Showing papers on "Sandwich panel published in 2002"

Cellular Metals Manufacturing

Abstract: Open cell, stochastic nickel foams are widely used for the electrodes and current collectors of metal – metal hydride batteries. Closed cell, periodic aluminum honeycomb is extensively used for the cores of light, stiff sandwich panel structures. Interest is now growing in other cell topologies and potential applications are expanding. For example cellular metals are being evaluated for impact energy absorption, for noise and vibration damping and for novel approaches to thermal management. Numerous methods for manufacturing cellular metals are being developed. As a basic understanding of the relationships between cell topology and the performance of cellular metals in each application area begins to emerge, interest is growing in processes that enable an optimized topology to be reproducibly created. For some applications, such as acoustic attenuation, stochastic metal foams are likely to be preferred over their periodically structured counterparts. Nonetheless, the average cell s ize, the cell size standard deviation, the relative density and the microstructure of the ligaments are all important to control. The invention of more stable processes and improved methods for on-line control of the cellular structure via in-situ sensing and more sophisticated control algorithms are likely to lead to significant improvements in foam topology. For load supporting applications, sandwich panels containing honeycomb cores are much superior to those utilizing stochastic foams, but they are more costly than stochastic foam core materials. Recently, processes have begun to emerge for making open cell periodic cell materials with triangular or pyramidal truss topologies. These have been shown to match the stiffness and strength of honeycomb in sandwich panels. New cellular metals manufacturing processes that use metal textiles and deformed sheet metal are being explored as potentially low cost manufacturing processes for these applications. These topologically optimized systems are opening up new multifunctional applications for cellular metals.

Failure Modes of Composite Sandwich Beams

Abstract: The overall performance of sandwich structures depends in general on the properties of the facesheets, the core, the adhesive bonding the core to the skins, as well as geometrical dimensions. Sandwich beams under general bending, shear and in-plane loading display various failure modes. Their initiation, propagation and interaction depend on the constituent material properties, geometry, and type of loading. Failure modes and their initiation can be predicted by conducting a thorough stress analysis and applying appropriate failure criteria in the critical regions of the beam. This analysis is difficult because of the nonlinear and inelastic behavior of the constituent materials and the complex interactions of failure modes. Possible failure modes include tensile or compressive failure of the facesheets, debonding at the core/facesheet interface, indentation failure under localized loading, core failure, wrinkling of the compression facesheet, and global buckling.

Fatigue of Composite Materials and Substructures for Wind Turbine Blades

Abstract: This report presents the major findings of the Montana State University Composite Materials Fatigue Program from 1997 to 2001, and is intended to be used in conjunction with the DOE/MSU Composite Materials Fatigue Database. Additions of greatest interest to the database in this time period include environmental and time under load effects for various resin systems; large tow carbon fiber laminates and glass/carbon hybrids; new reinforcement architectures varying from large strands to prepreg with well-dispersed fibers; spectrum loading and cumulative damage laws; giga-cycle testing of strands; tough resins for improved structural integrity; static and fatigue data for interply delamination; and design knockdown factors due to flaws and structural details as well as time under load and environmental conditions. The origins of a transition to increased tensile fatigue sensitivity with increasing fiber content are explored in detail for typical stranded reinforcing fabrics. The second focus of the report is on structural details which are prone to delamination failure, including ply terminations, skin-stiffener intersections, and sandwich panel terminations. Finite element based methodologies for predicting delamination initiation and growth in structural details are developed and validated, and simplified design recommendations are presented.

Predicting Progressive Delamination of Stiffened Fibre-Composite Panel and Repaired Sandwich Panel by Decohesion Models:

Abstract: An approach employing decohesive models with mixed damaged scale and using total fracture energy was developed to simulate the delamination process of a stiffened fibre-composite panel and a repaired composite sandwich panel. Two decohesive material models – a bilinear interfacial decohesive function and the other a cubic polynomial interfacial decohesive function – were developed by using total fracture energy Gc, and based on using interface elements. In comparison with traditional numerical methods in fracture mechanics, this approach automatically predicts the failure load, crack path and the residual stiffness in the fracture process. Applications in this article are delamination analysis of a stiffened fibre-composite panel under four-point bending conditions and a repaired composite sandwich panel under four-point bending test. Comparisons between modeling predictions and experimental observations show that these decohesive models perform well. This article compares the problem of numerical convergent failure between two decohesive material models.

Indentation failure in composite sandwich structures

Abstract: A combined analytical and experimental investigation of the indentation failure of a composite sandwich panel has been undertaken. Two cases have been studied: a sandwich panel with carbon/epoxy facing and a PVC foam layer supported on a rigid base and indented at the center with a cylindrical indentor; and a sandwich beam with symmetrical facing and core materials as in the sandwich panels. The load-deflection behavior of the loaded facing was monitored during the test. Strains were also measured near the load on both surfaces of the facing using embedded strain gages. A full-field analysis of the in-plane displacements in the foam was conducted using the moire method. The problem was modeled as an elastic beam resting on an elastic-plastic foundation. Initiation of indentation failure occurs when the foundation yields, while catastrophic failure takes place when the compression facing fractures. The experimental results are in good agreement with the results of the analytical modeling based on the Winkler foundation.

Structural Optimization of a Hat-Stiffened Panel Using Response Surfaces

Abstract: A design study for structural optimization of a hat-stiffened laminated composite panel concept for an upper cover panel of a typical passenger bay of a blended wing-body transport airplane configuration is described. The feasibility of a hat-stiffened composite panel concept for an upper cover panel is investigated and is compared on the basis of weight to a thick sandwich panel concept that has been selected by designers as the baseline concept for the upper cover panel. The upper cover panel is designed for two load cases: internal pressure only and combined internal pressure and spanwise compression due to wing bending. The structural optimization problem is formulated using the panel weight as the objective function, with constraints on stress and buckling. The spacing of the hat stiffeners, the thickness of the skin, and the thickness of the components of the hat stiffener are used as design variables. The initial geometry of the hat-stiffened panel design is determined using the PANDA2 program by restricting the design to have uniform cross section in the spanwise direction. Because of the pressure loading, a more efficient design has variable cross section. Such designs are obtained by combining the STAGS finite element program with the optimization program in the Microsoft EXCEL spreadsheet program using response surfaces. Buckling and stress response surfaces are constructed from multiple STAGS analyses and are used as constraints in the optimization. The optimization conducted with the response surfaces results in considerable weight savings compared to the uniform cross section design, albeit a more complex design. Initial optimization cycles identify a design space where simple approximate analyses, such as Euler-Bernoulli beam theory and Kirchhoff plate theory applied to laminated composites, can be used to predict the behavior of the structure.

Composite, Grid-Stiffened Panel Design for Post Buckling Using Hypersizer

Abstract: Due to weight and cost goals, a grid-stiffened panel concept is being used for redesign of a structural component on the Minotaur OSP space launch vehicle. By designing the structural panels to carry operational loads past the point of initial buckling (local postbuckling), the resulting grid stiffened panel concept is lighter and 30% less costly to manufacture than other design candidates such as the existing honeycomb sandwich panel concept flown today. During June 2001 in Seattle, Boeing performed a structural certification experiment of a composite, grid stiffened, cylindrical panel loaded in axial compression. Pretest predictions were made for linear elastic (bifurcation) buckling, and non-linear post buckling. The tools used for pretest analysis were HyperSizer, and the FEM based tools MSC/NASTRAN and STAGS. Local buckling of the facesheet triangular shaped skin pocket occurred at a load of around 230 (lb/in). The test panel was able to sustain considerable additional loading, with post buckling failure occurring at 1320 (lb/in). The HyperSizer post buckling pretest prediction was 1300 (lb/in), the STAGS pretest prediction was 1250 (lb/in), and the MSC/NASTRAN pretest prediction ranged from 1425 to 2000 (lb/in). HyperSizer’s implementation of local post buckling based on an effective width approach is presented.

Analysis, Design and Optimization of Non-Cylindrical Fuselage for Blended-Wing-Body (BWB) Vehicle

Abstract: Initial results of an investigation towards finding an efficient non-cylindrical fuselage configuration for a conceptual blended-wing-body flight vehicle were presented. A simplified 2-D beam column analysis and optimization was performed first. Then a set of detailed finite element models of deep sandwich panel and ribbed shell construction concepts were analyzed and optimized. Generally these concepts with flat surfaces were found to be structurally inefficient to withstand internal pressure and resultant compressive loads simultaneously. Alternatively, a set of multi-bubble fuselage configuration concepts were developed for balancing internal cabin pressure load efficiently, through membrane stress in inner-stiffened shell and inter-cabin walls. An outer-ribbed shell was designed to prevent buckling due to external resultant compressive loads. Initial results from finite element analysis appear to be promising. These concepts should be developed further to exploit their inherent structurally efficiency.

Sandwich panel structural joint

Abstract: A sandwich panel structure comprises a slotted panel and a tabbed panel The slotted panel includes a slotted panel core body and a first slotted panel facesheet The slotted panel core body defines a first slotted panel side with the first slotted panel facesheet disposed adjacent thereto and including a plurality of first slots The tabbed panel includes a tabbed panel core body and a first tabbed panel facesheet, the tabbed panel core body defining a first tabbed panel side with the first tabbed panel facesheet disposed thereto The first tabbed panel facesheet includes a plurality of first tabs disposed along a first joint edge with each of the first tabs respectively sized and configured to be received by a respective one of the first slots upon insertion of the first tabs into the first slots for joining the tabbed panel to the slotted panel at the first joint edge

Special behavior of unidirectional sandwich panels with transversely flexible core under statical loading

Abstract: Special features inherent in the response of ordinary (fully bonded) and delaminated sandwich panels with a transversely flexible (“soft”) core subjected to external in-plane and vertical statical loads are analyzed. The analytical formulation is based on a higher-order theory for sandwich panels with non-rigid bond layers between the face sheets and the core. The central finite difference scheme is used for discretizing the continuous formulation. The deflated iterative Arnoldi scheme for solution of a large-scale generalized eigenvalue problem is employed, as well as the quasi-Newton global framework for the natural parameter and the arc-length continuation procedures. The numerical higher-order analysis reveals that the ordinary sandwich panel behaves as a compound structure in which the local/localized, overall or interactive forms of the response can take place depending on the geometry, mechanical properties, and boundary conditions of the structure. The non-sinusoidal modes confined to the support zones of the panel may occur at critical loads much lower than those predicted on the basis of presumed sinusoidal modes. Soft-core sandwich panels possess a complex branching behavior with limit points and secondary bifurcations. The thin-film-delamination approach used in the field of the composite plates is unsuitable for the analysis of delaminated sandwich panels and consideration of the interaction between the face sheets and the core is required. The complex response of the soft-core sandwich panels can be predicted only with the aid of the enhanced higher-order theory.

Built-in damage detection system for sandwich structures under cryogenic temperatures

Abstract: A built-in diagnostic system is being developed to identify de-bond between the skins and the honeycomb core of a sandwich structure. The system will be totally automated which will greatly reduce the time needed to inspect sandwich structures. The project is divided into two parts: Design and manufacturing of the sensors to detect damage and development of software to interpret the sensor data. Due to the extreme temperatures, most sensors will not survive the cryogenic temperatures of the inner skin where the damage is located. An array of sensors integrated in the sandwich panel is used to detect the damage. These sensors are embedded on the warmer side of the structure, but are able to probe for damage on the colder side of the tank. A cost-effective method is being developed to install these sensors without modifying the traditional sandwich manufacturing technique. The software compares the sensor and the baseline data. Based on the change in signal, it outputs the location and size of the damage.

Equivalent Level of Safety Approach to Damage-Tolerant Aircraft Structural Design

Abstract: A theoretical approach to damage-tolerant structural design has been proposed based on a probabilistic characterization of relative structural safety. The equations necessary to quantify damage-tolerant structural safety are developed, and their use in the design of a generic composite sandwich panel are demonstrated. Structural safety is identified by the term level of safety, which is defined, for a single inspection event, as the compliment of the probability that a single flaw size larger than the critical flaw size for residual strength of the structure exists and that the flaw will not be detected. The equations derived from this definition incorporate a probabilistic treatment of damage sizes and inspection capabilities. Utilizing damage size data from existing composite aircraft components along with the level of safety, formulas, design charts for residual strength vs safety of a generic composite sandwich panel were constructed. An example design problem is presented that demonstrates the sensitivity of the facesheet thickness sizing parameters to the relative safety of the design. Bayesian statistical techniques are also incorporated to enable the subsequent use of service inspection data to reduce uncertainty in the damage size distributions and to update the structural level of safety value as service experience is acquired.

Modeling of Tapered Sandwich Panels Using a High-Order Sandwich Theory Formulation

Abstract: A newly developed high-order sandwich theory formulation is presented, which enables the analysis of sandwich beams or plates with variable core thickness. The faces are assumed to be of constant thickness and may be inclined arbitrary angles α 1 and α 2 , respectively, relative to the sandwich panel reference plane. The core thickness may change linearly over the length of the sandwich panel. The core is modeled as a specially orthotropic solid possessing stiffness in the out-of-plane direction only, thus including the transverse core flexibility in the modeling. The faces are modeled as laminated beams or plates including bending-stretching coupling and transverse shear effects. To validate the proposed high-order theory, the numerical results are compared with results obtained from finite element analysis, and a close match is observed. Furthermore, to demonstrate the features of the developed high-order sandwich theory formulation, numerical results obtained for two different types of tapered sandwich beams in three-point bending are presented. The characteristics of the elastic responses of the two sandwich panel configurations are compared with special emphasis on the complicated interaction between the faces through the core material. The analyses show that severe localized bending effects are displayed in the vicinity of load introduction and support points and in the vicinity of points/locations of abrupt geometric changes. These localized bending effects induce severe stress concentrations and may severely endanger the structural integrity of the sandwich panels under consideration.

Design of microstrip antennas with composite laminates considering their structural rigidity

Abstract: Two types of conformal load-bearing antenna structures (CLAS) were designed with microwave composite laminates and Nomex honeycomb cores to secure both the structural rigidity and a good electrical performance One was a 4 × 8 array for the synthetic-aperture radar (SAR) system and the other was a 5 × 2 array for the wireless local-area network (LAN) system The design was based on a wide bandwidth, high polarization purity, low losses, and high structural rigidity The design, fabrication, and structural/electrical performances of the antenna structures were studied Their flexural behavior was examined by three-point bending, impact, and buckling tests The electrical measurements were in a good agreement with simulation results The complex antenna structures obtained have good flexural characteristics The design of this antenna structure is extended to give a useful guide for sandwich panel manufacturers as well as antenna designers

Weight optimization of sandwich panel with acoustic constraints, experimental verification

Abstract: Weight optimization of a sandwich panel was performed. The constraints were both mechanical, in the form of stiffness and strength constraints, and acoustic, in the form of a required sound reducti ...

Finite Element Modeling of the Buckling Response of Sandwich Panels

Abstract: A comparative study of different modeling approaches for predicting sandwich panel buckling response is described. The study considers sandwich panels with anisotropic face sheets and a very thick core. Results from conventional analytical solutions for sandwich panel overall buckling and face-sheet-wrinkling type modes are compared with solutions obtained using different finite element modeling approaches. Finite element solutions are obtained using layered shell element models, with and without transverse shear flexibility, layered shell/solid element models, with shell elements for the face sheets and solid elements for the core, and sandwich models using a recently developed specialty sandwich element. Convergence characteristics of the shell/solid and sandwich element modeling approaches with respect to in-plane and through-the-thickness discretization, are demonstrated. Results of the study indicate that the specialty sandwich element provides an accurate and effective modeling approach for predicting both overall and localized sandwich panel buckling response. Furthermore, results indicate that anisotropy of the face sheets, along with the ratio of principle elastic moduli, affect the buckling response and these effects may not be represented accurately by analytical solutions. Modeling recommendations are also provided.

Damage resistance characterization of sandwich composites using response surfaces.

Abstract: : The influence of material configuration and impact parameters on the damage tolerance characteristics of sandwich composites comprised of carbon-epoxy woven fabric facesheets and Nomex honeycomb cores was investigated using empirically based response surfaces A series of carefully selected tests were used to isolate the coupled influence of various combinations of the number of facesheet plies, core density, core thickness, impact energy, impactor diameter, and impact velocity on the residual strength degradation due to vertical impact The ranges of selected material parameters were typical of those found in common aircraft applications Quadratic response surface estimates of the compressive residual strength as a continuous function of sandwich configuration and impact parameters did not correlate as well as impact resistance because of bifurcations between failure modes For a fixed set of impact parameters, regression results suggest that impact damage development and residual strength degradation is highly sandwich configuration dependent Increasing the number of facesheet plies and the thickness of the core material resulted in the greatest improvement in the damage tolerance characteristics An increase in the impactor diameter and impact energy results in a significant decrease in the estimated residual strength, particularly for those sandwich panels with thicker facesheets The effects of impact velocity on damage formation and loss of strength were also addressed The developed response surfaces for the compressive residual strength may have limited use, particularly as the measured data at low- energy impact levels, small impactor diameter, and thin facesheets exhibit significant scatter Employing the methodology outlined here, it may be possible to tailor sandwich composite designs in order to obtain enhanced damage tolerance characteristics over a range of expected impacts Such efforts may facilitate sandwich panel design by establishing

The Effect of a Slow Impact on Sandwich Plates

Abstract: The purpose of this paper is to study the relation between the impact force and the corresponding elastic indentation produced by a rigid sphere falling on a sandwich plate. The analysis of the problem is articulated in three steps. Firstly, an explicit elastic solution for a sandwich panel, statically loaded by a distribution of surface pressures, reproducing Hertz’s contact law of a rigid sphere on an elastic half-space, is derived. Then, a certain number of quasi-static impact tests made on sandwich specimens constituted by two layers of graphite epoxy and a core of cellular foam are collected and discussed. Finally, the results are compared with those obtained from a finite element analysis having considered the plate as a three-dimensional elastic body. The conclusion is that the Hertzian contact pressure distribution is appropriate to describe the response of sandwich panels with a high-density core, but the assumption is not appropriate when the core has low-density.

Development of a sandwich material with polypropylene/natural fibre skins and paper honeycomb core

Abstract: Honeycomb sandwich materials are well-known in many aerospace applications. However, honeycombs are also used as door fillings, as packaging protection elements and in the automotive industry. Those honeycombs are made from unimpregnated low cost papers. The high production costs and the low production capacities limit the use of those paper honeycombs in semistructural applications like the automotive interior. More cost efficient paper honeycombs could replace foam cores (polyurethane foams) in many automotive applications. Recently a new cost efficient paper honeycomb material and its continuous production process have been developed, which enables an automated in-line production of paper based honeycombs. The combination of this low cost paper honeycomb core and a polypropylene/natural fibre material for the skins is the subject of the present study. This material combination was investigated because the resulting sandwich panel is not only light weight and cost efficient but also renewable resource based and fully recyclable.

Thermoplastic sandwich panel and twin sheet moulding method of making same

Abstract: A method for molding panels (44) in a sheet molding system comprised of a thermoplastic sheet delivery station (10) and two mold halves (18), the method comprising the steps of delivering two sheeting layers (12, 14) of thermoplastic material between the two mold halves; inserting a regidizing insert (40) between the two sheeting layers of thermoplastic material; closing the mold halves to bring the two sheeting layers of thermoplastic material in contact with the insert; and compressing the thermoplastic sandwich between the mold halves to form a panel. A thermoplastic-insert sandwich and a laminate panel comprised of thermoplastic layers abutting a pre-manufactured insert are also claimed.

Shape-memory-based multifunctional structural actuator panels

Abstract: A multifunctional panel concept is presented in which a lightweight, structural sandwich panel is able to undergo a reversible change in shape upon application of a localized thermal stimulus. The shape change is effected by metallic shape memory face sheet elements, which employ a 'one-way' SM effect only. Unlike other designs, no external or bias forces are required to completed the full cycle of shape change. This is accomplished by a core design which, when one of the two face sheets is activated and thus undergoes a length change, forces the opposite face sheet to martensitically deform in tension. By alternately heating one face sheet and then the other, to transform the martensitic structure to austenite, the sandwich beam or panel is able to perform fully reversible cyclic shape changes. Heating can be accomplished electrically or by other means. The performance of the sandwich panel, in terms of required thermal power, actuation frequency, peak load bearing capacity, stiffness, and weight, can be optimized by proper selection of face sheet material and this thickness, the overall core thickness, core member thickness and length, and the design of the joint connecting cor membranes and face sheet.© (2002) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Heat Transfer in Compression Molding of Thermoplastic Composite Laminates and Sandwich Panels

Abstract: The key to finding relevant process parameters in manufacturing of thermoplastic sandwich panels is an accurate prediction of the temperature at the interface between core and face during manufacturing, as this temperature is critical for the bond between the constituents. In this paper a model for prediction of the temperature distribution within a sandwich panel during manufacturing is presented. The process of face manufacturing by compression molding also is modeled, both for a one-dimensional case using finite difference methods to investigate effects of varying thermal contact conductances, and for a three-dimensional (3D) case using finite element methods to study temperature edge effects. The models are verified by experiments, in which a preconsolidated glass fiber/polyamide 12 laminate is used, partly as a ply in a thicker laminate, partly as face sheet for a sandwich panel. The core material used is polymethacrylimide foam.

Shear Strength of Sandwich Panel Systems

Abstract: The use of sandwich panels as roof and wall claddings has increased considerably in recent times. For the designers to take advantage of the diaphragm action of stronger sandwich panel systems under in-plane shear forces due to wind loading appropriate data on shear strength and stiffness of these systems is required. An experimental investigation was therefore conducted to investigate the shear behaviour of commonly used crest-fixed sandwich panel systems. An improved fastening system was developed which resulted in approximately 2.5 times greater shear strength, and improved ductility. Shear strength and stiffness data were developed for sandwich panel systems with the improved fastening system for varying aspect ratio. Analytical formulae were also developed to predict the shear strength of sandwich panel systems using the basic tearing loads obtained from simple connection tests. This paper presents the details of both experimental and analytical studies on the shear behaviour of sandwich panel systems and the results.

Method for producing sandwich panel and body component

Abstract: The invention relates to methods for producing a light component (a sandwich panel or a component for a vehicle body) that is easy to construct. A plurality of cup-like cavities (18) are embodied in a flexible metal foil (16). An outer layer (12, 24) is applied to each side of the flexible metal foil (16). The opening of each cup-like cavity (18) is first covered by an outer layer (12) and the remaining outer layer (24) is then applied to the free ends of the cavities (18). An already formed sheet metal (24) of the body can be connected to the tip surfaces (26) of the cup-like cavities (18) by means of an adhesive (26) for producing a car body component (22), whereby said sheet metal is used as the remaining outer layer.

Semi sandwich panel

Abstract: The present invention relates to a semi sandwich panel (10) in which a core material (14) is partially formed in a portion which needs a strength and intensity between two surface finishing materials (12) formed of a textile for thereby implementing a certain strength and intensity of a sandwich panel, so that it is possible to obtain strength and intensity as a structure member which is a feature of a sandwich panel, and the number of fabrication processes is decreased, and the fabrication cost is decreased.

Method for manufacturing moisture absorbing and acoustic sandwich panel

Abstract: A method of manufacturing a sandwich panel structure having a moisture absorption, sound absorption, and heat insulation structure having a honeycomb structure is provided. [Means for Solving] Moisture absorption, sound absorption, heat insulation, using a honeycomb material made of paper, metal, ceramics, plastic material as the core layer material, using open-cell phenol foam as cells, and using the honeycomb core layer material as a sandwich panel -Non-flammable panel structure. [Selection diagram] FIG.