Laminates can have extraordinary in-plane mechanical properties. However, laminates are relatively thin which in case of having to support normal loads, they cannot withstand a great amount of force. The only way to make a laminate stronger in the normal direction is laminating several plies, however the main disadvantage is the increase in the weight. That’s why it has been using core materials to avoid this inconvenient. Sandwich constructions in composite materials is used to obtain a high specific bending stiffness in comparison to monolithic laminate construction. In contrast, the production of this panels are relatively higher than monolithic.
Sandwich construction are two strong skins or faces separated by a lightweight core. Due to this separation of faces, it increases the moment of inertia, with little increase in weight. This is the main reason that sandwich constructions in composite materials is used, to obtain a high specific bending stiffness in comparison to monolithic laminate construction. In contrast, the production cost of these panels is relatively higher than monolithic.
The materials which compose the sandwich construction are really important as its mechanical properties will determine the final properties of the sandwich. The bending deflection depends of the tensile and compressive Young Modulus of the skin materials while the shear deflection depends on the shear modulus of the core. Faces are usually made of fibre reinforced polymers (frp) or metals (steel, aluminium). The material used for the core is honeycomb, foams or balsa. More information about core materials in this post.
Benefits of using sandwich constructions
Sandwich constructions are usually used in aerospace industries because the benefits it has:
- Very low weight
- High stiffness
- Durability
The faces carry bending moments as tensile and compressive stresses and the core carries transverse forces as shear stresses
As it can be seen, the facing skins of a sandwich panels are compared with the flanges of the I-beam, which carry the bending stresses. One skin or flange is subjected to tension and the other compression. The core works equally as the web of an I-beam. It has to transmit the shear loads. The adhesive in the sandwich components joints all the element together so they can work as an unit of high torsional and bending rigidity.
Tyes of failure of sandwich constructions in composite materials
a) Facing failure: failure in the facing due to the it cannot withstand the tensile, compressive and shear stresses. This can be due to insufficient panel thickness, facing thickness or facing strength that may result in failure occurring in either the compression of tensile face.
b) Transverse shear failure: the core or panel thickness is insufficient for the shear strength.
c&d) Face wrinkling: also known as skin wrinkling, core compression failure or adhesive bond failure may occur depending on the relative strength of the core in compression and adhesive in flatwise tension.
e) Panel buckling: core thickness and shear modulus must be adequate to prevent the panel from buckling under end compression loads.
f) Shear crimping: occurs because of general bucliking, the shear modulus of the core material or the shear strength of the adhesive is low.
g) Intracell buckling: the skins are vey thin and the cell size is large. This effect may cause failure by propagation across adjacent cells, thereby inducing face wrinkling.
h) Local crushing core: caused due to the compressive strength of the material is too low.
