SANDWICH CONSTRUCTION TODAY AND TOMORROW

Howard G. Allen

Department of Civil Engineering, Southampton University.

The materials and fabrication processes currently used for sandwich construction are briefly reviewed, and promising lines of development are suggested. Comments are offered on the theory of sandwich construction and areas where further research would be valuable. The importance of a more systematic approach to testing for fatigue, impact and creep is emphasised. Some interesting examples of sandwich construction are described briefly.

COMPARISON OF OPTIMIZED SANDWICH PANELS OF VARIOUS CONSTRUCTIONS SUBJECTED TO IN-PLANE LOADS

J. R. Vinson

H. Fletcher Brown Professor of Mechanical andAerospace

Engineering, Department of Mechanical Engineering, University of Delaware, Newark, Delaware, 19716, U.S.A.

Sandwich construction plays a very important role in structural composites because of its exceptional stiffness and buckling resistance. Composite materials are increasing in usage because of their high stiffness and/ or strength. Therefore the use of composite material sandwich construction provides structural components of truly superior performance. Discussed herein are the means by which to analyze and structurally optimize sandwich panels of honeycomb, truss core,web core and solid core construction to inplane compressive loads and in-plane shear loads. A comparison of the various configurations is given for one material and loading system.

MECHANICAL BEHAVIOR OF STRUCTURAL SANDWICHES

R.Teti and G. Caprino

Department of Materials and Production Engineering,University of Naples, P.le Tecchio 80, 80125 Naples, Italy.

The core shear modulus of a sandwich may yield, under load, deformations much higher than those predictable on the basis of the usual elastic theory. Creep phenomena may increase these high deformations with time. The presence of ribs in the sandwich highly increases the apparent core modulus. Experimental results are shown to confirm the predictions on the sandwich elastic behavior. Wrinkling and crimping failures are shown to occur in a catastrophic way; an experimental verification of the reliability of the predictions on this type of failure is attempted.

FINITE ELEMENT ANALYSIS OF SANDWICH CONSTRUCTION: A CRITICAL REVIEW

K. H. Ha

Centre for Building Studies, Concordia University, Montreal,Quebec, Canada, H3G lM8.

Many finite element models have been proposed for the analysis of sandwich plates. In general, these elements can be classified into two broad streams. The first is based on the assumed displacement approach, and the second on the assumed-stress hybrid approach. Within each stream, the characteristics of the elements vary greatly in terms of formulation complexity, accuracy and applicability. This paper gives an overview of the state-of-the-art finite element analysis applied to sandwich plate structures.

COMPUTERIZED ANALYSIS AND DESIGN OF SANDWICH CONSTRUCTIONS

J. Backlund K-A. Olsson and F. Maartmann

Dept. of Aeronautical Structures and Materials, The Royal Institute of Technology, Stockholm, Sweden.

Sandwich constructions with high quality cellular plastic as core material and either metal or fibre composite faces are finding applications in an increasing number of vehicle systems. Simultaneously, computer based structural design methods are being developed which significantly facilitate the work for the designer. In this paper, a finite shell element for sandwich constructions is presented, along with applications in static and dynamic analysis and design. A discussion about the future development in this area is also included.

REMARKS ON HIGHER ORDER SANDWICH STRESS AND DEFLECTION ANALYSES

H.-R. Meyer-Piening

Prof. Dr.-Ing., ETH Zurich

Recently, a number of papers have been published on higher order sandwich analyses. Although some contributions may appear to be of only academic interest, there are important conclusions to be drawn with reference to local stress distributions in faces and core, applicability of F.E. results obtained with rather coarse meshes, local deflection and stresses near local load introduction zones, validity of 3- and 4-point bending measurements for stiffness evaluation, edge analysis and design, and large deflection analyses for point and area loaded plates. The comments are substantiated by a discussion of test results obtained from pressure loaded foam core sandwich plates.

ANALYSIS OF SANDWICH PANELS WITH THICK OR FORMED FACES

J.A. Hartsock

Consultant. 619 Sater Lane, Edmonds, WA 98020 USA

A method of analysis is presented for sandwich panels with thick or formed faces. A differential equation is developed and solved. Values are given for computing deflections and stresses on single spans.

NEWER INVESTIGATIONS INTO THE LOAD BEARING BEHAVIOR OF SANDWICH ELEMENTS

A. Burkhardt

Versuchsanstalt fur Stahl, Holz und Steine,University of Karlsruhe.

The determination of the creasing stress of sandwich elements with thin metal facings and supporting cores made of polyurethane solid foam is discussed. Theoretical values are compared to test results. The behavior of each viscoelastic material under a continuous load is investigated. Creep functions are indicated and are used to facilitate the calculation of the time dependent deformation The determination of the foam's characteristic values is treated. Critical items which still need attention are briefly mentioned.

THE USE OF MODELS FOR FOAM CORE BEHAVIOUR IN THE DESIGN OF SANDWICH PANELS

L. J. Gibson

Department of Civil Engineering, Massachusetts Institute of Technology,Cambridge, Massachusetts 02139, U.S.A.

Foam core sandwich panels are increasingly being considered for use in building applications because of their high stiffness per unit weight and their excellent thermal properties. The optimum design is that which gives the lightest or least expensive panel for the given requirements (eg. stiffness, strength or thermal conductivity). The optimization can be carried out with respect to the face and core materials, the face and core thicknesses, and, since foams have a continuously variable density, the core density. Models for the behaviour of foamed materials, describing their properties in terms of the solid cell wall properties, the volume fraction of solid and the cell geometry, provide the basis for optimising panel design with respect to the core density. Here, the models for the behaviour of foam core materials are first reviewed. We then show how the models can be used in the minimum weight design of sandwich panels for a given stiffness or strength. Finally, we describe a case study on the design of roof panels for housing.

MICROCOMPUTER SOFTWARE FOR DIMENSIONING COMPOSITE SANDWICH STRUCTURES

O.J. Saarela

Faculty of Mechanical Engineering, Helsinki University of Technology.

A microcomputer software package for dimensioning composite sandwich structures is under development. Ease of use, efficiency and a wide variety of analysis capabilities were the requirements set for the software. Linear, analytical solutions for laminate and sandwich structures are the basis for the analyses. The software is developed for IBM PC/XT/AT computers and compatibles. The programming language is Microsoft C.

WAVE PROPAGATION IN SANDWICHPLATES. SOME DYNAMICAL AND ACOUSTICAL ASPECTS

A.C. Nilsson

Dept technical AcousticsRoyal Institute of Technology, Stockholm

Some dynamical and acoustical properties of sandwich plates are investigated. The types of sandwich elements discussed are three layered plates with a thick light weight core with thin and comparatively stiff laminates bonded to each side of the core. Wave-numbers, loss factors and apparent bending stiffness for symmetric and asymmetric plates are derived. In addition the sound transmission loss for sandwich plates is discussed. Measured and predicted results are compared. It is found that bending stiffness and loss factor not only depend on material parameters and plate geometries but also on frequency.

MEETING FUTURE DESIGN CHALLENGES WITH ADVANCED CORE MATERIALS

F.BRINKEN AND G.REIF

AIREX Ltd

The paper describes the state of the art of foamed core sandwich technology. Aspects of design constraints and hints for optimum choice of core materials are discussed. Special emphasis is laid upon damage tolerance and flammability/environmental aspects.

CORRELATION BETWEEN PHYSICAL CHARACTERISTICS AND CHEMICAL COMPONENTS OF EXPANDED PVC

L.Lauri

Director of Pelf factory -Villanova di Longarone (Belluno-Italy), Pelf Headquarters : Via Frigimelica,2 Padova - Italy

In our report at the Seminar held in Padova last April on "CORE MATERIAL IN BOAT-BUILDING SANDWICH CONSTRUCTIONS", we dedicated a few words to the physical features of a core material, taking into account: effects of excessive solvent absorption; -concepts of resistance to distortion due to temperature and dimensional stability in relation to temperature;-use of plasticizers;-foaming agents, especially with regard to CFC used in PVC foam. As our efforts were appreciated by many designers and builders, we have decided to go into these aspects in more depth, analyzing the connection between the chemical formula and the PVC foam end-product in view of different final uses. This connection is of the utmost importance because it avoids a core material, developed for a certain use, presenting totally different and unwanted final features, giving rise to: -complications and even severe problems. -accusations of unsuitability for a given use of the core material, when the answer lies in the original chemical formula.

SUPERPLASTICALLY FORMED SANDWICH CORES

C.E.S. Ueng

Professor, School of Civil Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0355, USA

This paper presents a survey on the superplastic behavior of certain sheet alloys which can be used for the forming of sandwich cores. The state of the art of this method is presented. The potential of this approach in conjunction with diffusion bonding technique is discussed, the important related research work is suggested, and the future trend is projected.

ONE- AND TWO-COMPONENT POLYURETHANE ADHESIVES FOR BONDING SANDWICH ELEMENTS

C.Strobech

Casco Nobel A/S, Praestemoseve 2-4, DK-3480 Fredensborg, Denmark, Phone:+45 42281066,Fax: +45 42 282990, Telex: 40251 Casco dk

The different types of one- and two-component polyurethane adhesives are described with respect to open time, pressing time, consistency and application. The method for determination of the pressing time using flexural strength methods combined with tensile strength of the foam is shown as is the long-term resistance of two different adhesives in water at 600C.

THE USE OF TOUGH RESIN SYSTEMS FOR IMPROVED FRAME TO HULL BONDING IN GRP SHIPS

L.S. Norwood

Scott Bader Co Ltd, Wollaston, wellingborough, Northants, UK.

A major concern in any ability to resist peel and shear loading; this becomes a more difficult problem where long delays in laminating have occurred or when dissimilar materials are joined together. In the case of mine countemeasure vessels, adhesion between stiffener and hull becomes critical when the ship is subjected to underwater shock loading. In order to reduce costs and improve the hull-stiffener bond, tough resin system based on urethane acrylates have been developed to replace the mechanical crack-stopping system currently employed. This paper describes the benefits of urethane acrylates in the fram-hull application discussing the level of improvemnt over the mechanical system.

TEST PROCEDURES FOR FOAM CORE MATERIALS

K-A. Olsson and A. Lonno

Department of Aeronautical Structures and Materials. The Royal Institute of Technology, Stockholm, Sweden

Research and development concerning sandwich design, especially for marine applications, has been going on for many years at the Department. A special interest has been focused on the structural requirements and the testing of cellular rigid foams as core materials. It must be kept in mind that these foams are a construction of thin cell walls. The maximum capacity in the different load cases is limited by the buckling strength of the cell walls. The structural properties of a polymeric material is always dependent on the loading rate and on the temperature and sometimes dependent on humidity. These factors cannot be neglected for cellular foams, and have to be considered in the test specifications and even in the selection of test equipment.

EVALUATION OF THE MECHANICAL PROPERTIES OF POLYMERS USING ADVANCED TESTING TECHNOLOGY

R. Bardenheier and M. Cordes

Carl Schenck AG, D-6100 Darmstadt, P.O.Box 4ol8,W. Germany

Acoustic emission monitoring and analysis of fiber reinforced thermoplastics are described here, as well as the mechanical boundary conditions to run proper tests. A newly developed, high speed, servohydraulically driven impact test machine, suitable for studying the impact behaviour of polymers, is presented.

THERMOGRAPHIC NDT OF FOAM CORE MATERIALS

M. Vikstrom

Research Associate,Department of Aeronautical Structures and Materials, Royal Institute of Technology, S - 100 44 Stockholm, Sweden.

As a result of development in materials and of design concepts, composite structures have become more advanced and sometimes quite complex. During manufacturing of rigid PVC foams, commonly used as sandwich core, cavities will at times occur. These cavities cause stress concentrations and must therefore be located. Non-Destructive Testing (NDT) is a key ingredient in quality control. This paper suggests an NDT method for detecting cavities in rigid PVC foam. The method, passive thermography, is based on a scanning InfraRed (IR) detector. Experiments with simulated cavities of different sizes and with different depths beneath the surface have been carried out on materials of various densities.

THE DESIGN OF FRP SANDWICH PANELS FOR SHIP AND BOAT HULLS

R P. Reichard

Structural Composites Lab, Florida Institute of Technology,Melbourne, Florida, USA 32901

Naval architects presently rely on beam analysis techniques to predict the flexural response of panels. This has proven to be acceptable for isotropic materials, but serious problems are encountered when attempting to apply these methods to FRP sandwich laminates. Beam equations do not account for skin membrane effects, and shear stress levels in the core material of sandwich beams are much higher than those found in the core of sandwich panels. Several examples of data from beam and panel tests are presented to illustrate the differences between them. The limitations of the beam approach to designing sandwich panels are discussed, and an alternative approach using panel tests and models is suggested.

THE INFLUENCE OF FLAWED BUTT-JOINTS IN FOAM CORE SANDWICH BEAMS

D. ZENKERT and H. L. GROTH

Research and Senior Research Engineer, Department of Aeronautical Structures and Materials, The Royal Institute of Technology, S - 100 44 Stockholm, Sweden.

In the manufacturing of foam core sandwich structures, butt-joints are frequently used for the core. However, in the manufacturing process such butt-joints might become flawed. Hence, stress concentrations, such as sharp comers, will appear that may have fatal influence on structural behaviour. The present paper deals with the analysis of such flawed butt-joints. The analysis is based on a fracture mechanics approach. It is assumed that sharp bi-material corners will be present, either between the core and face material or between the core and the joint material. Such corners create singular stress fields which are used together with a suitable fracture criterion to predict structural failure. The results from standard shear tests are used to predict fracture loads of sandwich beams with simulated flaws.

WAYS OF ENHANCING THE IMPACT RESISTANCE OF CONVENTIONAL SANDWICH MATERIALS

P.A.GRADIN, P G. HOWGATE

Swedish Plastics And Rubber Institute, S-851 71 Sundsvall, Sweden

Investigations into the impact behaviour of elastomer modified sandwich panels have shown a promising improvement. The elastomer modification took the form of a thin, aramid reinforced elastomer layer placed between an outer glass fibre laminate and a foamed PVC core. A numerical analysis, using a second order FEM model shows the reduction in local shear stresses for the modified construction. Experimental examination of the residual strength and stiffness of panels under various impact loading conditions gave inconclusive results for both the modified and unmodified cases, although, general trends point to an improvement.

THE ROYAL AUSTRALIAN NAVY INSHORE MINEHUNTER - LESSONS LEARNED

B.L. Robson

Naval Engineering Services Branch, Department of Defence, Canberra, Australia

Lessons learned from the design, construction and evaluation of two Prototype Inshore Minehunters for the Royal Australian Navy are presented. These lessons are consequent upon the selection of a glass reinforced plastic foam sandwich composite as a construction material.

THE DEVELOPMENT OF THE GRP-SANDWICH TECHNIQUE FOR LARGE MARINE STRUCTURES.

S.-E. Hellbratt, O. Gullberg

KARLSKRONAVARVET AB, S-371 82 Karlskrona, Sweden

A review of the research and development program of the GRP-Sandwich concept is given. Examples of both theoretical and experimental investigations are presented as well as full scale panel tests and ship trials. The production experience of a series of six minesweepers, the LANDSORT-class, built in GRP-Sandwich is also discussed.

A number of different types of vessels built according to the GRP-Sandwich method are presented. One is the STANDARD FLEX 300 for the Danish Navy, and another is the light weight Surface Effect Ship M RIDER. Examples are given of other applications that are under development or already in production.

STRUCTURAL OPTIMIZATION OF A SURFACE EFFECT SHIP

G.O.F. Romell et al

Department of Aeronautical Structures and Materials, The Royal Institute of Technology, Stockholm, Sweden.

A design study, using the structural optimization system OASIS-ALADDIN, has been performed on the hull of a Surface Effect Ship made of GRP/PVC sandwich. The object was to minimize the weight of the ship by determining the optimal web spacing, as well as the skin and core thicknesses, in different parts of the hull. Constraints on panel stiffness and maximum stress were included to meet the rules of Det Norske Veritas. Four different load cases complying with classification rules were considered. The twostep approach that was used is described together with results and comments on the analysis.

EXPERIENCE OF FRP SANDWICH IN NORWEGIAN SHIP BUILDING

U Kristiansen

A.S. DICAB, P 0 Box 228, N-1364 HVALSTAD, Norway

An overview is presented explaining why composite materials are in use and why they will be in common use in marine transportation systems in the future.

SUPERSTRUCTURES OF LARGE SHIPS AND FLOATING OFFSHORE PLATFORMS BUILT IN FRP-SANDWICH - A FEASIBILITY STUDY

A.Y.J. Ulfvarson

Department of Marine Structural Engineering, Chalmers University of Technology

Low weight for vehicles has always been important. FRP-sandwich offers a combination of light weight and insulation that seems to be cost effective. Widespread application is prevented by rules of international agreements on safety at sea. In this study advantages and problems with FRP-sandwich are discussed. Further research and development work to support proposals to change present regulations are needed.

RATIONAL DESIGN OF LARGE SANDWICH STRUCTURES

J Rasmussen & J. Baatrup

Department of Ocean Engineering, The Technical University of Denmark

A rational computer aided design procedure for large sandwich structures (e.g. maritime structures) dealing with material characterisation, structural response, failure assessment and sensitivity analysis is discussed. In order to obtain an integrated procedure, the material characterisation is facilitated by micro mechanics inodels. Examples of these are given. In the procedure presented here the structural response is calculated by static and dynamic FEM analysis. Failure assessment is based on various local strength and stability criteria. Sensitivity analysis based on design variables relevant to composite sandwich elements is discussed. As an example a structural analysis of a composite sandwich structure analysed by the procedure is presented.

CONSTRUCTION AND DESIGN DETAILS OF THE 1988, AMERICA'S CUP YACHT HULL AND DECK STRUCTURES

C.Riley

Erskine-Johns Company Los Angeles, California, USA

In the 1988 event, the hull and deck of "Stars & Stripes" and "New Zealand" were primarily of sandwich construction. The authors were responsible, in part, for the laminate engineering of the two very different contenders. They present here an overview of the design constraints under which they worked and the resultant solutions adopted.

THE APPLICATION OF EXTRUDED POLYSTYRENE FOAM AS AN INSULATING CORE MATERIAL IN SANDWICH CONSTRUCTIONS.

K. Bukowski, P. Conway

DOW Europe, Foams & Construction Products Department, Rheinmanster, West Germany.

The paper describes the physical properties of a range of Styrofoam extruded polystyrene foam and how these properties can be used to a good effect in the core of sandwich panels. The results of large scale panel testing are discussed in detail and are presented together with an interpretation of the actual performance versus theoretical predictions. Results are also presented on dynamic fatigue load testing of Styrofoam core sandwich panels compared to similar panels with polyurethane foam cores and on full-scale testing of a refrigerated truck.

DEVELOPMENT OF THE TELE-X RECEIVING ANTENA MAIN REFLECTOR

H.Wilhelmsson

Ericsson Radar Electronics, Antenna Systems,

Large reflector antennas for space applications usually require the use of composites and sandwich construction. The TELE-X receiving antenna utilizes UHMS carbon fibre reinforced face skins and aluminium honeycomb. This design provides sufficient stiffness and profile accuracy for the launch and space environment in the TELE-X mission. The receiving nain reflector is a part of a non rotational synrmtric paraboloid. The size is 1.7 x 0.8 m with an elliptical contour. The reflector was manufactured using the prepreg technique and a thermlly stable autoclave mold. Three models have been made, one STM for structural testing, one ETM for electrical testing, and one PFM for flight. The reflector has been tested and evaluated using NDT,vibration testing, 3-D coordinate profile measurements and thermal vacuum testing.

CHARACTERIZATION OF FACE SHEET/CORE DEBONDING OF COMPOSITE SANDWICH BEAMS

L.A. Carlsson

Department of Mechanical Engineering, Florida Atlantic UniversityBoca Raton, FL 33431, U.S.A.

Debonding shear fracture of composite sandwich beams is investigated herein. A new test specimen that subjects the facing(core interface to a shear stress singularity is introduced. This specimen is examined analytically by shear deformation beam theory and experimentally on glass/polyester on balsa core sandwich beams.

A NEW STEP TO EASIER PRODUCTION OF HIGH QUALITY SANDWICH STRUCTURES

M.J.L. van Tooren

Faculty of Aerospace Engineering, Delft University of Technology.

The main topic of this paper is the production of sandwich panels with thermoplastic materials. A new technique has been developed which utilizes foaming hot-melt films as a replacement for epoxy adhesive films. These films also make feasible a simple, one-step production of sandwich panels with foamed cores. The production of the films will be discussed, and some applications will be shown. The new technique is attractive for application in both secondary and primary aircraft structures and may find wide application in many other engineering areas.

AUTOMATED AND CONTINUOUS PRODUCTION OF SANDWICH PANELS

C.Nilsson and P Svensson

Man. Dir of A-SANDWICH AB, Soraker.Box 135, S-312 22 Laholm, Sweden.

A new method for continuous production of large scale sandwich panels is presented. Many common manufacturing problems are overcome by the direct lamination/bonding in a vertical process. Hereby, panels can be manufactured with a uniform high quality.