Transcript Document 7175827

Lecture 2
SANDWICH COMPOSITES
Fig.1. A typical sandwich structure which consists of a core bonded
in between two faceplates using adhesive [1].
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.
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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.
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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
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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).
Design aspect
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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.
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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).
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Design aspect
Compressive modulus of the core and
facings should be sufficient to prevent
wrinkling of the faces under design load
(Fig.4).
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Design aspect
The core cells should be small enough to
prevent intracell dimpling of the faceplates
under design load.
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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
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Design aspect
The overall structure should have sufficient
flexural and shear rigidity to avoid excessive
deflections under design load
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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, fibrereinforced 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.
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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
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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
Lecture 2
PART III
SANDWICH COMPOSITES