JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013 - 50

laminate

How C-PlyTM can change the way
we design and manufacture
C-PlyTM is a recent patent pending invention by Stanford
University and Chomarat that offers significant weight
and cost reductions.

By
Stephen W. Tsai, Professor Research Emeritus,
Aeronautics & Astronautics, Stanford University
Michel Cognet, Chomarat Group Managing
Director

arbon/epoxy laminates are significantly stronger and stiffer than aluminium, but offer only 20% weight
savings. There is definitely something
wrong in the current practice. Once
we move beyond the traditional black
aluminium design rules, new design and
manufacturing concepts will be possible.

Black aluminium
When advanced composites emerged
out of the discovery of boron and then
carbon fibre in the 1960s, only designers
who had experience with aluminium
aircraft were called upon to design composites. Concepts like anisotropy, lamination, micromechanics and brand-new
manufacturing processes were foreign
to them. It was their natural instinct to
make composites behave as much like
aluminium as possible. Many rules were
established then, and they have dominated composite design for half a century.
Examples include the use of unitape
as the building block; the use of four
ply angles (0, ±45, and 90); symmetry
with respect to the mid-plane (to avoid
thermal warpage); balanced laminates
(to avoid anisotropy); the 10% rule

(each of 4 angles must be 10% of the ply
combinations to cover diversified loads);
ply drop rules (to achieve tapering); and
the strain allowable (of 0.4%, analogous
to that for aluminium at 0.8%).
All these rules may make composites
similar to aluminium, but they generate
additional costs. If 90-degree plies are
not needed, they should be left out. This
extra ply not only adds extra weight and
cost, but also an additional failure mode.
It also makes manufacturing difficult
because it causes bridging, i.e. it refuses
to bend in sharp corners. The requirement for symmetrical laminates is
not without cost,
either. While only
one ply would be
required, two need
to be added to
maintain the symmetry. Balanced
laminates are
needed only when
twisting moments
are fully reversible.
An unbalanced
laminate is anisotropic, which facifig. 1: C-PLY™
litates bend-twist

50 jec composites magazine / No78 January - February 2013

coupling. This unique behaviour does
not exist in a balanced laminate.
If a laminate is built from many repeated sub-laminates, many of these rules
are no longer necessary. The laminate
automatically becomes homogenized
and thus symmetrical, depending only
on ply combinations and not on stacking
sequence or permutation. Moreover,
such a laminate is structurally simple
to understand and easy to calculate.
Anisotropy should not be prohibited (by
permitting the use of balanced laminates
only), because bend-twist coupling can

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

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #78 - January/February 2013