1. EBB 324 /4 PART III SANDWICH COMPOSITES

2. EBB 324 /3 ASSESSMENTS ONE GROUP ASSIGNMENT (10%) ATTENDENCE

3. EBB 324 /3 EXAMINATION FORMAT 2 SECTIONS PART A (COMPULSORY) 3 QUESTIONS FROM EACH PART (i, ii, iii) PART B (CHOOSE 2 FROM 4 QUESTIONS) 4 QUESTIONS (3 QUESTIONS FROM EACH PART + 1 MIXED QUESTION)

4. Syllabus 1. Introduction to sandwich composites 2. Material and materials properties Skin materials core materials 3. Fundamentals 4. Applications

5. Introduction What is sandwich composites? Where are they used? What are their benefits? What are their limitations?

6. What is sandwich composites? Sandwiches?

7. Sandwich panel

8. Application of sandwich structures

11. SANDWICH STRUCTURES Three main components: Face sheets Adhesive bonding Core Materials

12. 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

13. EBB 324 /3 CORES The core materials for a sandwich construction should satisfy a number of strength and stiffness requirements. Firstly, the core has to be sufficiently stiff in the direction normal to the plane of the faceplates, in order to always keep them at the correct distance apart. Firstly, the core has to be sufficiently stiff in the direction normal to the plane of the faceplates, in order to always keep them at the correct distance apart. It must also possess good shear strength, to ensure that the faceplates do not slide over each other when a bending moment is applied. It must also possess good shear strength, to ensure that the faceplates do not slide over each other when a bending moment is applied. The third requirement is that the core has to be stiff enough to keep the faceplates nearly flat, to prevent the faceplates from buckling locally (wrinkling) under the influence of a compressive stress The third requirement is that the core has to be stiff enough to keep the faceplates nearly flat, to prevent the faceplates from buckling locally (wrinkling) under the influence of a compressive stress

14. Foam Cores Foam cores consist of either metallic, polymeric or ceramic foams filling three-dimensional space, in contrast to the two- dimensional patterns of the honeycomb and truss cores. Foam cores consist of either metallic, polymeric or ceramic foams filling three-dimensional space, in contrast to the two- dimensional patterns of the honeycomb and truss cores. The foam structure may be open-cell, closed-cell or mixed-cell. Sandwich panel incorporating a foam core can be made simply by bonding the faceplates to the foam using adhesives. Otherwise, a pure metallic bonding can also be attained by roll-cladding the faceplates to a piece of foamable precursor material [4]. The foam structure may be open-cell, closed-cell or mixed-cell. Sandwich panel incorporating a foam core can be made simply by bonding the faceplates to the foam using adhesives. Otherwise, a pure metallic bonding can also be attained by roll-cladding the faceplates to a piece of foamable precursor material [4]. This composite can then be deformed in an optional step and finally subjected to a heat treatment, where the foamable core expands but the faceplates remain dense. It is therefore important that the faceplates material has a higher melting point to prevent melting during foaming process. Fig.4 shows an example of a sandwich panel incorporating an aluminium foam core. This composite can then be deformed in an optional step and finally subjected to a heat treatment, where the foamable core expands but the faceplates remain dense. It is therefore important that the faceplates material has a higher melting point to prevent melting during foaming process. Fig.4 shows an example of a sandwich panel incorporating an aluminium foam core.

16. HONEYCOMB CORES Currently, 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.BoeingNomex¨

17. HONEYCOMB 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.

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

19. CORRUGATED CORES The 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.

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

21. Adhesive Bonding Lecture 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