Lecture 2 SANDWICH COMPOSITES Fig.1. A typical sandwich structure which consists of a core bonded in between two faceplates using adhesive [1].

Principles of Sandwich Structures Lecture 2 Principles of Sandwich Structures Typical sandwich materials always exhibit a particular fundamental pattern ö two faceplates (facings), which are comparatively thin but of high strength and stiffness, enclosing a core structure, which is relatively thick but light-weight, and possesses sufficient stiffness in the direction normal to the plane of the faceplates. The components of the sandwich material must also be bonded together, using either adhesives or mechanical fastenings, such that they can act as a composite load-bearing unit. In principle, the basic concept of a sandwich panel is that the faceplates carry the bending stresses whereas the core carries the shear stresses.

Lecture 2 In most cases, an efficient sandwich panel is obtained when the weight of the core is almost equivalent to the combined weight of the faceplates. By separating the faceplates using a low density core, the moment of inertia of the panel is increased and hence resulted in improved bending stiffness.

Lecture 2 Therefore, the bending stiffness of a sandwich structure greatly exceeds that of a solid structure having the same total weight and made of the same material as the facings. Furthermore, due to the porous nature of the core material, sandwich structure has inherent exceptional thermal insulation and acoustic damping properties.

Design aspect Lecture 2 The faceplates should be thick enough to withstand the tensile, compressive and shear stresses induced by the design load, as depicted in Figs.1(a) and (b).

Lecture 2 Design aspect The core should have sufficient strength to withstand the shear stresses induced by the design loads. The adhesive must have sufficient strength to carry shear stress into the core, as shown in Fig.2.

Lecture 2 Design aspect The core should be thick enough and have sufficient shear modulus to prevent overall buckling of the sandwich under load, and to prevent crimping (Fig.3).

Lecture 2 Design aspect Compressive modulus of the core and facings should be sufficient to prevent wrinkling of the faces under design load (Fig.4).

Lecture 2 Design aspect The core cells should be small enough to prevent intracell dimpling of the faceplates under design load.

Lecture 2 Design aspect The core should have sufficient compressive strength to resist crushing by design loads acting normal to the panel facings or by compressive stresses induced through flexure

Lecture 2 Design aspect The overall structure should have sufficient flexural and shear rigidity to avoid excessive deflections under design load

Lecture 2 Design aspect Based on the aforementioned principles and criteria, a wide range of sandwich structures can be constructed by combining various faceplates and core materials. The faceplates may be steel, aluminium, fibre-reinforced plastic, wood or even concrete. On the other hand, the core may be made of low density solid materials, such as polyethylene, rubber, balsa wood or porous materials, such as metallic foams, plastic foams (polyurethane, polystyrene, phenolic), honeycomb, and also truss assembly.

Advantages/disadvantages Lecture 2 Advantages/disadvantages Sandwich materials generally exhibit the following favourable properties: high load bearing capacity at low weight excellent thermal insulation surface finished faceplates provide good resistance against aggressive environments long life at low maintenance cost good water and vapour barrier excellent acoustic damping properties

Advantages/disadvantages Lecture 2 Advantages/disadvantages Naturally, the less favourable properties of sandwich materials can be identified as follows: creep under sustained load with rigid foam cores low thermal capacity poor fire resistance with rigid plastic foam cores deformation when one side of faceplate is exposed to intense heat

PART III SANDWICH COMPOSITES Lecture 2 PART III SANDWICH COMPOSITES