Showing papers on "Sandwich-structured composite published in 1996"

Computational Models for Sandwich Panels and Shells

Abstract: The focus of this review is on the hierarchy of computational models for sandwich plates and shells, predictor-corrector procedures, and the sensitivity of the sandwich response to variations in the different geometric and material parameters. The literature reviewed is devoted to the following application areas: heat transfer problems; thermal and mechanical stresses (including boundary layer and edge stresses); free vibrations and damping; transient dynamic response; bifurcation buckling, local buckling, face-sheet wrinkling and core crimping; large deflection and postbuckling problems; effects of discontinuities (eg, cutouts and stiffeners), and geometric changes (eg, tapered thickness); damage and failure of sandwich structures; experimental studies; optimization and design studies. Over 800 relevant references are cited in this review, and another 559 references are included in a supplemental bibliography for completeness. Extensive numerical results are presented for thermally stressed sandwich panels with composite face sheets showing the effects of variation in their geometric and material parameters on the accuracy of the free vibration response, and the sensitivity coefficients predicted by eight different modeling approaches (based on two-dimensional theories). The standard of comparison is taken to be the analytic three-dimensional thermoelasticity solutions. Some future directions for research on the modeling of sandwich plates and shells are outlined.

Localized Load Effects in High-Order Bending of Sandwich Panels with Flexible Core

Abstract: Localized load effects using a high-order theory for the bending behavior of a sandwich panel with a “soft” core (i.e., flexible) in the vertical direction that is based on variational principles are presented. The theory embodies a rigorous approach for the small-deformation analysis of sandwich plates having high-order effects owing to the nonlinear patterns of the in-plane and vertical deformations of the core through its height. Thus, the high-order and local effects are an inherent part of the high-order theory and improve on the available classical and high-order theories. The formulation details the governing equations and associated boundary conditions for a general construction of a sandwich panel with unidentical skins and a “soft” core made of foam or aramid honeycomb. The theory uses a classical thin-plate theory for the skins and a three-dimensional elasticity theory for the core. The behavior is presented in terms of internal resultants and displacements in skins, peeling and shear stresses in skin-core interfaces, and stress and displacement fields in the core, even in the vicinity of localized loads. The analysis handles any type of load and distinguishes among loads applied at different skins. A parametric study has been conducted on a simply supported sandwich panel with identical skins that are subjected to both a concentrated load applied at the middle of the panel with a transversely flexible, stiff core and distributed on a square region with various dimensions for various panel aspect ratios and to a fully uniform distributed load with various modulus of elasticity ratios of skin panel to core (in vertical direction).

Indentation of composite sandwich beams

Abstract: Indentation of a sandwich beam is analysed as linear elastic bending of the top skin on a rigid-perfectly plastic foundation (the core). The theoretical predictions of fracture load from this simple theory are shown to be in good agreement with experimental results from indentation tests on strips of sandwich panel, with glass-fibre-reinforced plastic skins and foam core, supported on a rigid base.

Impact Testing on Composites Laminates and Sandwich Panels

Abstract: The purpose of this work was the evaluation of the behavior of fiber-reinforced composites and sandwich panels for aeronautical applications under impact.Experimental tests were performed on several specimen configurations, based on different quasi-isotropic lay-up and materials such CFRP, CFRTP and a syntactic foam as the core of sandwich panels. Considering the high specific mechanics characteristics of such a foam and its cocurability with the facesheet material, symmetric and non-symmetric sandwich configurations have been tested. The study has pointed out the advantages for aeronautic constructions of different materials and the influence of the core position along the panel thickness.The impact tests were performed with a falling weight machine, which allowed the most important dynamic and kinetic parameters, such as the contact force, impactor velocity and displacement and perforation energy to be measured. Delamination areas of each specimen were also measured using N.D.I. reflection ultrasonic te...

Shear Stiffness D Qy for C-Core Sandwich Panels

Abstract: A C-core sandwich panel consists of two facing plates connected top and bottom to a core of C-shaped stiffeners. Sandwich panels of this type have high bending and shear-stiffness characteristic in the direction of the core. In the transverse direction, however, both stiffness properties are relatively weak, in particular the shear stiffness. In this paper, the shear stiffness in the weaker direction for C-core sandwich panels is derived and studied. Due to the geometry and discontinuity of the core configuration, the shear stiffness in the weaker direction must be derived from the first principle. By assuming the recurrence condition, the writers previously derived an approximate expression for the shear stiffness in the weaker direction. The work described here presents a more accurate formulation, taking into account the contact interaction between the facing plates and the flanges of the C-core stiffeners. The compatibility equations are obtained from Castigliano's second theorem. The shear strain of ...

Improved Theory for Contact Indentation of Sandwich Panels

Abstract: Core crushing and large face-sheet deflections are incorporated in a theory for contact indentation of sandwich panels. The model is based on the assumption of axisymmetric indentation of an infinite elastic face sheet bonded to an elastic-ideally plastic core on a rigid foundation. After core yielding, the problem is separated into an outer region where the core is elastic and an inner region where the core exerts a constant reactive pressure on the face sheet. The outer region is modeled as a plate on an elastic foundation. The inner region is modeled using small-deflection plate theory, first-order large-deflection plate theory, and membrane theory. The unknown plastic radius is found by matching the boundary conditions for the two regions. The extension of the model to orthotropic face sheets is indicated and demonstrated by examples. The predictions are in close agreement with experiments, which contrasts to previously published models where membrane effects and core crushing were neglected. The model can be used for improved modeling of impact on sandwich panels.

Use of genetic algorithms for optimal design of laminated composite sandwich panels with bending-twisting coupling

Abstract: A genetic algorithm approach is used for optimization of a cantilever sandwich plate (an idealization of an airfoil) with a vertical force applied to one free corner. The number of face sheet plies and the ply orientations are designed so as to minimize the weight of the structure while maximizing the twist in the direction opposite that caused by the loading, subject to stiffness, strength, and ply clustering penalties. A new type of GA topology called island injection, developed in a previous study for design of composite beams, is extended to the present case of sandwich panel design. Island injection GAs showed slightly better performance over ring and singlenode topologies with identical parameters and population sizes. In all cases, the GA successfully identified designs with the desired structural response.

Characterization of Localized Failure Modes in Honeycomb Sandwich Panels Using Indentation

Abstract: The indentation behavior ofaircraft floor panels (honeycomb sandwich structures) was investigated from the viewpoints of test methodology, effects of panel construction, and failure mechanisms. An indentation test method using a simply-supported plate was developed to replicate observed in-use failure modes. The localization and type of damage induced with this test method correlated extremely well with damage present in panels returned from airlines suggesting that highly-concentrated loads, such as from high heels, cause most panel damage. Furthermore, damage appeared to spread due to an accumulation of multiple damage events more so than propagating from only a few. Both aramid- and aluminum-cored panels showed failures that occurred just below the resin fillet on the top skin. Aramid failed in brittle compression and post-buckling whereas aluminum failed due to ductile buckling. These results strongly indicate that near-surface properties dominate indentation behavior in contrast to the core-dominated stabilized core compression test as well as the skin-dominated failure criteria used in ASTM-type impact tests. Indentation tests using various skin and core constructions demonstrated the influence of changing core density, core material, and skin construction as well as the synergistic effect of skin and core on indentation damage resistance. Several analytical models were examined to aid in understanding the failure mechanisms due to indentation. These suggest that large local skin deflections are important in local load redistribution leading to core failure.

Hybrid composite articles and missile components, and their fabrication

Abstract: A surface-protected composite article, such as a missile component, is prepared with a heat-sink substrate, a first composite layer of an pre-ceramic-matrix structural composite material, and a second composite layer of a reinforced silicone pre-ceramic material. The silicone material is co-curable with the organic matrix of the first composite layer. The silicone at the surface of the article is thereafter converted to a silica refractory by an appropriate treatment such as exposure to an oxygen-rich plasma or a high-surface temperature. The silica protects the surface of the composite material.

Z-pin reinforced bonds for connecting composite structures

Abstract: I improve the impact shock resistance of bonds between composite elements by including Z-pin reinforcement. I prepare stubbled composite structure by using peel plys over the appropriate surface of the composite during pin insertion using conventional processes. I then use the stubbled composite structure with padups, as necessary, to produce the Z-pin reinforced joint or bond between composite elements using any of adhesive bonding, cocuring, or thermoplastic welding.

Mechanical Performance of Composite Sandwich Beams with Syntactic Foam Cores

Abstract: A design concept for composite wind tunnel compressor blades at NASA Ames Research Center was based on a sandwich construction composed of carbon fiber-reinforced polymer (CFRP) skins bonded to a syntactic foam core. The detailed design and analysis of this blade required the input of the elastic and strength properties of the sandwich constituents. Available standards that apply to regular foams and honeycomb cores were found to be partly inadequate for this purpose. An improved testing procedure and its analysis that provided the basic elastic and strength properties are reported in this paper. Tensile, compressive, and shear properties of the foam and skin are identified using beam deflection and failure mode analysis. Constituent properties derived from beam flexure tests culminate in a general strategy of comparing analytical parameters to results found experimentally. The beam failure modes are found to be either in compressive failure of the skin or in shear failure of the core. This matched the analytical predictions. The deflection of the beam in a three-point flexure test was also found to correlate well with the analytical model. The study concludes that the correlation between experimental results and the analytical predictions will enable the designer to predict the mechanical behavior and strength of a sandwich beam design. Furthermore, it allows for the experimental analysis of a sandwich beam without having to separate its constituent materials.

The point force response of sandwich panels and its application to impact problems

Abstract: The point force response of polymer matrix composite (PMC) sandwich panels is simulated analytically. Direct formulations based on Fourier series expansions usually have a slow convergence rate because the core dent zone is typically only a few square inches, which is two to three orders smaller than the panel size. To enhance the convergence rate, the loading system is divided into symmetrical and anti-symmetrical modes. The anti-symmetrical mode simulates panel bending deflection, including the effects of core shear and flatwise stiffness. The symmetrical mode simulates the core dent. The purpose of this derivation is to simulate impact response. Preliminary success has been achieved in correlating the measured responses of impact tests. Results are also verified for simplified cases such as panels with isotropic facesheets. The model can easily extract the elastic strain energy contribution from the overall impact energy, which is quite essential to distinguish the difference between panel and coupon level impact response. Approximate formulae for the shape of the core dent zone are also proposed and compared with simulation. It is also reasoned that the model can be used to estimate the maximum core crush zone when a severe point impact does not result in an extensive core shear rupture zone. Dij = bending stiffness of the top facesheet Dave= see eq(22b) Df = for isotropic facesheet DQ= see eq(24) Ds= bending stiffness of a sandwich panel Eeiastic = elastic strain energy, see eqs(3,4) EZZ = flatwise modulus of core GXZ & Gyz = shear moduli of core Ka = contact stiffness of anti-symmetrical mode Ks = contact stiffness of symmetrical mode KSoo= calculated by an infinite plate model Ksf = calculated by a finite plate model P = magnitude of a point force P = 2Ezz/tc 5= deflection under an indentor 5total = total deflection 8a= related to anti-symmetrical mode 8s=related to symmetrical mode A, = characteristic length, see eq(9). °xz & yz = core shear stresses

Mode Shape Experimental Holographic Technique for Spacecraft Sandwich Panels

Abstract: Mode shapes acquired using holographic interferometry with both stroboscopic real time and time average techniques and mode shapes retrieved with a broad band approach are compared. The test structure is a sandwich panel made with an aluminum honeycomb core and two CFRP skins. Such materials are usedfor light spacecraft structures subjected to dynamic loads especially during the launch. In the first approach acitation will be obtained driving a tmnsducer by a single frequency sine-wave, while in the second one the structure will be excited by an impact hammer. In general the holographic interferometry is conducted on clamped structures while impact testing, whenever possible, on free-free configurations. In this work we have succeded in comparing the holographic interferometry experimental data with the ezperimental ones from the broad band approach, both obtained in free-free condition. The problem how to correctly realize the constraints and to approach the free-free configumtion, maintaining the rigid motion of the whole structure small enough not to confuse the interferometric pattern, has been successfully OIJWcome.

Composite, sandwich panel with reinforced anchorage point for automobile construction

Abstract: A reinforced sandwich structure composite panel with a rigid foam core enclosed between two skins of resin impregnated glass fibre for at least one open housing (7) in the core (1). In the housing is formed a reinforcing stud of complementary shape in resin (8) coming from direct moulding with at least one of the skins. Also claimed is an automobile using at least one such panel in its bodywork.

Deformation of sandwich panels in cylindrical bending by point forces. 1. Analytical construction

Abstract: A refined model for bending of a three-layered panel with a soft filler is proposed. The modified model permits us to consider the asymmetry of elastic properties and thickness of the outer layer relative to the middle plane of the panel in a composite sandwich structure. In constructing the deformation mechanism, a heterogeneous kinematic model was adopted, which, in contrast to the assumptions for the deformation of the whole stack of layers, features four degrees of displacement freedom permitting consideration of the separate nature of the deformation of the outer layers in bending and of the intermediate layer in transverse compression combined with shear. This approach is postulated according to an energetic evaluation of the deformation of the layers [2]. The specific features of the stress from point forces in cylindrical bending are considered using the operational Laplace method, which avoids the additional difficulties in analyzing the solution convergence arising when it is represented by a series of eigenfunctions of the boundary value problem. The fundamental functions of a twelfth-order set of equations are used to construct the boundary problem reduced to a Cauchy problem. Various boundary effects of the point stress are described using a generalized Dirac function. Variants are examined for the limiting transformation of the model parameters leading to a qualitative change in its kinematics and the corresponding simplified bending models.

Gulfstream V floors : primary aircraft structure in advanced thermoplastics

Abstract: Carbon fiber reinforced thermoplastic floor panels have been developed for the new Gulfstream V corporate jet, the primary target being weight reduction compared to previous aluminum designs. The Gulfstream V panels are divided into moderately loaded panels and more heavily loaded panels in the wing area. The moderately loaded sandwich panels are a continued development of existing thermoplastic floor panels, such as those of the Fokker 100 and the Airbus A300/600 Super Transporter aircraft. They are found in the forward cabin and baggage areas and feature a simple and cost-effective edge folding technique and ultrasonically welded thermoplastic inserts. The primary structure floors, a first for thermoplastic composites, serve as a pressure bulkhead. They were designed around material allowables, obtained from extensive damage tolerance testing. The GV panels, together with other thermoplastic aircraft component technologies developed in parallel, open the road to a vast array of primary aircraft structure components, such as rudders, elevators and pressure bulkheads.

Marine applications for structural adhesives

Abstract: Modern structural adhesives are now available that are potentially suitable for the bonding of both metallic components and fibre reinforced polymer materials for structural marine applications. Pultruded grp specimens are increasingly available which together with mass produced grp panels form the building blocks for a wide range of fabricated composite polymer structures. The trends in advanced marine technology are to make greater use of prefabricated components. A major problem in the use of polymer composite materials for large structures is the joining of sub assemblies and laminate panels. This thesis considers the understanding and development of the use of bonded structures and adhesive applications in the offshore and marine industries. The study includes the production, thermal and fatigue performance of bonded grp components and the problem of butt joining of laminates as well as the performance of steel sandwich structures. The results of a small scale experimental study of various detailed design options for panels are supported by finite element analysis. The discussion highlights the advantages and disadvantages of various types of butt joints. Possible failure mechanisms are discussed and include a ranking of joining methods with respect to the strength of the basic laminate. Sandwich panels offer a practical substitute for traditional stiffened plating and, with careful consideration of face and core parameters may be more structurally efficient than stiffened single skin structures. Both analytical and experimental techniques were utilised to study the performance characteristics of steel corrugated core sandwich beam elements under static and fatigue loading. Comparisons between the viable alternative fabrication methods showed adhesive bonding to be very acceptable, especially where structures are subjected to fatigue loading. Transverse to the corrugations, the failure modes are complex and dependent on the combination of geometry of the face and core material. The performance of the necessary bonded joints is particularly influenced by the type of the adhesive used and the form of surface preparation prior to bonding. Small scale experiments have highlighted the importance of suitable surface preparation to promote short term static strength and durability. Investigation of the effect of aging in a wet environment is particularly important in a marine environment as a major concern is the sensitivity of the adhesive to the effects of water. The research has shown that a primer can make an important contribution to both the strength and durability of the joint. Other factors influencing overall joint performance are the type of surface preparation chosen for the substrate, geometric details including joint orientation, spew fillet removal and load. Adhesion at the interface between the adhesive and substrate and also bulk adhesive hydrolytic degradation were also investigated to determine the rote each has in influencing the overall durability of an adhesively bonded connection. A novel form of test was successfully developed to rapidly grade the adhesion durability performance at the interface. The results of this research should improve confidence regarding adhesive bonding of materials of types, sections and sizes suitable for marine structures.

An Evaluation of Low Energy Cure Glass Fabric Prepregs.

Abstract: : Low energy cure glass fabric prepregs were evaluated. These materials were formulated to cure at low temperatures, from 140 deg F to 220 deg F, and under vacuum bag pressure. Several of these material systems are now commercially available and have potential applications in high quality, low cost fabrication of surface ship structures. Most of the prepreg resins are epoxy, but polyester and vinyl ester laminates were included in the evaluation. The test laminates were fabricated with a warps parallel lay-up and tested in tension, compression, flexure, and impact. Limited in-plane shear testing was done, particularly to assess the advantage of a quasi-isotropic vs. warps parallel stacking sequence. Thermal analysis was used to determine as-fabricated, post-cured, and moisture conditioned values of Tg. Single-sided prepreg and no-tack prepreg were identified as useful material forms for achieving low void content under vacuum bag pressure. The low cure temperature of the prepregs allows vacuum bag fabrication of sandwich panels with a variety of foam cores.

Optimum design of hybrid shape memory alloy sandwich panels for maximum natural frequencies

Abstract: The paper presents a solution of the vibration problem for a sandwich panel with shape memory alloy (SMA) fibers embedded within resin sleeves and positioned at the middle plane of the panel. The fibers whose axial displacements are restricted, generate significant tensile stresses when working in the reverse transformation phase. The problem is formulated as follows: `Design such a system of SMA fibers that the fundamental frequency of the sandwich panel will not decrease below a prescribed value due to an increase of temperature within a specific range'. The solution of this problem that requires a minimum number of SMA fibers implies their nonuniform distribution. The design considered in the present paper is limited by the case where SMA fibers are oriented in one direction which may provide a better technological solution. The analysis is based on a new constitutive law proposed by the author. This law can accurately reflect the behavior of a constrained SMA fiber in the reverse transformation phase. It is shown that in SMA sandwich panels, the fundamental frequency can be kept equal or even higher than its room-temperature counterpart, in spite of the presence of compressive thermally-induced stresses.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Moisture Diffusion Analysis in Multilayer Composite Materials by Finite Difference Analysis.

Abstract: : This report provides a FORTRAN source code for calculating the moisture uptake, diffusion, and internal distribution through the thickness of multi-layered composite sandwich materials as a function of time. This finite difference diffusion code (FDDC) was designed as a handy tool for materials engineers and electronics engineers who have no easy access to a finite element diffusion code (FEDC), yet who have a need to study long-term moisture effects on composites or sandwich structures. A unidirectional diffusion analysis through laminated plates or sandwich panels with many different material layers, for many different environments, can be performed within a few seconds of CPU time. The code can be run on Macintosh or IBM-compatible computers. A sample run is provided which may serve as a tutorial and also as a comparison with a previously run finite element analysis of the same problems. The differences in the sample runs for the calculated moisture concentrations of the FDDC and FEDC analyses were insignificant (less than one-half percent).

Moisture Diffusion in Impact Damaged Face Sheets of Composite Sandwich Materials.

Abstract: : Moisture permeation can weaken composite sandwich material structures This report describes an investigation of the change in moisture diffusion coefficients in composite sandwich face sheet material which has sustained moderate damage Sandwich panels were subjected to impacts of 50, 75 and 100 foot-pounds using a one-inch diameter instrumented impact head After the panels were subjected to the impacts, the core and rear face material were removed, leaving only the impacted face sheet In the analysis, a sufficient undamaged area of the composite was retained in order to prevent further damage to the impact area Small squares of damaged and undamaged face sheet material were then dried in a vacuum oven and exposed to 80 percent RH at 22 deg C for a period of seven months The diffusion coefficients were determined through the damaged area and compared with that of undamaged face sheets The sorption curves of the damaged specimens were analyzed to estimate the moisture diffusion coefficients It was determined that small damage, barely noticeable, increased the diffusion coefficients of the damaged area by about two orders of magnitude