandwich est devenue aujourd’hui de plus en plus d’un grand intérêt dans les différents secteurs industriels. Cette tendance est principalement due aux propriétés mécaniques avantageuses des structures sandwich. L’un des principaux avantages de ce type de structures réside principalement dans le rapport rigidité-poids élevé. En revanche, acoustiquement la diminution de la masse du panneau avec une rigidité élevée conduit à un confort acoustique insatisfait. Pour cette raison, il y a une demande croissante pour des approches de modélisation du comportement vibroacoustique des structures sandwich avec une précision maximale. La présente thèse propose une approche méso-macro basée sur une méthode numérique pour la prédiction des caractéristiques dynamiques des structures sandwich. La méthode est principalement utilisée pour résoudre le problème de transparence acoustique considéré dans ce projet de thèse. Le travail présenté porte principalement sur la topologie du coeur du sandwich pour traiter le problème abordé. Le principal avantage du modèle proposé réside dans les effets du cœur prises en compte telle que l’effet du cisaillement et celle de l’orthotropie du panneau sandwich. L’approche de modélisation proposée est basée sur la méthode des éléments finis ondulatoire, qui combine la méthode des éléments finis classique et la théorie des structures périodiques. La structure sandwich a été modélisée comme un guide des ondes tridimensionnelles qui garde absolument les informations à l’échelle mésoscopique du panneau modélisé. La fréquence de transition définie la fréquence à laquelle le cisaillement du coeur devient important. Cette fréquence spéciale a été identifié via deux méthodes numériques. Une expression de transmission acoustique à travers un panneau sandwich a également été dérivée. Ensuite, une étude paramétrique a été menée dans le but de révéler l’effet des différents paramètres géométriques sur les indicateurs vibroacoustiques
Prediction of the flexural vibroacoustic behavior of honeycomb sandwich structures in the low-mid frequency is nowadays becoming of high interest in different industrial sectors. This trend is mainly owing to the advantageous mechanical properties of the sandwich structures. One of the main advantages of this kind of structures lies principally in the high stiffness-to-weight ratio. Even though, acoustically the decrease of the panel mass with a high stiffness leads to an unsuitable acoustic comfort. For this reason, there is an increasing demand for approaches modeling the vibroacoustic behavior of the sandwich structures with a maximum accuracy. The present thesis deals with a meso-macro approach based on a numerical method for modeling the vibroacoustic behavior of sandwich structures. The modeling description is mainly used to address the acoustic insulation problem considered in the thesis. The presented work focuses on the topology of the sandwich core to treat the addressed problem. The main advantage of the proposed model is that it takes into account the core shear and panel orthotropic effects. The modeling approach suggested here is based on the wave finite element method (WFE method), which combines the standard finite element method and the periodic structure theory. The sandwich structure has been modeled as a tridimensional waveguide which holds absolutely the meso-scale information of the modeled panel. The transition frequency, which indicates the frequency at which the core shear becomes important, was identified via two different numerical methods. An expression of the acoustic transmission for an equivalent isotropic sandwich panel was also derived. A parametric study was then conducted with a goal of revealing the effect of the geometric parameters of the sandwich core on the vibroacoustic indicators