SANDWICH COMPOSITES

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Lecture 2

• SANDWICH COMPOSITES

Fig.1. A typical sandwich structure which
consists of a core bonded in between two
faceplates using adhesive 1.
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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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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.

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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.

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Lecture 2
Design aspect
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).
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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.
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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).
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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).
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Lecture 2
Design aspect
The core cells should be small enough to prevent
intracell dimpling of the faceplates under design
load.
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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
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Lecture 2
Design aspect
The overall structure should have sufficient
flexural and shear rigidity to avoid excessive
deflections under design load
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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.

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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

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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

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Lecture 2

• PART III
• SANDWICH COMPOSITES