A sandwich panel is any structure made of three layers: a low-density core (PIR, mineral wool, XPS), and a thin skin-layer bonded to each side. Sandwich panels are used in applications where a combination of high structural rigidity and low weight is required.
The structural functionality of a sandwich panel is similar to the classic I-beam, where two face sheets primarily resist the in-plane and lateral bending loads (similar to flanges of an I- beam), while the core material mainly resists the shear loads (similar to the web of an I-beam).The idea is to use a light/soft but thick layer for the core and strong but thin layers for face sheets. This results in increasing the overall thickness of the panel, which often improves the structural attributes, like bending stiffness, and maintain or even reduce the weight.
Sandwich panels are an example of a sandwich structured composite: the strength and lightness of this technology makes it popular and widespread. Its versatility means that the panels have many applications and come in many forms: the core and skin materials can vary widely and the core may be a honeycomb or a solid filling. Enclosed panels are termed cassettes.
One obvious application is in aircraft, where mechanical performance and weight-saving are essential. Transportation and automotive applications also exist.
In building and construction, these prefabricated products designed for use as building envelopes. They appear in industrial and office buildings, in clean and cold rooms and also in private houses, whether renovation or new-build. They combine a high-quality product with high flexibility regarding design. They generally have a good energy-efficiency and sustainability.
In packaging, applications include fluted polypropylene boards and polypropylene honeycomb boards.
The qualities that have produced the rapid growth in the use of sandwich panels, particularly in construction, include:
Sandwich panels have λ-values from 0.024 W/(m·K) for polyurethane to 0.05 W/(m·K) for mineral wool. Therefore, they can achieve different U-values depending on the core and the thickness of the panel.
The installation of a system with sandwich panels minimizes thermal bridges through the joints.
The assessed sound reduction measurement lies at approx. 25 dB for PU elements and at approx. 30 dB for MW elements.
The space between the supports can be up to 11 m (walls), depending on the type of panel used. Normal applications have spaces between the supports that are approx. 3 m – 5 m.
The thickness of panels is from 40 mm up to more than 200 mm.
The density of sandwich panels range from 10 kg/m2 up to 35 kg/m2, depending on the foam and metal thickness, decreasing time and effort in: transportation, handling and installation.
All these geometric and material properties influence the global/local failure behavior of the sandwich panels under different loading conditions such as indentation, impact,fatigue and bending.
Sandwich panels have different fire behaviours, resistance and reaction, depending on: the foam, the metal thickness, the coating, etc. The user will need to choose between the different sandwich panel types, depending on the requirements.
Research by the Association of British Insurersand the Building Research Establishment in the UK highlighted that "sandwich panels do not start a fire on their own, and where these systems have been implicated in fire spread, the fire has often started in high risk areas such as cooking areas, subsequently spreading as a result of poor fire risk management, prevention and containment measures".
There is evidence that when sandwich panels are used to clad a building it can contribute to the rapid spread of fire up the outside of the building itself. As an architect put it, in choosing the core material for a sandwich panel "I only use the mineral wool ones because your gut tells you it is not right to wrap a building in plastic". In 2000 Gordon Cooke, a leading fire safety consultant, reported that "the use of plastic foam cored sandwich panels ... is difficult to justify when considering life safety". He said the panels "can contribute to the severity and speed of fire development" and this has led to "massive fire losses".
Design of a cavity between the cladding and the exterior wall of the building (or its sheath of insulation) is also significant: flames can occupy the cavity and be drawn upwards by convection, elongating to create secondary fires, and do so "regardless of the materials used to line the cavities".