by Jeff Wright
The Technical Staff at Gougeon
Brothers, Inc. regularly discuss material
properties in a variety of applications.
For example, it is not uncommon
for us to discuss with a customer
how to use carbon fiber to
stiffen a structure, such as the shaft
of a kayak paddle, and then within
minutes discuss with another how to
bond a dimensionally unstable
wood, such as oak, and ensure precautions
are taken so that the relative
movement of the wood will not
cause a failure.
For the kayak paddle, the customer's
concern is that the epoxy will allow
the stiff carbon fiber fabric to be as rigid as possible. For bonding the
unstable wood, the customer would like an epoxy that will be able to
elongate more than the wood to ensure that the movement of the substrates
will not fracture the epoxy. These examples highlight people's
interest in the flexibility of the epoxy.
However, this interest also often
leads them into the world of material
properties, a field they know
little about. Material properties form
a complicated web whose threads include
not only flexibility but also
elongation, stress, creep, ultimate
properties, yield properties and
We have previously discussed how
the flexibility of West System® 105
Resin mixed with 205, 206, 207, or
209 Hardener makes it very well
suited for use with common
boatbuilding materials (see
Epoxyworks 16). The many boats
that have been successfully built using
these 105 Resin-based epoxies
with wood, carbon fiber, and fiberglass
illustrate this epoxy's excellent
properties. However, since we have
introduced a new West System
product—G/flex® Epoxy—that has
increased flexibility, this is a good
time to review the relationship of
flexibility to other material properties.
The stiffness of a material is often
confused with its strength and
toughness. Stiffness, or modulus, is
simply a measurement of the ability
to resist elongating when a load is
applied. Stiffness and strength are
separate properties. Strength is the
amount of stress a material can withstand
before it fails. Strength and
stiffness cross paths when a material
exceeds its yield strength, leaves the
elastic region, and enters a plastic region.
When a material is in its elastic region,
it will always "return" to its
original state after stress is removed.
For example, when a wooden or fiberglass
boat is taken out for a ride
during the day, its hull deflects each
time it hits a wave, but when the
boat comes back to the dock, it has
the same shape as when it left. The
hull spent its entire day in the elastic
region, and it always snaps back to
where it started. In contrast, if you
see an old steel freighter with its
steel shell dented in around the
frames, the steel has been put into its
plastic region (beyond its yield
strength) and experienced enough
stress that the hull shell could not return
to its original shape.
Since wood and most composite materials,
such as fiberglass and carbon
fiber, are not appropriate for use in
the plastic region beyond their yield
strength, we did not formulate our
WEST SYSTEM epoxies to work outside
the elastic region. Instead, WEST
SYSTEM epoxies were formulated to
enhance the performance of wood
and composite materials under
real-world operating conditions. We
have spent over 30 years optimizing
WEST SYSTEM epoxies. Even when
competitors criticized our product
for not having the ability to operate
in the plastic region, we knew that
the elongation of WEST SYSTEM epoxy
was ideally suited for common
boat building materials. Simply making
the epoxy more flexible would
have reduced the overall performance
of the structures that were
laminated and bonded with it.
The concern about a resin's flexibility
may come from an inaccurate
perception that low elongation numbers
imply brittleness. Most production
boats are built using polyester
resins. Due to the brittle nature of
polyester resins, these boats often
have problems with the laminate
cracking. WEST SYSTEM 105
Resin-based epoxy is much tougher
than polyester resins, which makes it more resistant to cracking. As an engineer,
I am jealous that the makers
of pickup trucks can boast about the
stiffness of their truck frames without
raising any concern about brittleness.
G/flex vs. polyurethane
When something is bent, it must
elongate a certain amount. Since
joints can fail when they are flexed,
a high-elongation material would appear
to be a good choice for a strong
joint. Many of the polyurethane adhesive/
sealant materials advertise
elongations of +100%, and they
have often been used, inappropriately,
as bonding adhesives. Sealants
are designed to withstand the movement
in a gasketed joint, for example
between a bronze through-hull
fitting and a hull bottom. The sealant
has to deal with the relative
movement as the hull flexes around
the rigid thru-hull fitting or when a
careless mechanic steps on the
through-hull, also causing it to deflect.
Since the film of sealant may
be less than 1/32" thick, a 1/8" movement
would require 200% elongation.
The sealant only needs to adhere
well enough to stay in contact
with the through-hull fitting and the
hull bottom; the backing nut is keeping
the through-hull in place, not the
sealant. Although polyurethane adhesive/
sealants are great materials
when used as intended, these products
do not have nearly the strength
Increased flexibility and elongation
Our newest product, G/flex®, has utilized
a new chemistry that provides a
very tough epoxy with greater flexibility
and elongation than West
System® 105 Resin-based epoxies.
The need for a higher elongation material
was not driven by any shortcomings
with 105 Resin combination's.
Instead, G/flex was developed
for applications for which no products
are currently available. These applications
include substrates that are
prone to excessive flexing,
hard-to-bond tropical woods, flexible
laminates, and bonding to wet substrates.
G/flex is an extremely tough
epoxy that is more flexible than 105
Resin-based epoxies without sacrificing
We do not recommend G/flex for
building a stitch-and-glue kayak. In
this case, 105 Resin-based epoxy is a
perfect choice since the plywood will
never exceed the elongation of the
epoxy. However, G/flex will better
match the modulus of flexible substrates,
such as thin sheets of aluminum
or even canvas that need to be
G/flex also has the ability to
stretch after its ultimate strength has
been exceeded, which allows G/flex
to elongate beyond its elastic range.
This can provide a safety factor for
some bonding situations.
G/flex: resistance to creep stress
While we are proud that we developed
an epoxy with more elongation,high strength, and excellent adhesion,
G/flex's resistance to creep
rupture is possibly the most notable
achievement. Engineers typically use
dimensions, strength properties, and
predicted loads to calculate the stress
that an assembled joint will be subjected
to in service.
duration of the load also needs to be
considered. Structural engineers who
are Star Trek fans refer to this as the
"Stress/Time Continuum." It is more
often referred to as creep stress.
Creep stress is a material property
that describes how a material resists
the stress of a constant long-term
load. Creep stress problems can be
evident when something simply does
not hold its shape, such as the roof
on my 100-year-old home that has
sagged from its original shape.
Creep stress can also result in a complete
failure, such as when a highly
stressed glue joint at the stem of a
boat fails months after the glue has
fully cured when the boat is moored
at the dock. In these cases, it can be
said that the material has flowed or
"crept" due to constant stress. Creep
rupture often occurs at a stress below
the material's ultimate strength.
This is why creep loads are built into
design safety factors. Creep stress is
common in stitch-and-glue boats after
the plywood has been bent into
the desired shape and also in scarf
joints in boards that are constantly
Testing the creep characteristics of
105 Resin-based epoxies
Fortunately for users, our West
System 105 Resin-based epoxies are
formulated to be resistant to creep
stress. Our understanding of creep
stress spans over 20 years, including
studies of West System 105 epoxy-
bonded fasteners in 70' wind
turbine blades exposed to constant
high loads created by the centrifugal
force of the spinning blades. Over
the years, we have used several
methods to test the creep characteristics
of our epoxy. All the testing
was done in a 95°F environment to
accurately reflect the environment
that many epoxy joints will experience
in a boat's cabin or bilge.
Our first generation testing method
was the Notched Beam Test, which is
illustrated in Figure 1. The Notched
Beam Test applies bending stress to a
notch cut across a wooden beam that
has been filled with the adhesive that
is being tested. When the correct
load is applied to the beam, the resulting
bending moment will cause a
creep rupture. Too small a load results
in an infinitely long test. Too
much weight results in a sudden ultimate
strength failure. (For details on
conducting your own notched-beam
testing ask for 000-815 Notched-
Beam Test for Creep-Rupture.
The Notched Beam Test provided
very valuable data but had shortcomings.
The main one was that it took a
great deal of time to generate a substantial
set of data points. In the
1990s, we developed the next generation
creep test, affectionately referred
to as the Drip and Grip. The
test sample was constructed with
two ¾"x¾"x3" wood laminate coupons
that were bonded with a butt
joint using an 1
8" glue line. This test
utilized a tensile load that was increased
very slowly over the span of
12 to 24 hours. Results from the
Drip and Grip and the Notched Beam Test showed good correlation.
This new test provided a data point
every 24 hours.
Our current generation of creep test
machine has been in use for 5 years.
It uses the same concept as the Drip
and Grip, but the load is applied
pneumatically and data acquisition is
computerized. We can now test multiple
specimens at the same time
with different load rates and capture
data throughout the test. Recording
the data every minute of a 24-hour
test allows the engineer to have a full
record of stress, load, and temperature.
This ensures that any problems
during the test are easily recognized.
Figure 3 is a photograph of our current
creep stress test machine.
Gougeon Brothers, Inc. has developed
a large database of creep stress
performance of our epoxy, competitive
epoxies, and popular urethanes.
Constantly testing for creep stress
performance allows us to provide our
customers with products that perform
well under long duration loading.
Testing has also demonstrated
the risk of adding chemicals to the
epoxy that make it more flexible.
Testing also shows the poor creep
performance of common urethane.
Since many flexible adhesives have
poor creep performance, the challenge
has been to develop a formulation
which has more flexibility than
our 105 Resin-based epoxies without
a significant loss in creep stress performance.
Our chemists met that
challenge with the development of
our new G/flex Epoxy. G/flex offers
a higher degree of flexibility without
a significant loss in creep stress performance.
This is a significant breakthrough
in adhesive performance.
G/flex provides long-term performance
that common one-part elastic
urethane adhesive/sealants can not
Remember that the performance
of an adhesive joint can be
greatly improved by avoiding peel
loads, having the proper adhesive
thickness, and ensuring there is
enough surface area.
Stiffness of Neat Epoxies vs. Laminate
G/flex can be used to bond many different materials, but it can also be used to
create a more flexible fiberglass laminate. This is useful in unique applications
that must bend more than a typical fiberglass laminate, such as a scupper flapor
repairing a sheet-molded ATV fender. Although G/flex is not intended as a laminating
system, it is useful for small projects that need to be a bit more flexible.
This chart shows the Relative Stiffness of neat, or unmodified, West SYSTEM
105/205 and G/flex, and their respective fiberglass laminates. The stiffness of
High Density Polyethylene (HDPE) is shown as a comparison.
Copyright 2002, Gougeon Brothers, Inc. All rights reserved.
Last Modified on 10/28/02.
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.