Composite materials are mainly anisotropic; most of the composite materials parts can only stand in plane stress. So, if the components have to resist normal stress, their strength is minimal. That’s why the use of core materials increases laminate bending stiffness.
Core materials in sandwich constructions are the most essential component. They can carry the shear loads between the tension and compression face, just like an I-beam work.
Types of core materials
The most commonly used cores for sandwich panels are corrugated, honeycomb, balsa wood and cellular foams.
Properties of core materials
They have in common the following properties:
- Low density.
- Shear modulus.
- Shear strength.
- Normal Stiffness.
- Thermal insulation.
Honeycomb core material
Its main use are aerospace applications due to its low density. The honeycomb cores have cells with different shapes. These shapes can be hexagonal, square, over-expanded hexagonal and flex-core. In case the core is going to be used at curved geometries, the last two types of shapes are the most common used. The reason is because they reduce the anticlastic bending and cell wall buckling when curved.
Due to the nature of honeycomb manufacturing, they have different in-plane properties from each other due to the corrugation and the expansion process. The three principal directions to wich material properties of honeycombs are referred are: width (W), length (L) and transverse (T).
Aluminum honeycombs
Used mainly at the aerospace industries. The most common aluminum alloys are 5052, 5056 and 2024. For high strength applications aluminum 505 are used. In case for general purpose 5052 aluminum is employed.
Non-metallic honeycombs
The most common used are the Nomex, which is an aramid fiber-based fabric expanded in much the same way as aluminum alloy honeycomb. It is used because of its toughness and damage resistance. It has similar mechanical properties as aluminum alloys.
Balsa wood
Was the first cores to be used. Balsa is a type of wood that has closed-cell structure seen under microscope. A typical density for balsa wood is 100 to 300 kg/m3. However, it is highly sensitive to moisture, which makes reduce its mechanical properties rapidly.
Cellular foams
Cellular foams do not offer the same stiffness and strength to weight ratio than honeycombs. However, they are cheap, in comparison with honeycombs and it is a solid, which makes it easier during manufacturing to laminate the skins as it adapts to the core. Moreover, other advantages of cellular foams in composite materials for sandwich construction are the fact that they offer thermal insulation, water resistant and acoustical damping. Some of the types of cellular foams are described below.
Polyurethane foam (PUR)
It is produced when the blowing agent of the polymer expands at elevated temperatures. They are manufactured by extrusion or by expansion in closed moulds. PS has closed cells and is available in densities ranging from 15 to 300 kg/m3. PS foam has quite good mechanical and thermal insulation properties, and it is cheap. A drawback is its sensitivity to solvents, particularly styrene, and hence ester-based matrices cannot be used as adhesives.
Polyvinyl chloride foam (PVC)
PVC foam is a cross-linked iso-cyanate modified type. The cross-linked PVC is more rigid, has higher mechanical properties, is less heat sensitive, but more brittle. Still, even the cross-linked PVC has an ultimate elongation of about 10% in tension that is much higher than any PUR foam. PVC foam is available in finite size blocks with densities from 30 to 400 kg/m3. The mechanical properties of PVC are higher than those of both PUR and PS, but it is also more expensive than those.
Poly-methacryl-imide foam (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 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 180°C environments. The cell structure is very fine with closed cells and the densities available are from 30 to 300 kg/m3.
