Reinforcing Fiber Tapes
By Captain J.R. Watson
Composites are a blend of resin (in
this case mixed epoxy) and reinforcing
fiber. Folks often ask, "How
strong are they?" It is difficult to answer
this question due to many variables
including resin type, fiber type,
fiber orientation, and resin/fiber ratio.
To give a value for a laminate, we
reduce the variables. Values shown in
this article were done with test samples
using WEST SYSTEM 105 Epoxy
Resin/206 Slow Hardener at room
temperature (70°F). Reinforcing fibers
are Episize materials. Laminates
made for the test had fibers oriented
in one direction (unidirectional)
and were laminated using simple
contact pressure yielding consistent
resin/fiber content (below).
Test samples were prepared with reinforced
ends (below) that could be
gripped by our MTS test machine.
Testing then exposed the samples to
tension and compression.
WEST SYSTEM unidirectional carbon
Carbon fiber is typically produced in
an inert environment at temperatures
above 1,800°F. The process of
producing carbon fiber accounts for
its high price and classification as an
exotic fiber. Carbon fibers are
known for their light weight, high
strength, and high stiffness.
702 and 703 Unidirectional Carbon
Tapes are designed to be used with
WEST SYSTEM Brand epoxy. The
hardener choice is dependent on
laminate thickness (dimensionally),
the time required to perform the operation,
and ambient temperature
when cure will take place.
702 and 703 carbon fibers are in
tow form. A tow is like a ribbon.
The tow of this material consists of
12,000 fiber bundles per inch of
width, so it is referred to as 12K material.
The tow is continuous, and
unidirectional on the '0' axis (along
the tow length). The fibers are not
twisted or woven. The white strands
running on the 90 (degree) axis are
polyester threads that hold the fiber
bundles in place and are non-structural.
This 12K material is on the order
of 11 oz (per square yard) material
and measures .025" thick. Carbon
fibers are electrically conductive
and have a specific gravity of 1.75.
When applying carbon fiber, assess
the anticipated load path and orient
the fibers in that direction. For example,
a vertical flag pole is a cantilevered
structure which is subjected
to bending forces. Fiber orientation
would be along the pole's length to
stiffen it. Carbon fibers are effective
in compression or tension, but it's
best to apply fibers equally to both
sides of a structure to maintain balance.
Carbon fibers can be introduced
into wooden laminates, to augment between
laminates, but they are most effective placed furthest
from the neutral axis.
Off-axis plies result in a rapid loss of
strength and stiffness. A depression
in a lay up (such as a wrinkle or
bump) will attract forces and jeopardize
the laminate. If off-axis fibers
cross one another, it is best if the
laminate is resin rich (on the order
of 50%). A resin rich laminate helps
keep the fibers from contacting each
other, which we don't want.
Wet the substrate with epoxy. It's
best to place the first layer of carbon
fiber into the coating while that
coating is still wet as it will transfer
best to the reinforcing fiber. Keep
the fibers straight (in columns) as
best you can.
Locking sheet metal pliers with
sandpaper adhered to the flange allows
you to pre-tension as well as
align the fibers precisely. When
working with long objects, such as
spars, we will cut our tape to length
and attach the grips to both ends so
the tape can be pulled taut and then
lowered into position (below).
Once they are in place, wet the fibers
with epoxy using a brush and
squeegee. Hold the tool at a low angle
and thoroughly wet the carbon
fiber. If the laminate requires more
layers, continue to apply layers, wetting
the fibers as you proceed. As a
rule, don't apply more than six layers
at a time or you may experience
excessive exothermic heat from the
epoxy as it cures.
A method to determine how thick a
laminate will be is to take a number
of laminations (say four), place a
coin on the top and bottom of the
dry laminate stack and press lightly
together with thumb and index finger.
Offset the coins slightly (below).
Pass a sewing needle through the
stack until it touches the coin. Place
a piece of tape on the needle on the
other side of the laminate so you can
measure the dimension.
When all layers are applied, a good
approach is to place release fabric or
polyethylene plastic over the last
layer and squeegee firmly to remove
excess resin and trapped air bubbles.
Leave in place until the laminate
cures. Remove the release fabric or
plastic by pulling it back on itself as
close to 180° as possible. Pull
steadily and avoid jerking as you
peel this film from the laminate.
Cured mechanical properties of
and 703 Unidirectional Carbon
Tapes with WEST SYSTEM 105 Resin
and 206 Hardener at a 60/40
resin/fiber content are as follows:
Modulus(tensile) 13.9x106 psi
(compressive) 9.5x106 psi
Tensile strength 200,000 psi
702 (1.5" wide tow) 7,500 lb
703 (3" wide tow) 15,000 lb
Compressive strength 64,000 psi
Poisson's ratio1 .35
WEST SYSTEM unidirectional glass tape
713 Unidirectional Glass Tape is
electrical grade fiberglass referred to
as E glass. 713 fibers in tape form
are continuous, unidirectional on the -
axis. They are not twisted or woven.
The white strands running on
the 90 axis are polyester fill threads
that hold the fiber bundles in place
and are non-structural. This material
is on the order of 11 oz per square
yard, measures .018 inches in thickness,
and has a specific gravity of
You would apply fiberglass tape in
the same manner as described above
for carbon fiber.
Cured mechanical properties of 713
Unidirectional Glass Tape with
WEST SYSTEM epoxy at a 60/40
resin/fiber content are as follows:
Modulus (tensile) 3.7x106 psi
Tensile strength 78,000 psi
Poisson's ratio 1 .35
WEST SYSTEM woven fabric tapes
Woven Fabric Tapes is electrical
grade fiberglass that come in various
widths: 729 (2"), 731 (3"), 732 (4")
and 733 (6"). They weigh 9 oz per
square yard. These woven tapes have
a selvaged edge that prevents fraying.
Tapes are ideal to toughen and protect
corners as well as for a myriad of
other applications. Corners must first
be rounded about like the radius of a
dime (minimum) because the tape
will not make the turn of a sharp corner.
I take a short sample of the tape,
fold it in half to find the center and
then mark it. I center the mark over
the apex of the corner. I then mark
where the edges lie (below).
Now, I'll take a combination square
and mark the two edges along the
entire length of the corner (above). I
wet the substrate between the marks
with resin/hardener mix along the
entire length. Then I roll the tape,
placing the edge on the mark and
smooth it down. Usually, I'll place
just one surface (the easiest) down
first. Once its entire length is in
place, I press the remainder down.
As the glass tape is negotiating the
corner, it may not want to stay put.
I've found if I just let it set for half
an hour, the epoxy gets stickier as it
begins to cure and then the glass
stays down. I'll re-wet the glass tape
as required (it will be white if it is
not wetted properly).
When covering the corner of an arc
shape, the edge may still not stay
down. In these instances, I'll slit the
edge, producing a dart, until it cooperates.
I'll wait a few more hours
and then apply another coat to fill
the weave of the tape as well as to
inspect for wrinkles or bubbles
which can be resolved at this time.
Sand the cured tape with 50-grit
sandpaper on a sanding block to
feather the selvaged edge. If the edge
has got to blend, thicken a mixture
of epoxy with 410 Microlight and
apply it with a wide putty knife.
WEST SYSTEM biaxial tape
727 Biaxial Tape is 4" wide, 15 oz
(per square yard) electrical grade fiberglass
referred to as E glass. Biaxial
is essentially two layers of unidirectional
fiberglass placed 45° to
the '0' axis off the roll. The layers
are held together with a light stitching,
thus it is referred to as a stitched
tape. It should not be confused with
braid, which is woven. Biaxial has
good utility as the strands are oriented
to allow the tape to pass over
a tighter corner. Biaxial is generally
stronger on a per thickness basis
than a woven fabric. 727 is .018" per
tow in thickness and has a specific
gravity of 2.54. Woven tape has half
its fibers taking the full brunt of the
bend whereas with biaxial they pass
as a more gradual 45° angle. However,
Biaxial materials will take more
filling with resin or fillers to finish
Poisson ratio is the ratio of the transverse contracting strain to the elongation strain when a
rod is stretched by forces which are applied at
its ends and which are parallel to the rod's axis.
Epoxyworks 27 / Fall 2008
Copyright © 2008, Gougeon Brothers, Inc. All rights reserved.
Reproduction in any form, in whole or in part, is expressly forbidden without the consent of the publisher. EPOXYWORKS, Gougeon Brothers, WEST SYSTEM, Episize, Scarffer and Microlight as used throughout this publication, are trademarks of Gougeon Brothers, Inc., Bay City, Michigan, USA.