A sandwich 
composite is fabricated by attaching two thin stiff skins to a
lightweight thick core. The core material is a low strength material, but due
to high thickness of the sandwich composite yields with high bending stiffness
with overall low density. balsa wood, syntactic foam Open cell and closed cell
structured foam and composite honeycomb are mainly used core materials. Glass
or carbon fiber reinforced laminates are widely used as skin materials. Sheet
metal is also used as skin materials in some cases. They are commonly being
used in ship construction, building, bridges, trains, car doors, panels etc. In
construction new green sandwich structures are also being introduced where the
core is usually made from the natural materials (wood) and the skin is made
from earth (clay) instead of cement. Thus making it more environment
friendly.                                 

                                            Generally,
the sandwich composites are symmetric, the core configuration plays major role
in variations of sandwich component construction The core of a sandwich
structure can be any material, but in general they are classified into four
different types; honeycomb core, foam or solid core, web core and corrugated or
truss core. For the functioning of the sandwich structure as a whole the
binding of core and face sheets is another important criterion.

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                       The basic concept of a
sandwich structure is that the face sheets carry the bending loads while the
core carries the shear loads. The face sheets are strong and stiff in tension
and compression compared to the low density core material, whose primary
purpose is to keep the face sheets separated in order to maintain a high
section modulus (a high “moment of inertia” or “second moment of the area”)
.The core material has relatively low density (e.g., honeycomb or foam), which
results in high specific mechanical properties, in particular, high flexural
strength and stiffness properties relative to the overall panel density.
Therefore, sandwich panels are efficient in carrying bending loads. Also, they
provide increase in buckling resistance to shear panels and compression
members. Sandwich construction results in lower lateral deformations, higher
buckling resistance and higher natural frequencies than monocoque
constructions. A sandwich structure operates in the same way with the
traditional I-beam, which has two flanges and a web connecting the flanges. The
connecting web makes it possible for the flanges to act together and resist
shear stresses. Sandwich structure and an I-beam differ from each other that,
in a sandwich composite the core and laminates are different materials and the
core provides continuous support for the laminates rather than being
concentrated in a narrow web. When the structure is subjected to bending, the
laminates act together, resisting the external bending moment so that one
laminate is loaded in compression and the other in tension. The core resists
transverse forces, at the same time, supports the laminates and stabilizes them
against buckling and wrinkling. 

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