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2005 | Book

Sandwich Structures 7: Advancing with Sandwich Structures and Materials

Proceedings of the 7th International Conference on Sandwich Structures, Aalborg University, Aalborg, Denmark, 29–31 August 2005

Editors: O.T. Thomsen, E. Bozhevolnaya, A. Lyckegaard

Publisher: Springer Netherlands

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About this book

Sandwich structures represent a special form of a laminated composite material or structural elements, where a relatively thick, lightweight and compliant core material separates thin stiff and strong face sheets. The faces are usually made of laminated polymeric based composite materials, and typically, the core can be a honeycomb type material, a polymeric foam or balsa wood. The faces and the core are joined by adhesive bonding, which ensures the load transfer between the sandwich constituent parts. The result is a special laminate with very high bending stiffness and strength to weight ratios. Sandwich structures are being used successfully for a variety of applications such as spacecraft, aircraft, train and car structures, wind turbine blades, boat/ship superstructures, boat/ship hulls and many others.

The overall objective of the 7th International Conference on Sandwich Structures (ICSS-7) is to provide a forum for the presentation and discussion of the latest research and technology on all aspects of sandwich structures and materials, spanning the entire spectrum of research to applications in all the fields listed above.

Table of Contents

Frontmatter

Keynote Lectures

Sandwich Structures: Past, Present, and Future

The use of sandwich structures continues to increase rapidly for applications ranging from satellites, aircraft, ships, automobiles, rail cars, wind energy systems, and bridge construction to mention only a few. The many advantages of sandwich constructions, the development of new materials, and the need for high performance, low-weight structures insure that sandwich construction will continue to be in demand. The equations describing the behavior of sandwich structures are usually compatible with the equations developed for composite material thin-walled structures, simply by employing the appropriate in-plane, flexural, and transverse shear stiffness quantities. Only if a very flexible core is used, is a higher order theory needed.

Jack R. Vinson
Sandwich Structures Technology in Commercial Aviation
Present Applications and Future Trends

This paper gives a brief overview of sandwich application history in general and present composite sandwich structures at Airbus. Current R&D developments for sandwich in primary structures are being outlined followed by a discussion of potentials and challenges for composite sandwich structures.

Axel S. Herrmann, Pierre C. Zahlen, Ichwan Zuardy
Damage Assessment and Damage Tolerance of FRP Sandwich Structures

A review is made of production defects and in-service damage types that arise in sandwich structures having fibre reinforced polymer (FRP) face sheets. A brief overview is given of relevant defect and damage models and how these models can be used in an assessment of criticality with regard to local and global structure, as a basis for deciding on corrective measures, for the case of a naval ship. Challenges resulting from limitations in inspection techniques are discussed. The concept of damage tolerance is discussed in the light of the above. It is argued that the most suitable and economical approach to achieving damage tolerance is dependent on the application.

Brian Hayman

Sessions Organised by the ONR (Office of Naval Research, U.S. Navy, Program Manager Dr. Y. Rajapakse)

Geometrical Non-Linear Response of Modern Sandwich Panels — Localized Effects

The paper presents the results of an investigation on the role of localized effects in the geometrically non-linear response of modern sandwich panels made of a “soft” core. The adopted non-linear analysis approach incorporates the effects of the vertical flexibility of the core, and it is based on the approach of the High-Order Sandwich Panel Theory (HSAPT). The non-linear governing equations are solved using the multi-point shooting method along with parametric and arc-length continuation procedures. And the non-linear response is described in terms of deflections and stress resultants in the face sheets, as well as in terms of the interfacial stress components at the upper and lower face-core interfaces and equilibrium path curves of the load versus the extreme absolute values of some structural quantities. The numerical study investigates the localized effects and how do they attenuate with the deformations in the case of a panel with a reinforced core at its mid region and subjected to a uniform distributed load. The results are compared with a panel that has a core with uniform properties. One of the findings of this research is that a typical modern panel exhibits a limit point response as a result of a load that causes bending, with and without localized effects, even when subjected to uniformly distributed loads and the material discontinuity of the core leads to localized effects that significantly affects the non-linear response.

Y. Frostig
Global Buckling of Wide Sandwich Panels with Orthotropic Phases: An Elasticity Solution

There exist several formulas for the global buckling of sandwich plates, each based on a specific set of assumptions and a specific plate or beam model. It is not easy to determine the accuracy and range of validity of these rather simple formulas unless an elasticity solution exists. In this paper, we present an elasticity solution to the problem of global buckling of wide sandwich panels (equivalent to sandwich columns) subjected to axially compressive loading (along the short side). The emphasis on this study is on the global (single-wave) rather than the wrinkling (multi-wave) mode. The sandwich section is symmetric and all constituent phases, i.e., the facings and the core, are assumed to be orthotropic. The buckling problem is formulated as an eigen-boundary-value problem for differential equations, with the axial load being the eigenvalue. The complication in the sandwich construction arises due to the existence of additional “internal” conditions at the face sheet/core interfaces. Results are produced for a range of geometric configurations and these are compared with the different global buckling formulas in the literature.

George A. Kardomateas
Parametric Study of Structurally Graded Core Junctions

In practical sandwich constructions it is often necessary to reinforce the core with high strength inserts to facilitate concentrated loads. The junction between the main core and the core insert is a potential weak spot of the sandwich structure. This section of the sandwich structure is investigated using finite element analysis and the results are compared with experimental data. Furthermore, parameters concerning the shape of the core junction are investigated using finite element analysis.

Anders Lyckegaard, Elena Bozhevolnaya, Ole Thybo Thomsen
Local Effects Induced by Core Junctions in Sandwich Beams under General Loading Conditions

Local effects occurring near junctions between different cores in sandwich structures subjected to axial and transverse forces and bending moments are considered. These local effects are associated with large stress concentrations in the faces and the core near the core junctions. The local effects are studied for two typical cases representing industrial applications. Finite element analyses show that significant stress concentrations are induced near core junctions subjected to transverse, tension/compression as well as bending loads. Finally, improved designs of core junctions are discussed.

Elena Bozhevolnaya, Anders Lyckegaard, Ole Thybo Thomsen
Material Characterization of PVC Foam under Static and Dynamic Loading

To support the development of sandwich composite failure models, a series of material coupon tests were performed to characterize foam core material response under quasi-static and dynamic tension loading. Material stress-strain response was found to be highly nonlinear and dependent on the loading orientation relative to the axis of the foam sheet. At low to moderate strain rates, less than 150 sec

−1

, tension strength and modulus were both increased and ultimate strain at failure decreased.

D.C. Loup, R.C. Matteson, A.W.J. Gielen
Dynamic Response of Anisotropic Sandwich Panels to Underwater and In-Air Explosions

A study devoted to the dynamic response of sandwich panels to underwater and in-air explosions is presented. The study is carried out in the context of a geometrically nonlinear model of sandwich structures featuring anisotropic laminated face sheets and a transversally compressible orthotropic core. The unsteady pressure generated by the explosion and acting on the face of the sandwich panel includes the effect of the pressure wave transmission through the core. Its implications on the structural time-histories as corresponding to the underwater and in-air explosions are put into evidence. The effects of the transverse core compressibility on dynamic response are highlighted. In this sense, one of its major implications is the possibility to capture interactively the global and local (wrinkling) dynamic response of the panel. It is shown that implementation of the structural tailoring technique in the face sheets can constitute an important mechanism toward enhancing the dynamic load carrying capacity of sandwich panels when exposed to blast pulses. Effects of the core, as well as the ones due to the ply-thickness, combined with that of ply-angle and stacking sequence of face sheets, orthotropy of the material of the core, geometrical nonlinearities, initial geometric imperfections and of the damping ratio are investigated, and their implications upon the dynamic response are highlighted and pertinent conclusions are outlined.

Liviu Librescu, Sang-Yong Oh, Jörg Hohe
Enhancement of Blast Resistance of Sandwich Plates

This research examines the effect of design modifications on response of sandwich plates to impulse pressure loads. The objective is to limit damage by delamination of the laminated face sheets and by crushing of the structural foam core that dominates response of conventionally designed sandwich plates. This is achieved by introducing structural elements that store the incident energy and thus reduce damage-related energy dissipation. In particular, ductile interlayers inserted between the outer face sheet and the foam core, can absorb a significant part of the incident energy, and protect the foam core from excessive deformation. These design concepts have been developed in our earlier work on the effect of low and medium velocity impact on sandwich plates, where they enhanced resistance to local deflections of the face sheet, foam crushing and interface delaminations.

George J. Dvorak, Yehia A. Bahei-El-Din
A Two-Property Yield, Failure (Fracture) Criterion for Isotropic Matrix Materials

A yield/failure criterion is coordinated with a separate fracture criterion to provide a comprehensive three dimensional description of failure for isotropic materials. Two properties provide the entire calibration for the two criteria.

Richard M. Christensen
On Crack Extension in Foam Cored Sandwich Fracture Specimens

The crack propagation path in foam cored sandwich DCB specimen is analyzed using laminated beam analysis. The analysis determines whether the crack would propagate self-similarly or kink upwards or downwards, and the steady state position of the crack. Kink direction and crack propagation path predictions are in agreement with experimental observations. Fracture toughness data related to the various propagation paths is presented. The resistance curves recorded for various initial crack positions indicate some mixed mode influence on the toughness, with a minimum toughness in the range of 500–600 J/m

2

corresponding to pure mode I.

R.C. Matteson, L.A. Carlsson, F. Aviles, D.C. Loup
Effect of Geometric Constraint on Fracture Toughness of PVC Foam Core Sandwich Beams

Purpose of this study was to understand the effect of core material thickness (

t

c

) on the core deformation constraint and the associated mode I fracture toughness in Double Cantilever Sandwich Beam (DCSB) specimens. Specimens were made from woven roving glass fiber/vinyl ester composite face sheet with PVC core, whose thickness ranged from 3.18 mm to 40.6 mm. The specimens were tested in mode I loading and measured fracture initiation (

G

IC

) and resistance (

G

IR

) toughnesses. The

GIC

was found to be practically same for core thicknesses from 3.18 to 40.6 mm. The

G

IR

was found to be 1.02, 0.88 and 0.91 kJ/m

2

for

t

c

’s 3.18, 6.25, and 12.7 mm. For

t

c

≥ 25.4 mm, the crack grew by only few mm’s before it deflected to face sheet. Larger

G

IR

for

t

c

= 3.18 mm is probably due to resin densification of foam cells in the co-cure processing of panels. Three dimensional, material nonlinear finite element (FE) analysis very well simulated the test data. The

J

IC

integrals from FE analysis agreed well with

G

IC

from the test. The analysis revealed that the deformation constraint was nearly the same for all core thicknesses considered and thus resulting in nearly identical fracture toughness.

Kunigal Shivakumar, Huanchun Chen, Anil Bhargava
Damage Evaluation of Sandwich Structures Using Vibration and Thermal Signatures

Combining NDE techniques can prove to be beneficial in assessing the physical state of health of composite sandwich structures. Several NDE techniques were examined from which vibration and thermal responses were selected. A Neural Network (NN) was chosen as a means to interpret and classify the information such that the type of damage, severity and location could be identified. Numerical simulations were used to train the NN and experimental measurements were used to test and validate the approach.

Andres Cecchini, Frederick Just-Agosto, David Serrano, Basir Shafiq
Fatigue Performance and Size Effect in Sandwich Composites

Results of sandwich composite static and fatigue loading are presented. To discern cracking in various constituents of the sandwich composite, AE technique is used. Core damage has been found to be the predominant failure activity. Fiber rupture triggered the onset of catastrophic failure. Mode I cracking was observed in the core while fiber rupture took place in mode I.

Basir Shafiq, Amilcar Quispitupa, Frederick Just, Miguel Banos
Study of Fatigue Endurance of Conventional and Modified Core Junctions in Sandwich Beams

Local effects occurring near junctions between different cores in sandwich structures are considered. Two groups of sandwich beams with conventional butt and reinforced butt core junctions, respectively, were examined experimentally in three-point bending under static and fatigue loadings. A rigorous statistical treatment of the obtained data has shown unambiguously that sandwich beams with modified, that is reinforced, butt junctions display superior structural performance compared with sandwich beams with the traditional core junctions.

Elena Bozhevolnaya, Anders Lyckegaard, Ole Thybo Thomsen
Fatigue of Closed Cell Foams

This paper deals with fatigue of closed cell foams. The main idea is to use a few simple tests to predict the tension-tension fatigue properties of foams. The required testing consists of crack propagation rate measurements and one tension-tension fatigue test performed at yield stress for the foam. This data can then be combined to construct a synthetic S-N curve for the foam. Tests on three densities of Divinycell H-grade foam are performed and the results support this approach. Some preliminary results from two densities of Rohacell WF-grade are given as well. Static properties of foams scale with relative density and once this scaling can be obtained through various static tests and the same scaling appears to be valid for both crack propagation rates and fatigue properties of foams. The implication of this is that once the fatigue behaviour of one relative density foam is established, one can predict the fatigue behaviour of all other relative density foams within the same class of materials.

Dan Zenkert, Andrey Shipsha, 1Magnus Burman
Fatigue of Pure and Nanophased Sandwich Composites under Shear Loading

Sandwich structures are widely used in marine, automotive, and aerospace structures because of their high stiffness and strength to weight ratio. In all of these applications, core plays an important role in controlling the extent of damage in sandwich structures especially when subjected to repetitive dynamic loading. When a sandwich structure is subjected to transverse loads, the face sheets carry bending moments as tensile and compressive stresses and the core carries transverse forces as shear stresses. The core is typically the weakest part of the structure and is first to fail in shear. Hence strengthening of core materials will essentially enhance the overall performance of sandwich structures. In this study foam core materials have been strengthened with the infusion of acicular nanoparticles such as carbon nanotubes and carbon nanofibers in the polymer precursor. This infusion has been carried through a sonic cavitation process. Once the core was modified, sandwich composites were fabricated through a traditional resin transfer molding (RTM) process. Shear fatigue behavior of sandwich composites having both pure and nanophased polyurethane foams as core materials have been investigated. The density of the core materials was identical in both cases. Static shear tests reveal that nanophased foams are more ductile, have higher strength and stiffness, and better crack propagation resistance when compared to pure foams. Shear fatigue tests were conducted at room temperature, at a frequency of 3 Hz and at a stress ratio,

R

= 0.1. S-N curves were generated and shear fatigue characteristics were determined. The number of cycles to failure for the nanophased sandwich was substantially higher than that of the neat ones. SEM micrographs show that the cell structures of nanophased polyurethane foams are stronger and larger in size with thicker walls and edges. These stronger cell structures subsequently strengthen the sub interfaces when the sandwich composite is fabricated. The high intrinsic toughness of the sub interface delays the initiation of fatigue cracks and thereby increases the fatigue life of the nanophased sandwich composites. There was no volume change for either the neat or the nanophased foam during shear deformation, and the material failed by shearing in the vicinity of the centerline of the specimen along the longitudinal axis. In both cases numerous 45° shear cracks formed across the width and the cracks traversed through the entire thickness of the specimen signaling the final failure event during fatigue.

Hassan Mahfuz, Shaik Zainuddin, Mohammed F. Uddin, Vijaya K. Rangari, Shaik Jeelani
Sea Water Effects on Polymeric Foams and Their Sandwich Layups

The ingress of sea water and its damaging effects on polymeric foams was investigated experimentally and explained by a mechanics model. Similarly, the reduction in delamination toughness at the core/facing interface was recorded and explained by means of fracture mechanics.

Y.J. Weitsman, X. Li, A. Ionita

Modelling, Analysis and Design

Sandwich Plates: Stresses, Deflection, Buckling and Wrinkling Loads
A Case Study

This paper deals with the linear static and buckling analysis of an asymmetric and orthotropic stiffened 3-layered square sandwich plate. Three different analytical formulas (Model 1, 2 and 3) have been taken from Ref. [1], slightly modified to account for unequal faces and different Poisson’s ratios and applied to a square sandwich plate of fixed dimensions but different face layer thickness. The obtained deflections and buckling loads are compared with the results of a 3D analysis [2] and of Finite Element calculations. Objectives are to evaluate the limits of validity of the formulas in view of thick face layers and orthotropic properties. The properties of the two considered specimen are listed in Tables 1 and 2. The obtained results might serve as benchmark for further studies.

H.-R. Meyer-Piening
Modelling of Viscoelastically Damped Sandwich Beams: A Comparative Study

The aim of this paper is to evaluate classical models for sandwich beams. These models are compared in the static and the dynamic fields. In all cases the Finite-Element-based solution is considered as reference.

H. Hu, S. Belouettar, E. Daya, M. Potier-Ferry
A Homogenization Based Theory for Laminated and Sandwich Beams

A theory for laminated and sandwich beams is developed based on far-field stress and strain solutions called Fundamental State Solutions. Through-thickness stress and strain moments of the Fundamental Solutions are used to obtain homogenized axial, flexural and shear stiffness as well as a shear-strain moment correction. A sequence of beam models with similarity to the Timoshenko model are obtained. Excellent agreement is shown for all stress and strain components when compared to accurate two-dimensional finite element results.

Jorn S. Hansen, Graeme Kennedy, Sergio F.M. de Almeida
Modelling of Composite and Sandwich Plates by a Trigonometric Layerwise Theory and Multiquadrics

In this paper we use a trigonometric layerwise deformation theory for modelling symmetric composite plates. We use a meshless discretization method based on global multiquadric radial basis functions. The results obtained are compared with solutions derived from other models and numerical techniques. The results show that the use of trigonometric layerwise deformation theory discretized with multiquadrics provides very good solutions for composite plates and excellent solutions for sandwich plates.

Carla M.C. Roque, António J.M. Ferreira, Renato M.N. Jorge
Stability Behaviour of Cylindrical and Conical Sandwich Shells with Flexible Core

A higher-order theory for the analysis of cylindrical and conical sandwich shells with flexible core is presented. The governing differential equations are derived on the basis of a three-layer model and solved by numerical integration. The theory is veri.ed by comparison of achieved results to those published in the literature and to finite element computations.

Congying Zhong, Hans-G. Reimerdes
High Order Nonlinear Contact Effects in the Dynamic Behavior of Delaminated Sandwich Panels with a Flexible Core

The dynamic behavior of sandwich panels with a flexible core and debonded region with and without contact is studied. The investigation uses the high-order theory of sandwich panels and considers geometrical nonlinear effects and the nonlinearity associated with the contact characteristics of the delaminated surfaces. The rotary inertia of the face-sheets and the core and the high order velocity and acceleration fields through the depth of the core are included. The dynamic governing equations, boundary conditions and continuity requirements are derived through the Hamilton principle. Numerical results that reveal the influence of the contact characteristics on the dynamic response of the sandwich panel are also presented and discussed.

H. Schwarts-Givli, O. Rabinovitch, Y. Frostig
Bending Behavior of Sandwich Panels with a “Soft” Core and Embedded Rigid Inserts

The bending behavior of sandwich panels with a “soft” core hosting embedded rigid inserts is analytically investigated. Three approaches for the mathematical modeling of the sandwich panel hosting the rigid insert are discussed. In addition, the continuity conditions between the regions hosting the rigid insert and the adjacent sandwich regions are derived. Numerical results that demonstrate the capabilities of the model and highlight some of the effects that characterize the response of the panel are also presented.

Oded Rabinovitch, Ehab Hamed
Elastic Behaviour of Z Reinforced Sandwich Beams

Reinforced sandwich panels are frequently used in refrigerated trailer construction. There is a need to reinforce the sandwich panels because they can achieve some very large dimensions and consequently loose their ability to support the working loads. Work has been developed in the field of elastic and fracture behaviour of several sandwich beams with Z reinforcements. It was concluded that some reinforcements highly increase the beams rigidity. A few specialized analytical models were developed for linear elastic analysis, and some numerical models were built to analyze the different types of beams tested experimentally in three-point bending. The models contribute to a better understanding of the deformation phenomena involved and help in isolating and characterising the critical terms of divergence between theory and experiment. The present work contributes to the establishment of design and analysis parameters necessary to this type of construction.

Marco Leite, Arlindo Silva, Manuel de Freitas
Wrinkling of Shallow Sandwich Shells for the General Case of Multi-Axial Orthotropy, Curvature and Loading

The paper considers wrinkling of orthotropic faces of a shallow sandwich shell with an isotropic core for a general case of in-plane multi-axial loading. The principal axes of loading do not necessarily coincide with the principal axes of orthotropy and curvature. The technique of the obtained closed-form solutions are based on minimization of critical forces with respect to wavelengths and angles of wrinkling.

Vitaly Skvortsov, Ole Thybo Thomsen
Transverse Stresses of a Filler under Multipoint Loadings of a Sandwich Panel

Finite solution for deformation of a sandwich panel by two systems of forces applied symmetrically and nonsymmetrically to the upper and lower faces were examined. The simple estimates of transverse normal stresses in the midlayer of a sandwich panel at squeezing from the infinite system of ribs located on either face surface have been deduced.

V.A. Polyakov, R.P. Shlitsa, V.V. Khitrov, V.I. Zhigun
A Triangular Finite Element for Sandwich Plates Accounting for Transverse Core Compressibility

The present study is concerned with a triangular higher-order finite element for sandwich plates with transversely compressible core. The element formulation utilizes a v. Kármán type multilayer theory, adopting the discrete Kirchhoff approach for the contribution of the face sheets and a simplified three-dimensional formulation for the core. The discretized nonlinear problem is solved by the Newton-Raphson method.

Serkan Demiray, Wilfried Becker, Jörg Hohe
Finite Element for the Static and Stability Analysis of Sandwich Plates

A finite element for the static and stability analysis of sandwich plates is formulated based on a three-layer sandwich model and its underlying differential equations. The Kirchhoff-Love hypothesis is assumed for the face sheets. The core is modelled by a three-dimensional material law neglecting the in-plane core stiffnesses. A stability analysis is performed on the basis of the geometric stiffness matrix considering the membrane prestresses of the face sheets.

Markus Linke, Wolfgang Wohlers, Hans-Günther Reimerdes
Two Fe Formulations for a Rapid 3D Stress Analysis of Composite Sandwich Structures

A rapid 3D stress analysis of sandwich structures made from composite face sheets and a lightweight core is needed for an efficient simulation of impact damage tolerance and resistance. For that reason, two finite shell element formulations based on layer-wise theories are developed using pure displacement approaches. The number of layers is confined to three, one for each skin laminate and one for the core, accounting for the very different stiffness of skin and core material. In order to obtain reasonable transverse shear stiffness properties and also improved transverse shear and normal stresses the equilibrium approach by Rolfes and Rohwer [1] is extended to a three-layered sandwich structure.

Anja Wetzel, Luise Kärger
Sandwich Materials Selection Charts

This paper presents an approach to facilitate comparison and optimization of sandwich material combinations. Equivalent homogenised sandwich material properties (bending stiffness, density and cost) are presented graphically in materials selection charts to enable an efficient performance per cost evaluation. The effects of core shear deformations and panel production costs can be included in those sandwich materials selection charts.

Jochen Pflug, Ignaas Verpoest
Optimal Design of Laminated Extended & Honeycomb Cores Sandwich Panels under Out-of-Plane Load with Simply Supported Boundary Conditions Using Genetic Algorithm

This paper presents discrete optimization of laminated sandwich panels with simply supported edges, under out-of-plane load that include honeycomb and extended cores. Because of problem complexity and discontinuity of variables, evolutionary algorithm (GA) is preferred for this optimization

M.J. Jamali, I. Rajabi, M.H. Kadivar
Designing Sandwich Inserts and Core Junctions for Maximum Structural Stiffness Using Discrete Material Optimization

In the present paper the structural optimization approach Discrete Material Optimization (DMO) is introduced and applied to stiffness maximization of locally reinforced sandwich structures. The aim of the optimization is for each element to choose the material from the set of candidate materials that minimizes the objective the most. A design study of a sandwich panel in three-point bending shows that the DMO method is successful in providing valuable clues to efficient design of such structures.

Jan Stegmann, Erik Lund

Fracture and Damage Tolerance

On Residual Compressive Strength Prediction of Composite Sandwich Panels after Low-Velocity Impact Damage

This paper introduces a Nonlinear Finite Element Analysis on damage propagation behavior of composite panels under in-plane uniaxial quasi-static compression after a low-velocity impact. The major damage modes due to the impact were incorporated into the model. A consequential core crushing mechanism was incorporated into the analysis. The critical far field stress corresponding to the onset of damage propagation near the damage zone was captured successfully with a good correlation with experimental data. These values can be used to predict the residual compressive strength of low-velocity impacted composite sandwich panels.

Zonghong Xie, Anthony J. Vizzini, Mao Yang
Investigation of Parameters Dictating Damage and Energy Absorption Characteristics in Sandwich Panels

Bending behaviour of composite-skinned sandwich panels with both aluminium and nomex honeycombs have been investigated under a quasi-static loading with both HS and FE indenters. Damage mechanisms were identified as core crush, top-skin delamination and skin failure. A combination of skin thickness and indenter nose shape dictates not only the nature of these damage mechanisms but also their energy-absorbing capacity.

G. Zhou, M. Hill, N. Hookham
Real-Time Damage Detection of Honeycomb Sandwich Structures using Small-Diameter Fiber Bragg Grating Sensors

The authors developed new techniques to detect inside damages in honeycomb sandwich structures using small-diameter optical fiber sensors. We embedded sensors in the adhesive layer between the core and the facesheet. From a decrease in the transmitted optical power and the change in the form of the reflection spectra from fiber Bragg grating (FBG) sensors, the debonding and the impact damage could be detected sensitively in real-time.

Shu Minakuchi, Yoji Okabe, Nobuo Takeda
The Influence of Face Sheet Wrinkle Defects on the Performance of FRP Sandwich Structures

Wrinkle defects may reduce the compressive strength of a face laminate for in-plane loading applied perpendicularly to the line of the wrinkle. To be able to decide whether a repair is needed it is necessary to know the magnitude of the strength reduction for a given wrinkle geometry. In the studies reported here, the influence of wrinkle defects on the in-plane compressive strength of quasi-isotropic CFRP laminates used in PVC foam-cored sandwich panels has been investigated by three approaches: testing of sandwich beam specimens in four-point bending, testing of sandwich panels with in-plane compression, and finite element simulation. Wrinkles involving different numbers of plies have been considered. Two different sandwich lay-ups typical of deck and hull bottom panels in a naval ship have been included.

Brian Hayman, Christian Berggreen, Robert Pettersson
Residual Strength of Debonded Sandwich Panels Loaded with Lateral Pressure
Experimental Investigation and Fracture Mechanical Modeling

For the determination of debonded sandwich panel residual strength with lateral loading a parametric finite element model is developed. The parametric model allows an arbitrary positioning of the debond within the panel and consists of both solid and shell elements. A fracture mechanical approach using the crack flank displacements obtained from the FEA solution combined with measured mixed-mode fracture toughness values are used to determine the ultimate failure load. Experiments were conducted to compare against the analysis results. The comparison of numerical and experimentally achieved results showed that the used modeling approach predicts the failure load and failure mode well.

Perttu Jolma, Sebastian Segercrantz, Christian Berggreen
Measurement of Interface Fracture Toughness of Sandwich Structures

Sandwiches are analyzed by the application of linear elastic fracture mechanics. An expression for the energy release rate is found by analytical evaluation of the J-integral. Also, a method for determining the mode mixity is described and applied. The theory presented is applied to a test method and the fracture toughness of two sandwiches are measured as function of the mode mixity.

Rasmus C. Østergaard, Bent F. Sørensen
Numerical Study of Fractured Sandwich Composites under Flexural Loading

Fatigue crack growth of foam core sandwich beams loaded in flexure has been investigated numerically. Extensive fatigue data from foam core sandwich composites under flexural loading were analyzed. A first core-skin debond parallel to the beam axis is considered. A static non-linear elastic two-dimensional finite element analysis of the sandwich beam is performed to evaluate the stress intensity factors at the crack tips.

E.E. Theotokoglou, L.A. Carlssson, C.D. Vrettos, H. Mahfuz
Dependence of Sandwich Damage Initiation and Crack Propagation on Core Material Fracture Properties

In this investigation, three-point bend tests were performed on pre-cracked and unnotched sandwich structures offering a wide range of fracture toughnesses. It has been shown that an areas-based method is the most appropriate for characterizing the fracture toughness of the sandwich structures. It has also been shown that tough core materials are less sensitive to delaminations between core and skin than their more brittle counterparts.

Lukas Berger, Markus Morgenthaler, Wesley Cantwell, Kurt Feichtinger, Russell Elkin
Dependence of Inplane Sandwich Shear Deformation on Core Material Type and Thickness

In this study in plane shear tests on sandwich plates were performed using two different experimental set-ups. Failure modes, as well as maximum failure loads were compared with predictions using sandwich calculation equations given by Zenkert and Plantema. It could be shown, that the prediction of failure load and mode was quite consistent with measurements for panels of 10mm thickness or more. Very thin panels could not be predicted correctly.

Markus Morgenthaler, Lukas Berger, Kurt Feichtinger, Russell Elkin
Residual Strength of In-Plane Loaded Debonded Sandwich Panels: Experimental Investigation

Face/core debond damaged sandwich panels exposed to uniform and non-uniform compression loads are studied experimentally. The panel geometry is full-scale rectangular with a centrally located circular prefabricated debond. The results show a considerable strength reduction with increasing debond diameter, with the failure mechanisms varying between buckling driven debond propagation and face compression failure for large and small debonds respectively.

Christian Lundsgaard-Larsen, Christian Berggreen, Alexandros Nøkkentved
Residual Strength of In-Plane Loaded Debonded Sandwich Panels: Fracture Mechanical Modelling

This paper presents a FEM based numerical model for prediction of residual strength of damaged sandwich panels. As demonstrated, the model can predict the maximum load carrying capacity of real-life panels with debond damages, where the failure is governed by face-sheet buckling followed by debond growth. Comparison of the theoretical predictions is carried out against a series of large-scale experiments described in Lundsgaard-Larsen et al. [1].

Christian Berggreen, Bo Cerup Simonsen
Resistance of Fastenings of Sandwich Panels

Fastenings are an essential part of the sandwich constructions. Mechanical f fastenings are needed to fix the panels to the frame of a building and fasteners are used to mount additional covering and components to a face of a sandwich panel. The paper studies failure modes on typical fastenings and introduces models to evaluate the resistance and to adjust the test results for design equations.

Paavo Hassinen
Experimental Failure Mode Determination of GRP/PVC-Foam Sandwich T-Joints
Static and Dynamic Experiments

Application of GRP/PVC-Foam sandwich structures on naval ships strongly depends on the mechanical strength and collapse behaviour of the joints of the structure. Quasi-static and dynamic experiments were executed to determine the failure modes and failure progression of T-joints. The results were used to determine the focus of the material tests, and to determine the main properties of failure models that were developed.

Alexander W.J. Gielen, Wim Trouwborst, Douglas C. Loup
Flexural Behaviour of Aluminium Foam/Composite Structures
An Investigation into the Deformation Mechanism and Strain Distribution under 4 Point Bending in Comparison with Polymer Foam Core Structures

The deformation mechanism of an aluminium foam core/thermoplastic composite facing sandwich structure was compared with an equivalent polymer foam core sandwich structure under 4pt bend loading. Full field strain analysis showed the metal core deformed extensively mid-beam while the polymer foam had high strain concentrations under the load rollers.

M. Styles, P. Compston, S. Kalyanasundaram
Repair Classification for Sandwich Panels with Hail Damage

Hail strikes of possibly exceeding an energy level of 50 Joules may cause multiple-site damage to thin gauged composite airplane structures. If not repaired properly, they may trigger an extensive damage to airplane structures and disruptions to airline operations, and therefore posing a major maintenance and repair concern for the airlines. It is important both for the OEM and the airlines to be able to classify the hail strike damage for appropriate repair procedure. Therefore, this study presents a methodology to assist engineers in the classification of the repair type for hail strike damage. The methodology involves an accurate prediction of the stress and strain fields, and residual strength prediction.

M. Das, E. Madenci, H. Razi
Damage Tolerance Assessment of Repaired Composite Sandwich Structures

Polymer sandwich configuration is now extensively used in many engineering structural applications such as ships, boats, spacecraft and aircraft. The design and manufacturing aspects of such materials are well known and regulations and codes exist to help designers, producers and operators of such artifacts. However, maintenance and repair are issues that have received relatively little attention. The aim of the research is to understand the influence of BVID upon structural repairs made to advanced composite sandwich structures and to determine whether a tapered scarf repair is as damage resistant and damage tolerant as the original structure.

Richard Trask, Bob Cripps, Ajit Shenoi

Dynamics, Vibration and Sound

Dynamical Transient Response of Shallowsandwich Panel Subjected to Pressure Field

Dynamical transient behavior of a shallow sandwich panel subjected to impulsive pressure load is studied. Governing equations and boundary conditions are derived from the Hamiltonian of this mechanical system. The problem is solved by the use of finite difference (FD) technique, which is illustrated by an example

Sergey Kadyrov, Vitalij Skvortsov
Effect of Transverse Core Compressibility on Dynamic Buckling of Sandwich Structures

The present study is concerned with the analysis of the transient response of softcore sandwich structures in the postbuckling range using a higher-order geometrically nonlinear shell model including the transverse core compressibility. The transverse compressibility results in the development of a chaotic vibration with unpredictable wrinkling amplitudes and has significant effects on the eigenfrequency.

Jörg Hohe, Liviu Librescu, Sang Yong Oh
Dynamic Characterisation of Marine Sandwich Structures

Dynamic experimental and theoretical methods for sandwich panel structures subjected to water slamming are described, including a unique servo-hydraulic controlled slam test system and a pressure based transient finite element technique. The pressure simulation method accurately represents the pressures observed in slamming tests, and the transient dynamic finite element modelling can simulate sandwich panel responses to a slamming load.

Mark Battley, Ivan Stenius, Johan Breder, Susan Edinger
Local Free Vibration Analysis of Initially Stressed Curved Sandwich Beams

A theoretical approach is provided to investigate the problem of local free vibration of an initially stressed curved sandwich beam. The equation of motion of the skin is derived from energy principles and based upon small displacements away from an initial, loaded equilibrium state. The core material is simplified as an elastic foundation and the skins are considered as composite beams on an elastic foundation. The investigation shows that the vibration frequencies and modes are dependent not only on the flexural rigidity and length of beam and foundation modulus, but also on geometry of curvature and axial compressive/tensile force.

W. Wang, R.A. Shenoi
Vibration Analysis of Composite Sandwich Plates and Layup Optimization

This paper is concerned with the vibration analysis of rectangular, symmetric composite sandwich plates and the layup optimization of top and bottom laminated FRP composite faces. The fundamental frequency of the composite sandwich plate is discussed in the subspace of four in-plane lamination parameters of the laminated FRP composite face, and the optimum laminate configuration which maximizes the fundamental frequency is determined.

Hideki Sekine, Hiroshi Shirahata, Mariko Matsuda
Flexural Vibrations af A Three-Layer Sandwich Beam
Using Ordinary Fourth Order Beam Theory in Combination with Frequency Dependent Parameters to Predict the Flexural Dynamics of a Sandwich Beam

The purpose of this work has been to evaluate the possibility of using modified lower order methods — such as the Bernoulli-Euler or Timoshenko beam theories with frequency dependent parameters — to calculate the response of sandwich beams subject to different end conditions. The models have been verified by measurements on a freely suspended asymmetric sandwich beam with aluminium laminates and a plastic foam core, indicating good agreement.

Daniel Backström, Anders Nilsson
An Effective 2D Linear Elasticity Vibrational Model for Layered and Sandwich Clamped-Clamped Unidirectional Strips

An effective new model for vibration analysis of the layered, and in particular sandwich, one-span, clamped-clamped (C-C) strips and beams is presented in the paper. The model is derived directly from the local linear elasticity model (solution) for the corresponding simply supported (S-S) structures after replacing the Fourier sinus series eigenfunctions with the Bernoulli-Euler eigenfunctions. The approach is verified by comparisons of numerical results obtained with the data available in the literature. Number of layers is unlimited but the model was elaborated for the structures composed of five layers.

Stanislaw Karczmarzyk
Minimization of Acoustic Radiation from Composite Sandwich Structures

This paper presents a study of structural-acoustic optimizations of sandwich structures for minimal sound radiation. A sublaminate modeling approach of sandwich structures with a proper balance of accuracy and computational efficiency is adopted in this work. Finite elements are used to compute the vibration response and acoustic radiation of the structure. The material and geometric properties of an anisotropic sandwich are treated as design parameters. The objective of optimization is to minimize the acoustic power radiated by the structure. A piecewise objective function is studied that includes nonlinear constraints of the optimization problem in a nature way by taking advantages of the pattern search algorithm. Numerical examples are presented to show the effectiveness of the structural-acoustic optimization in terms of the reduction of the radiated sound power.

H. Denli, J.Q. Sun
On Wave Propagation in Sandwich Plates Under Heavy Fluid Loading

This talk addresses wave propagation in an unbounded fluid-loaded elastic sandwich plate of symmetric composition. Several aspects are highlighted, including the interaction between shear and flexural waves and the coupling between the “in-phase” and “anti-phase” waves (with respect to transverse deflections of skins).

Sergey Sorokin, Nigel Peake
Wave Propagation in A Sandwich Plate Loaded by A Viscous Compressible Fluid

This paper addresses wave propagation in an unbounded elastic sandwich plate loaded by a layer of viscous compressible fluid. The dispersion equation is derived and the location of dispersion curves as a function of the depth parameter is analyzed. The viscosity-induced attenuation of waves in this wave guide is compared with the wave attenuation in a rigid duct filled with a viscous fluid and in a similar wave guide, where a sandwich plate is replaced by a conventional Kirchhoff plate.

A.V. Chubinskiy, S.V. Sorokin

Impact

Impact Damage on Lightweight Sandwich Panels

Collision experiments and finite element analysis were carried out to investigate damage on sandwich panels struck by spherical missiles at small velocities. Analytical models based on either quasi-static or dynamic behaviour of structures were developed to calculate impact force during low speed impact on circular sandwich panels. The results of the analytical and numerical models and the experimental measurement were compared. The dependence of damage on both structural parameters and impact variables was investigated.

Daowu Zhou, W.J. Stronge
Ballistic Resistance of 2D & 3D Woven Sandwich Composites

In the present study, ballistic resistance of sandwich composite structures for vehicle armor panel applications was investigated. The core material of the sandwich structure was a layer of Alumina ceramic and a layer of composite backing, sandwiched between 2D plain weave composite skins. The ballistic performance of sandwich materials with 3D backing was compared to the baseline 2D plain weave backed composites. An IMACON 200 high-speed camera was used to obtain high-speed photographs of the ballistic events of penetration and damage. These images were analyzed to study real time damage mechanism of the strike face surface of several targets and subsequently to obtain average resistive force of target points during impact. Velocities of projectile (armor piercing bullets) were recorded in all the experiments and were found to be in the range of 915 – 975 m/s. Post mortem analyses, which included sectioning of panel, were performed. Results showed that armor panels with 3D woven backing had a higher ballistic efficiency than the 2D baseline panels, strike face damage mechanics were predominantly axi-symmetric about the impact point and panels with 3D backing had controlled delamination and fewer complete penetrations.

Arun Shukla, Joseph Grogan, Srinivasan A. Tekalur, Alex Bogdanovich, Robert A. Coffelt
Impact and Indentation Behavior of Sandwich Panels
Modeling and Experimental Testing

The behavior of sandwich panels subjected to local loads and low-velocity impact is studied with analytical and numerical methods as well as quasi-static testing and low-sped impact testing. The results indicate that the core thickness has no influence on the initial part of the indentation behavior for a panel if the core thickness is larger than a certain critical core thickness. Both the analytical model and the numerical calculations agreed well with both the quasi-static testing and the low-velocity impact test.

Carl-Johan Lindholm
A Comparison of Low Energy Impact Behaviour in Aluminium Foam and Polymer Foam Sandwich Structures

Energy absorption of polymer and aluminum foam sandwich structures with glass-fibre composite skins was similar for 5-25J impacts. The polymer foam structure exhibited localized fibre fracture and core crushing as impact energy increased. The aluminum foam structure exhibited extensive plastic deformation, radiating from the impact point, at all impact energies.

P. Compston, M. Styles, S. Kalyanasundaram
Numerical Modeling of Sandwich Panel Response to Ballistic Loading
Energy Balance for Varying Impactor Geometries

A sandwich panel is described by an axisymmetric lumped mass/spring model. The panel compliance is simplified, considering only core shear deformation. Transverse penetrating impact is modeled; impactor diameter is significantly smaller than panel size. Experimental data for the total loss in impactor kinetic energy and momentum and estimated damage energy are given. For a selection of impactor tip shapes, the numerical model is used to evaluate different force-histories between the impactor and the panel during penetration

Jørgen Kepler, Michael Rygaard Hansen
Low Velocity Impact Investigations of Sandwich Panels with Different Cores

The behaviour of sandwich panels with different core structures after low velocity impact damage was investigated. The material properties are measured and the damage extensions are detected by ultra sonic testing. To assess the strength of the damaged sandwich structures, the panels are investigated in 4-point-bending tests until failure.

Wilfried Göttner, Hans-G. Reimerdes
Mechanical Behavior of Rubberfilled Multifunctional Honeycomb Sandwich Composite

A rubber-filled multifunctional honeycomb sandwich composite was developed in this paper. This structure was composed of facesheets, honeycomb core and vulcanized liquid silicon rubber (LSR) in the honeycomb cells. The rubber fillings were designed to support honeycomb cell walls, act as viscoelastic dampers and dissipate impact energy functionally. In order to investigate the impact and damping performance of this new developed composite, low-velocity impact and vibration tests were conducted to the fabricated specimens in two groups, with and without rubber filled. Each group had three kinds of specimens with various stacked carbon/epoxy laminate facesheets, [0/90]

4s

, [0/45/-45/90]

2s

, [45/-45]

4s

. Damage areas of each impacted specimen were inspected by ultrasonic C-scan. For vibration tests, displacement response and damping ratio were checked and compared. The experimental results provided a good agreement with our material design concept.

Huang Hao, Chee-Ryong Joe, Dong-Uk Kim
Stitching Effect on Static and Dynamic Behaviour of Sandwich Structures

Beyond the improvement of the structure stiffness, the stitches reinforce adhesion between the core and the skin and allows the structure to tolerate impacts of low energies. Even if we consider the increase of the mass of the panels which moderates the mechanical performances (specific properties), the interest of such reinforcements is considerable. The materials then created presents a real structural potential.

B. Lascoup, Z. Aboura, K. Khellil, M. Benzeggagh

Processing and Fabrication

Continuous Manufacturing and Performance of 3D Reinforced Sandwich Structures

The patented Napcoê technology presented in this paper is designed to create in a continuous way 3D tailored sandwich structures while maintaining the production efficiency. The through-thickness reinforcement is obtained from regular fabrics. This process allows the production of complex preforms that can be post formed and impregnated with liquid resin using a closed molding production method or thermoformed in the case of thermoplastic composites.

G. Le Roy, C. Binetruy, P. Krawczak
A Novel Extrusion-Welded Sandwich Structure for Thermoplastic Composite Storage Tanks

A new concept of thermoplastic sandwich structures is presented. It relies on a fibre-reinforced core based on glass/thermoplastic commingled yarn and neat thermoplastic skins. Such a structure has been developed so as to manufacture a new range of storage tanks. The use of composites in the tank core structure results in a significant reduction of the total wall thickness, at identical industrial performances with the neat thermoplastic solution.

E. Lagardere, M.-F. Lacrampe, O. Skawinski, P. Krawczak, C. Ducret, M. Giletti
The Opportunities of Flexible Foam Processing for Rigid Foam Sandwich Cores
Blending Two Foam Classes into One

In order to benefit from the processing advantages of soft foam, as well as the high mechanical properties of rigid foam, a new type of polyurethane foam StructUre™ has been developed. It can, after fast and easy processing in its flexible state, be further cross-linked to a rigid state by means of high energy electrons. This results in a high-level mechanical property profile, making the originally soft foam suitable for application as rigid sandwich core material.

Henri Mispreuve, Leendert den Haan
Formability of Lightweight, Vibration Damping and Medium Perfused Sandwich Sheets
Analysis through Practical Experiments and FEM-Simulations

The unique optimization potential of sandwich sheets with respect to a defined criterion, e.g. weight reduction, originates from the combination of different materials into one sheet. Due to this material combination, the sheet behaves differently from conventional sheet materials during forming and in addition unique failure modes like delamination might occur. Within the framework of two, separate research projects these unique features of sandwich sheets were analyzed through practical experiments and FEM-analyses.

Reiner Kopp, Marc Nutzmann, Johan van Santen
Fabrication and Mechanical Properties of Functionally Graded Micro Porous Metals by Mim-Base Powder Space Holder Method

This study describes the manufacturing method based on metal powder injection moulding for micro porous metal components with high functionally graded sandwich structure. The effectiveness of sandwich structure produced by co-sintering and co-injection moulding processes to compensate the deficiencies on the mechanical property of porous structures, was investigated.

Kazuaki Nishiyabu, Satoru Matsuzaki, Shigeo Tanaka
Sandwich Injection Moulding and Physical Properties of Cup-Stacked Carbon Nanotube / Polypropylene and Polypropylene

This study describes the manufacturing method and physical properties of sandwich structure tubes using cup-stacked carbon nanotube/polypropylene and polypropylene. The effectiveness of sandwich structure produced by sequential injection moulding to enhance the axial and transverse compressive properties of thermoplastics was investigated. It was shown that compressive properties of the sandwich tubes effectively improved by proportionally positioning rich nanotube materials to the outer layer.

Kenji Okubo, Shigeo Tanaka, Naoyuki Oya, Kazuaki Nishiyabu, Takashi Yanagisawa, Masanori Tomita
New Rohacellê Development For Resin Infusion Processes

Two new types of ROHACELLê foam specially designed for resin infusion processes are now commercially available. In comparison to the heat resistant ROHACELLê type the new foams have smaller cell sizes leading to significantly lower surface resin uptakes of the sandwich core in resin infusion processes. The mechanical properties of the new foams are excellent and even better than those of the common heat resistant type.

Jonas Scherble, Thorsten Jahn
Development and Validation of A Continuous Production Concept for Thermoplastic Honeycomb

The continuous production of thermoplastic honeycomb cores is a recent development at K.U.Leuven. The production equipment has been improved to deliver high quality material. This equipment is used for the determination of optimum process parameters. This paper describes the production equipment, its development and the experiments that have been done.

J. Philipp Bratfisch, Dirk Vandepitte, Jochen Pflug, Ignaas Verpoest
Sandwich Panel With a Periodical and Graded Core
Manufactured Using Rapid Prototyping

This paper presents a technique for the production of graded sandwich cores. A significant failure mode of sandwich panels is the indentation of the foam core in regions at which local forces are introduced into the sandwich panel. The aim of this work is to synthesise a microstructure that can diminish this failure mode. By applying a rapid prototyping technique a periodic, open beam structure has been synthesised. Additionally the structure has been graded in the longitudinal direction by alternating over the beam diameter. The obtained structure was joined with aluminium face sheets to form a sandwich beam. The panel was subjected to a three point bending test and the results were compared to a finite element model.

Jens Nygaard, Anders Lyckegaard, Jesper Christiansen
Investigation of the Cure Process for Thick Composite Sandwich Panels

A numerical and experimental investigation of the temperature distribution during the cure process of a composite sandwich panel manufactured from thick thermoset carbon/epoxy prepreg skin layers and a thermally stable closed cell foam core is presented. The temperature distribution at solidification is subsequently determined from the numerical analysis. The results from the numerical and experimental analyses are compared and good agreement is found.

Daniel K. Jensen

NDE and Environmental Degradation

Automated Ultrasonic Inspection of Large-Scale Sandwich Structures

This paper represents the results obtained from the successful ultrasonic developing work performed within the EUCLID RTP3.21 [1] and the JP 3.23 THALES [2] projects. New automated ultrasonic method for large-scale inspection of sandwich structures applied in the marine industry has been developed. On basis of several laboratory and field tests it can be concluded that the method shows great potential in revealing typical manufacturing and in-service defects.

Thomas Mathiasen Wulf
Evaluation of Sandwich Materials Using Ultrasonic Air-Coupled Scanning Technique

The ultrasonic Air-coupled scanning technique was used to evaluate the quality of sandwich panels. Scans of panels with defects such as debond between skin laminate and foam core, core crack and defects arising from mechanical tests are presented. The air-coupled technique is an easy technique to use since it can be performed directly in atmospheric air.

Kaj K. Borum
Structural NDE of CFRP Composite Materials Using Fiber Bragg Grating Sensors

In this paper, the Fiber Bragg Grating (FBG) sensors are real time employed to simultaneously monitoring the cure process of CFRP composite laminates with and without damage. Furthermore, the NDE of smart composite laminates embedded FBG sensors are performed by using the 3-point bending test.

Jinsong Leng, Yanju Liu, Xingli Xu, Shanyi Du
Study of the Hygro-Mechanical Behavior of Corrugated Cardboard

Corrugated cardboard is very sensitive to atmospheric conditions. The aim of this work is to study the effects of these parameters, especialy the relative humidity (RH), on the mechanical behavior of corrugated cardboard sandwich structure. Tensile and three-point bending tests were used under various rates of relative humidity. An analytical model based on the classical laminate plate theory is used to predict the elastic behavior of the corrugated cardboard under different atmosphere conditions. The model is then extended to predict the inelastic behavior of the corrugated cardboard from the behavior of its components.

Allaoui Samir, Aboura Zoheir, Benzeggagh Malk

New Materials and Materials Characterisation

New Concepts for Sandwich Structures

This paper summarizes a number of presumably new concepts within the general area of sandwich structures. The concepts have been analyzed numerically and/or analytically, manufactured, and tested. The concepts include strip or grid skin sandwich for light weight structures; graded or laminated sandwich cores; “artificial” balsa cores with higher strength and stiffness than conventional end grain balsa; and steel truss / sandwich skin hybrid ship hulls.

Joachim L. Grenestedt
Magnesium Integral Foam — A New Metallic Sandwich Structure

Structural foams or integral foams are monolithic components with a solid skin and a cellular core. We developed a new method to produce metallic integral foam with conventional casting machines. The advantages of the material are a low density, a high energy absorption capacity, a high weight specific bending stiffness and a remarkable damping capacity.

Markus Hirschmann, Carolin Körner, Robert F. Singer
New Improved Foam Core Materials for Advanced Processing

Closed moulding processes put new requirements on the core materials for sandwich structures. Two new foam core material grades have been developed to meet these requirements. The first will replace the existing standard grades but with improved strength, ductility and thermal properties while maintaining the excellent stiffness of today’s product. This grade is oriented towards closed manufacturing processes with processing at lower temperatures like infusion, RTM and VARTM and post cure up to 90°C. The second grade will provide full compatibility with low and medium temperature pre-preg and RFI systems for processing up to 160°C.

In addition, for the DIAB core infusion process with a grooved core material, the making up has been improved for weight sensitive applications by using an optimized finer groove pattern in the core surface to minimize resin uptake while still providing an even distribution and wet-out of the fibers.

Stefan Reuterlöv
Characterisation of Novel K-Cor Sandwich Structures

Sandwich beams reinforced with K-Cor™ are characterized in terms of stiffness, strength and fracture by means of static tests, and compared to traditional sandwich materials. Stiffness is evaluated with a series of three flexure tests applied to each sample. Strength tests show different failure mechanisms for the two architectures. Mode I fracture properties are measured and a significant increase in the core toughness of reinforced samples is revealed.

Pascal Casari, Denis Cartié, Peter Davies
Finite Element Analysis on Out-of-Plane Compression Properties of Thermoplastic Honeycomb

In this paper, the out-of-plane compressive properties of thermoplastic hexagonal honeycombs are investigated by finite element analysis (FEA). Both linear (eigenvalue problem) and nonlinear buckling analyses are performed correspondingly on honeycomb perfect models and a hexagonal imperfect unit-cell model to determine actual honeycomb compressive properties. The numerical FEA results show much better agreement with the experimental results than other conventional theories.

Xinyu Fan, Ignaas Verpoest, Dirk Vandepitte
Deformation of Foam Cores in Uniaxial Compression-Tension Cycle

This paper deals with experimental and analytical analysis of the mechanical behaviour of crushed foam cores. Three rigid cellular core materials are tested in uniaxial compression-tension-compression. An analytical model is proposed describing the stress-strain curve in tension and secondary compression. A special emphasis is laid on the relaxation behaviour of a crushed foam core in tension.

Vitaly Koissin, Andrey Shipsha
Loading Rate Effects on Foam Cores for Marine Sandwich Structures

This paper presents first results from a project which aims to generate foam core properties under loading rates representative of those encountered during wave impact of racing yachts. First, special instrumentation enabled shear strain rates to be measured in-situ. Then a symmetrical shear test was designed to allow shear stress-strain behaviour to be determined at rates corresponding to those measured at sea. Image analysis was used to validate the test set-up. First results for high density foams indicate that quasi-static data may suffice for design.

Eric Lolive, Pascal Casari, Peter Davies
Characterization of the Thermal Expansion Behaviour of a PVC Foam Core: Non-linearities and Gradients Across the Width

A speckle full field interferometric technique is used for characterizing the Coefficient of Thermal Expansion of a PVC foam core through the thickness. A significant gradient is highlighted and is correlated to a gradient in density. A simulation shows the effect of taking such gradient into account on stress level in the core of a sandwich submitted to a temperature variation.

Pascal Casari, Carlos Ferreira, Frédéric Jacquemin
Determination of In-plane and Out-of-plane Elastic Constants For Medium-thickness Sandwich Composite Skins

A full set of elastic constants was determined/estimated for two sandwich composite skin laminates. Out-of-plane compression and shear specimens were cut from skin laminates bonded together to avoid making thick laminates. The approach gives relatively low scatter for the elastic constants and little influence from the bonds as judged from finite element analysis.

Helmuth Toftegaard, Stergios Goutianos
Stochastic Homogenization of Polymeric Foams

The present study is concerned with a probabilistic homogenization analysis of polymeric cellular media to be used as core materials for sandwich structures. The approach is based on a randomized representative volume element in conjunction with a Monte Carlo simulation. The results for stiffness and strength are evaluated by stochastic methods.

Jörg Hohe
Stochastic Finite Element Models of Foam Materials

Driven by the desire to understand the influence of various mechanical properties and geometric features of foam materials, on the stiffness, failure mechanisms and fatigue life of sandwich materials, 3D finite element models of amorphous cellular structures have recently been developed and analyzed. Different algorithms were used to generate seed points of Voronoi tessellations, such as randomly distorted regular lattice distributions and totally random distributions. Periodic boundary conditions were used on representative volume elements containing hundreds of cells. Some comparison with experimental data from real foam materials was made and the agreement was found to be good. In the presented paper discrepancies between the used models and real foams are discussed and it is concluded that the adopted approach has substantial potential although there are some obvious routes for further improvement.

Stefan Hallström, Steven Ribeiro-Ayeh

Applications and Design Solutions

Application of Load Carrying Sandwich Elements in Large Wind Turbine Blades

The present work investigates the possibilities and drawbacks when applying sandwich as opposed to single skin composites in the flanges of the load carrying spar in a future 180 m wind turbine rotor. FEA is applied to investigate two basic designs with single skin and sandwich flanges respectively. For a single skin design, buckling is critical compared to other design criterions. By introducing sandwich, a significant weight reduction and increased buckling capacity is obtained. Tower clearance now becomes critical. Proper choice of core material and thickness is important to prevent face wrinkling and large tip deflection. Geometric non-linear analysis showed sensitivity to imperfections.

Jacob Fisker Jensen, Jacob Pagh Schultz, Christian Berggreen, Kim Branner
Thermoplastic Composite Sandwich Structure for Sportive Applications

Hurling is one of the fastest team games and related to a long term tradition in rules and practice. Former attempts to manufacture composite hurleys by the process of liquid composite molding (RTM) or injection molding failed. With the compression molding process the mechanical properties required, the traditional game performance, and target costs were matched.

M.M. Sommer, M. Päβler, R. Schledjewski, M. Stack
Study of Snowboard Sandwich Structures

The aim of the present research is to extend the knowledge of mechanical properties both on single components and on complete structure employed for snowboard. Flexural and torsion tests are performed to acquire important comparison parameters between snowboard sandwich structures that differ for the core material employed (wood, PVC foam core). A simplified FEM model is proposed to simulate the flexural tests of the sandwich structure showing good predictive capability.

C. Borsellino, L. Calabrese, R. Passari, A. Valenza
Development and Evaluation of an RTM Bicycle Frame

In this paper the complete development and testing of an innovative composite bicycle frame is presented. The frame comprises epoxy resin, glass and carbon fabrics, foam core and metallic inserts produced by the resin transfer moulding technique in a closed mould. The new frame is lighter, stiffer and cheaper from the corresponding aluminium tube frame.

Nikos G. Pantelelis
Development of New Reinforced Load Introductions for Sandwich Structures

This article deals with the design, manufacturing and testing of new innovative load introductions into sandwich structures consisting of fibre reinforced plastics. Vitally new to these load introductions is the principle of local through-the-thickness reinforcements by threads, resulting in an excellent connection of both face sheets and core and, if necessary, also the load introduction element itself. The threads are introduced into textile face sheets and the core by stitching process. After stitching, the textile face sheets and the threads are impregnated with the polymer matrix using a liquid composite moulding process. The resulting mechanical out-of-plane properties of the so-called IDAK load introductions outperform current state-of-the-art load introductions.

M. Alexander Roth
Sandwich Structures with Composite Inserts: Experimental Studies

Studies are presented on the performance of insert assemblies of the sandwich structures with localized through-the-thickness compressive loading. Through-the-thickness and partially inserted fully potted inserts are studied. Insert material considered are: aluminum and 3D woven composite. Experimental results are compared with analytical predictions. It is observed that the specific strength of 3D woven composite inserts is more than that of aluminum inserts.

G. Nageswara Rao, Ullas Agrawal, N.K. Naik
Development and Evaluation of FRP Sandwich Beams Containing Glass Fibers Into Phenolic Foam Core

Phenolic resin has excellent properties of fire resistance, low smoke during burning, and it also has a good balance between its cost and mechanical properties compared with other types of resin used in FRPs. If phenolic resin can be employed as a matrix of FRP, such FRP can have a higher fire safety factor which will be a desirable property in the structures of vessels and railway carriages. However, for the case of the resole type of phenolic resin, water due to condensation reaction remains in the matrix, and this water evaporates resulting in the formation of voids during curing process. In order to develop a new type of phenolic composite that be able to overcome this weakness, we used a foam type of phenolic resin and glass fibers as the matrix and as the reinforcement, respectively. We then developed a new pultrusion technique for the new composite, namely the phenolic foam composite (PFC) and examined its mechanical properties and thermal conductivity. In this paper, we reported a new technology to mold not only a phenolic foam composite but also a sandwich beam in which the PFC as a core and a thin phenolic GFRP or CFRP as facelayers were used. We also examined thermal and bending properties of this sandwich beam.

Goichi Ben, Akiko Shoji, Mituru Souma
Experimental and Numerical Analysis of Hollow and Foam-Filled A-stringer/A-former Under Axial Compression Load and Bending Moment

The topic of this investigation deals with experimental and numerical analysis of different configurations of hollow and of PMI foam-filled A-stringers/A-formers under axial compression load and bending moment. In this connection the Finite-Element-Analysis (FEA) program ANSYS® is used for the numerical approach. The result of this analysis is that PMI foam-filled A-profiles show significant higher buckling loads in comparison to hollow ones. These improvements depend on the loading conditions, e.g. bending moment and axial compression, as well as the geometry of the A-profile.

René Krämer, M. Alexander Roth
Backmatter
Metadata
Title
Sandwich Structures 7: Advancing with Sandwich Structures and Materials
Editors
O.T. Thomsen
E. Bozhevolnaya
A. Lyckegaard
Copyright Year
2005
Publisher
Springer Netherlands
Electronic ISBN
978-1-4020-3848-8
Print ISBN
978-1-4020-3444-2
DOI
https://doi.org/10.1007/1-4020-3848-8