Sandwich-structured composite
In materials science, a sandwich-structured composite is a special class of composite material that is fabricated by attaching two thin-but-stiff skins to a lightweight-but-thick core. The core material is normally of low strength, but its greater thickness provides the sandwich composite with high bending stiffness with overall low density.
Open- and closed-cell-structured foams like polyethersulfone, polyvinylchloride, polyurethane, polyethylene, or polystyrene foams, balsa wood, syntactic foams, and honeycombs are commonly used core materials. Sometimes, the honeycomb structure is filled with other foams for added strength. Open- and closed-cell metal foams can also be used as core materials.
Laminates of glass- or carbon-fiber-reinforced thermoplastics or thermoset polymers are widely used as skin materials. Sheet metal is also used as skin material in some cases.
The core is bonded to the skins with an adhesive or, with metal components, by brazing.
History
A summary of the important developments in sandwich structures is given below.- 230 BCE – Archimedes describes the laws of levers and a way to calculate density.
- 25 BCE – Vitruvius reports about the efficient use of materials in Roman truss roof structures.
- 1493 – Leonardo da Vinci discovers the neutral axis and load-deflection relation in three-point bending.
- 1570 – Palladio presents truss-beam constructions with diagonal beams to prevent shear deformations.
- 1638 – Galileo Galilei describes the efficiency of tubes versus solid rods.
- 1652 – Wendelin Schildknecht reports about sandwich beam structures with curved wooden-beam reinforcements.
- 1726 – Jacob Leupold documents tubular bridges with compression-loaded roofs.
- 1786 – Victor Louis uses iron sandwich beams in the galleries of the Palais-Royal in Paris.
- 1802 – Jean-Baptiste Rondelet analyses and documents the sandwich effect in a beam with spacers.
- 1820 – Alphonse Duleau discovers and publishes the moment of inertia for sandwich constructions.
- 1830 – Robert Stephenson builds the Planet locomotive using a sandwich beam frame made of wood plated with iron.
- 1914 – R. Höfler and S. Renyi patent the first use of honeycomb structures for structural applications.
- 1915 – Hugo Junkers patents the first honeycomb cores for aircraft application.
- 1934 – Edward G. Budd patents welded steel honeycomb sandwich panel from corrugated metal sheets.
- 1937 – Claude Dornier patents a honeycomb sandwich panel with skins pressed in a plastic state into the core cell walls.
- 1938 – Norman de Bruyne patents the structural adhesive bonding of honeycomb sandwich structures.
- 1940 – The de Havilland Mosquito was built with sandwich composites—a balsawood core with plywood skins.
Types of sandwich structures
Recycled paper is also now being used over a closed-cell recycled kraft honeycomb core, creating a lightweight, strong, and fully repulpable composite board. This material is being used for applications including point-of-purchase displays, bulkheads, recyclable office furniture, exhibition stands, wall dividers and terrace boards.
To fix different panels, among other solutions, a transition zone is normally used, which is a gradual reduction of the core height, until the two fiber skins are in touch. In this place, the fixation can be made by means of bolts, rivets, or adhesive.
With respect to the core type and the way the core supports the skins, sandwich structures can be divided into the following groups: homogeneously supported, locally supported, regionally supported, unidirectionally supported, bidirectionally supported. The latter group is represented by honeycomb structure which, due to an optimal performance-to-weight ratio, is typically used in most demanding applications including aerospace.