EBB 324 /4EBB 324 /4

PART IIIPART III

SANDWICH SANDWICH COMPOSITESCOMPOSITES

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ASSESSMENTSASSESSMENTS

ONE GROUP ASSIGNMENT (10%)ONE GROUP ASSIGNMENT (10%)

ATTENDENCEATTENDENCE

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EXAMINATION FORMATEXAMINATION FORMAT2 SECTIONS2 SECTIONS

PART A (COMPULSORY)PART A (COMPULSORY)3 QUESTIONS FROM EACH PART (i, ii, iii)3 QUESTIONS FROM EACH PART (i, ii, iii)

PART B (CHOOSE 2 FROM 4 QUESTIONS)PART B (CHOOSE 2 FROM 4 QUESTIONS)4 QUESTIONS (3 QUESTIONS FROM EACH PART + 1 4 QUESTIONS (3 QUESTIONS FROM EACH PART + 1

MIXED QUESTION)MIXED QUESTION)

SyllabusSyllabus1.1. Introduction to sandwich Introduction to sandwich

compositescomposites2.2. Material and materials propertiesMaterial and materials properties

Skin materialsSkin materialscore materialscore materials

3.3. FundamentalsFundamentals4.4. ApplicationsApplications

IntroductionIntroductionWhat is sandwich What is sandwich

composites?composites?

Where are they used?Where are they used?

What are their benefits?What are their benefits?

What are their limitations?What are their limitations?

What is sandwich What is sandwich composites?composites?Sandwiches?Sandwiches?

Sandwich panelSandwich panel

Application of sandwich Application of sandwich structuresstructures

Application of sandwich Application of sandwich structuresstructures

Application of sandwich Application of sandwich structuresstructures

SANDWICH STRUCTURESSANDWICH STRUCTURES

Three main components:

Face sheets

Adhesive bonding

Core Materials

FACE MATERIALS:FACE MATERIALS:

Any structural material which is available in the form of thin sheet can be used

Requirements:

High stiffness

High tensile and compressive strength

Good impact resistance

Good surface finish

Environmental resistance

Wear resistance

EBB 324 /3EBB 324 /3CORESCORES

The core materials for a sandwich construction The core materials for a sandwich construction should satisfy a number of strength and stiffness should satisfy a number of strength and stiffness

requirements. requirements. •Firstly, the core has to be sufficiently stiff in the Firstly, the core has to be sufficiently stiff in the direction normal to the plane of the faceplates, in direction normal to the plane of the faceplates, in order to always keep them at the correct order to always keep them at the correct distance apart. distance apart. •It must also possess good shear strength, to It must also possess good shear strength, to ensure that the faceplates do not slide over each ensure that the faceplates do not slide over each other when a bending moment is applied. other when a bending moment is applied. •The third requirement is that the core has to be The third requirement is that the core has to be stiff enough to keep the faceplates nearly flat, to stiff enough to keep the faceplates nearly flat, to prevent the faceplates from buckling locally prevent the faceplates from buckling locally (wrinkling) under the influence of a compressive (wrinkling) under the influence of a compressive stressstress

Foam CoresFoam Cores

•Foam cores consist of either Foam cores consist of either metallic, polymeric metallic, polymeric oror ceramicceramic foams filling three-dimensional space, in contrast to foams filling three-dimensional space, in contrast to the two-dimensional patterns of the honeycomb and truss the two-dimensional patterns of the honeycomb and truss cores. cores.

•The foam structure may be open-cell, closed-cell or mixed-The foam structure may be open-cell, closed-cell or mixed-cell. Sandwich panel incorporating a foam core can be made cell. Sandwich panel incorporating a foam core can be made simply by bonding the faceplates to the foam using simply by bonding the faceplates to the foam using adhesives. Otherwise, a pure metallic bonding can also be adhesives. Otherwise, a pure metallic bonding can also be attained by roll-cladding the faceplates to a piece of attained by roll-cladding the faceplates to a piece of foamable precursor material [4]. foamable precursor material [4].

•This composite can then be deformed in an optional step This composite can then be deformed in an optional step and finally subjected to a heat treatment, where the and finally subjected to a heat treatment, where the foamable core expands but the faceplates remain dense. It is foamable core expands but the faceplates remain dense. It is therefore important that the faceplates material has a higher therefore important that the faceplates material has a higher melting point to prevent melting during foaming process. melting point to prevent melting during foaming process. Fig.4 shows an example of a sandwich panel incorporating an Fig.4 shows an example of a sandwich panel incorporating an aluminium foam core.aluminium foam core.

HONEYCOMB CORESHONEYCOMB CORESCurrently, almost all of the sandwich panels

employed in the commercial and military aircrafts are made of metal or fibre-reinforced composite

facings incorporating honeycomb cores, as shown in Fig.1. Boeing commercial jets, from the 747 to 767, heavily make use of honeycomb panels in

their construction. Sandwich panels incorporating honeycomb structures are fast replacing the

conventional Îsheet and stringerâ air-foil construction, because it offers significantly

improved resistance against buckling at working loads. More spectacular exploits of honeycomb structure can be seen in the Beech Starship and Voyager aircrafts, as depicted in Fig.2., where

Nomex¨ honeycomb core is utilised extensively throughout the structure.

HONEYCOMB CORESHONEYCOMB CORES

The basic honeycomb has a two-dimensional hexagonal pattern and may be made of sheet metals (normally steel and aluminium), fibre-reinforced polymers, unreinforced polymers, and aramid fibre paper dipped in phenolic resin (Nomex¨). Other variants of the honeycomb core are illustrated in Fig.1(b) and (c), and unlike the hexagonal pattern, these cores are capable of deforming to form sandwich panels with curved surfaces.

HONEYCOMB CORESHONEYCOMB CORES

Fig.2. (a) The Beech Starship and (b) Voyager [2]

CORRUGATED CORESCORRUGATED CORESThe corrugated cores composed of a fluted metal sheet attached alternatively to the faceplates. Corrugated core

panels are less widely utilised in the aircraft construction, but most of their applications can be found in packaging,

transportation containers and building elements, owing to the low manufacturing cost involved. Different variations of this

core are illustrated in Fig.3.

                                                                                               

                                        

Adhesive BondingAdhesive Bonding

Various adhesive systems can be used to bond the face sheets and the core material for sandwich structure fabrication                                                       

Adhesive BondingAdhesive Bonding

Lecture 1Lecture 1

The requirements on the adhesive are somewhat different from normal use

Bonding of sandwich structure involves the bonding two very dissimilar constituents, one solid component to a softer cellular one

For honeycomb component, adhesive tape is commonly employed