BUCKLING ADD BENDING OF RECTANGULAR ORTHOTROPIC SANDWICH PLATES

J C Roberts, G Bao, M Critchfield, J Beach and S Bartlett

The Johns Hopkins University , Applied Physics Laboratory and Department of Mechanical Engineering.

Sandwich orthotropic fiber reinforced plastic (FPP) plates were tested for buckling by in-plane compression and for stresses and deflections tunder uniform out-of-plane pressure. The sandwich plates were 122 cm (48 inches) long x 91 cm (36 inches) wide and 2.54 cm (I inch) thick with a 1.91 cm (0.75 inch) thick balsa core. The two loaded edges of the unstiffened and sandwich plates were clamped, while the two long edges were simply supported. The buckling load, as well as stresses and deflections from the tests were then compared to those from finite element analysis (FEA) and analvtic solutions. Approximate solutions to the coupled sandwich/face plate equations are available. However it was thought that for this analysis averaging the flexural rigidities (Dij) of the balsa core/face sheet plate through-the-thickness by integration of the individual stiffness matrices Qij would give better result when used as average flexural rigidities in solid plate equations with the same boundary conditions. There was poor agreement between FEA and experimental elastic buckling results for the sandwich plates because they failed in local buckling prior to global buckling. Even though deflections and stresses are more of a point function within the laminate there was better agreement between anaIytic and FEA solutions for stresses and deflections due to out-of-plane uniform pressure. There was also better agreement between FEA and experimental results in the stresses and deflections due to uniform out-of-plane pressure. Experimental error could be traced, in part to the pressure bags used to apply out-of-plane pressure creating non-uniform pressures in the corners, binding at the plate edges during in-plane loads due to the simple supports, and a coarse resolution bag pressure gage. In future work, when thinner sandwich plates of balsa wood and foam cores are evaluated for in-plane compression and out-of-plane pressure, a new pressure gage with greater resolution will be installed. A solid lubricant filled grease will be used along the simply supported edges to prevent binding, and higher bag pressures will be applied to ensure uniform pressure across the entire plate area.

NONLINEAR DESIGN SCHEDULE AND PROPER FAILURE CRITERIONS FOR LARGE SANDWICH STRUCTURES.

Ame H. Jorgensen

DANYARD AALBORG A/S, Speditorvej 1, DK-9000 Alborg.

Resent research within nonlinear and high order behavior of sandwich structures have resulted in several analytical formulations describing large sandwich panels consisting of orthotropic core and composite plies of laminate. The results of nonlinear analysis have generally shown that the margin to failure is higher than anticipated by linear analysis. This method, which is based on the principle of minimum potential energy, is simplified into a spreadsheet post processor for FEM analysis, that may allow full analytical calculation of all significant failure modes, resulting in a comprehensive evaluation of large sandwich structures.

NON-LINEAR FORMULATION FOR SANDWICH SHELLS

A J M Ferreira, J T Barbosa,J C de Sa and A T Marques

Departamento de Engenharia Mecanica e Gestao Industrial, Rua dos Bragas, 4099 Porto Codex PORTUGAL

A non-linear formulation for general, arbitrary sandwich shells is presented. The formulation accounts for geometric non-linear and material non-linear behaviour of the shell. The first and the third order shear deformation theories are presented as far as the kinematics are concerned.The formulation is implemented in both the Marguerre and the Ahmad shell elements. The first one is more adequate for shallow shell, and the second one is a more general element. The formulation is based on the displacement formulation of the finite element method and relies on a five-degree of freedom per node (first-order) and on seven degrees of freedom per node (third order). The formulation is discussed on some examples, namely on a cylindrical shell subjected to compressive loads (buckling behaviour) and on a three point bending beam.

ANALYSIS OF SANDWICH BEAMS BY 2D ELASTO-PLASTIC FORMULATION AND A JOINT ELEMENT

L C Sousa, A J M Ferreira*and J C Si

Departamento de Engenharia Mecanica e Gestao Industrial, Universidade do Porto, Rua dos Bragas, 4099 Porto Codex,

Sandwich structures are usually analysed by different methods of finite element representation. Most of them apply an ideal interface that will not collapse during deformation. As the core and the adhesive are weaker than the skins, it is expected that the performance of the adhesive will play a significant role. In this paper, it is proposed an analysis of sandwich beams that considers the plasticity of the core and of tile skin. The 2D analysis considers an elasto-plastic formulation based on an associative formulation. It is proposed also the inclusion of a joint element at the skin-core interface. This element may collapse plastically, and debonding will occur. Results so far illustrate the need of such refined analysis.

DYNAMIC ANALYSIS OF SANDWICH STRUCTURES WITH DAMPING

E. Barkanov, R. Rikards and A. Chate

Institute of Computer Analysis of Structures, Riga Technical University, Latvia

Finite element analysis of sandwich structures with viscoelastic layers is performed. Structures like these are introduced to obtain systems with high energy absorbing properties. Damping properties of the structure have been evaluated by the energy method, the method of complex eigenvalues and from the resonant peaks of the frequency response function. The time domain behaviour of the structure has been presented from the frequency domain response by the fast Fourier transform technique. The frequency dependent damping is examined. Numerical examples of the free vibration, frequency and transient response analyses are presented for the sandwich beam and sandwich cylindrical panel.

FATIGUE CRACK GROWTH IN RIGID PVC CELLULAR FOAM UNDER COMBINED MODE-I AND MODE-II LOADING

P.M.C. Noury, R.A. Shenoi, and I. Sinclair

Department of Ship Science, University ot'Southampton Department of Engineerina tvlaterials, University of Southampton

The paper report on experiments performed on sharp-cracked specimens using the compact-tension-shear (CTS) test technique. Loading conditions ranging from pure mode-I (opening mode), mixed-mode, to pure mode-II (shearing mode) are generated using a simple loading device. Linear elastic fracture mechanics (LEFM) and plane strain conditions are assumed. Crack path g , da/dN versus D K and crack growth behaviour for each loading condition is ascertained. Fatigue data are compared with results obtained from static tests in similar configurations.

HIGH-PERFORMANCE FOAM CORES BASED ON PMI. DEFINITION OF STATUS AND RECENT DEVELOPMENTS

Hermann F. Seibert

Rohm GmbH, Darmstadt, Department H7-T-S

The present paper defines the position of a family of already very well established rigid foams based on PMI. It will be shown that PMI is widely used as a core material for demanding sandwich structures in many fields of application, e.g. air- and spacecraft, railway rolling stock and ships. Typical applications will be discussed to characterize the performance of these particular foam plastics and to show that they significantly contribute to weight and cost savings. Most recently, two new PMI grades have been developed which will be discussed in this paper.

ON FATIGUE CHARACTERISATION OF LOW DENSITY RIGID PVC/POLYUREA FOAM

M. Danielsson L Ottosson and M. Stibe

Albany Nordiskafilt AB, Box 510, 302 880 Halmstad, Sweden

Fatigue evaluation on rigid foam core material in three different modes, compression, tensile and shear has been made. It was shown that fatigue compression or buckling occurs at lower stress levels than the fatigue cohesive tensile failures for the same material. Comparing the relative stress levels for failure, foam material subjected to tension fail at lower numbers. But when looking at the absolute stress levels it was clear that the material could stand higher loads when tested in tensile mode. This confirms the general thought that buckling or compression is the limiting property of such foam in shear fatigue. It was also shown that cell geometry and the properties of the polymer matrix characteristics might have an influence on fatigue resistance. However, density variation within the materials was found to be one of the key parameters to consider in controlling fatigue failures since most material properties for rigid foam materials varies linearly with density. A low degree of plastification of the foam core matrix material has a significant negative effect on shear fatigue resistance. This affect is caused by heat generation in plasticised areas from hysteresis creating failure areas close to the skin material.

VACUUM INSULATION CORE

J. Carrere

Dow Deutschland Inc.

INSTILL™ Vacuum Insulation Core, developed by The Dow Chemical company, is a patented open cell, extruded polystyrene foam. Used as the core material in VIPs, it delivers 3 to 7 times the insulation capability of conventional systems. The INSTILL™ solution allows to produce VIPs at the lowest cost: the pressure necessary to reach the thermal performance (~ 5 mW/m. ° K) is higher than with other materials and therefore cheaper to achieve; pre-conditioning treatment is limited and contributes also to the lower cost of VIPs production. The application domain of INSTILL™ based VIPs is the one of low temperatures (negative temperature up to 70'C) where only limited space is available: refrigerators, refrigerated trucks, isothermal boxes, building applications. The list is not exhaustive.

lMPROVED STONE WOOL CORE FOR METAL FACED BUILDING SANDWICH PANELS

J. Rasmussen

Department Manager, Product Development, Rockwool International A/S

A general comparison between stone wool and plastic foam core material for metal faced building sandwich panel is given. It is a well known fact that stone wool cored sandwich panels provide a superior fire safety in buildings compared to foam cored panels. But what have then been the main obstacles until now for even further expansion of the use of stone wool core materials for sandwich panels in building construction? One of the major items have been the production difficulties compared to e.g. continues production of polyurethane foam cored panels on high capacity production lines. The development of a stone wool core material that solves this problem is the main Subject for the present paper. Related improvements regarding strength, aesthetics and moisture are also presented.

INNOVATIVE SANDWICH STRUCTURES BASED ON WOVEN AND KNITTED DISTANCE FABRICS.

I. Verpoest, J. lvens, D. Philips andH Judawisastra

Department of Metallurgy and Materials Engineering, Katholieke Universiteft Leuven de Croylaan 2, 3001 LEUVEN (Belgium)

Some ten years ago, we have introduced the concept of sandwich structures, based on three-dimensional or distance woven fabrics. Five years later, we have shown that distance knitted fabrics show some additional advantages for application in composite sandwich structures. In this paper, we will present a review of the most recent developments in the processing, properties and applications of this now class of sandwich structures. Woven sandwich structures offer two important advantages. which are both related to the integral core-skin structure. First, the processing cost is drastically reduced. Second, skin-core debonding is virtually impossible. Both advantages have been essential in opening new application areas for sandwich structures, in the marine, transport and construction area; an overview of these applications will be presented. Secondary properties like fatigue and impact resistance, thermal and acoustical isolation and fire resistance will be addressed as well. Knitted distance fabrics offer two additional advantages. First, the knitted loop structure provides a build-in deformability, and hence, double curved structures can be produced easily. Second, the knitting process allows a wide variety of skin structures, from fully closed skins to very open (honeycomb-type) net structures. It will be explained how a proper choice of knitting yarns can lead to strong, stiff and impact resistant sandwich stuctures, which are still very open and hence offer the additional advantage of ventilation. Some provisional applications will be highlighted,

SHEAR STRENGTH OF BALSA-CORED SANDWICH PANELS

D. McGeorge and B. Haynian

Det Norske Veritas, N-1322 Hovik, Norway

The results of an experimental study of the shear strength of balsa-cored sandwich beams and panels are presented. The main findings of the study were: there are important size effects in balsa cores; bending superimposed on the shear load reduces the shear strength; good agreement was shown between tests on beams and panels. The shear strength of balsa measured according to the block shear test method is higher than for beams and panels by a factor of 2 to 3 for typical high-speed craft configurations. Based on the test results, a simple model for the shear strength of balsa-cored sandwich beams and panels is proposed.

ASSEMBLY OF SANDWICH PANELS BASED ON LOW-COST RECYCLED POLYPROPYLENE EXPANDED FOAM.

D. Hearn, E. Valot & G. Verchery,

ISAT, University of Burgundy, BP 31, 58027 NEVERS CEDEX, France.

This study examines the possibility of using a recycled thermoplastic foam type material in the assembly of industrial sandwich panels. Several specimens of panel were made both with aluminium and GFRP skin materials bonded to 40 mm thick moulded polypropylene core blocks of approximately 60 kg/m³ density. Identical specimens were made based on 50 kg/m³ density rigid polyurethane foam, which is an industry standard, for direct comparison purposes. All panels were assembled through an intermediate bonding step with polyurethane adhesive, which is again widely accepted industrial practice. Static 3-point bending, oligocyclic 3-point bending and adhesive tensile strength tests were used in assessing overall performance. A fairly good match was found between core materials in static testing. However the other test results indicated that bonding on the thermoplastic material would need to be improved by either some mechanical or physical surface treatment or some modification to the moldcd surface aspect. Work is continuing on this and the shock absorption characteristics of sandwich material.

MODELLING OF METAL FOAMS

J.L. Grenestedt

Department of Aeronautics, KungI Tekniska Hogskolan, S-100 44 Stockholm, Sweden.

The ability to manufacture metal foams, in combination with the potential for very exciting mechanical properties including stiffness and strength, have aroused quite a bit of interest recently. However experimentally measured stiffnesses and strengths of closed cell aluminum foams fall short of what may be expected by extrapolating properties of common closed cell PVC based foams. The shortcomings are believed mainly to be due to geometric "imperfections" in the cell morphologies of the aluminum foams. In this paper, a "perfect" model is first introduced and shown to predict properties of PVC foams well. Three different kinds of imperfections are then introduced, one of which appears to successfully explain the poor properties of aluminum foams.

PASSIVE FIRE PROTECTION OF STRUCTURAL COMPOSITES

B. Hoynin and J Taby

FiReCo AS, Fredrikstad, Nonvay

The International Maritime Organisation (IMO) Code of Safety for High Speed Craft (HSC-code) has introduced very strict requirements for the passive fire safety for vessels made of composite materials. This paper summarises the IMO requirements related to fire reaction and fire resistance for a composite vessel. It is shown that requirements in many cases are much stricter than for conventional vessels. This paper also discusses the relation between different structural materials and structural integrity during a fire. It is shown that sandwich structures can meet the new requirements with little or no extra weight.

BASIC SANDWICH PANEL TESTS

T. N. Bitzer

HexceI Corporation, Dublin, California

This paper deals with the basic mechanical property tests for sandwich panels and honeycomb cores. The ASTM Standard Test Methods used in the United States are discussed and potential test problems pointed out. Tests discussed include flatwise compression, edgewise compression, flatwise tension, flexural bending, beam shear, plate shear, and climbing drum peel. The objective of this paper is to provide the layman with the basic knowledge of the various test methods used to test sandwich panels.

FOAM-CORED SANDWICH PANELS UNDER STATIC PRESSURE LOADING: SOME NEW 'TESTS AND ANALYSES.

B. Hayman, M. Wiese, P. Davies, D. Choqueuese, B. Hoyning and P. Mitusch

Det Norske Veritas, Norway

Four sandwich panels with PVC foam cores and GRP skins have been tested under increasing lateral pressure loading. The panels were extensively instrumented with strain gauges both on the skin laminates and in the core. Acoustic emission sensors were also installed. The tests were carefully designed to provide data suitable for use as reference data in benchmark trials of analysis methods. The results were extensively analysed and compared with a series of finite element simulations in both linear and nonlinear ranges. The paper describes the tests and analyses and presents conclusions and recommendations regarding both experimental techniques and FE analysis methods.

FRACTURE TOUGHNESS OF CORE SKIN INTERFACES OF SANDWICH MATERIALS

A.T. Etchermeyer and D. McGeorge

Det Norske Veritas, Veritasveien 1, N-1322 Hovik, Norway

This paper investigates how damages in the form of core/skin delamination propagate. The results can be used to classify the resistance of materials against spread of this type of damage. They could also be used as a basis for deciding whether to repair a structure. Mode I fracture toughness data was measured for the interface between GRP skins and cores of PVC foam and balsa wood in sandwich beams. A new test method was developed. Depending on core material and interface quality, the cracks propagated in or close to the interface. Due to nonlinear behaviour observed in the tests, a special analysis procedure was used to provide the fracture toughness. When the crack propagates inside a PVC core (along the interface) this method gives G ic values similar to published values for the core material.

A TEST SPEClMEN FOR DETERMINING THE FRACTURE RESISTANCE OF A FACING/CORE INTERFACE

Xiaoming Li and L. A. Carlsson

Department of Mechanical Engineering, Florida Atlantic University (FAU), FL 33431, USA

Debond fracture of foam core sandwich is approached by using a mixed mode fracture test called Tilted Debond Sandwich (TDS) test. The purpose of the test is to promote debonding and suppress crack deflection into the core (kinking). For certain specimen and precrack geometries, it was found experimentally that there exists a unique critical tilt angle 0_c. A crack initiated at the facing/core interface will kink into the core if the tilt angle 0 < 0_c, and it will maintain its debonding path when 0 = 0_c The angle 0, decreases with increased crack length. The fracture toughness for debonding, expressed as the critical strain energy release rate, increased with increased magnitude of the shear component.

EXPERIMENTAL ANALYSIS OF SANDWICH PLATES UNDER COMPRESSION, SHEAR AND COMBINED LOADING

B. Castanie, J. J. Barrau, S. Creze, andJ. P. Jaouen

Sup'Aero, Laboratoire Structures, 31055 Toulouse, France

An original testing procedure specifically used for asymmetrical sandwich technology is described. Tests have been conducted successfully on nonimpacted plates as well as on impacted plates. The test plates were subjected to either compressive loading either shear loading or combined compression/ shear loading. The results obtained by the tests showed validity of the procedure and showed the structural efficiency of asymmetric sandwich structures. The procedure also revealed a stabilized crack growth witn impacted plates.

A SHEAR GAGE DEVICE FOR MEASURING OF THE SHEAR STRAIN IN THE CORE OF SANDWICH PANELS

Anders Lonno

FMV, Swedish Defence Material Administration, S- 1 15 88 Stockholm Sweden

The paper presents a device for measuring the shear strain in the core of a sandwich construction. The device, that has been invented, developed and patented by FMV, is based on the insertion of a stick through one of the skins into the core of the sandwich panel. The angle between the plane of the sandwich panel and the stick will change when the core of the sandwich panel is loaded in shear. The angle alteration is proportional to the shear strain in the core. The geometric angle is transduced to an electrical signal i.e. by the use of a position transducer. The device can be used for all types of core materials such as foam, honeycomb and balsa. The paper presents the device, test results, calibrations, and experiences from use in full scale shock tests and boat hull wave impact tests in marine environment.

NONDESTRUCTIVE INSPECTION AND EVALUATION IVIETHODS FOR SANDWICH PANELS

J. R. Weitzenbock, A. T. Echtermeyer, F. Artiga-Dubois and M. Parmar

Det Norske Veritas, Norway

The paper reports on a test and evaluation programme to compare different non-destructive inspection and evaluation methods for the inspection of sandwich panels. A large number of panels were made for benchmark testing. The specimens contained defects of various sizes which were located at different depths. These specimens were tested in the laboratory using ultrasound, thermographic, microwave and mechanical methods. In addition field tests were carried out in ship yards to determine the performance of the chosen NDE/NDI methods in a production environment. The paper summarises the results, comparing the different methods and showing their respective capabilities and limitations.

SANDWICH MANUFACTURING: PAST, PRESENT, AND FUTURE

B. T. Astrom

Department of Aeronautics, Kungl Tekniska Hogskolan, Stockholm, Sweden

Sandwich components and structures have traditionally been manufactured through very labor-intensive techniques, meaning that few sandwich products have ever been produced in long series. The norm is one-of-a-kind products such as competition yachts and ships, or components for limited-series products, such as aircraft, boats, and competition cars. Even in such applications, traditional manufacturing techniques are graduaIIy being replaced by semi-automatic or more rational alternatives offering cost reductions and propertv improvements. Much higher production rates are nevertheless necessary to make the sandwich concept economicallv attractive for mass production of automobiles and other commodity applications, meaning, that other materials and completely different manufacturing techniques tend to be required. This paper briefly reviews mature manufacturing techniques, discusses techniques which are currently facilitating a gradual transition to significantly shorter cycle times, and finally elaborates upon materials and techniques which are thought to represent the future.

DESIGN AND MANUFACTURE OF MONOILITIHIC SANDWICH STRUCTURES WITH CELLULAR CORES

A.S. Herrmann

German Aerospace Center, Institute of Structural Mechanics, Braunschweig

One of the most major disadvantages of sandwich structures with foam core is the limited shear strength of the core. Also for space applications any air enclosed in the core as well as in the skin laminates must be avoided, otherwise low pressure environmental conditions may cause a destruction of the surface and the whole component by expanding of the enclosed air. This leads to the requirement for additional manufacturing step. All these disadvantages are avoided by using a new monolithic Sandwich design with cellular cores. The design is promoted by a new manufacturing method the so called Differential-Pressure Resin-Transfer-Moulding-Technique (DP-RTM).

AN ANALYTICAL AND EXPERIMENTAL INVESTIGATION OF SANDWICH COMPOSITES SUBJECTED TO LOW-VELOCITY IMPACT

T. Anderson, F. Madenci, and J. C. Fish

Department of Aerospace and Mechanical Engineering, The University of Arizona, Tucson, Arizona 8572 1, U.S.A

Previous studies have established that the low-velocity impact phenomenon can be studied without explicitly including the dynamic contact event for a known contact force and depth of indentation relation. Therefore, this study presents a three dimensional analytical solution for the stress and displacement fields in a sandwich panel subjected to quasi-static indentation through a rigid sphere. Unlike the usual assumption of a Hertzian-type distribution, the sphere's unknown contact area and pressure distribution due to indentation are determined as part of the analysis utilizing an iterative Solution method. Sandwich panels are also subjected to low-velocity impact in order to characterize their impact response. The peak value of a contact force history during the impact event is used to represent the quasi-static indentation force for the computation of the stress and strain fields.

IMPACT RESPONSE OF SANDWICH PANELS WITH TWO P.V.C. FOAMS.

F. Collombet, L. Guillaumat, J.L. Lataillade, P. Davies and A..Torres-Marques

LA.M.E.F.-E.N.S.A.M., Bordeaux, France

The analysis of the mechanical response of sandwich structures loaded in low velocity high mass impacts is a multiparameter problem. One way of quantifying the influence of factors on one or more responses is to use experimental design methods. A Doehlert matrix was used with three parameters: the span of the sandwich panels, the velocity and the mass of the projectile. The sandwich skins are made of glass rovimat in polyester with 2mm thickness. Two PVC different core foams of the same density, 80 kg/rn³, were used, each with 20 mm thickness. These were nominally linear and crosslinked foams. For each material the test matrix associated with these variables requires 13 tests. The evolution of responses as a function of variables is modelled by simple mathematical expressions (empirical polynomials) but these are not based on physical mechanisms. The damage processes are: upper skin cracking on the impacted side, fibre fracture, then under some test conditions perforation. The core is dynamically compressed. In these sandwiches, the delaminated area of the lower skin depend on the values both of mass and velocity of the striker for a given energy. Shear cracks through the thickness are only noted in the rigid foam. From the test results the coefficients related to the mass, the velocity and span variables can be determined, together with their coupling for a given response. For example, the study of the contact force model exhibits three areas of influence which are respectively controlled by mass, velocity or both of them.

TWO-DIMENSIONAL SOLUTION TO FORCED VIBRATION PROBLEMS OF MULTI-SPAN, LAYERED, VISCOELASTIC BEAMS AND STRIPS

S. Karczmarzyk

Institute of Machine Design Fundamentals, Warsaw University of Technology.

A new, two-dimensional solution to forced vibration problem of layered multi-span beams and strips is proposed in the paper. The vibration considered is caused by excitations of the supports. The solution has been obtained within the linear theory of elasticity in a closed form. Both the cross-sectional warping in each layer as well as all the continuity conditions between adjoining layers have been satisfied. Two kinds of materials of the layers have been considered namely, the isotropic material and so-called monoclinic material. Any limitations on parameters of the layers have not been imposed. Final (numerical) form of the solution consists of three sets of linear algebraic equations. Two of them are the homogeneous sets and imply two transcendental characteristic equations.

A 1-D FLUID-SANDWICH INTERACTION MODEL FOR THE EARLY TIME UNDERWATER SHOCK LOADING

K.Makinen and Sergey Kadyrov

Kockums AB, Karlskronavarvet, S-37182 Karlskrona, Sweden

When a marine structure is exposed to an Underwater Explosion, the load on the structure is influenced by what is happening at the interface between the fluid and the structure. The fluid-structure interaction between a homogenous structure and a sandwich structure is significantly different due to the core material in the sandwich. In this paper a 1-D numerical method is described for determining the displacement, velocity and pressure in the sandwich structure and velocity and pressure in the fluid. This 1-D load model has also been applied to a 3D model of a sandwich panel and the agreement between the numerical and experimental results are good.

SLAMMING ON A FLAT CFRP / BALSA SANDWICH STRUCTURE

G. Katsaounis, M. Samuelides, B. Hayman and R. Wallat

Dept of Naval Arch and Marine Eng, NTUA. Athens. Greece

The paper presents an experimental study on flat bottom slamming, which is based on an extensive series of drop tests, The tests are performed with a sandwich model representing in full scale a wet deck panel of a large SES vessel. The effect of the impacting mass and impact velocity has been investigated. Results are shown in terms of pressure, strain and acceleration histories. Further the pressure measurements have been related to the drop height, the impact energy and the induced strains. The paper compares the results with those of a finite element analysis of the sandwich panel under static pressure loading. Finally conclusions are drawn concerning the induced pressures, the relation between these pressures and the measured strains, and the air cushioning effect.

EVALUATION OF OBLIQUE IMPACT DA.MAGE ON SANDWICH PANELS WITH PVC AND BALSA CORE MATERIALS

M Wiese, A. Echtermeyer and B. Hayman

Det Norske Veritas, Norway

This paper reports on oblique impact tests performed on a series of GRP sandwich panels. The panels had polyester/glass combimat skins of different thicknesses in combination with cores of HI00 and H200 PVC foams and DI00 end-grained balsa. The panels were impacted at an oblique angle by a free falling cylindrical impactor with up to 6kJ of kinetic energy. For each panel layup the impact resistance was established, in terms of the energy required to penetrate the outer (impacted) skin. The study also considered the delamination areas for both the outer and inner skins and the differences in failure modes for the panels with the two basic types of core materials.

STRENGTH PREDICTION OF NOTCHIED THIN-SKIN HONEYCOMB SANDWICH STRUCTURES

H. Razi, S. H. Ward and M. D. Bickford

Boeing Commercial Airplane Group, M/S 67-FE, P.0, Box 3707, Seattle, Washington 98124

In a composite material under tension or compression loading, a damage zone (DZ) is developed in front of a notch prior to failure. Recent research indicates that the DZ manifests itself in the form of strain-softening material behavior. This paper describes a generalized analysis method based on a bilinear strain-softening material law combined with a damage zone model (DZM) to predict residual strength of thin-skin honeycomb sandwich structures with damage such as holes and slots. This method was developed as an engineering too] for evaluating the strength of damaged composite aircraft structures. The method can be extended to determine the strength of impact damaged composite structures.

PA RTIAL INSERTS IN SANDWICH PANELS - FATIGUE TESTS

J. Kepler

Institute of Mechanical Engineering, Aalborg University, Denmark

Inserts are used for transferring concentrated loads to a sandwich panel. A partial insert penetrates one face sheet only, as opposed to through-the-thickness inserts, which penetrates both face sheets. A total of 33 partial inserts were tested by subjecting them to a pullout load. 27 of these were fatigue tests. The load ranges and insert geometries were varied. Specimen manufacturing, test parameters (geometries, materials and load conditions) and results are described.

SHA PE OPTIMISATION OF INTERNAL METAL DOUBLER FOR LOAD INTRODUCTION IN SANDWICH STRUCTURES

A. Shipsha, J. Soderlund and D. Zenkert

Department of Aeronautics, Division of Lightweight Structures, Royal Institute of Technology, Stockholm.

This paper deals with numerical shape optimisation of "pre-fitted" internal metal doublers (IMD), potentially suitable for serial production of sandwich structural components. The shape of an IMD is optimised by using the structural optimisation code OASIS-ALADDIN in order to obtain a more even stress distribution in the core and to minimise its weight. Numerical shape optimisation is performed for sandwich panels subjected to a transverse tension loading. Bi- and trimaterial corners, causing singular stress fields, are treated using a point stress criterion.

DELAMINATION FAILURE AT PLY DROPS IN CFRP/SANDWICH PANELS LOADED IN TENSION

0. T. Thomsen, Y. Frostig, and F. Mortensen

Institute of Mechanical Engineering, Aalborg University, Aalborg, Denmark

This paper presents the results of an investigation of delamination failure at "exterior" ply drops in tension loaded sindwich panels with laminated composite face sheets. The thickness of composite face sheet laminates in sandwich panels is often increased locally around highly loaded locations such as joints, rivets, bolts or inserts. At the stations where such ply drops are made, the local bending stiffness of the face sheet laminates change discontinuously, thus inducing severe local bending effects. These local bending effects induce severe stress concentrations in the interfaces between the dropped plies/ sublaminates and the remaining face sheet laminates, and this may initiate delamination failure. The present paper suggests a simple theoretical approach, based on phenomenological considerations, for the prediction of such delamination failures. An experimental investigation has been carried out, and the measured delamination loads compare well the theoretical predictions.

FAILURE OF SANDWICH BEA.MS WITH DISCONTINUOUS CORE STIFFNESS

S. Hallstrom, J.L. Grenestedt and P.H. Bull

Department of Aeronautics, Division of Lightweight Structures, Royal Institute of Technology, Stockholm, Sweden.

Sandwich beams containing core joints are investigated in order to determine whether discontinuities in core stiffness have a decreasing effect on the bending strength of the beams. A study on the influence of various stiffness ratios over core joints, and various skin/core thickness ratios is performed, using the finite element method. The core materials are assumed to be isotropic and the results are quantified by applying material properties from a commonly used PVC foam. It is concluded and partly verified in experiments that the influence of core joints is insignificant for many practical cases, i.e. when the core stiffness ratio is moderate or when the skins are relatively thin, independent of the applied loads.

FLEXURAL CHARACTERISTICS OF IN-PLANE ADHESIVELY BONDED SANDWICH JOINTS

N.H. Cossich, R.A. Shenoi, P.A. Wilson

Department of Ship Science, University of Southampton, UK

This paper is concerned with the mechanical behaviour of in-plane joints in sandwich structures under flexural loads. The impetus behind research stems from the growing development in modular construction techniques that offer competitive global access to higher technology sandwich fabrications. Characterisation of flexural response is therefore important, as in most instances the primary role of a sandwich is to impart flexural rigidity. A test programme involving several commonly used joining methods, including scarf, internal spline and external tape, coupled with flexible and inflexible adhesives is used to demonstrate the mode of failure and strength of the joined sandwich.