-> Competencies -> Material science -> Sandwich structures
A sandwich-structured composite is a special class of composite materials that is fabricated by attaching two thin but stiff facesheets to a lightweight but thick core. The core material is normally low strength material, but its higher thickness provides the sandwich composite with high bending stiffness with overall low density. Open- and closed-cell-structured foams like polyvinylchloride, polyurethane, polyethylene or polystyrene foams, balsa wood, syntactic foams, metal foams and honeycombs are commonly used core materials. The selection of the optimal sandwich structure is usually based on the different conditions associated with load-bearing capacity, production, and component design. Lightweight and stiff sandwich panels are a vital element of many modern aircraft and spacecraft designs. Reinforcing and edge finishing of such panels can be costly and time consuming, but it is essential.
Honeyomb Sandwich structures
The honeycomb sandwich construction is one of the most valued structural engineering innovations developed by the composites industry. Sometimes, the honeycomb structure is filled with other foams for added strength. Although they are light and stiff, honeycomb sandwich structures need local reinforcement in load-bearing areas or in fixation spots like inserts. Naturally, open sandwich edges need reinforcement and protection as well to prevent from impact damages and moisture ingress.Used extensively in aerospace and many other industries, the honeycomb sandwich provides the following key benefits over conventional materials:
- Very low weight
- High stiffness
- Production cost savings
Admatis has experience in fabricating honeycomb sandwich structures. The honeycomb core and the facesheet raw material are
also aluminium alloy.
Aluminium foam sandwich structures
Aluminium foam sandwich (AFS) is a sandwich panel product which is made of two metallic dense face sheets and a metal foam core made of an aluminium alloy. AFS is an engineering structural material owing to its stiffness-to-mass ratio and energy absorption capacity ideal for application such as the shell of a high-speed train or debree protection structure of spacecrafts. In terms of the bonding between facesheets and foam core the processing of AFS is categorised into two ways - ex-situ and in-situ bonding.
Ex-situ bonded AFS
Ex-situ bonding is achieved by gluing face sheets with an aluminium foam by adhesive bonding, brazing or diffusion bonding. Foams used in this method are either closed-cell or open-cell. When a closed-cell foam is used then it is produced from aluminium alloys either by liquid metal route or by powder metallurgy route. Open-cell foam core is made of aluminium and other metals as well. Face sheets are chosen from a variety of aluminium alloy, and other metals such as steel.
In-situ bonded AFS
For in-situ bonded face sheets the core is closed-cell foam. The goal of in-situ bonding is to create a metallic bonding between the foam core and face sheets. This is achieved in three ways. A foamable precursor is expanded between two face sheets. When the liquid foam comes in contact with the solid face sheets a metallic bond is established. This is difficult to realize as the oxidation of both aluminium face sheets and foam prevent forming a sound bonding. Another strategy is to rapidly solidify the surface of a foamable molten metal before it can foam into to a dense skin while the interior of the metal evolves to a foam structure. The third way to achieve in-situ bonding consists of compaction of metal powders together with face sheets. This sandwich-compact assembly goes through several rolling steps to attain desired precursor and face sheet thickness. After which this three-layer composite is heated to transform the core layer into foam.
Admatis has experience in fabricating metal foam sandwich panels with ex-situ bonded AFS.
ROHACELL foam sandwich panels
ROHACELL is a closed-cell rigid foam based on polymethacrylimide (PMI) chemistry, which does not contain any CFC's developed. The aerospace industry has long made use of as a lightweight, structural core material like Rohacell that also reduces the cost of sandwich materials because of its process reliability. As an example, helicopter rotor blades are increasingly produced from Rohacell sandwich composite materials.
Admatis using Rohacell foam to fabricating sandwich panels.
Sandwich panel inserts
Inserts are commonly used to transfer loads to sandwich composite structures. Local stress concentrations due to inserts are known to cause structural failure, and experimental pull-out tests show that the failure load can vary by 20% between batches of sandwich panels. Various configurations of inserts are employed to join panels together and to attach external objects. The insert can be attached by an adhesive potting compound to a panel consisting of two facesheets and a honeycomb or foam core. Structutral type inserts offer maximum shear and column strength by gripping and supporting the face sheets from within the core area of the sandwich panel. These inserts are available in threaded and non-threaded, through or blind, locking or non locking.
Admatis has experience in designing, manufacturing, and testing sandwich panels.
- Composite sandwich panels (with panel inserts) were manufactured from aluminium honeycomb, aluminium foam and rohacell foam,
- Composite Telescope baffles were developed and manufactured from aluminium honeycomb and rohacell foam.
Sandwich panel test samples (left), panel inserts (lower left), vibration test sandwich panels
(middle) and composite telescope baffles (right) developed by Admatis