JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013 - 51

offer gust load alleviation in wind turbine
blades.

angles and thick unitape which is 600
gsm, or 4 times heavier. Homogenization is achieved when the total laminate
consists of many identical sub-laminates.
The resulting laminate is stronger and
tougher than a laminate made of thick
unitape (at 150 gsm or higher) or thicker
sub-laminates with many more than 2
or 3 ply angles. There are 12 different
ways to stack a 4-ply-angle sub-laminate,
e.g. [0/90/45/-45], [0/45/90/-45],
etc. When the number of plies in a
sub-laminate increases further, there are
hundreds or thousands of stacking possibilities. Ply stacking is not optimized
in practice, so there is no rational way to
determine how optimum a composite
structure is if it is not homogenized. On
the other hand, topology optimization
of a homogenized composite structure
is practical. C-Ply TM is suited to offer
the best sub-laminate, due to its easily
achieved combinations of materials (carbon versus glass), ply thicknesses (thick
or thin combinations) and continuous
variable in the off-axis angles (from 20 to
30, and 45 and up).

Ply drop as it is practiced today is a black
art. The rules governing ply drops are
more concerned over the local stress
concentration as stipulated by the ply
thickness drop, the distance between
drops. Ply drop also requires the maintenance of mid-plane symmetry. Thus,
mid-plane symmetrical plies are dropped
simultaneously. We would recommend
that ply drop be treated as a topology
optimization issue. The same methodology for determining the tapering in
aluminium structures should be used.
Finally, strain allowable is treated incorrectly as a scalar quantity (like density,
temperature, voids, thickness and safety
factor). Although this simple number
is easy to accept, it is mathematically
wrong. Strain, like stress, is dependent
on the reference coordinates. It does not
remain constant as the coordinate axes
change. The safety factor is a scalar, and
thus more appropriate, measure; failure
strain or its allowable is not. Because of
anisotropy, composite laminates do not
behave like aluminium. Their Poisson’s
ratios can be larger than 0.5, and stress
and strain respond to loads in totally
different ways. It is a huge disservice to
express allowable strain by a single number irrespective of the applied load and
the ply combination of a laminate.

Why does C-PlyTM use
two ply angles?
C-Ply TM is a non-crimp fabric (NCF) developed by Chomarat. Among its unique
features are the thin plies of carbon fibres
made by spreading large tows into highquality plies before stitching. Each ply
weighs 75 grams per square meter (gsm),
equal to half the weight of conventional
unitape. Thinner plies are also possible.
C-Ply TM is unique in that it can have
2 plies, including one [0] ply and an
off-axis ply with an angle of either 20 to
30 degrees or 45 degrees or higher. In
the case of shallow angles (30 degrees or
less), two plies can be better matched to

Fig. 2: C-PLY™ Bi-Angle construction

take the applied load; i.e., the resulting
ply failures are closer to each other so the
first-ply-failure envelopes are higher than
in the case of a cross-ply laminate such
as [0/90], where one of the plies would
fail at a much lower applied load than
the other. So [0] and [90] plies have
totally different, and incompatible, loadcarrying capabilities that make them
mismatched plies. C-Ply TM has two better-matched ply angles. This matching is
particularly useful for combined bending
and twisting loads that are common in
wings, shafts and other beam-like structures. Shallow angles allow designing
stronger micro-crack-free laminates.

Homogenized laminates
The combination of two angles and thin
plies results in a 150 gsm sub-laminate,
as compared with the conventional 4

A homogenized laminate automatically
becomes symmetric. Sub-laminates can
be stacked continuously without concern
about the location of the mid-plane, and
about the mirror image of the stacking
below and above the mid-plane. Laminates become increasingly insensitive to
thermal warpage as the number of sublaminates increases. The residual curing
stress across the laminate is uniform in
all laminate orientation directions. The
in-plane and flexural stiffness is equal
and the governing equations of motion,
vibration and buckling are all much
simplified. Many closed-form solutions
are available. Confidence in analysis
and design thus increases. There is no
effect of stacking sequences and only ply
combinations matter, so that ply layups
with gaps and laps are less of a concern.
This makes composites less sensitive to
defects.
All these features make optimization
simple and achievable. These advantages
do not exist in the black aluminium

No78 January - February 2013

/ jec composites magazine 51
JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013

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