Material composed of two thin, stiff skins around a lightweight core
Composite sandwich structure panel used for testing at NASA
1) In materials science, a sandwich-structured composite is a special class of composite materials 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.
Metal composite material (MCM) is a type of sandwich formed from two thin skins of metal bonded to a plastic core in a continuous process under controlled pressure, heat, and tension.[3]
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.[4]
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.[5] 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.
Properties of sandwich structures
The strength of the composite material is dependent largely on two factors:
The outer skins: If the sandwich is supported on both sides, and then stressed by means of a downward force in the middle of the beam, then the bending moment will introduce shear forces in the material. The shear forces result in the bottom skin in tension and the top skin in compression. The core material spaces these two skins apart. The thicker the core material the stronger the composite. This principle works in much the same way as an I-beam does.[6]
The interface between the core and the skin: Because the shear stresses in the composite material change rapidly between the core and the skin, the adhesive layer also sees some degree of shear force. If the adhesive bond between the two layers is too weak, the most probable result will be delamination. The failure of the interface between the skin and core is critical and the most common damage mode. The propensity of this damage to propagate through the interface or dive either into the skin or core is governed by the shear component.[7]
Application of sandwich structures
The composite honeycomb structure of a helicopter nozzle
Sandwich structures can be widely used in sandwich panels, with different types such as FRP sandwich panel, aluminium composite panel, etc. FRP polyester reinforced composite honeycomb panel (sandwich panel) is made of polyester reinforced plastic, multi-axial high-strength glass fiber and PP honeycomb panel in special antiskid tread pattern mold through the process of constant temperature vacuum adsorption & agglutination and solidification.
Sandwich theory[8][9] describes the behaviour of a beam, plate, or shell which consists of three layers - two face sheets and one core. The most commonly used sandwich theory is linear and is an extension of first order beam theory. Linear local buckling sandwich theory is of importance for the design and analysis of Sandwich plates or sandwich panels, which are of use in building construction, vehicle construction, airplane construction and refrigeration engineering.
^Cutler, John Henry; Koppel, Ivan; Liber, Jeremy (10 February 2006). Understanding Aircraft Structures. Blackwell Publishing Limited. p. 14. ISBN1-4051-2032-0.
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