| This page
will attempt to shed some light on the mechanics of composite sandwich
panels. The main points will illustrate: |
-the structure of a 'Composite sandwich panel
-the role of the skin and core in resisting loads
-the distribution of forces in such panel (bending only)
-why composite sandwich cores are stiffer and stronger than the same weight single skin panels.
-examples of different kinds of panels |
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This
page is intended for those who do not have any engineering background but
would still like to get a peek at the inner workings of sandwich cores.
Derivations of Flexure formulas, Moments of Inertia and other
complexities are beyond the scope of this page. A lot of assumptions are
made for the sake of simplicity. All cross sections are symmetrical about
a neutral axis(centriod) and the material is subject to pure
bending(flexure). |
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The best way to visualize the
structure of a 'sandwich core panel' is to use the analogy of a simple "I"
beam. (see above)
Like the 'I' beam, a sandwich core panel
consists of strong skins (flanges) bonded to a core (web).
The skins are subject to tension/compression and are largely responsible
for the strength of the 'sandwich'. The function of the core
is to support the thin skins so that they don't buckle (deform) and stay
fixed relative to each other. The core experiences mostly shear stresses
(sliding) as well as some degree of vertical tension and compression. Its
material properties and thickness determine the stiffness of
such a panel.
Unlike
the simple beam, which is designed to withstand stresses mostly along the
x axis and bending about the y axis, the sandwich panel can be stressed
along and about any axis laying in the x-y plane. The implication
is that such panel can extend 'infinitely', forming a strong and
continuous self-sustaining plate or shell such as a wood strip kayak. No
reinforcing elements are needed because they are already built into the
structure.
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The
Core |
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The easiest way to illustrate how
the core supports shear stresses is to take a deck of cards or a telephone
book and bend it. You will notice how the individual layers slide or
'shear' past each other.
Now, suppose that the sheets were all
glued together. The pages are no longer free to move and the deck becomes
very stiff. At this point, the only way the deck could bend is if the
layers on the 'tension' side of the 'neutral axis' (red dashed line)
stretched and the 'compressed' side squeezed
together. |
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This picture illustrates the shear in a weak core such as the
unglued deck of cards or a sheet of elastic material like rubber.
The
skins experience very little stress because the core deforms easily. Such
cores are said to have low 'Shear Modulus of
Elasticity' |
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Materials
with very low Shear Modulus are unsuitable as structural cores
because they cannot withstand shear stress. Boats made with such cores
would be weak, excessively flexible, and easily deformed. This would defy
the whole point of this construction. |
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The core in this illustration would be the equivalent of the deck
of cards glued together. The material resists shear (high Shear Modulus)
very well. Note that the sections throughout the core are perpendicular to
the neutral axis (dashed red line).
This means that the 'layers' in the
core resists sliding (shear deformation) and the core and skins are forced
to stretch and compress. |
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Skins made of material of high 'Modulus of
Elasticity' are best used in conjunction with cores of high
'Shear Modulus'. This balance is important so that neither material
fails long before the other is stressed to acceptable level.
For
instance, strong Graphite or Kevlar skins bonded to a 'Styrofoam
insulation' core would be a complete waste because such 'Low Shear
Modulus' core would always fail long before the skin could be stressed to
1% of its potential strength. Of course, for this reason Styrofoam is not
considered a structural core material. |
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Production line
Single Board 
DLF-6 technical instructions
