Lecture 4

1. The cores used in load carrying sandwich constructions can be divided into four main groups: • Corrugated • Honeycomb (Various shapes and materials) • Balsa wood • Cellular foams (Polymeric, metallic and Ceramic) Lecture 4 CORE MATERIALS

2. Core should have low density in order to add as little as possible to the total weight of the sandwich • Young’s modulus perpendicular to the faces should be fairly high to prevent a decrease in the core thickness and therefore a rapid decrease in the flexural rigidity • The core is mainly subjected to shear so that the core shear strains produce global deformations and core shear stresses • Thus, a core must be chosen that would not fail under the applied transverse load and with a shear modulus high enough to give the required shear stiffness • The critical wrinkling load depends on both Young’s modulus and the shear modulus of the core Lecture 4 CORE MATERIALS

3. The properties of primary interest for the core may be summarised as: • Low density • Shear modulus • Shear strength • Stiffness perpendicular to the faces • Thermal insulation Lecture 4 CORE MATERIALS

4. Core materials of honeycomb type have been developed and used mainly in aerospace applications • However, cheap honeycomb materials made from impregnated paper are also used in building applications • Honeycomb cores can be manufactured in a variety of cell shapes but the most commonly used shape is the hexagonal • Others are square, over-expanded hexagonal, flex-core. • Over-expanded and flex-core are mainly used when the core needs to be curved in the manufacturing of the sandwich element Lecture 4 HONEYCOMB CORES

5. Over-expanded hexagonal and flex-core shapes reduce the anticlastic bending and cell wall buckling when curved • There are other cell shapes used such as rectangular, and reinforced hexagonal. • The manufacturing of metal honeycombs is performed in two different ways: Corrugating and expansion processes • Corrugating implies that pre-corrugated metal sheets are bonded together and stacked into blocks • When the adhesive has cured, blocks with the required thickness can be cut from the stack • The process is commonly used in manufacture of high-density metal honeycombs Lecture 4 HONEYCOMB CORES

6. The expansion process begins with the stacking of thin plane sheets of web material on which adhesive nodes have been printed • By stacking many thin layers in this way a block is made • Each block may then be cut into desired thickness (T-direction). • When the adhesive has cured it may be expanded by pulling in the W-direction until a desired cell shape has been achieved Lecture 4 HONEYCOMB CORES

7. Various honeycomb cores may be found such as: • Aluminium alloy honeycomb • Kraft paper honeycombs • Non-metallic honeycombs Lecture 4 HONEYCOMB CORES

8. Aluminium alloy honeycomb • Extensivly used in aerospace applications during the past decades • They are commonly made of the aluminium alloys 5052, 5056, and 2024 • 5052 is a general purpose alloy, 5056 a high strength version of 5052 and 2024 a heat treated aluminium alloy with good properties even at elevated temperature • The 5052 and 5056 alloy honeycombs can be used in environments up to 180°C and the 2024 up to 210°C. Lecture 4 HONEYCOMB CORES

9. Kraft paper honeycombs • Manufactured by impregnating paper with resin to make it water resistant • This provides cheap, but still mechanically very good sandwich core • Some manufacturers can even fill the cells of Kraft paper honeycomb with a light weight foam (usually PUR or phenolic) for improved thermal insulation Lecture 4 HONEYCOMB CORES

10. Non-metallic honeycomb • Similar to fibre-reinforced plastics but with honeycomb shape • Produced by impregnating a pre-fabricated cell-shaped fabric in a bath of resin • Different honeycombs are available with glass, aramid or even carbon fibre fabric reinforcement • The matrix which the fabric is impregnated with usually phenolic, heat resistant phenolic, polyimide or polyester • Phenolic impregnated have maximum working temperature up to 180°C, polyimide 250°C, polyester 80°C Lecture 4 HONEYCOMB CORES

11. Non-metallic honeycomb cont’d • A well-known type of fibre-impregnated honeycomb is made of NOMEX paper, which is an aramid fibre based fabric expanded in much the same way as aluminium alloy honeycomb before being coated with resin • It is widely used because of its high toughness and damage resistance and since it has almost as high mechanical properties as aluminium alloy honeycomb. • Nomex honeycomb can be used up to 180°C at which its strength still approximately 75% of its room temperature value Lecture 4 HONEYCOMB CORES

12. First material used as cores in load carrying sandwich structures • Balsa is a wood but under the microscope it can be seen as a high-aspect-ratio closed-cell structure • The fibres or grains are oriented in the direction of growth producing cells with a typical length of 0.5-1.0 mm and with a diameter of about 0.05 mm, thus giving the cell ratio of approximately 1:25. • The properties of balsa are therefore high in direction of growth but much lower in the others • Balsa exists in different qualities with densities in the regime 100 to 300 kgm-3. Lecture 4 BALSA WOOD CORE

13. Balsa is also very sensitive to humidity with the properties rapidly declining with the water content • To overcome the above problem balsa is most commonly utilised in its “end-grain” shape. • This means that the balsa wood is cut up in cubic pieces and bonded together edge wise so that a block is produced where the fibre direction is located perpendicular to the plane of the block. • In this way, principal direction of stiffness is perpendicular to the faces, and humidity is spread along the fibres and hence damage would only cause localised humidity damage • The drawback is that all the small balsa blocks have different densities and the design limit must be taken from the piece of having the lowest properties Lecture 4 BALSA WOOD CORE cont’d

14. Cellular foams do not offer the same high stiffness and strength-to-weight ratios as honeycombs but have other very important advantages • Firstly, cellular foams are in general less expensive than honeycombs but more importantly, a foam is a solid on a macroscopic level making the manufacturing of sandwich element easier; the foam surface is easy to bond to, surface preparation and shaping is simple and connections of block are easily performed by adhesive bonding • In addition, cellular foams offer high thermal insulation, acoustal damping, and the closed cell structure of most foams ensure that the structure will become bouyant and resistant to water penetration Lecture 4 CELLULAR FOAMS

15. There exist a variety of foams, with different advantages and disadvantages. Some of these are (polymer-based): • Polyurethane foam (PUR) • Polystyrene foam (PS) • Polyvinyl chloride foam (PVC) • Poly-methacryl-imide foam (PMI) Lecture 4 CELLULAR FOAMS cont’d

16. Polyurethane foam (PUR) • The urethane polymer is formed through the reaction between iso-cyanate and polyol, and tri-chloro-fluoro-methane or carbon dioxide used as blowing agent • Produced in many variations from soft with more or less open cells to rigid types with predominantly closed cells and in a wide range of density • They can be made fire resistant by using additive containing phosphorous • Due to high molecular weight, PUR foams have low thermal conductivity and diffusion coefficients giving them very good insulation properties Lecture 4 CELLULAR FOAMS

17. Polyurethane foam (PUR) • Rigid PUR foams generally have quite brittle cell walls and hence the PUR core has low toughness and low ultimate elongation • The mechanical properties are lower than most other cellular plastic core but PUR foams are probably the cheapest of all available core materials • The primary use of PUR is for insulation purposes or in less critical load bearing elements • An advantage is that PUR foam can be produced in finite size blocks as well as being formed in-situ thus giving an integrated manufacturing process in conjunction with the manufacturing of sandwich elements Lecture 4 CELLULAR FOAMS

18. Polystyrene foam • Produced either by extrusion or by expansion in closed moulds • In both cases the plastic is mixed with the blowing agent which then expands at elevated temperature • A major obstacle was that CFC was used as blowing agent, but recently PS foams have been expanded without the use of environmentally dangerous CFC-gases • PS has closed cells and is available in densities ranging from 15 to 300 kgm-3. • Ps foam has quite good mechanical and thermal insulation properties, and its cheap Lecture 4 CELLULAR FOAMS

19. Polystyrene foam cont’d • A drawback is its sensitivity to solvents, particularly styrene, and hence ester-based matrices can not be used as adhesives • PS is primarily used as thermal insulation material but lately it has also been used in load carrying structures such as refrigerated tanks and containers Lecture 4 CELLULAR FOAMS

20. Polyvinyl chloride foam (PVC) • Exists in two different forms; one purely thermoplastic also called linear PVC foam, and one cross-linked iso-cyanide modified type • The linear PVC has great ductility, quite good mechanical properties but softens at elevated temperatures • The cross-linked PVC is more rigid, has higher mechanical properties, is less heat sensitive, but more brittle. • Still, even cross-linked PVC has an ultimate elongation of about 10% in tension which is much higher than PUR foam Lecture 4 CELLULAR FOAMS

21. Polyvinyl chloride foam (PVC) cont’d • PVC foam is available in finite size blocks with densities from 30 to 400 kgm-3 • The mechanical properties of PVC are higher than those of both PUR and PS, but is also expensive than those • It is non-flammable foam but when burnt a hydrochloric acid gas is released • PVC foam are used in almost every type of application varying from pure insulation applications to aerospace structures and hence the almost widely used of all foams and perhaps of all core materials • PVC has about 95% closed cells for the lower densities and almost entirely closed cell for higher, which is much appreciated in applications where water absorption is a problem Lecture 4 CELLULAR FOAMS

22. Poly-methacryl-imide (PMI) • Acryl-imide cellular plastics are made from expanded imide-modified polyacrylates • The mechanical properties are good, perhaps the best of all commercially available cellular foams, but the price is also the highest • PMI is fairly brittle with an ultimate elongation in tension of approximately 3% in tension. • The main advantage is the temperature resistance making it possible to use PMI foam in conjunction with epoxy prepregs in autoclave manufacturing in up to 180C environments • The cell structure is very fine with closed cells and the densities available are from 30 to 300 kgm-3 Lecture 4 CELLULAR FOAMS

23. In most cases, an efficient sandwich panel is obtained when the weight of the core is almost equivalent to the combined weight of the faceplates [2]. By separating the faceplates using a low density core, the moment of inertia of the panel is increased and hence resulted in improved bending stiffness. Therefore, the bending stiffness of a sandwich structure greatly exceeds that of a solid structure having the same total weight and made of the same material as the facings. Furthermore, due to the porous nature of the core material, sandwich structure has inherent exceptional thermal insulation and acoustic damping properties. Lecture 4