Showing papers on "Sandwich-structured composite published in 1998"

Composite Materials: Mechanical Behavior and Structural Analysis

Abstract: Preface.- Translators Preface.- Part I: Composite Materials. Basic Features of Composite Materials. The Constituents of a Composite Material. Molding processes and Architecture of Composite Materials.- Part II: Basic Concepts of the Mechanical Behavior of Materials. Mathematical Basics. Stresses. Strains. The Elastic Behavior of Materials. The Mechanics of Deformable Solids.- Part III: Mechanical Behavior of Composite Materials. Elastic Behavior of Unidirectional Composite Materials. Elastic Behavior of an Orthotropic Composite. Off-Axis Behavior of Composite Materials. Fracture Mechanisms and Damage of Composite Materials.- Part IV: Modeling the Mechanical Behavior of Laminates and Sandwich Plates. Basics of Laminate Theory. Classical Laminate Theory. Effect of the Stacking Sequence. Mat and Cloth Reinforced Materials. Governing Equations and Energy Formulation of Classical Laminate Theory. Including Transverse Shear Deformation in Laminate Theory. Theory of Sandwich Plates.- Part V: Analysis of the Mechanical Behavior of Composite Material Structures. Cylindrical bending. Bending of Laminate and Sandwich Beams. Bending of Orthotropic Laminate Plates. Bending of Plates.

Metal foaming by a powder metallurgy method: Production, properties and applications

Abstract: A method for fabricating metal foams based on the powder metallurgy process is presented. This foaming process allows for the production of complex-shaped foam parts, metal foam sandwich panels and foam filled hollow profiles. A range of alloys can be foamed using this method including aluminum, zinc, tin, lead and steel. The as-produced part has a closed-cell microstructure and a high fraction of porosity (typical range from 40–90% porosity). Selected mechanical properties of metal foams are evaluated, including the loading of foam samples with and without face skins and the axial crushing of tubular structures with foam reinforcement. Potential applications are discussed such as light-weight construction and energy absorption for both military and civilian uses.

Metal sandwich panels

Abstract: It has been discovered that improved poured-in-place sandwich panels (10) may be made which have a planar, rigid, cellular polyisocyanurate or polyurethane foam core (12), an interior facer (18) on one side of the polyisocyanurate foam core and a metal skin (20) on the other, opposite side of the polyisocyanurate foam core. Adhesion of the metal skin to the polyisocyanurate or polyurethane foam core is improved through the use of a primer layer (22), such as a polyurea or a polyepoxide. Preferably, glass fibers are provided in the polyisocyanurate or polyurethane foam core as reinforcement and as fire retardant.

Flexural analysis of prestressed concrete sandwich panels with truss connectors

Abstract: A closed form elastic continuum approach was modified to account for discrete truss connectors to estimate service load deflections and bending stresses of non-loadbearing semi-composite sandwich panels. Predictions of maximum deflections and bending stresses were compared to finite element results and experimental data. Close agreement was found between the two analytical approaches, and both methods produced conservative predictions as compared to experimental data. Sensitivity analyses indicate the presence of unidentified additional shear transfer mechanisms in actual panels that are not fully represented in the elastic models. However, the theory holds promise for design and for understanding the behavior of semi-composite sandwich panels.

Thermoplastic seam welds

Abstract: The present invention is a method for joining thermoplastic composite sandwich panels with thermoplastic welds (fusion bonds) made without autoclave processing of the joint. The preferred joint is a double interleaf staggered joint with supporting titanium doublers providing a tensile strength of at least 12,000 lb/in. The joint is particularly suited for joining sections of a cryogenic tank for spacecraft.

Pitch-based carbon foam and composites

Abstract: A process for producing carbon foam or a composite is disclosed which obviates the need for conventional oxidative stabilization. The process employs mesophase or isotropic pitch and a simplified process using a single mold. The foam has a relatively uniform distribution of pore sizes and a highly aligned graphic structure in the struts. The foam material can be made into a composite which is useful in high temperature sandwich panels for both thermal and structural applications.

Penetration and Perforation of Composite Sandwich Panels by Hemispherical and Conical Projectiles

Abstract: The results of penetration and perforation tests carried out on composite sandwich panels with GRP skins and PVC foam cores using hemispherical-ended and conical-nosed indenters/projectiles under quasi-static, drop-weight, and ballistic impact conditions, with impact velocities up to 305 m/s, are described. Load-displacement characteristics under quasi-static loading are presented and the ballistic limits as well as perforation energies are determined. A classification of the sandwich panel responses based on the panel thickness-to-projectile diameter ratio is deduced. General empirical formulas that predict the dynamic perforation energies for fiber-reinforced plastic (FRP) laminates and composite sandwich panels loaded by hemispherical-ended projectiles are derived. The empirical equations correlate well with available experimental data. It is shown that, to a first approximation, the formulas obtained for hemispherical-ended projectiles are also applicable to conical-nosed projectiles.

Forward and Inverse Analysis for Impact on Sandwich Panels

Abstract: A Green’ s function approach is presented for forward and inverse analyses of sandwich panels that were struck transversely at low velocities To model the dynamic response of the panel, both membrane and bending deformations of the laminated face plates were considered, and the core, in this case made of honeycomb, was treated to deform by transverse shear force The displacement e eld of the panel as a whole was then modeled using beam functions along with the Rayleigh ‐Ritzmethod Thederived coupled equations of motion were further transferred to the eigenspace to form Green’ s function In the forward analysis, the solution was obtained directly, and the agreement between the prediction and the measured data was found to be very satisfactory when the impact location was beyond a distance that was a few times the panel thickness On the other hand, a constrained optimization method was employed in the inverse analysis to search for the optimal impact force history from the recorded strain histories The determined force history was also found to have a very satisfactory agreement with the measured one

Compression Behavior of FRP Sandwich Specimens with Interface Debonds

Abstract: Local regions of bond failure between face sheet and core are often observed when sandwich structures are exposed to low energy impacts. Similarly, impact loads frequently result in delaminations w...

Non-linear response of geometrically imperfect sandwich curved panels under thermomechanical loading

Abstract: This paper deals with the non-linear response of sandwich curved panels exposed to thermomechanical loadings The mechanical loads consist of compressive/tensile edge loads, and a lateral pressure while the temperature field is assumed to exhibit a linear variation through the thickness of the panel Towards obtaining the equations governing the postbuckling response, the Extended Galerkin’s Method is used The numerical illustrations concern doubly curved, circular cylindrical and as a special case, flat panels, all the edges being simply supported Moveable and immoveable tangential boundary conditions in the directions normal to the edges are considered and their implications upon the thermomechanical load-carrying capacity are emphasized Effects of the radii of curvature and of initial geometric imperfections on the load-carrying capacity of sandwich panels are also considered and their influence upon the intensity of the snap-through buckling are discussed It is shown that in special cases involving the thermomechanical loading and initial geometric imperfection, the snap-through phenomenon can occur also in the case of flat sandwich panels

Shear Stiffness for Z-Core Sandwich Panels

Abstract: In this paper, the shear stiffness in the weaker direction of Z-core sandwich panels is studied. The recurrence condition is assumed, and the shear stiffness is derived from the first principle. The contact interaction between the facing plates and the flanges of the Z-sections is considered. Compatibility equations and the shear strain of a typical segment are obtained from the Principle of Virtual Work. Formulas for the shear stiffness in the weaker direction are derived explicitly. It is shown that there are two shear stiffness values associated with the Z-core sandwich panels, depending on the direction of the shear force. The stronger value of a typical panel can be 60% higher than the weaker value. To verify this phenomenon experimentally, two steel sandwich panels with different stiffener spacing were fabricated and tested. Good agreement was obtained between the theoretical predictions and the experimental results.

Damage characteristics and residual strength of composite sandwich panels impacted with and without compression loading

Abstract: The results of an experimental study of the impact damage characteristics and residual strength of composite sandwich panels impacted with and without a compression loading are presented. Results of impact damage screening tests conducted to identify the impact-energy levels at which damage initiates and at which barely visible impact damage occurs in the impacted facesheet are discussed. Parametric effects studied in these tests include the impactor diameter, dropped-weight versus airgun-launched impactors, and the effect of the location of the impact site with respect to the panel boundaries. Residual strength results of panels tested in compression after impact are presented and compared with results of panels that are subjected to a compressive preload prior to being impacted.

Method for producing a composite structure including a piezoelectric element

Abstract: A composite structure (3) includes a piezoelectric element (1) sandwiched between fiber composite panels (2). The composite structure can be used as an actuator element. In a method for producing the composite structure, the fiber composite panels are pre-stressed and pre-strained by applying respective pre-stressing forces (F1, F2) in opposite directions along two substantially perpendicular axes in the plane of the panel. The pre-stressed and pre-strained condition of the fiber composite panels (2) is maintained, while the panels are adhesively bonded (5) surfacially onto the piezoelectric element (1). After the bonding is completed, the pre-stressing forces are removed from the fiber composite panels. As a result, in the finished composite structure (3), the fiber composite panels (2) are under internal tension, while the piezoelectric element (1) is under internal compression. The pre-stressing forces (F1, F2) are particularly selected in magnitude and direction to achieve the required resultant balance of tension and compression within the composite structure (3). The pre-compression of the piezoelectric element (1) should be sufficient so that operating loads on the composite structure do not result in tension loading of the piezoceramic element, while substantially maintaining the useful active strain range of the overall composite structure.

Polymer foams as core materials in sandwich laminates (comparison with honeycomb)

Abstract: The mechanical properties of sandwich laminates with two different closed cell polymer foams (PEI and PMI) as core materials were examined compared with sandwich laminates with honeycomb as core material. The applied test methods were compression, tension, four-point bending and instrumented falling weight impact. To examine the influence of the adhesive bonding between the face sheet and the core material, the sandwich laminates were tested with and without adhesive bonding. For comparison purposes the processing procedure for all sandwich laminates was kept constant. Therefore all sandwich laminates were produced at the same time in the same autoclave. In comparison to a honeycomb core, sandwich laminates with foam as core material showed superior surface quality and impact energy absorption. The influence of the adhesive bonding between face sheet and core material was found to be very small under tensile loading conditions. From microscopical investigations of the bond line it was deduced that in the case of direct bonding, sufficient resin was able to flow during processing from the face laminate into the micropores at the foam core surface without adhesive.

Comprehensive overview of theories for sandwich panels

Abstract: This paper gives a comprehensive overview of theories for sandwich panels, dealing with the mechanical behaviour. The classical theories are mainly formulated during the fifties and sixties. The derived analytical solutions are based on simplified assumptions. To study local effects near supports, load points and other discontinuities, superposition approaches and higher-order theories have been developed during recent years. Mostly these derivations can only be solved with the use of numerical solving techniques. Finite-element methods developed during the last two decades, are an alternative for the analytical solutions. The three-layer models seems to be very popular, but also more detailed three-dimensional models are usefull. One application of sandwich panels is discussed more in detail, for the building sector the development of design rules is reviewed. It is observed that the rules used nowadays are mainly based on classical theories. To illustrate the advantages of the other theories in case of local sandwich behaviour, an example of a sandwich beam under three point bending is presented. This paper concludes with suggestions how to make use of these recently developed sandwich theories.

Pitch-based carbon foam and composites

Abstract: A process for producing carbon foam or a composite is disclosed which obviates the need for conventional oxidative stabilization. The process employs mesophase or isotropic pitch and a simplified process using a single mold. The foam has a relatively uniform distribution of pore sizes and a highly aligned graphitic structure in the struts. The foam material can be made into a composite which is useful in high temperature sandwich panels for both thermal and structural applications.

Homogenization of Periodic Sandwiches

Abstract: Metallic sandwich plates (ribbed, honeycomb or corrugated plates) are widely used in aerospace and other industries due to their light weight and high stiffness. A three-dimensional accurate finite element model of such sandwich panels can become computationally expensive, and an equivalent plate formulation is therefore particularly useful.

Effects of Boundary Conditions in High-Order Buckling of Sandwich Panels with Transversely Flexible Core

Abstract: The effects of boundary conditions on the buckling behavior and bifurcation load level of sandwich panels/beams with a “soft” core due to in-plane loads are presented. The study is conducted using a closed-form high-order linearized buckling analysis, that includes the influence of the transverse flexibility of the core as well as of the localized effects on the overall sandwich panel/beam behavior, and allows the use of different boundary conditions for the upper and the lower skin at the same section. The panel/beam construction is general and consists of two skins (not necessary identical), metallic or composite laminated symmetric, and a “soft” core made of foam or a low strength honeycomb which is flexible in the vertical direction. The closed-form high-order analysis yields the general buckling behavior of the structure which means that the solutions obtained allow for interaction between the skins and the core. The solutions are general and are not based on separation of the buckling response on several types of uncoupled buckling phenomena, such as overall buckling, skins wrinkling, etc., as commonly used in the literature. The finite differences technique has been applied to approximate the governing equations of the closed-form high-order formulation and transform the set of the linearized governing differential equations to an eigenvalue problem that is solved using the deflated iterative Arnoldi procedure. The influence of various boundary conditions, including the different support types throughout the height of the same section and non-identical conditions at the upper and the lower skin, as well as of the core properties, on the buckling behavior of the sandwich panels/beams is considered. The discrepancy between the Timoshenko-Reissner model and the present closed-form high-order formulation is discussed. In particular, a partial fixity phenomenon due to the existence of the pinned boundary conditions, i.e. simply-supported conditions, at the upper and the lower skins at the edge is demonstrated. It is shown that the core properties affect the buckling loads and the corresponding modes of the panel/beam in such a way that the structures with identical boundary conditions but with different cores may undergo different types of buckling such as overall and local as well as interactive loss of stability. The effect of edge concentrated moment induced by a couple of forces exerted on the skins only is also studied.

Testing of sandwich panels under uniform pressure

Abstract: The properties of a testing device for transverse loading of simply supported sandwich panels using a uniformly distributed load are investigated. Differences between hard and soft simply supported boundary conditions are verified. Shear stream gages are used to measure the shear stress in the core of the sandwich. The deflection, strains on the upper face, and the load distribution and measured on an isotropic sandwich panel and the same properties are calculated using an analytical method and a finite-element method (FEM). The boundary conditions are investigated by letting the FEM program calculate the solution for both soft and hard simply supported boundaries. The results show that the type of simply supported boundary conditions has a considerable effect on the solution. It is shown that the test panel used in this research better represents the soft boundary conditions. It is noted that the pressure can vary substantially close to the corners of the panel but is uniformly distributed over the central part of the panel.

Prediction of Elastic Properties of Sandwich Panels Using a Homogenization Computational Model

Abstract: One of the major difficulties in the analysis of composite materials is to characterize its average or equivalent elastic mechanical properties. There are many different approaches to address the problem, ranging from experimental to analytical and computational methods. A description of some general classical methods, and methods for laminate theory can be found, for example, in [1] and [2] and references therein.